diff --git a/reftable/LICENSE b/reftable/LICENSE
new file mode 100644
index 0000000000..402e0f9356
--- /dev/null
+++ b/reftable/LICENSE
@@ -0,0 +1,31 @@
+BSD License
+
+Copyright (c) 2020, Google LLC
+All rights reserved.
+
+Redistribution and use in source and binary forms, with or without
+modification, are permitted provided that the following conditions are
+met:
+
+* Redistributions of source code must retain the above copyright notice,
+this list of conditions and the following disclaimer.
+
+* Redistributions in binary form must reproduce the above copyright
+notice, this list of conditions and the following disclaimer in the
+documentation and/or other materials provided with the distribution.
+
+* Neither the name of Google LLC nor the names of its contributors may
+be used to endorse or promote products derived from this software
+without specific prior written permission.
+
+THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+"AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+(INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
diff --git a/reftable/README.md b/reftable/README.md
new file mode 100644
index 0000000000..1c52d55833
--- /dev/null
+++ b/reftable/README.md
@@ -0,0 +1,17 @@
+
+The source code in this directory comes from https://github.com/google/reftable.
+
+The VERSION file keeps track of the current version of the reftable library.
+
+To update the library, do:
+
+   ((cd reftable-repo && git fetch origin && git checkout origin/master ) ||
+    git clone https://github.com/google/reftable reftable-repo) && \
+   cp reftable-repo/c/*.[ch] reftable/ && \
+   cp reftable-repo/LICENSE reftable/ &&
+   git --git-dir reftable-repo/.git show --no-patch origin/master \
+    > reftable/VERSION && \
+   echo '/* empty */' > reftable/config.h
+
+Bugfixes should be accompanied by a test and applied to upstream project at
+https://github.com/google/reftable.
diff --git a/reftable/VERSION b/reftable/VERSION
new file mode 100644
index 0000000000..627f9c49f6
--- /dev/null
+++ b/reftable/VERSION
@@ -0,0 +1,5 @@
+commit c616d53b88657c3a5fe4d2e7243a48effc34c626
+Author: Han-Wen Nienhuys <hanwen@google.com>
+Date:   Mon Jan 27 15:05:43 2020 +0100
+
+    C: ban // comments
diff --git a/reftable/basics.c b/reftable/basics.c
new file mode 100644
index 0000000000..791dcc867a
--- /dev/null
+++ b/reftable/basics.c
@@ -0,0 +1,196 @@
+/*
+Copyright 2020 Google LLC
+
+Use of this source code is governed by a BSD-style
+license that can be found in the LICENSE file or at
+https://developers.google.com/open-source/licenses/bsd
+*/
+
+#include "basics.h"
+
+#include "system.h"
+
+void put_u24(byte *out, uint32_t i)
+{
+	out[0] = (byte)((i >> 16) & 0xff);
+	out[1] = (byte)((i >> 8) & 0xff);
+	out[2] = (byte)((i)&0xff);
+}
+
+uint32_t get_u24(byte *in)
+{
+	return (uint32_t)(in[0]) << 16 | (uint32_t)(in[1]) << 8 |
+	       (uint32_t)(in[2]);
+}
+
+void put_u32(byte *out, uint32_t i)
+{
+	out[0] = (byte)((i >> 24) & 0xff);
+	out[1] = (byte)((i >> 16) & 0xff);
+	out[2] = (byte)((i >> 8) & 0xff);
+	out[3] = (byte)((i)&0xff);
+}
+
+uint32_t get_u32(byte *in)
+{
+	return (uint32_t)(in[0]) << 24 | (uint32_t)(in[1]) << 16 |
+	       (uint32_t)(in[2]) << 8 | (uint32_t)(in[3]);
+}
+
+void put_u64(byte *out, uint64_t v)
+{
+	int i = 0;
+	for (i = sizeof(uint64_t); i--;) {
+		out[i] = (byte)(v & 0xff);
+		v >>= 8;
+	}
+}
+
+uint64_t get_u64(byte *out)
+{
+	uint64_t v = 0;
+	int i = 0;
+	for (i = 0; i < sizeof(uint64_t); i++) {
+		v = (v << 8) | (byte)(out[i] & 0xff);
+	}
+	return v;
+}
+
+void put_u16(byte *out, uint16_t i)
+{
+	out[0] = (byte)((i >> 8) & 0xff);
+	out[1] = (byte)((i)&0xff);
+}
+
+uint16_t get_u16(byte *in)
+{
+	return (uint32_t)(in[0]) << 8 | (uint32_t)(in[1]);
+}
+
+/*
+  find smallest index i in [0, sz) at which f(i) is true, assuming
+  that f is ascending. Return sz if f(i) is false for all indices.
+*/
+int binsearch(int sz, int (*f)(int k, void *args), void *args)
+{
+	int lo = 0;
+	int hi = sz;
+
+	/* invariant: (hi == sz) || f(hi) == true
+	   (lo == 0 && f(0) == true) || fi(lo) == false
+	 */
+	while (hi - lo > 1) {
+		int mid = lo + (hi - lo) / 2;
+
+		int val = f(mid, args);
+		if (val) {
+			hi = mid;
+		} else {
+			lo = mid;
+		}
+	}
+
+	if (lo == 0) {
+		if (f(0, args)) {
+			return 0;
+		} else {
+			return 1;
+		}
+	}
+
+	return hi;
+}
+
+void free_names(char **a)
+{
+	char **p = a;
+	if (p == NULL) {
+		return;
+	}
+	while (*p) {
+		free(*p);
+		p++;
+	}
+	free(a);
+}
+
+int names_length(char **names)
+{
+	int len = 0;
+	for (char **p = names; *p; p++) {
+		len++;
+	}
+	return len;
+}
+
+/* parse a newline separated list of names. Empty names are discarded. */
+void parse_names(char *buf, int size, char ***namesp)
+{
+	char **names = NULL;
+	int names_cap = 0;
+	int names_len = 0;
+
+	char *p = buf;
+	char *end = buf + size;
+	while (p < end) {
+		char *next = strchr(p, '\n');
+		if (next != NULL) {
+			*next = 0;
+		} else {
+			next = end;
+		}
+		if (p < next) {
+			if (names_len == names_cap) {
+				names_cap = 2 * names_cap + 1;
+				names = realloc(names,
+						names_cap * sizeof(char *));
+			}
+			names[names_len++] = strdup(p);
+		}
+		p = next + 1;
+	}
+
+	if (names_len == names_cap) {
+		names_cap = 2 * names_cap + 1;
+		names = realloc(names, names_cap * sizeof(char *));
+	}
+
+	names[names_len] = NULL;
+	*namesp = names;
+}
+
+int names_equal(char **a, char **b)
+{
+	while (*a && *b) {
+		if (strcmp(*a, *b)) {
+			return 0;
+		}
+
+		a++;
+		b++;
+	}
+
+	return *a == *b;
+}
+
+const char *error_str(int err)
+{
+	switch (err) {
+	case IO_ERROR:
+		return "I/O error";
+	case FORMAT_ERROR:
+		return "FORMAT_ERROR";
+	case NOT_EXIST_ERROR:
+		return "NOT_EXIST_ERROR";
+	case LOCK_ERROR:
+		return "LOCK_ERROR";
+	case API_ERROR:
+		return "API_ERROR";
+	case ZLIB_ERROR:
+		return "ZLIB_ERROR";
+	case -1:
+		return "general error";
+	default:
+		return "unknown error code";
+	}
+}
diff --git a/reftable/basics.h b/reftable/basics.h
new file mode 100644
index 0000000000..e2115bb879
--- /dev/null
+++ b/reftable/basics.h
@@ -0,0 +1,38 @@
+/*
+Copyright 2020 Google LLC
+
+Use of this source code is governed by a BSD-style
+license that can be found in the LICENSE file or at
+https://developers.google.com/open-source/licenses/bsd
+*/
+
+#ifndef BASICS_H
+#define BASICS_H
+
+#include "system.h"
+
+#include "reftable.h"
+
+#define true 1
+#define false 0
+#define ARRAYSIZE(a) sizeof(a) / sizeof(a[0])
+
+void put_u24(byte *out, uint32_t i);
+uint32_t get_u24(byte *in);
+
+uint64_t get_u64(byte *in);
+void put_u64(byte *out, uint64_t i);
+
+void put_u32(byte *out, uint32_t i);
+uint32_t get_u32(byte *in);
+
+void put_u16(byte *out, uint16_t i);
+uint16_t get_u16(byte *in);
+int binsearch(int sz, int (*f)(int k, void *args), void *args);
+
+void free_names(char **a);
+void parse_names(char *buf, int size, char ***namesp);
+int names_equal(char **a, char **b);
+int names_length(char **names);
+
+#endif
diff --git a/reftable/block.c b/reftable/block.c
new file mode 100644
index 0000000000..9fe4f8a080
--- /dev/null
+++ b/reftable/block.c
@@ -0,0 +1,401 @@
+/*
+Copyright 2020 Google LLC
+
+Use of this source code is governed by a BSD-style
+license that can be found in the LICENSE file or at
+https://developers.google.com/open-source/licenses/bsd
+*/
+
+#include "block.h"
+
+#include "system.h"
+
+#include "blocksource.h"
+#include "constants.h"
+#include "record.h"
+#include "reftable.h"
+#include "zlib.h"
+
+int block_writer_register_restart(struct block_writer *w, int n, bool restart,
+				  struct slice key);
+
+void block_writer_init(struct block_writer *bw, byte typ, byte *buf,
+		       uint32_t block_size, uint32_t header_off, int hash_size)
+{
+	bw->buf = buf;
+	bw->hash_size = hash_size;
+	bw->block_size = block_size;
+	bw->header_off = header_off;
+	bw->buf[header_off] = typ;
+	bw->next = header_off + 4;
+	bw->restart_interval = 16;
+	bw->entries = 0;
+}
+
+byte block_writer_type(struct block_writer *bw)
+{
+	return bw->buf[bw->header_off];
+}
+
+/* adds the record to the block. Returns -1 if it does not fit, 0 on
+   success */
+int block_writer_add(struct block_writer *w, struct record rec)
+{
+	struct slice empty = {};
+	struct slice last = w->entries % w->restart_interval == 0 ? empty :
+								    w->last_key;
+	struct slice out = {
+		.buf = w->buf + w->next,
+		.len = w->block_size - w->next,
+	};
+
+	struct slice start = out;
+
+	bool restart = false;
+	struct slice key = {};
+	int n = 0;
+
+	record_key(rec, &key);
+	n = encode_key(&restart, out, last, key, record_val_type(rec));
+	if (n < 0) {
+		goto err;
+	}
+	out.buf += n;
+	out.len -= n;
+
+	n = record_encode(rec, out, w->hash_size);
+	if (n < 0) {
+		goto err;
+	}
+
+	out.buf += n;
+	out.len -= n;
+
+	if (block_writer_register_restart(w, start.len - out.len, restart,
+					  key) < 0) {
+		goto err;
+	}
+
+	free(slice_yield(&key));
+	return 0;
+
+err:
+	free(slice_yield(&key));
+	return -1;
+}
+
+int block_writer_register_restart(struct block_writer *w, int n, bool restart,
+				  struct slice key)
+{
+	int rlen = w->restart_len;
+	if (rlen >= MAX_RESTARTS) {
+		restart = false;
+	}
+
+	if (restart) {
+		rlen++;
+	}
+	if (2 + 3 * rlen + n > w->block_size - w->next) {
+		return -1;
+	}
+	if (restart) {
+		if (w->restart_len == w->restart_cap) {
+			w->restart_cap = w->restart_cap * 2 + 1;
+			w->restarts = realloc(
+				w->restarts, sizeof(uint32_t) * w->restart_cap);
+		}
+
+		w->restarts[w->restart_len++] = w->next;
+	}
+
+	w->next += n;
+	slice_copy(&w->last_key, key);
+	w->entries++;
+	return 0;
+}
+
+int block_writer_finish(struct block_writer *w)
+{
+	int i = 0;
+	for (i = 0; i < w->restart_len; i++) {
+		put_u24(w->buf + w->next, w->restarts[i]);
+		w->next += 3;
+	}
+
+	put_u16(w->buf + w->next, w->restart_len);
+	w->next += 2;
+	put_u24(w->buf + 1 + w->header_off, w->next);
+
+	if (block_writer_type(w) == BLOCK_TYPE_LOG) {
+		int block_header_skip = 4 + w->header_off;
+		struct slice compressed = {};
+		uLongf dest_len = 0, src_len = 0;
+		slice_resize(&compressed, w->next - block_header_skip);
+
+		dest_len = compressed.len;
+		src_len = w->next - block_header_skip;
+
+		if (Z_OK != compress2(compressed.buf, &dest_len,
+				      w->buf + block_header_skip, src_len, 9)) {
+			free(slice_yield(&compressed));
+			return ZLIB_ERROR;
+		}
+		memcpy(w->buf + block_header_skip, compressed.buf, dest_len);
+		w->next = dest_len + block_header_skip;
+	}
+	return w->next;
+}
+
+byte block_reader_type(struct block_reader *r)
+{
+	return r->block.data[r->header_off];
+}
+
+int block_reader_init(struct block_reader *br, struct block *block,
+		      uint32_t header_off, uint32_t table_block_size,
+		      int hash_size)
+{
+	uint32_t full_block_size = table_block_size;
+	byte typ = block->data[header_off];
+	uint32_t sz = get_u24(block->data + header_off + 1);
+
+	if (!is_block_type(typ)) {
+		return FORMAT_ERROR;
+	}
+
+	if (typ == BLOCK_TYPE_LOG) {
+		struct slice uncompressed = {};
+		int block_header_skip = 4 + header_off;
+		uLongf dst_len = sz - block_header_skip;
+		uLongf src_len = block->len - block_header_skip;
+
+		slice_resize(&uncompressed, sz);
+		memcpy(uncompressed.buf, block->data, block_header_skip);
+
+		if (Z_OK !=
+		    uncompress2(uncompressed.buf + block_header_skip, &dst_len,
+				block->data + block_header_skip, &src_len)) {
+			free(slice_yield(&uncompressed));
+			return ZLIB_ERROR;
+		}
+
+		block_source_return_block(block->source, block);
+		block->data = uncompressed.buf;
+		block->len = dst_len; /* XXX: 4 bytes missing? */
+		block->source = malloc_block_source();
+		full_block_size = src_len + block_header_skip;
+	} else if (full_block_size == 0) {
+		full_block_size = sz;
+	} else if (sz < full_block_size && sz < block->len &&
+		   block->data[sz] != 0) {
+		/* If the block is smaller than the full block size, it is
+                   padded (data followed by '\0') or the next block is
+                   unaligned. */
+		full_block_size = sz;
+	}
+
+	{
+		uint16_t restart_count = get_u16(block->data + sz - 2);
+		uint32_t restart_start = sz - 2 - 3 * restart_count;
+
+		byte *restart_bytes = block->data + restart_start;
+
+		/* transfer ownership. */
+		br->block = *block;
+		block->data = NULL;
+		block->len = 0;
+
+		br->hash_size = hash_size;
+		br->block_len = restart_start;
+		br->full_block_size = full_block_size;
+		br->header_off = header_off;
+		br->restart_count = restart_count;
+		br->restart_bytes = restart_bytes;
+	}
+
+	return 0;
+}
+
+static uint32_t block_reader_restart_offset(struct block_reader *br, int i)
+{
+	return get_u24(br->restart_bytes + 3 * i);
+}
+
+void block_reader_start(struct block_reader *br, struct block_iter *it)
+{
+	it->br = br;
+	slice_resize(&it->last_key, 0);
+	it->next_off = br->header_off + 4;
+}
+
+struct restart_find_args {
+	struct slice key;
+	struct block_reader *r;
+	int error;
+};
+
+static int restart_key_less(int idx, void *args)
+{
+	struct restart_find_args *a = (struct restart_find_args *)args;
+	uint32_t off = block_reader_restart_offset(a->r, idx);
+	struct slice in = {
+		.buf = a->r->block.data + off,
+		.len = a->r->block_len - off,
+	};
+
+	/* the restart key is verbatim in the block, so this could avoid the
+	   alloc for decoding the key */
+	struct slice rkey = {};
+	struct slice last_key = {};
+	byte unused_extra;
+	int n = decode_key(&rkey, &unused_extra, last_key, in);
+	if (n < 0) {
+		a->error = 1;
+		return -1;
+	}
+
+	{
+		int result = slice_compare(a->key, rkey);
+		free(slice_yield(&rkey));
+		return result;
+	}
+}
+
+void block_iter_copy_from(struct block_iter *dest, struct block_iter *src)
+{
+	dest->br = src->br;
+	dest->next_off = src->next_off;
+	slice_copy(&dest->last_key, src->last_key);
+}
+
+/* return < 0 for error, 0 for OK, > 0 for EOF. */
+int block_iter_next(struct block_iter *it, struct record rec)
+{
+	if (it->next_off >= it->br->block_len) {
+		return 1;
+	}
+
+	{
+		struct slice in = {
+			.buf = it->br->block.data + it->next_off,
+			.len = it->br->block_len - it->next_off,
+		};
+		struct slice start = in;
+		struct slice key = {};
+		byte extra;
+		int n = decode_key(&key, &extra, it->last_key, in);
+		if (n < 0) {
+			return -1;
+		}
+
+		in.buf += n;
+		in.len -= n;
+		n = record_decode(rec, key, extra, in, it->br->hash_size);
+		if (n < 0) {
+			return -1;
+		}
+		in.buf += n;
+		in.len -= n;
+
+		slice_copy(&it->last_key, key);
+		it->next_off += start.len - in.len;
+		free(slice_yield(&key));
+		return 0;
+	}
+}
+
+int block_reader_first_key(struct block_reader *br, struct slice *key)
+{
+	struct slice empty = {};
+	int off = br->header_off + 4;
+	struct slice in = {
+		.buf = br->block.data + off,
+		.len = br->block_len - off,
+	};
+
+	byte extra = 0;
+	int n = decode_key(key, &extra, empty, in);
+	if (n < 0) {
+		return n;
+	}
+	return 0;
+}
+
+int block_iter_seek(struct block_iter *it, struct slice want)
+{
+	return block_reader_seek(it->br, it, want);
+}
+
+void block_iter_close(struct block_iter *it)
+{
+	free(slice_yield(&it->last_key));
+}
+
+int block_reader_seek(struct block_reader *br, struct block_iter *it,
+		      struct slice want)
+{
+	struct restart_find_args args = {
+		.key = want,
+		.r = br,
+	};
+
+	int i = binsearch(br->restart_count, &restart_key_less, &args);
+	if (args.error) {
+		return -1;
+	}
+
+	it->br = br;
+	if (i > 0) {
+		i--;
+		it->next_off = block_reader_restart_offset(br, i);
+	} else {
+		it->next_off = br->header_off + 4;
+	}
+
+	{
+		struct record rec = new_record(block_reader_type(br));
+		struct slice key = {};
+		int result = 0;
+		int err = 0;
+		struct block_iter next = {};
+		while (true) {
+			block_iter_copy_from(&next, it);
+
+			err = block_iter_next(&next, rec);
+			if (err < 0) {
+				result = -1;
+				goto exit;
+			}
+
+			record_key(rec, &key);
+			if (err > 0 || slice_compare(key, want) >= 0) {
+				result = 0;
+				goto exit;
+			}
+
+			block_iter_copy_from(it, &next);
+		}
+
+	exit:
+		free(slice_yield(&key));
+		free(slice_yield(&next.last_key));
+		record_clear(rec);
+		free(record_yield(&rec));
+
+		return result;
+	}
+}
+
+void block_writer_reset(struct block_writer *bw)
+{
+	bw->restart_len = 0;
+	bw->last_key.len = 0;
+}
+
+void block_writer_clear(struct block_writer *bw)
+{
+	free(bw->restarts);
+	bw->restarts = NULL;
+	free(slice_yield(&bw->last_key));
+	/* the block is not owned. */
+}
diff --git a/reftable/block.h b/reftable/block.h
new file mode 100644
index 0000000000..bb42588111
--- /dev/null
+++ b/reftable/block.h
@@ -0,0 +1,71 @@
+/*
+Copyright 2020 Google LLC
+
+Use of this source code is governed by a BSD-style
+license that can be found in the LICENSE file or at
+https://developers.google.com/open-source/licenses/bsd
+*/
+
+#ifndef BLOCK_H
+#define BLOCK_H
+
+#include "basics.h"
+#include "record.h"
+#include "reftable.h"
+
+struct block_writer {
+	byte *buf;
+	uint32_t block_size;
+	uint32_t header_off;
+	int restart_interval;
+	int hash_size;
+
+	uint32_t next;
+	uint32_t *restarts;
+	uint32_t restart_len;
+	uint32_t restart_cap;
+	struct slice last_key;
+	int entries;
+};
+
+void block_writer_init(struct block_writer *bw, byte typ, byte *buf,
+		       uint32_t block_size, uint32_t header_off, int hash_size);
+byte block_writer_type(struct block_writer *bw);
+int block_writer_add(struct block_writer *w, struct record rec);
+int block_writer_finish(struct block_writer *w);
+void block_writer_reset(struct block_writer *bw);
+void block_writer_clear(struct block_writer *bw);
+
+struct block_reader {
+	uint32_t header_off;
+	struct block block;
+	int hash_size;
+
+	/* size of the data, excluding restart data. */
+	uint32_t block_len;
+	byte *restart_bytes;
+	uint32_t full_block_size;
+	uint16_t restart_count;
+};
+
+struct block_iter {
+	struct block_reader *br;
+	struct slice last_key;
+	uint32_t next_off;
+};
+
+int block_reader_init(struct block_reader *br, struct block *bl,
+		      uint32_t header_off, uint32_t table_block_size,
+		      int hash_size);
+void block_reader_start(struct block_reader *br, struct block_iter *it);
+int block_reader_seek(struct block_reader *br, struct block_iter *it,
+		      struct slice want);
+byte block_reader_type(struct block_reader *r);
+int block_reader_first_key(struct block_reader *br, struct slice *key);
+
+void block_iter_copy_from(struct block_iter *dest, struct block_iter *src);
+int block_iter_next(struct block_iter *it, struct record rec);
+int block_iter_seek(struct block_iter *it, struct slice want);
+void block_iter_close(struct block_iter *it);
+
+#endif
diff --git a/reftable/block_test.c b/reftable/block_test.c
new file mode 100644
index 0000000000..694333f80f
--- /dev/null
+++ b/reftable/block_test.c
@@ -0,0 +1,151 @@
+/*
+Copyright 2020 Google LLC
+
+Use of this source code is governed by a BSD-style
+license that can be found in the LICENSE file or at
+https://developers.google.com/open-source/licenses/bsd
+*/
+
+#include "block.h"
+
+#include "system.h"
+
+#include "basics.h"
+#include "constants.h"
+#include "record.h"
+#include "reftable.h"
+#include "test_framework.h"
+
+struct binsearch_args {
+	int key;
+	int *arr;
+};
+
+static int binsearch_func(int i, void *void_args)
+{
+	struct binsearch_args *args = (struct binsearch_args *)void_args;
+
+	return args->key < args->arr[i];
+}
+
+void test_binsearch()
+{
+	int arr[] = { 2, 4, 6, 8, 10 };
+	int sz = ARRAYSIZE(arr);
+	struct binsearch_args args = {
+		.arr = arr,
+	};
+
+	int i = 0;
+	for (i = 1; i < 11; i++) {
+		args.key = i;
+		int res = binsearch(sz, &binsearch_func, &args);
+
+		if (res < sz) {
+			assert(args.key < arr[res]);
+			if (res > 0) {
+				assert(args.key >= arr[res - 1]);
+			}
+		} else {
+			assert(args.key == 10 || args.key == 11);
+		}
+	}
+}
+
+void test_block_read_write()
+{
+	const int header_off = 21; /* random */
+	const int N = 30;
+	char *names[N];
+	const int block_size = 1024;
+	struct block block = {};
+	block.data = calloc(block_size, 1);
+	block.len = block_size;
+
+	struct block_writer bw = {};
+	block_writer_init(&bw, BLOCK_TYPE_REF, block.data, block_size,
+			  header_off, SHA1_SIZE);
+	struct ref_record ref = {};
+	struct record rec = {};
+	record_from_ref(&rec, &ref);
+
+	int i = 0;
+	for (i = 0; i < N; i++) {
+		char name[100];
+		snprintf(name, sizeof(name), "branch%02d", i);
+
+		byte hash[SHA1_SIZE];
+		memset(hash, i, sizeof(hash));
+
+		ref.ref_name = name;
+		ref.value = hash;
+		names[i] = strdup(name);
+		int n = block_writer_add(&bw, rec);
+		ref.ref_name = NULL;
+		ref.value = NULL;
+		assert(n == 0);
+	}
+
+	int n = block_writer_finish(&bw);
+	assert(n > 0);
+
+	block_writer_clear(&bw);
+
+	struct block_reader br = {};
+	block_reader_init(&br, &block, header_off, block_size, SHA1_SIZE);
+
+	struct block_iter it = {};
+	block_reader_start(&br, &it);
+
+	int j = 0;
+	while (true) {
+		int r = block_iter_next(&it, rec);
+		assert(r >= 0);
+		if (r > 0) {
+			break;
+		}
+		assert_streq(names[j], ref.ref_name);
+		j++;
+	}
+
+	record_clear(rec);
+	block_iter_close(&it);
+
+	struct slice want = {};
+	for (i = 0; i < N; i++) {
+		slice_set_string(&want, names[i]);
+
+		struct block_iter it = {};
+		int n = block_reader_seek(&br, &it, want);
+		assert(n == 0);
+
+		n = block_iter_next(&it, rec);
+		assert(n == 0);
+
+		assert_streq(names[i], ref.ref_name);
+
+		want.len--;
+		n = block_reader_seek(&br, &it, want);
+		assert(n == 0);
+
+		n = block_iter_next(&it, rec);
+		assert(n == 0);
+		assert_streq(names[10 * (i / 10)], ref.ref_name);
+
+		block_iter_close(&it);
+	}
+
+	record_clear(rec);
+	free(block.data);
+	free(slice_yield(&want));
+	for (i = 0; i < N; i++) {
+		free(names[i]);
+	}
+}
+
+int main()
+{
+	add_test_case("binsearch", &test_binsearch);
+	add_test_case("block_read_write", &test_block_read_write);
+	test_main();
+}
diff --git a/reftable/blocksource.h b/reftable/blocksource.h
new file mode 100644
index 0000000000..f3ad3a4c22
--- /dev/null
+++ b/reftable/blocksource.h
@@ -0,0 +1,20 @@
+/*
+Copyright 2020 Google LLC
+
+Use of this source code is governed by a BSD-style
+license that can be found in the LICENSE file or at
+https://developers.google.com/open-source/licenses/bsd
+*/
+
+#ifndef BLOCKSOURCE_H
+#define BLOCKSOURCE_H
+
+#include "reftable.h"
+
+uint64_t block_source_size(struct block_source source);
+int block_source_read_block(struct block_source source, struct block *dest,
+			    uint64_t off, uint32_t size);
+void block_source_return_block(struct block_source source, struct block *ret);
+void block_source_close(struct block_source source);
+
+#endif
diff --git a/reftable/bytes.c b/reftable/bytes.c
new file mode 100644
index 0000000000..e69de29bb2
diff --git a/reftable/config.h b/reftable/config.h
new file mode 100644
index 0000000000..40a8c178f1
--- /dev/null
+++ b/reftable/config.h
@@ -0,0 +1 @@
+/* empty */
diff --git a/reftable/constants.h b/reftable/constants.h
new file mode 100644
index 0000000000..cd35704610
--- /dev/null
+++ b/reftable/constants.h
@@ -0,0 +1,27 @@
+/*
+Copyright 2020 Google LLC
+
+Use of this source code is governed by a BSD-style
+license that can be found in the LICENSE file or at
+https://developers.google.com/open-source/licenses/bsd
+*/
+
+#ifndef CONSTANTS_H
+#define CONSTANTS_H
+
+#define SHA1_SIZE 20
+#define SHA256_SIZE 32
+#define VERSION 1
+#define HEADER_SIZE 24
+#define FOOTER_SIZE 68
+
+#define BLOCK_TYPE_LOG 'g'
+#define BLOCK_TYPE_INDEX 'i'
+#define BLOCK_TYPE_REF 'r'
+#define BLOCK_TYPE_OBJ 'o'
+#define BLOCK_TYPE_ANY 0
+
+#define MAX_RESTARTS ((1 << 16) - 1)
+#define DEFAULT_BLOCK_SIZE 4096
+
+#endif
diff --git a/reftable/dump.c b/reftable/dump.c
new file mode 100644
index 0000000000..acabe18fbe
--- /dev/null
+++ b/reftable/dump.c
@@ -0,0 +1,97 @@
+/*
+Copyright 2020 Google LLC
+
+Use of this source code is governed by a BSD-style
+license that can be found in the LICENSE file or at
+https://developers.google.com/open-source/licenses/bsd
+*/
+
+#include "system.h"
+
+#include "reftable.h"
+
+static int dump_table(const char *tablename)
+{
+	struct block_source src = {};
+	int err = block_source_from_file(&src, tablename);
+	if (err < 0) {
+		return err;
+	}
+
+	struct reader *r = NULL;
+	err = new_reader(&r, src, tablename);
+	if (err < 0) {
+		return err;
+	}
+
+	{
+		struct iterator it = {};
+		err = reader_seek_ref(r, &it, "");
+		if (err < 0) {
+			return err;
+		}
+
+		struct ref_record ref = {};
+		while (1) {
+			err = iterator_next_ref(it, &ref);
+			if (err > 0) {
+				break;
+			}
+			if (err < 0) {
+				return err;
+			}
+			ref_record_print(&ref, 20);
+		}
+		iterator_destroy(&it);
+		ref_record_clear(&ref);
+	}
+
+	{
+		struct iterator it = {};
+		err = reader_seek_log(r, &it, "");
+		if (err < 0) {
+			return err;
+		}
+		struct log_record log = {};
+		while (1) {
+			err = iterator_next_log(it, &log);
+			if (err > 0) {
+				break;
+			}
+			if (err < 0) {
+				return err;
+			}
+			log_record_print(&log, 20);
+		}
+		iterator_destroy(&it);
+		log_record_clear(&log);
+	}
+	return 0;
+}
+
+int main(int argc, char *argv[])
+{
+	int opt;
+	const char *table = NULL;
+	while ((opt = getopt(argc, argv, "t:")) != -1) {
+		switch (opt) {
+		case 't':
+			table = strdup(optarg);
+			break;
+		case '?':
+			printf("usage: %s [-table tablefile]\n", argv[0]);
+			return 2;
+			break;
+		}
+	}
+
+	if (table != NULL) {
+		int err = dump_table(table);
+		if (err < 0) {
+			fprintf(stderr, "%s: %s: %s\n", argv[0], table,
+				error_str(err));
+			return 1;
+		}
+	}
+	return 0;
+}
diff --git a/reftable/file.c b/reftable/file.c
new file mode 100644
index 0000000000..b2ea90bf94
--- /dev/null
+++ b/reftable/file.c
@@ -0,0 +1,97 @@
+/*
+Copyright 2020 Google LLC
+
+Use of this source code is governed by a BSD-style
+license that can be found in the LICENSE file or at
+https://developers.google.com/open-source/licenses/bsd
+*/
+
+#include "system.h"
+
+#include "block.h"
+#include "iter.h"
+#include "record.h"
+#include "reftable.h"
+#include "tree.h"
+
+struct file_block_source {
+	int fd;
+	uint64_t size;
+};
+
+static uint64_t file_size(void *b)
+{
+	return ((struct file_block_source *)b)->size;
+}
+
+static void file_return_block(void *b, struct block *dest)
+{
+	memset(dest->data, 0xff, dest->len);
+	free(dest->data);
+}
+
+static void file_close(void *b)
+{
+	int fd = ((struct file_block_source *)b)->fd;
+	if (fd > 0) {
+		close(fd);
+		((struct file_block_source *)b)->fd = 0;
+	}
+
+	free(b);
+}
+
+static int file_read_block(void *v, struct block *dest, uint64_t off,
+			   uint32_t size)
+{
+	struct file_block_source *b = (struct file_block_source *)v;
+	assert(off + size <= b->size);
+	dest->data = malloc(size);
+	if (pread(b->fd, dest->data, size, off) != size) {
+		return -1;
+	}
+	dest->len = size;
+	return size;
+}
+
+struct block_source_vtable file_vtable = {
+	.size = &file_size,
+	.read_block = &file_read_block,
+	.return_block = &file_return_block,
+	.close = &file_close,
+};
+
+int block_source_from_file(struct block_source *bs, const char *name)
+{
+	struct stat st = {};
+	int err = 0;
+	int fd = open(name, O_RDONLY);
+	if (fd < 0) {
+		if (errno == ENOENT) {
+			return NOT_EXIST_ERROR;
+		}
+		return -1;
+	}
+
+	err = fstat(fd, &st);
+	if (err < 0) {
+		return -1;
+	}
+
+	{
+		struct file_block_source *p =
+			calloc(sizeof(struct file_block_source), 1);
+		p->size = st.st_size;
+		p->fd = fd;
+
+		bs->ops = &file_vtable;
+		bs->arg = p;
+	}
+	return 0;
+}
+
+int fd_writer(void *arg, byte *data, int sz)
+{
+	int *fdp = (int *)arg;
+	return write(*fdp, data, sz);
+}
diff --git a/reftable/iter.c b/reftable/iter.c
new file mode 100644
index 0000000000..e306a5d465
--- /dev/null
+++ b/reftable/iter.c
@@ -0,0 +1,230 @@
+/*
+Copyright 2020 Google LLC
+
+Use of this source code is governed by a BSD-style
+license that can be found in the LICENSE file or at
+https://developers.google.com/open-source/licenses/bsd
+*/
+
+#include "iter.h"
+
+#include "system.h"
+
+#include "block.h"
+#include "constants.h"
+#include "reader.h"
+#include "reftable.h"
+
+bool iterator_is_null(struct iterator it)
+{
+	return it.ops == NULL;
+}
+
+static int empty_iterator_next(void *arg, struct record rec)
+{
+	return 1;
+}
+
+static void empty_iterator_close(void *arg)
+{
+}
+
+struct iterator_vtable empty_vtable = {
+	.next = &empty_iterator_next,
+	.close = &empty_iterator_close,
+};
+
+void iterator_set_empty(struct iterator *it)
+{
+	it->iter_arg = NULL;
+	it->ops = &empty_vtable;
+}
+
+int iterator_next(struct iterator it, struct record rec)
+{
+	return it.ops->next(it.iter_arg, rec);
+}
+
+void iterator_destroy(struct iterator *it)
+{
+	if (it->ops == NULL) {
+		return;
+	}
+	it->ops->close(it->iter_arg);
+	it->ops = NULL;
+	free(it->iter_arg);
+	it->iter_arg = NULL;
+}
+
+int iterator_next_ref(struct iterator it, struct ref_record *ref)
+{
+	struct record rec = {};
+	record_from_ref(&rec, ref);
+	return iterator_next(it, rec);
+}
+
+int iterator_next_log(struct iterator it, struct log_record *log)
+{
+	struct record rec = {};
+	record_from_log(&rec, log);
+	return iterator_next(it, rec);
+}
+
+static void filtering_ref_iterator_close(void *iter_arg)
+{
+	struct filtering_ref_iterator *fri =
+		(struct filtering_ref_iterator *)iter_arg;
+	free(slice_yield(&fri->oid));
+	iterator_destroy(&fri->it);
+}
+
+static int filtering_ref_iterator_next(void *iter_arg, struct record rec)
+{
+	struct filtering_ref_iterator *fri =
+		(struct filtering_ref_iterator *)iter_arg;
+	struct ref_record *ref = (struct ref_record *)rec.data;
+
+	while (true) {
+		int err = iterator_next_ref(fri->it, ref);
+		if (err != 0) {
+			return err;
+		}
+
+		if (fri->double_check) {
+			struct iterator it = {};
+
+			int err = reader_seek_ref(fri->r, &it, ref->ref_name);
+			if (err == 0) {
+				err = iterator_next_ref(it, ref);
+			}
+
+			iterator_destroy(&it);
+
+			if (err < 0) {
+				return err;
+			}
+
+			if (err > 0) {
+				continue;
+			}
+		}
+
+		if ((ref->target_value != NULL &&
+		     !memcmp(fri->oid.buf, ref->target_value, fri->oid.len)) ||
+		    (ref->value != NULL &&
+		     !memcmp(fri->oid.buf, ref->value, fri->oid.len))) {
+			return 0;
+		}
+	}
+}
+
+struct iterator_vtable filtering_ref_iterator_vtable = {
+	.next = &filtering_ref_iterator_next,
+	.close = &filtering_ref_iterator_close,
+};
+
+void iterator_from_filtering_ref_iterator(struct iterator *it,
+					  struct filtering_ref_iterator *fri)
+{
+	it->iter_arg = fri;
+	it->ops = &filtering_ref_iterator_vtable;
+}
+
+static void indexed_table_ref_iter_close(void *p)
+{
+	struct indexed_table_ref_iter *it = (struct indexed_table_ref_iter *)p;
+	block_iter_close(&it->cur);
+	reader_return_block(it->r, &it->block_reader.block);
+	free(slice_yield(&it->oid));
+}
+
+static int indexed_table_ref_iter_next_block(struct indexed_table_ref_iter *it)
+{
+	if (it->offset_idx == it->offset_len) {
+		it->finished = true;
+		return 1;
+	}
+
+	reader_return_block(it->r, &it->block_reader.block);
+
+	{
+		uint64_t off = it->offsets[it->offset_idx++];
+		int err = reader_init_block_reader(it->r, &it->block_reader,
+						   off, BLOCK_TYPE_REF);
+		if (err < 0) {
+			return err;
+		}
+		if (err > 0) {
+			/* indexed block does not exist. */
+			return FORMAT_ERROR;
+		}
+	}
+	block_reader_start(&it->block_reader, &it->cur);
+	return 0;
+}
+
+static int indexed_table_ref_iter_next(void *p, struct record rec)
+{
+	struct indexed_table_ref_iter *it = (struct indexed_table_ref_iter *)p;
+	struct ref_record *ref = (struct ref_record *)rec.data;
+
+	while (true) {
+		int err = block_iter_next(&it->cur, rec);
+		if (err < 0) {
+			return err;
+		}
+
+		if (err > 0) {
+			err = indexed_table_ref_iter_next_block(it);
+			if (err < 0) {
+				return err;
+			}
+
+			if (it->finished) {
+				return 1;
+			}
+			continue;
+		}
+
+		if (!memcmp(it->oid.buf, ref->target_value, it->oid.len) ||
+		    !memcmp(it->oid.buf, ref->value, it->oid.len)) {
+			return 0;
+		}
+	}
+}
+
+int new_indexed_table_ref_iter(struct indexed_table_ref_iter **dest,
+			       struct reader *r, byte *oid, int oid_len,
+			       uint64_t *offsets, int offset_len)
+{
+	struct indexed_table_ref_iter *itr =
+		calloc(sizeof(struct indexed_table_ref_iter), 1);
+	int err = 0;
+
+	itr->r = r;
+	slice_resize(&itr->oid, oid_len);
+	memcpy(itr->oid.buf, oid, oid_len);
+
+	itr->offsets = offsets;
+	itr->offset_len = offset_len;
+
+	err = indexed_table_ref_iter_next_block(itr);
+	if (err < 0) {
+		free(itr);
+	} else {
+		*dest = itr;
+	}
+	return err;
+}
+
+struct iterator_vtable indexed_table_ref_iter_vtable = {
+	.next = &indexed_table_ref_iter_next,
+	.close = &indexed_table_ref_iter_close,
+};
+
+void iterator_from_indexed_table_ref_iter(struct iterator *it,
+					  struct indexed_table_ref_iter *itr)
+{
+	it->iter_arg = itr;
+	it->ops = &indexed_table_ref_iter_vtable;
+}
diff --git a/reftable/iter.h b/reftable/iter.h
new file mode 100644
index 0000000000..f497f2a27e
--- /dev/null
+++ b/reftable/iter.h
@@ -0,0 +1,56 @@
+/*
+Copyright 2020 Google LLC
+
+Use of this source code is governed by a BSD-style
+license that can be found in the LICENSE file or at
+https://developers.google.com/open-source/licenses/bsd
+*/
+
+#ifndef ITER_H
+#define ITER_H
+
+#include "block.h"
+#include "record.h"
+#include "slice.h"
+
+struct iterator_vtable {
+	int (*next)(void *iter_arg, struct record rec);
+	void (*close)(void *iter_arg);
+};
+
+void iterator_set_empty(struct iterator *it);
+int iterator_next(struct iterator it, struct record rec);
+bool iterator_is_null(struct iterator it);
+
+struct filtering_ref_iterator {
+	struct reader *r;
+	struct slice oid;
+	bool double_check;
+	struct iterator it;
+};
+
+void iterator_from_filtering_ref_iterator(struct iterator *,
+					  struct filtering_ref_iterator *);
+
+struct indexed_table_ref_iter {
+	struct reader *r;
+	struct slice oid;
+
+	/* mutable */
+	uint64_t *offsets;
+
+	/* Points to the next offset to read. */
+	int offset_idx;
+	int offset_len;
+	struct block_reader block_reader;
+	struct block_iter cur;
+	bool finished;
+};
+
+void iterator_from_indexed_table_ref_iter(struct iterator *it,
+					  struct indexed_table_ref_iter *itr);
+int new_indexed_table_ref_iter(struct indexed_table_ref_iter **dest,
+			       struct reader *r, byte *oid, int oid_len,
+			       uint64_t *offsets, int offset_len);
+
+#endif
diff --git a/reftable/merged.c b/reftable/merged.c
new file mode 100644
index 0000000000..7e32bb5d8a
--- /dev/null
+++ b/reftable/merged.c
@@ -0,0 +1,288 @@
+/*
+Copyright 2020 Google LLC
+
+Use of this source code is governed by a BSD-style
+license that can be found in the LICENSE file or at
+https://developers.google.com/open-source/licenses/bsd
+*/
+
+#include "merged.h"
+
+#include "system.h"
+
+#include "constants.h"
+#include "iter.h"
+#include "pq.h"
+#include "reader.h"
+
+static int merged_iter_init(struct merged_iter *mi)
+{
+	int i = 0;
+	for (i = 0; i < mi->stack_len; i++) {
+		struct record rec = new_record(mi->typ);
+		int err = iterator_next(mi->stack[i], rec);
+		if (err < 0) {
+			return err;
+		}
+
+		if (err > 0) {
+			iterator_destroy(&mi->stack[i]);
+			record_clear(rec);
+			free(record_yield(&rec));
+		} else {
+			struct pq_entry e = {
+				.rec = rec,
+				.index = i,
+			};
+			merged_iter_pqueue_add(&mi->pq, e);
+		}
+	}
+
+	return 0;
+}
+
+static void merged_iter_close(void *p)
+{
+	struct merged_iter *mi = (struct merged_iter *)p;
+	int i = 0;
+	merged_iter_pqueue_clear(&mi->pq);
+	for (i = 0; i < mi->stack_len; i++) {
+		iterator_destroy(&mi->stack[i]);
+	}
+	free(mi->stack);
+}
+
+static int merged_iter_advance_subiter(struct merged_iter *mi, int idx)
+{
+	if (iterator_is_null(mi->stack[idx])) {
+		return 0;
+	}
+
+	{
+		struct record rec = new_record(mi->typ);
+		struct pq_entry e = {
+			.rec = rec,
+			.index = idx,
+		};
+		int err = iterator_next(mi->stack[idx], rec);
+		if (err < 0) {
+			return err;
+		}
+
+		if (err > 0) {
+			iterator_destroy(&mi->stack[idx]);
+			record_clear(rec);
+			free(record_yield(&rec));
+			return 0;
+		}
+
+		merged_iter_pqueue_add(&mi->pq, e);
+	}
+	return 0;
+}
+
+static int merged_iter_next(struct merged_iter *mi, struct record rec)
+{
+	struct slice entry_key = {};
+	struct pq_entry entry = merged_iter_pqueue_remove(&mi->pq);
+	int err = merged_iter_advance_subiter(mi, entry.index);
+	if (err < 0) {
+		return err;
+	}
+
+	record_key(entry.rec, &entry_key);
+	while (!merged_iter_pqueue_is_empty(mi->pq)) {
+		struct pq_entry top = merged_iter_pqueue_top(mi->pq);
+		struct slice k = {};
+		int err = 0, cmp = 0;
+
+		record_key(top.rec, &k);
+
+		cmp = slice_compare(k, entry_key);
+		free(slice_yield(&k));
+
+		if (cmp > 0) {
+			break;
+		}
+
+		merged_iter_pqueue_remove(&mi->pq);
+		err = merged_iter_advance_subiter(mi, top.index);
+		if (err < 0) {
+			return err;
+		}
+		record_clear(top.rec);
+		free(record_yield(&top.rec));
+	}
+
+	record_copy_from(rec, entry.rec, mi->hash_size);
+	record_clear(entry.rec);
+	free(record_yield(&entry.rec));
+	free(slice_yield(&entry_key));
+	return 0;
+}
+
+static int merged_iter_next_void(void *p, struct record rec)
+{
+	struct merged_iter *mi = (struct merged_iter *)p;
+	if (merged_iter_pqueue_is_empty(mi->pq)) {
+		return 1;
+	}
+
+	return merged_iter_next(mi, rec);
+}
+
+struct iterator_vtable merged_iter_vtable = {
+	.next = &merged_iter_next_void,
+	.close = &merged_iter_close,
+};
+
+static void iterator_from_merged_iter(struct iterator *it,
+				      struct merged_iter *mi)
+{
+	it->iter_arg = mi;
+	it->ops = &merged_iter_vtable;
+}
+
+int new_merged_table(struct merged_table **dest, struct reader **stack, int n)
+{
+	uint64_t last_max = 0;
+	uint64_t first_min = 0;
+	int i = 0;
+	for (i = 0; i < n; i++) {
+		struct reader *r = stack[i];
+		if (i > 0 && last_max >= reader_min_update_index(r)) {
+			return FORMAT_ERROR;
+		}
+		if (i == 0) {
+			first_min = reader_min_update_index(r);
+		}
+
+		last_max = reader_max_update_index(r);
+	}
+
+	{
+		struct merged_table m = {
+			.stack = stack,
+			.stack_len = n,
+			.min = first_min,
+			.max = last_max,
+			.hash_size = SHA1_SIZE,
+		};
+
+		*dest = calloc(sizeof(struct merged_table), 1);
+		**dest = m;
+	}
+	return 0;
+}
+
+void merged_table_close(struct merged_table *mt)
+{
+	int i = 0;
+	for (i = 0; i < mt->stack_len; i++) {
+		reader_free(mt->stack[i]);
+	}
+	free(mt->stack);
+	mt->stack = NULL;
+	mt->stack_len = 0;
+}
+
+/* clears the list of subtable, without affecting the readers themselves. */
+void merged_table_clear(struct merged_table *mt)
+{
+	free(mt->stack);
+	mt->stack = NULL;
+	mt->stack_len = 0;
+}
+
+void merged_table_free(struct merged_table *mt)
+{
+	if (mt == NULL) {
+		return;
+	}
+	merged_table_clear(mt);
+	free(mt);
+}
+
+uint64_t merged_max_update_index(struct merged_table *mt)
+{
+	return mt->max;
+}
+
+uint64_t merged_min_update_index(struct merged_table *mt)
+{
+	return mt->min;
+}
+
+static int merged_table_seek_record(struct merged_table *mt,
+				    struct iterator *it, struct record rec)
+{
+	struct iterator *iters = calloc(sizeof(struct iterator), mt->stack_len);
+	struct merged_iter merged = {
+		.stack = iters,
+		.typ = record_type(rec),
+		.hash_size = mt->hash_size,
+	};
+	int n = 0;
+	int err = 0;
+	int i = 0;
+	for (i = 0; i < mt->stack_len && err == 0; i++) {
+		int e = reader_seek(mt->stack[i], &iters[n], rec);
+		if (e < 0) {
+			err = e;
+		}
+		if (e == 0) {
+			n++;
+		}
+	}
+	if (err < 0) {
+		int i = 0;
+		for (i = 0; i < n; i++) {
+			iterator_destroy(&iters[i]);
+		}
+		free(iters);
+		return err;
+	}
+
+	merged.stack_len = n, err = merged_iter_init(&merged);
+	if (err < 0) {
+		merged_iter_close(&merged);
+		return err;
+	}
+
+	{
+		struct merged_iter *p = malloc(sizeof(struct merged_iter));
+		*p = merged;
+		iterator_from_merged_iter(it, p);
+	}
+	return 0;
+}
+
+int merged_table_seek_ref(struct merged_table *mt, struct iterator *it,
+			  const char *name)
+{
+	struct ref_record ref = {
+		.ref_name = (char *)name,
+	};
+	struct record rec = {};
+	record_from_ref(&rec, &ref);
+	return merged_table_seek_record(mt, it, rec);
+}
+
+int merged_table_seek_log_at(struct merged_table *mt, struct iterator *it,
+			     const char *name, uint64_t update_index)
+{
+	struct log_record log = {
+		.ref_name = (char *)name,
+		.update_index = update_index,
+	};
+	struct record rec = {};
+	record_from_log(&rec, &log);
+	return merged_table_seek_record(mt, it, rec);
+}
+
+int merged_table_seek_log(struct merged_table *mt, struct iterator *it,
+			  const char *name)
+{
+	uint64_t max = ~((uint64_t)0);
+	return merged_table_seek_log_at(mt, it, name, max);
+}
diff --git a/reftable/merged.h b/reftable/merged.h
new file mode 100644
index 0000000000..b8d3572e26
--- /dev/null
+++ b/reftable/merged.h
@@ -0,0 +1,34 @@
+/*
+Copyright 2020 Google LLC
+
+Use of this source code is governed by a BSD-style
+license that can be found in the LICENSE file or at
+https://developers.google.com/open-source/licenses/bsd
+*/
+
+#ifndef MERGED_H
+#define MERGED_H
+
+#include "pq.h"
+#include "reftable.h"
+
+struct merged_table {
+	struct reader **stack;
+	int stack_len;
+	int hash_size;
+
+	uint64_t min;
+	uint64_t max;
+};
+
+struct merged_iter {
+	struct iterator *stack;
+	int hash_size;
+	int stack_len;
+	byte typ;
+	struct merged_iter_pqueue pq;
+} merged_iter;
+
+void merged_table_clear(struct merged_table *mt);
+
+#endif
diff --git a/reftable/merged_test.c b/reftable/merged_test.c
new file mode 100644
index 0000000000..2a46c5ffbd
--- /dev/null
+++ b/reftable/merged_test.c
@@ -0,0 +1,258 @@
+/*
+Copyright 2020 Google LLC
+
+Use of this source code is governed by a BSD-style
+license that can be found in the LICENSE file or at
+https://developers.google.com/open-source/licenses/bsd
+*/
+
+#include "merged.h"
+
+#include "system.h"
+
+#include "basics.h"
+#include "block.h"
+#include "constants.h"
+#include "pq.h"
+#include "reader.h"
+#include "record.h"
+#include "reftable.h"
+#include "test_framework.h"
+
+void test_pq(void)
+{
+	char *names[54] = {};
+	int N = ARRAYSIZE(names) - 1;
+
+	int i = 0;
+	for (i = 0; i < N; i++) {
+		char name[100];
+		snprintf(name, sizeof(name), "%02d", i);
+		names[i] = strdup(name);
+	}
+
+	struct merged_iter_pqueue pq = {};
+
+	i = 1;
+	do {
+		struct record rec = new_record(BLOCK_TYPE_REF);
+		record_as_ref(rec)->ref_name = names[i];
+
+		struct pq_entry e = {
+			.rec = rec,
+		};
+		merged_iter_pqueue_add(&pq, e);
+		merged_iter_pqueue_check(pq);
+		i = (i * 7) % N;
+	} while (i != 1);
+
+	const char *last = NULL;
+	while (!merged_iter_pqueue_is_empty(pq)) {
+		struct pq_entry e = merged_iter_pqueue_remove(&pq);
+		merged_iter_pqueue_check(pq);
+		struct ref_record *ref = record_as_ref(e.rec);
+
+		if (last != NULL) {
+			assert(strcmp(last, ref->ref_name) < 0);
+		}
+		last = ref->ref_name;
+		ref->ref_name = NULL;
+		free(ref);
+	}
+
+	for (i = 0; i < N; i++) {
+		free(names[i]);
+	}
+
+	merged_iter_pqueue_clear(&pq);
+}
+
+void write_test_table(struct slice *buf, struct ref_record refs[], int n)
+{
+	int min = 0xffffffff;
+	int max = 0;
+	int i = 0;
+	for (i = 0; i < n; i++) {
+		uint64_t ui = refs[i].update_index;
+		if (ui > max) {
+			max = ui;
+		}
+		if (ui < min) {
+			min = ui;
+		}
+	}
+
+	struct write_options opts = {
+		.block_size = 256,
+	};
+
+	struct writer *w = new_writer(&slice_write_void, buf, &opts);
+	writer_set_limits(w, min, max);
+
+	for (i = 0; i < n; i++) {
+		uint64_t before = refs[i].update_index;
+		int n = writer_add_ref(w, &refs[i]);
+		assert(n == 0);
+		assert(before == refs[i].update_index);
+	}
+
+	int err = writer_close(w);
+	assert(err == 0);
+
+	writer_free(w);
+	w = NULL;
+}
+
+static struct merged_table *merged_table_from_records(struct ref_record **refs,
+						      int *sizes,
+						      struct slice *buf, int n)
+{
+	struct block_source *source = calloc(n, sizeof(*source));
+	struct reader **rd = calloc(n, sizeof(*rd));
+	int i = 0;
+	for (i = 0; i < n; i++) {
+		write_test_table(&buf[i], refs[i], sizes[i]);
+		block_source_from_slice(&source[i], &buf[i]);
+
+		int err = new_reader(&rd[i], source[i], "name");
+		assert(err == 0);
+	}
+
+	struct merged_table *mt = NULL;
+	int err = new_merged_table(&mt, rd, n);
+	assert(err == 0);
+	return mt;
+}
+
+void test_merged_between(void)
+{
+	byte hash1[SHA1_SIZE];
+	byte hash2[SHA1_SIZE];
+
+	set_test_hash(hash1, 1);
+	set_test_hash(hash2, 2);
+	struct ref_record r1[] = { {
+		.ref_name = "b",
+		.update_index = 1,
+		.value = hash1,
+	} };
+	struct ref_record r2[] = { {
+		.ref_name = "a",
+		.update_index = 2,
+	} };
+
+	struct ref_record *refs[] = { r1, r2 };
+	int sizes[] = { 1, 1 };
+	struct slice bufs[2] = {};
+	struct merged_table *mt =
+		merged_table_from_records(refs, sizes, bufs, 2);
+
+	struct iterator it = {};
+	int err = merged_table_seek_ref(mt, &it, "a");
+	assert(err == 0);
+
+	struct ref_record ref = {};
+	err = iterator_next_ref(it, &ref);
+	assert_err(err);
+	assert(ref.update_index == 2);
+}
+
+void test_merged(void)
+{
+	byte hash1[SHA1_SIZE];
+	byte hash2[SHA1_SIZE];
+
+	set_test_hash(hash1, 1);
+	set_test_hash(hash2, 2);
+	struct ref_record r1[] = { {
+					   .ref_name = "a",
+					   .update_index = 1,
+					   .value = hash1,
+				   },
+				   {
+					   .ref_name = "b",
+					   .update_index = 1,
+					   .value = hash1,
+				   },
+				   {
+					   .ref_name = "c",
+					   .update_index = 1,
+					   .value = hash1,
+				   } };
+	struct ref_record r2[] = { {
+		.ref_name = "a",
+		.update_index = 2,
+	} };
+	struct ref_record r3[] = {
+		{
+			.ref_name = "c",
+			.update_index = 3,
+			.value = hash2,
+		},
+		{
+			.ref_name = "d",
+			.update_index = 3,
+			.value = hash1,
+		},
+	};
+
+	struct ref_record *refs[] = { r1, r2, r3 };
+	int sizes[3] = { 3, 1, 2 };
+	struct slice bufs[3] = {};
+
+	struct merged_table *mt =
+		merged_table_from_records(refs, sizes, bufs, 3);
+
+	struct iterator it = {};
+	int err = merged_table_seek_ref(mt, &it, "a");
+	assert(err == 0);
+
+	struct ref_record *out = NULL;
+	int len = 0;
+	int cap = 0;
+	while (len < 100) { /* cap loops/recursion. */
+		struct ref_record ref = {};
+		int err = iterator_next_ref(it, &ref);
+		if (err > 0) {
+			break;
+		}
+		if (len == cap) {
+			cap = 2 * cap + 1;
+			out = realloc(out, sizeof(struct ref_record) * cap);
+		}
+		out[len++] = ref;
+	}
+	iterator_destroy(&it);
+
+	struct ref_record want[] = {
+		r2[0],
+		r1[1],
+		r3[0],
+		r3[1],
+	};
+	assert(ARRAYSIZE(want) == len);
+	int i = 0;
+	for (i = 0; i < len; i++) {
+		assert(ref_record_equal(&want[i], &out[i], SHA1_SIZE));
+	}
+	for (i = 0; i < len; i++) {
+		ref_record_clear(&out[i]);
+	}
+	free(out);
+
+	for (i = 0; i < 3; i++) {
+		free(slice_yield(&bufs[i]));
+	}
+	merged_table_close(mt);
+	merged_table_free(mt);
+}
+
+/* XXX test refs_for(oid) */
+
+int main()
+{
+	add_test_case("test_merged_between", &test_merged_between);
+	add_test_case("test_pq", &test_pq);
+	add_test_case("test_merged", &test_merged);
+	test_main();
+}
diff --git a/reftable/pq.c b/reftable/pq.c
new file mode 100644
index 0000000000..62cb75c5c9
--- /dev/null
+++ b/reftable/pq.c
@@ -0,0 +1,124 @@
+/*
+Copyright 2020 Google LLC
+
+Use of this source code is governed by a BSD-style
+license that can be found in the LICENSE file or at
+https://developers.google.com/open-source/licenses/bsd
+*/
+
+#include "pq.h"
+
+#include "system.h"
+
+int pq_less(struct pq_entry a, struct pq_entry b)
+{
+	struct slice ak = {};
+	struct slice bk = {};
+	int cmp = 0;
+	record_key(a.rec, &ak);
+	record_key(b.rec, &bk);
+
+	cmp = slice_compare(ak, bk);
+
+	free(slice_yield(&ak));
+	free(slice_yield(&bk));
+
+	if (cmp == 0) {
+		return a.index > b.index;
+	}
+
+	return cmp < 0;
+}
+
+struct pq_entry merged_iter_pqueue_top(struct merged_iter_pqueue pq)
+{
+	return pq.heap[0];
+}
+
+bool merged_iter_pqueue_is_empty(struct merged_iter_pqueue pq)
+{
+	return pq.len == 0;
+}
+
+void merged_iter_pqueue_check(struct merged_iter_pqueue pq)
+{
+	int i = 0;
+	for (i = 1; i < pq.len; i++) {
+		int parent = (i - 1) / 2;
+
+		assert(pq_less(pq.heap[parent], pq.heap[i]));
+	}
+}
+
+struct pq_entry merged_iter_pqueue_remove(struct merged_iter_pqueue *pq)
+{
+	int i = 0;
+	struct pq_entry e = pq->heap[0];
+	pq->heap[0] = pq->heap[pq->len - 1];
+	pq->len--;
+
+	i = 0;
+	while (i < pq->len) {
+		int min = i;
+		int j = 2 * i + 1;
+		int k = 2 * i + 2;
+		if (j < pq->len && pq_less(pq->heap[j], pq->heap[i])) {
+			min = j;
+		}
+		if (k < pq->len && pq_less(pq->heap[k], pq->heap[min])) {
+			min = k;
+		}
+
+		if (min == i) {
+			break;
+		}
+
+		{
+			struct pq_entry tmp = pq->heap[min];
+			pq->heap[min] = pq->heap[i];
+			pq->heap[i] = tmp;
+		}
+
+		i = min;
+	}
+
+	return e;
+}
+
+void merged_iter_pqueue_add(struct merged_iter_pqueue *pq, struct pq_entry e)
+{
+	int i = 0;
+	if (pq->len == pq->cap) {
+		pq->cap = 2 * pq->cap + 1;
+		pq->heap = realloc(pq->heap, pq->cap * sizeof(struct pq_entry));
+	}
+
+	pq->heap[pq->len++] = e;
+	i = pq->len - 1;
+	while (i > 0) {
+		int j = (i - 1) / 2;
+		if (pq_less(pq->heap[j], pq->heap[i])) {
+			break;
+		}
+
+		{
+			struct pq_entry tmp = pq->heap[j];
+			pq->heap[j] = pq->heap[i];
+			pq->heap[i] = tmp;
+		}
+
+		i = j;
+	}
+}
+
+void merged_iter_pqueue_clear(struct merged_iter_pqueue *pq)
+{
+	int i = 0;
+	for (i = 0; i < pq->len; i++) {
+		record_clear(pq->heap[i].rec);
+		free(record_yield(&pq->heap[i].rec));
+	}
+	free(pq->heap);
+	pq->heap = NULL;
+	pq->len = pq->cap = 0;
+}
diff --git a/reftable/pq.h b/reftable/pq.h
new file mode 100644
index 0000000000..5f7018979d
--- /dev/null
+++ b/reftable/pq.h
@@ -0,0 +1,34 @@
+/*
+Copyright 2020 Google LLC
+
+Use of this source code is governed by a BSD-style
+license that can be found in the LICENSE file or at
+https://developers.google.com/open-source/licenses/bsd
+*/
+
+#ifndef PQ_H
+#define PQ_H
+
+#include "record.h"
+
+struct pq_entry {
+	struct record rec;
+	int index;
+};
+
+int pq_less(struct pq_entry a, struct pq_entry b);
+
+struct merged_iter_pqueue {
+	struct pq_entry *heap;
+	int len;
+	int cap;
+};
+
+struct pq_entry merged_iter_pqueue_top(struct merged_iter_pqueue pq);
+bool merged_iter_pqueue_is_empty(struct merged_iter_pqueue pq);
+void merged_iter_pqueue_check(struct merged_iter_pqueue pq);
+struct pq_entry merged_iter_pqueue_remove(struct merged_iter_pqueue *pq);
+void merged_iter_pqueue_add(struct merged_iter_pqueue *pq, struct pq_entry e);
+void merged_iter_pqueue_clear(struct merged_iter_pqueue *pq);
+
+#endif
diff --git a/reftable/reader.c b/reftable/reader.c
new file mode 100644
index 0000000000..754114b58b
--- /dev/null
+++ b/reftable/reader.c
@@ -0,0 +1,710 @@
+/*
+Copyright 2020 Google LLC
+
+Use of this source code is governed by a BSD-style
+license that can be found in the LICENSE file or at
+https://developers.google.com/open-source/licenses/bsd
+*/
+
+#include "reader.h"
+
+#include "system.h"
+
+#include "block.h"
+#include "constants.h"
+#include "iter.h"
+#include "record.h"
+#include "reftable.h"
+#include "tree.h"
+
+uint64_t block_source_size(struct block_source source)
+{
+	return source.ops->size(source.arg);
+}
+
+int block_source_read_block(struct block_source source, struct block *dest,
+			    uint64_t off, uint32_t size)
+{
+	int result = source.ops->read_block(source.arg, dest, off, size);
+	dest->source = source;
+	return result;
+}
+
+void block_source_return_block(struct block_source source, struct block *blockp)
+{
+	source.ops->return_block(source.arg, blockp);
+	blockp->data = NULL;
+	blockp->len = 0;
+	blockp->source.ops = NULL;
+	blockp->source.arg = NULL;
+}
+
+void block_source_close(struct block_source *source)
+{
+	if (source->ops == NULL) {
+		return;
+	}
+
+	source->ops->close(source->arg);
+	source->ops = NULL;
+}
+
+static struct reader_offsets *reader_offsets_for(struct reader *r, byte typ)
+{
+	switch (typ) {
+	case BLOCK_TYPE_REF:
+		return &r->ref_offsets;
+	case BLOCK_TYPE_LOG:
+		return &r->log_offsets;
+	case BLOCK_TYPE_OBJ:
+		return &r->obj_offsets;
+	}
+	abort();
+}
+
+static int reader_get_block(struct reader *r, struct block *dest, uint64_t off,
+			    uint32_t sz)
+{
+	if (off >= r->size) {
+		return 0;
+	}
+
+	if (off + sz > r->size) {
+		sz = r->size - off;
+	}
+
+	return block_source_read_block(r->source, dest, off, sz);
+}
+
+void reader_return_block(struct reader *r, struct block *p)
+{
+	block_source_return_block(r->source, p);
+}
+
+const char *reader_name(struct reader *r)
+{
+	return r->name;
+}
+
+static int parse_footer(struct reader *r, byte *footer, byte *header)
+{
+	byte *f = footer;
+	int err = 0;
+	if (memcmp(f, "REFT", 4)) {
+		err = FORMAT_ERROR;
+		goto exit;
+	}
+	f += 4;
+
+	if (memcmp(footer, header, HEADER_SIZE)) {
+		err = FORMAT_ERROR;
+		goto exit;
+	}
+
+	{
+		byte version = *f++;
+		if (version != 1) {
+			err = FORMAT_ERROR;
+			goto exit;
+		}
+	}
+
+	r->block_size = get_u24(f);
+
+	f += 3;
+	r->min_update_index = get_u64(f);
+	f += 8;
+	r->max_update_index = get_u64(f);
+	f += 8;
+
+	r->ref_offsets.index_offset = get_u64(f);
+	f += 8;
+
+	r->obj_offsets.offset = get_u64(f);
+	f += 8;
+
+	r->object_id_len = r->obj_offsets.offset & ((1 << 5) - 1);
+	r->obj_offsets.offset >>= 5;
+
+	r->obj_offsets.index_offset = get_u64(f);
+	f += 8;
+	r->log_offsets.offset = get_u64(f);
+	f += 8;
+	r->log_offsets.index_offset = get_u64(f);
+	f += 8;
+
+	{
+		uint32_t computed_crc = crc32(0, footer, f - footer);
+		uint32_t file_crc = get_u32(f);
+		f += 4;
+		if (computed_crc != file_crc) {
+			err = FORMAT_ERROR;
+			goto exit;
+		}
+	}
+
+	{
+		byte first_block_typ = header[HEADER_SIZE];
+		r->ref_offsets.present = (first_block_typ == BLOCK_TYPE_REF);
+		r->ref_offsets.offset = 0;
+		r->log_offsets.present = (first_block_typ == BLOCK_TYPE_LOG ||
+					  r->log_offsets.offset > 0);
+		r->obj_offsets.present = r->obj_offsets.offset > 0;
+	}
+	err = 0;
+exit:
+	return err;
+}
+
+int init_reader(struct reader *r, struct block_source source, const char *name)
+{
+	struct block footer = {};
+	struct block header = {};
+	int err = 0;
+
+	memset(r, 0, sizeof(struct reader));
+	r->size = block_source_size(source) - FOOTER_SIZE;
+	r->source = source;
+	r->name = strdup(name);
+	r->hash_size = SHA1_SIZE;
+
+	err = block_source_read_block(source, &footer, r->size, FOOTER_SIZE);
+	if (err != FOOTER_SIZE) {
+		err = IO_ERROR;
+		goto exit;
+	}
+
+	/* Need +1 to read type of first block. */
+	err = reader_get_block(r, &header, 0, HEADER_SIZE + 1);
+	if (err != HEADER_SIZE + 1) {
+		err = IO_ERROR;
+		goto exit;
+	}
+
+	err = parse_footer(r, footer.data, header.data);
+exit:
+	block_source_return_block(r->source, &footer);
+	block_source_return_block(r->source, &header);
+	return err;
+}
+
+struct table_iter {
+	struct reader *r;
+	byte typ;
+	uint64_t block_off;
+	struct block_iter bi;
+	bool finished;
+};
+
+static void table_iter_copy_from(struct table_iter *dest,
+				 struct table_iter *src)
+{
+	dest->r = src->r;
+	dest->typ = src->typ;
+	dest->block_off = src->block_off;
+	dest->finished = src->finished;
+	block_iter_copy_from(&dest->bi, &src->bi);
+}
+
+static int table_iter_next_in_block(struct table_iter *ti, struct record rec)
+{
+	int res = block_iter_next(&ti->bi, rec);
+	if (res == 0 && record_type(rec) == BLOCK_TYPE_REF) {
+		((struct ref_record *)rec.data)->update_index +=
+			ti->r->min_update_index;
+	}
+
+	return res;
+}
+
+static void table_iter_block_done(struct table_iter *ti)
+{
+	if (ti->bi.br == NULL) {
+		return;
+	}
+	reader_return_block(ti->r, &ti->bi.br->block);
+	free(ti->bi.br);
+	ti->bi.br = NULL;
+
+	ti->bi.last_key.len = 0;
+	ti->bi.next_off = 0;
+}
+
+static int32_t extract_block_size(byte *data, byte *typ, uint64_t off)
+{
+	int32_t result = 0;
+
+	if (off == 0) {
+		data += 24;
+	}
+
+	*typ = data[0];
+	if (is_block_type(*typ)) {
+		result = get_u24(data + 1);
+	}
+	return result;
+}
+
+int reader_init_block_reader(struct reader *r, struct block_reader *br,
+			     uint64_t next_off, byte want_typ)
+{
+	int32_t guess_block_size = r->block_size ? r->block_size :
+						   DEFAULT_BLOCK_SIZE;
+	struct block block = {};
+	byte block_typ = 0;
+	int err = 0;
+	uint32_t header_off = next_off ? 0 : HEADER_SIZE;
+	int32_t block_size = 0;
+
+	if (next_off >= r->size) {
+		return 1;
+	}
+
+	err = reader_get_block(r, &block, next_off, guess_block_size);
+	if (err < 0) {
+		return err;
+	}
+
+	block_size = extract_block_size(block.data, &block_typ, next_off);
+	if (block_size < 0) {
+		return block_size;
+	}
+
+	if (want_typ != BLOCK_TYPE_ANY && block_typ != want_typ) {
+		reader_return_block(r, &block);
+		return 1;
+	}
+
+	if (block_size > guess_block_size) {
+		reader_return_block(r, &block);
+		err = reader_get_block(r, &block, next_off, block_size);
+		if (err < 0) {
+			return err;
+		}
+	}
+
+	return block_reader_init(br, &block, header_off, r->block_size,
+				 r->hash_size);
+}
+
+static int table_iter_next_block(struct table_iter *dest,
+				 struct table_iter *src)
+{
+	uint64_t next_block_off = src->block_off + src->bi.br->full_block_size;
+	struct block_reader br = {};
+	int err = 0;
+
+	dest->r = src->r;
+	dest->typ = src->typ;
+	dest->block_off = next_block_off;
+
+	err = reader_init_block_reader(src->r, &br, next_block_off, src->typ);
+	if (err > 0) {
+		dest->finished = true;
+		return 1;
+	}
+	if (err != 0) {
+		return err;
+	}
+
+	{
+		struct block_reader *brp = malloc(sizeof(struct block_reader));
+		*brp = br;
+
+		dest->finished = false;
+		block_reader_start(brp, &dest->bi);
+	}
+	return 0;
+}
+
+static int table_iter_next(struct table_iter *ti, struct record rec)
+{
+	if (record_type(rec) != ti->typ) {
+		return API_ERROR;
+	}
+
+	while (true) {
+		struct table_iter next = {};
+		int err = 0;
+		if (ti->finished) {
+			return 1;
+		}
+
+		err = table_iter_next_in_block(ti, rec);
+		if (err <= 0) {
+			return err;
+		}
+
+		err = table_iter_next_block(&next, ti);
+		if (err != 0) {
+			ti->finished = true;
+		}
+		table_iter_block_done(ti);
+		if (err != 0) {
+			return err;
+		}
+		table_iter_copy_from(ti, &next);
+		block_iter_close(&next.bi);
+	}
+}
+
+static int table_iter_next_void(void *ti, struct record rec)
+{
+	return table_iter_next((struct table_iter *)ti, rec);
+}
+
+static void table_iter_close(void *p)
+{
+	struct table_iter *ti = (struct table_iter *)p;
+	table_iter_block_done(ti);
+	block_iter_close(&ti->bi);
+}
+
+struct iterator_vtable table_iter_vtable = {
+	.next = &table_iter_next_void,
+	.close = &table_iter_close,
+};
+
+static void iterator_from_table_iter(struct iterator *it, struct table_iter *ti)
+{
+	it->iter_arg = ti;
+	it->ops = &table_iter_vtable;
+}
+
+static int reader_table_iter_at(struct reader *r, struct table_iter *ti,
+				uint64_t off, byte typ)
+{
+	struct block_reader br = {};
+	struct block_reader *brp = NULL;
+
+	int err = reader_init_block_reader(r, &br, off, typ);
+	if (err != 0) {
+		return err;
+	}
+
+	brp = malloc(sizeof(struct block_reader));
+	*brp = br;
+	ti->r = r;
+	ti->typ = block_reader_type(brp);
+	ti->block_off = off;
+	block_reader_start(brp, &ti->bi);
+	return 0;
+}
+
+static int reader_start(struct reader *r, struct table_iter *ti, byte typ,
+			bool index)
+{
+	struct reader_offsets *offs = reader_offsets_for(r, typ);
+	uint64_t off = offs->offset;
+	if (index) {
+		off = offs->index_offset;
+		if (off == 0) {
+			return 1;
+		}
+		typ = BLOCK_TYPE_INDEX;
+	}
+
+	return reader_table_iter_at(r, ti, off, typ);
+}
+
+static int reader_seek_linear(struct reader *r, struct table_iter *ti,
+			      struct record want)
+{
+	struct record rec = new_record(record_type(want));
+	struct slice want_key = {};
+	struct slice got_key = {};
+	struct table_iter next = {};
+	int err = -1;
+	record_key(want, &want_key);
+
+	while (true) {
+		err = table_iter_next_block(&next, ti);
+		if (err < 0) {
+			goto exit;
+		}
+
+		if (err > 0) {
+			break;
+		}
+
+		err = block_reader_first_key(next.bi.br, &got_key);
+		if (err < 0) {
+			goto exit;
+		}
+		{
+			int cmp = slice_compare(got_key, want_key);
+			if (cmp > 0) {
+				table_iter_block_done(&next);
+				break;
+			}
+		}
+
+		table_iter_block_done(ti);
+		table_iter_copy_from(ti, &next);
+	}
+
+	err = block_iter_seek(&ti->bi, want_key);
+	if (err < 0) {
+		goto exit;
+	}
+	err = 0;
+
+exit:
+	block_iter_close(&next.bi);
+	record_clear(rec);
+	free(record_yield(&rec));
+	free(slice_yield(&want_key));
+	free(slice_yield(&got_key));
+	return err;
+}
+
+static int reader_seek_indexed(struct reader *r, struct iterator *it,
+			       struct record rec)
+{
+	struct index_record want_index = {};
+	struct record want_index_rec = {};
+	struct index_record index_result = {};
+	struct record index_result_rec = {};
+	struct table_iter index_iter = {};
+	struct table_iter next = {};
+	int err = 0;
+
+	record_key(rec, &want_index.last_key);
+	record_from_index(&want_index_rec, &want_index);
+	record_from_index(&index_result_rec, &index_result);
+
+	err = reader_start(r, &index_iter, record_type(rec), true);
+	if (err < 0) {
+		goto exit;
+	}
+
+	err = reader_seek_linear(r, &index_iter, want_index_rec);
+	while (true) {
+		err = table_iter_next(&index_iter, index_result_rec);
+		table_iter_block_done(&index_iter);
+		if (err != 0) {
+			goto exit;
+		}
+
+		err = reader_table_iter_at(r, &next, index_result.offset, 0);
+		if (err != 0) {
+			goto exit;
+		}
+
+		err = block_iter_seek(&next.bi, want_index.last_key);
+		if (err < 0) {
+			goto exit;
+		}
+
+		if (next.typ == record_type(rec)) {
+			err = 0;
+			break;
+		}
+
+		if (next.typ != BLOCK_TYPE_INDEX) {
+			err = FORMAT_ERROR;
+			break;
+		}
+
+		table_iter_copy_from(&index_iter, &next);
+	}
+
+	if (err == 0) {
+		struct table_iter *malloced =
+			calloc(sizeof(struct table_iter), 1);
+		table_iter_copy_from(malloced, &next);
+		iterator_from_table_iter(it, malloced);
+	}
+exit:
+	block_iter_close(&next.bi);
+	table_iter_close(&index_iter);
+	record_clear(want_index_rec);
+	record_clear(index_result_rec);
+	return err;
+}
+
+static int reader_seek_internal(struct reader *r, struct iterator *it,
+				struct record rec)
+{
+	struct reader_offsets *offs = reader_offsets_for(r, record_type(rec));
+	uint64_t idx = offs->index_offset;
+	struct table_iter ti = {};
+	int err = 0;
+	if (idx > 0) {
+		return reader_seek_indexed(r, it, rec);
+	}
+
+	err = reader_start(r, &ti, record_type(rec), false);
+	if (err < 0) {
+		return err;
+	}
+	err = reader_seek_linear(r, &ti, rec);
+	if (err < 0) {
+		return err;
+	}
+
+	{
+		struct table_iter *p = malloc(sizeof(struct table_iter));
+		*p = ti;
+		iterator_from_table_iter(it, p);
+	}
+
+	return 0;
+}
+
+int reader_seek(struct reader *r, struct iterator *it, struct record rec)
+{
+	byte typ = record_type(rec);
+
+	struct reader_offsets *offs = reader_offsets_for(r, typ);
+	if (!offs->present) {
+		iterator_set_empty(it);
+		return 0;
+	}
+
+	return reader_seek_internal(r, it, rec);
+}
+
+int reader_seek_ref(struct reader *r, struct iterator *it, const char *name)
+{
+	struct ref_record ref = {
+		.ref_name = (char *)name,
+	};
+	struct record rec = {};
+	record_from_ref(&rec, &ref);
+	return reader_seek(r, it, rec);
+}
+
+int reader_seek_log_at(struct reader *r, struct iterator *it, const char *name,
+		       uint64_t update_index)
+{
+	struct log_record log = {
+		.ref_name = (char *)name,
+		.update_index = update_index,
+	};
+	struct record rec = {};
+	record_from_log(&rec, &log);
+	return reader_seek(r, it, rec);
+}
+
+int reader_seek_log(struct reader *r, struct iterator *it, const char *name)
+{
+	uint64_t max = ~((uint64_t)0);
+	return reader_seek_log_at(r, it, name, max);
+}
+
+void reader_close(struct reader *r)
+{
+	block_source_close(&r->source);
+	free(r->name);
+	r->name = NULL;
+}
+
+int new_reader(struct reader **p, struct block_source src, char const *name)
+{
+	struct reader *rd = calloc(sizeof(struct reader), 1);
+	int err = init_reader(rd, src, name);
+	if (err == 0) {
+		*p = rd;
+	} else {
+		free(rd);
+	}
+	return err;
+}
+
+void reader_free(struct reader *r)
+{
+	reader_close(r);
+	free(r);
+}
+
+static int reader_refs_for_indexed(struct reader *r, struct iterator *it,
+				   byte *oid)
+{
+	struct obj_record want = {
+		.hash_prefix = oid,
+		.hash_prefix_len = r->object_id_len,
+	};
+	struct record want_rec = {};
+	struct iterator oit = {};
+	struct obj_record got = {};
+	struct record got_rec = {};
+	int err = 0;
+
+	record_from_obj(&want_rec, &want);
+
+	err = reader_seek(r, &oit, want_rec);
+	if (err != 0) {
+		return err;
+	}
+
+	record_from_obj(&got_rec, &got);
+	err = iterator_next(oit, got_rec);
+	iterator_destroy(&oit);
+	if (err < 0) {
+		return err;
+	}
+
+	if (err > 0 ||
+	    memcmp(want.hash_prefix, got.hash_prefix, r->object_id_len)) {
+		iterator_set_empty(it);
+		return 0;
+	}
+
+	{
+		struct indexed_table_ref_iter *itr = NULL;
+		err = new_indexed_table_ref_iter(&itr, r, oid, r->hash_size,
+						 got.offsets, got.offset_len);
+		if (err < 0) {
+			record_clear(got_rec);
+			return err;
+		}
+		got.offsets = NULL;
+		record_clear(got_rec);
+
+		iterator_from_indexed_table_ref_iter(it, itr);
+	}
+
+	return 0;
+}
+
+static int reader_refs_for_unindexed(struct reader *r, struct iterator *it,
+				     byte *oid, int oid_len)
+{
+	struct table_iter *ti = calloc(sizeof(struct table_iter), 1);
+	struct filtering_ref_iterator *filter = NULL;
+	int err = reader_start(r, ti, BLOCK_TYPE_REF, false);
+	if (err < 0) {
+		free(ti);
+		return err;
+	}
+
+	filter = calloc(sizeof(struct filtering_ref_iterator), 1);
+	slice_resize(&filter->oid, oid_len);
+	memcpy(filter->oid.buf, oid, oid_len);
+	filter->r = r;
+	filter->double_check = false;
+	iterator_from_table_iter(&filter->it, ti);
+
+	iterator_from_filtering_ref_iterator(it, filter);
+	return 0;
+}
+
+int reader_refs_for(struct reader *r, struct iterator *it, byte *oid,
+		    int oid_len)
+{
+	if (r->obj_offsets.present) {
+		return reader_refs_for_indexed(r, it, oid);
+	}
+	return reader_refs_for_unindexed(r, it, oid, oid_len);
+}
+
+uint64_t reader_max_update_index(struct reader *r)
+{
+	return r->max_update_index;
+}
+
+uint64_t reader_min_update_index(struct reader *r)
+{
+	return r->min_update_index;
+}
diff --git a/reftable/reader.h b/reftable/reader.h
new file mode 100644
index 0000000000..599a90028e
--- /dev/null
+++ b/reftable/reader.h
@@ -0,0 +1,52 @@
+/*
+Copyright 2020 Google LLC
+
+Use of this source code is governed by a BSD-style
+license that can be found in the LICENSE file or at
+https://developers.google.com/open-source/licenses/bsd
+*/
+
+#ifndef READER_H
+#define READER_H
+
+#include "block.h"
+#include "record.h"
+#include "reftable.h"
+
+uint64_t block_source_size(struct block_source source);
+
+int block_source_read_block(struct block_source source, struct block *dest,
+			    uint64_t off, uint32_t size);
+void block_source_return_block(struct block_source source, struct block *ret);
+void block_source_close(struct block_source *source);
+
+struct reader_offsets {
+	bool present;
+	uint64_t offset;
+	uint64_t index_offset;
+};
+
+struct reader {
+	struct block_source source;
+	char *name;
+	int hash_size;
+	uint64_t size;
+	uint32_t block_size;
+	uint64_t min_update_index;
+	uint64_t max_update_index;
+	int object_id_len;
+
+	struct reader_offsets ref_offsets;
+	struct reader_offsets obj_offsets;
+	struct reader_offsets log_offsets;
+};
+
+int init_reader(struct reader *r, struct block_source source, const char *name);
+int reader_seek(struct reader *r, struct iterator *it, struct record rec);
+void reader_close(struct reader *r);
+const char *reader_name(struct reader *r);
+void reader_return_block(struct reader *r, struct block *p);
+int reader_init_block_reader(struct reader *r, struct block_reader *br,
+			     uint64_t next_off, byte want_typ);
+
+#endif
diff --git a/reftable/record.c b/reftable/record.c
new file mode 100644
index 0000000000..657e0e0d91
--- /dev/null
+++ b/reftable/record.c
@@ -0,0 +1,1110 @@
+/*
+Copyright 2020 Google LLC
+
+Use of this source code is governed by a BSD-style
+license that can be found in the LICENSE file or at
+https://developers.google.com/open-source/licenses/bsd
+*/
+
+#include "record.h"
+
+#include "system.h"
+
+#include "constants.h"
+#include "reftable.h"
+
+int is_block_type(byte typ)
+{
+	switch (typ) {
+	case BLOCK_TYPE_REF:
+	case BLOCK_TYPE_LOG:
+	case BLOCK_TYPE_OBJ:
+	case BLOCK_TYPE_INDEX:
+		return true;
+	}
+	return false;
+}
+
+int get_var_int(uint64_t *dest, struct slice in)
+{
+	int ptr = 0;
+	uint64_t val;
+
+	if (in.len == 0) {
+		return -1;
+	}
+	val = in.buf[ptr] & 0x7f;
+
+	while (in.buf[ptr] & 0x80) {
+		ptr++;
+		if (ptr > in.len) {
+			return -1;
+		}
+		val = (val + 1) << 7 | (uint64_t)(in.buf[ptr] & 0x7f);
+	}
+
+	*dest = val;
+	return ptr + 1;
+}
+
+int put_var_int(struct slice dest, uint64_t val)
+{
+	byte buf[10] = {};
+	int i = 9;
+	buf[i] = (byte)(val & 0x7f);
+	i--;
+	while (true) {
+		val >>= 7;
+		if (!val) {
+			break;
+		}
+		val--;
+		buf[i] = 0x80 | (byte)(val & 0x7f);
+		i--;
+	}
+
+	{
+		int n = sizeof(buf) - i - 1;
+		if (dest.len < n) {
+			return -1;
+		}
+		memcpy(dest.buf, &buf[i + 1], n);
+		return n;
+	}
+}
+
+int common_prefix_size(struct slice a, struct slice b)
+{
+	int p = 0;
+	while (p < a.len && p < b.len) {
+		if (a.buf[p] != b.buf[p]) {
+			break;
+		}
+		p++;
+	}
+
+	return p;
+}
+
+static int decode_string(struct slice *dest, struct slice in)
+{
+	int start_len = in.len;
+	uint64_t tsize = 0;
+	int n = get_var_int(&tsize, in);
+	if (n <= 0) {
+		return -1;
+	}
+	in.buf += n;
+	in.len -= n;
+	if (in.len < tsize) {
+		return -1;
+	}
+
+	slice_resize(dest, tsize + 1);
+	dest->buf[tsize] = 0;
+	memcpy(dest->buf, in.buf, tsize);
+	in.buf += tsize;
+	in.len -= tsize;
+
+	return start_len - in.len;
+}
+
+int encode_key(bool *restart, struct slice dest, struct slice prev_key,
+	       struct slice key, byte extra)
+{
+	struct slice start = dest;
+	int prefix_len = common_prefix_size(prev_key, key);
+	uint64_t suffix_len = key.len - prefix_len;
+	int n = put_var_int(dest, (uint64_t)prefix_len);
+	if (n < 0) {
+		return -1;
+	}
+	dest.buf += n;
+	dest.len -= n;
+
+	*restart = (prefix_len == 0);
+
+	n = put_var_int(dest, suffix_len << 3 | (uint64_t)extra);
+	if (n < 0) {
+		return -1;
+	}
+	dest.buf += n;
+	dest.len -= n;
+
+	if (dest.len < suffix_len) {
+		return -1;
+	}
+	memcpy(dest.buf, key.buf + prefix_len, suffix_len);
+	dest.buf += suffix_len;
+	dest.len -= suffix_len;
+
+	return start.len - dest.len;
+}
+
+static byte ref_record_type(void)
+{
+	return BLOCK_TYPE_REF;
+}
+
+static void ref_record_key(const void *r, struct slice *dest)
+{
+	const struct ref_record *rec = (const struct ref_record *)r;
+	slice_set_string(dest, rec->ref_name);
+}
+
+static void ref_record_copy_from(void *rec, const void *src_rec, int hash_size)
+{
+	struct ref_record *ref = (struct ref_record *)rec;
+	struct ref_record *src = (struct ref_record *)src_rec;
+	assert(hash_size > 0);
+
+	/* This is simple and correct, but we could probably reuse the hash
+           fields. */
+	ref_record_clear(ref);
+	if (src->ref_name != NULL) {
+		ref->ref_name = strdup(src->ref_name);
+	}
+
+	if (src->target != NULL) {
+		ref->target = strdup(src->target);
+	}
+
+	if (src->target_value != NULL) {
+		ref->target_value = malloc(hash_size);
+		memcpy(ref->target_value, src->target_value, hash_size);
+	}
+
+	if (src->value != NULL) {
+		ref->value = malloc(hash_size);
+		memcpy(ref->value, src->value, hash_size);
+	}
+	ref->update_index = src->update_index;
+}
+
+static char hexdigit(int c)
+{
+	if (c <= 9) {
+		return '0' + c;
+	}
+	return 'a' + (c - 10);
+}
+
+static void hex_format(char *dest, byte *src, int hash_size)
+{
+	assert(hash_size > 0);
+	if (src != NULL) {
+		int i = 0;
+		for (i = 0; i < hash_size; i++) {
+			dest[2 * i] = hexdigit(src[i] >> 4);
+			dest[2 * i + 1] = hexdigit(src[i] & 0xf);
+		}
+		dest[2 * hash_size] = 0;
+	}
+}
+
+void ref_record_print(struct ref_record *ref, int hash_size)
+{
+	char hex[SHA256_SIZE + 1] = {};
+
+	printf("ref{%s(%" PRIdMAX ") ", ref->ref_name, ref->update_index);
+	if (ref->value != NULL) {
+		hex_format(hex, ref->value, hash_size);
+		printf("%s", hex);
+	}
+	if (ref->target_value != NULL) {
+		hex_format(hex, ref->target_value, hash_size);
+		printf(" (T %s)", hex);
+	}
+	if (ref->target != NULL) {
+		printf("=> %s", ref->target);
+	}
+	printf("}\n");
+}
+
+static void ref_record_clear_void(void *rec)
+{
+	ref_record_clear((struct ref_record *)rec);
+}
+
+void ref_record_clear(struct ref_record *ref)
+{
+	free(ref->ref_name);
+	free(ref->target);
+	free(ref->target_value);
+	free(ref->value);
+	memset(ref, 0, sizeof(struct ref_record));
+}
+
+static byte ref_record_val_type(const void *rec)
+{
+	const struct ref_record *r = (const struct ref_record *)rec;
+	if (r->value != NULL) {
+		if (r->target_value != NULL) {
+			return 2;
+		} else {
+			return 1;
+		}
+	} else if (r->target != NULL) {
+		return 3;
+	}
+	return 0;
+}
+
+static int encode_string(char *str, struct slice s)
+{
+	struct slice start = s;
+	int l = strlen(str);
+	int n = put_var_int(s, l);
+	if (n < 0) {
+		return -1;
+	}
+	s.buf += n;
+	s.len -= n;
+	if (s.len < l) {
+		return -1;
+	}
+	memcpy(s.buf, str, l);
+	s.buf += l;
+	s.len -= l;
+
+	return start.len - s.len;
+}
+
+static int ref_record_encode(const void *rec, struct slice s, int hash_size)
+{
+	const struct ref_record *r = (const struct ref_record *)rec;
+	struct slice start = s;
+	int n = put_var_int(s, r->update_index);
+	assert(hash_size > 0);
+	if (n < 0) {
+		return -1;
+	}
+	s.buf += n;
+	s.len -= n;
+
+	if (r->value != NULL) {
+		if (s.len < hash_size) {
+			return -1;
+		}
+		memcpy(s.buf, r->value, hash_size);
+		s.buf += hash_size;
+		s.len -= hash_size;
+	}
+
+	if (r->target_value != NULL) {
+		if (s.len < hash_size) {
+			return -1;
+		}
+		memcpy(s.buf, r->target_value, hash_size);
+		s.buf += hash_size;
+		s.len -= hash_size;
+	}
+
+	if (r->target != NULL) {
+		int n = encode_string(r->target, s);
+		if (n < 0) {
+			return -1;
+		}
+		s.buf += n;
+		s.len -= n;
+	}
+
+	return start.len - s.len;
+}
+
+static int ref_record_decode(void *rec, struct slice key, byte val_type,
+			     struct slice in, int hash_size)
+{
+	struct ref_record *r = (struct ref_record *)rec;
+	struct slice start = in;
+	bool seen_value = false;
+	bool seen_target_value = false;
+	bool seen_target = false;
+
+	int n = get_var_int(&r->update_index, in);
+	if (n < 0) {
+		return n;
+	}
+	assert(hash_size > 0);
+
+	in.buf += n;
+	in.len -= n;
+
+	r->ref_name = realloc(r->ref_name, key.len + 1);
+	memcpy(r->ref_name, key.buf, key.len);
+	r->ref_name[key.len] = 0;
+
+	switch (val_type) {
+	case 1:
+	case 2:
+		if (in.len < hash_size) {
+			return -1;
+		}
+
+		if (r->value == NULL) {
+			r->value = malloc(hash_size);
+		}
+		seen_value = true;
+		memcpy(r->value, in.buf, hash_size);
+		in.buf += hash_size;
+		in.len -= hash_size;
+		if (val_type == 1) {
+			break;
+		}
+		if (r->target_value == NULL) {
+			r->target_value = malloc(hash_size);
+		}
+		seen_target_value = true;
+		memcpy(r->target_value, in.buf, hash_size);
+		in.buf += hash_size;
+		in.len -= hash_size;
+		break;
+	case 3: {
+		struct slice dest = {};
+		int n = decode_string(&dest, in);
+		if (n < 0) {
+			return -1;
+		}
+		in.buf += n;
+		in.len -= n;
+		seen_target = true;
+		r->target = (char *)slice_as_string(&dest);
+	} break;
+
+	case 0:
+		break;
+	default:
+		abort();
+		break;
+	}
+
+	if (!seen_target && r->target != NULL) {
+		free(r->target);
+		r->target = NULL;
+	}
+	if (!seen_target_value && r->target_value != NULL) {
+		free(r->target_value);
+		r->target_value = NULL;
+	}
+	if (!seen_value && r->value != NULL) {
+		free(r->value);
+		r->value = NULL;
+	}
+
+	return start.len - in.len;
+}
+
+int decode_key(struct slice *key, byte *extra, struct slice last_key,
+	       struct slice in)
+{
+	int start_len = in.len;
+	uint64_t prefix_len = 0;
+	uint64_t suffix_len = 0;
+	int n = get_var_int(&prefix_len, in);
+	if (n < 0) {
+		return -1;
+	}
+	in.buf += n;
+	in.len -= n;
+
+	if (prefix_len > last_key.len) {
+		return -1;
+	}
+
+	n = get_var_int(&suffix_len, in);
+	if (n <= 0) {
+		return -1;
+	}
+	in.buf += n;
+	in.len -= n;
+
+	*extra = (byte)(suffix_len & 0x7);
+	suffix_len >>= 3;
+
+	if (in.len < suffix_len) {
+		return -1;
+	}
+
+	slice_resize(key, suffix_len + prefix_len);
+	memcpy(key->buf, last_key.buf, prefix_len);
+
+	memcpy(key->buf + prefix_len, in.buf, suffix_len);
+	in.buf += suffix_len;
+	in.len -= suffix_len;
+
+	return start_len - in.len;
+}
+
+struct record_vtable ref_record_vtable = {
+	.key = &ref_record_key,
+	.type = &ref_record_type,
+	.copy_from = &ref_record_copy_from,
+	.val_type = &ref_record_val_type,
+	.encode = &ref_record_encode,
+	.decode = &ref_record_decode,
+	.clear = &ref_record_clear_void,
+};
+
+static byte obj_record_type(void)
+{
+	return BLOCK_TYPE_OBJ;
+}
+
+static void obj_record_key(const void *r, struct slice *dest)
+{
+	const struct obj_record *rec = (const struct obj_record *)r;
+	slice_resize(dest, rec->hash_prefix_len);
+	memcpy(dest->buf, rec->hash_prefix, rec->hash_prefix_len);
+}
+
+static void obj_record_copy_from(void *rec, const void *src_rec, int hash_size)
+{
+	struct obj_record *ref = (struct obj_record *)rec;
+	const struct obj_record *src = (const struct obj_record *)src_rec;
+
+	*ref = *src;
+	ref->hash_prefix = malloc(ref->hash_prefix_len);
+	memcpy(ref->hash_prefix, src->hash_prefix, ref->hash_prefix_len);
+
+	{
+		int olen = ref->offset_len * sizeof(uint64_t);
+		ref->offsets = malloc(olen);
+		memcpy(ref->offsets, src->offsets, olen);
+	}
+}
+
+static void obj_record_clear(void *rec)
+{
+	struct obj_record *ref = (struct obj_record *)rec;
+	free(ref->hash_prefix);
+	free(ref->offsets);
+	memset(ref, 0, sizeof(struct obj_record));
+}
+
+static byte obj_record_val_type(const void *rec)
+{
+	struct obj_record *r = (struct obj_record *)rec;
+	if (r->offset_len > 0 && r->offset_len < 8) {
+		return r->offset_len;
+	}
+	return 0;
+}
+
+static int obj_record_encode(const void *rec, struct slice s, int hash_size)
+{
+	struct obj_record *r = (struct obj_record *)rec;
+	struct slice start = s;
+	int n = 0;
+	if (r->offset_len == 0 || r->offset_len >= 8) {
+		n = put_var_int(s, r->offset_len);
+		if (n < 0) {
+			return -1;
+		}
+		s.buf += n;
+		s.len -= n;
+	}
+	if (r->offset_len == 0) {
+		return start.len - s.len;
+	}
+	n = put_var_int(s, r->offsets[0]);
+	if (n < 0) {
+		return -1;
+	}
+	s.buf += n;
+	s.len -= n;
+
+	{
+		uint64_t last = r->offsets[0];
+		int i = 0;
+		for (i = 1; i < r->offset_len; i++) {
+			int n = put_var_int(s, r->offsets[i] - last);
+			if (n < 0) {
+				return -1;
+			}
+			s.buf += n;
+			s.len -= n;
+			last = r->offsets[i];
+		}
+	}
+	return start.len - s.len;
+}
+
+static int obj_record_decode(void *rec, struct slice key, byte val_type,
+			     struct slice in, int hash_size)
+{
+	struct slice start = in;
+	struct obj_record *r = (struct obj_record *)rec;
+	uint64_t count = val_type;
+	int n = 0;
+	r->hash_prefix = malloc(key.len);
+	memcpy(r->hash_prefix, key.buf, key.len);
+	r->hash_prefix_len = key.len;
+
+	if (val_type == 0) {
+		n = get_var_int(&count, in);
+		if (n < 0) {
+			return n;
+		}
+
+		in.buf += n;
+		in.len -= n;
+	}
+
+	r->offsets = NULL;
+	r->offset_len = 0;
+	if (count == 0) {
+		return start.len - in.len;
+	}
+
+	r->offsets = malloc(count * sizeof(uint64_t));
+	r->offset_len = count;
+
+	n = get_var_int(&r->offsets[0], in);
+	if (n < 0) {
+		return n;
+	}
+
+	in.buf += n;
+	in.len -= n;
+
+	{
+		uint64_t last = r->offsets[0];
+		int j = 1;
+		while (j < count) {
+			uint64_t delta = 0;
+			int n = get_var_int(&delta, in);
+			if (n < 0) {
+				return n;
+			}
+
+			in.buf += n;
+			in.len -= n;
+
+			last = r->offsets[j] = (delta + last);
+			j++;
+		}
+	}
+	return start.len - in.len;
+}
+
+struct record_vtable obj_record_vtable = {
+	.key = &obj_record_key,
+	.type = &obj_record_type,
+	.copy_from = &obj_record_copy_from,
+	.val_type = &obj_record_val_type,
+	.encode = &obj_record_encode,
+	.decode = &obj_record_decode,
+	.clear = &obj_record_clear,
+};
+
+void log_record_print(struct log_record *log, int hash_size)
+{
+	char hex[SHA256_SIZE + 1] = {};
+
+	printf("log{%s(%" PRIdMAX ") %s <%s> %lu %04d\n", log->ref_name,
+	       log->update_index, log->name, log->email, log->time,
+	       log->tz_offset);
+	hex_format(hex, log->old_hash, hash_size);
+	printf("%s => ", hex);
+	hex_format(hex, log->new_hash, hash_size);
+	printf("%s\n\n%s\n}\n", hex, log->message);
+}
+
+static byte log_record_type(void)
+{
+	return BLOCK_TYPE_LOG;
+}
+
+static void log_record_key(const void *r, struct slice *dest)
+{
+	const struct log_record *rec = (const struct log_record *)r;
+	int len = strlen(rec->ref_name);
+	uint64_t ts = 0;
+	slice_resize(dest, len + 9);
+	memcpy(dest->buf, rec->ref_name, len + 1);
+	ts = (~ts) - rec->update_index;
+	put_u64(dest->buf + 1 + len, ts);
+}
+
+static void log_record_copy_from(void *rec, const void *src_rec, int hash_size)
+{
+	struct log_record *dst = (struct log_record *)rec;
+	const struct log_record *src = (const struct log_record *)src_rec;
+
+	*dst = *src;
+	dst->ref_name = strdup(dst->ref_name);
+	dst->email = strdup(dst->email);
+	dst->name = strdup(dst->name);
+	dst->message = strdup(dst->message);
+	if (dst->new_hash != NULL) {
+		dst->new_hash = malloc(hash_size);
+		memcpy(dst->new_hash, src->new_hash, hash_size);
+	}
+	if (dst->old_hash != NULL) {
+		dst->old_hash = malloc(hash_size);
+		memcpy(dst->old_hash, src->old_hash, hash_size);
+	}
+}
+
+static void log_record_clear_void(void *rec)
+{
+	struct log_record *r = (struct log_record *)rec;
+	log_record_clear(r);
+}
+
+void log_record_clear(struct log_record *r)
+{
+	free(r->ref_name);
+	free(r->new_hash);
+	free(r->old_hash);
+	free(r->name);
+	free(r->email);
+	free(r->message);
+	memset(r, 0, sizeof(struct log_record));
+}
+
+static byte log_record_val_type(const void *rec)
+{
+	return 1;
+}
+
+static byte zero[SHA256_SIZE] = {};
+
+static int log_record_encode(const void *rec, struct slice s, int hash_size)
+{
+	struct log_record *r = (struct log_record *)rec;
+	struct slice start = s;
+	int n = 0;
+	byte *oldh = r->old_hash;
+	byte *newh = r->new_hash;
+	if (oldh == NULL) {
+		oldh = zero;
+	}
+	if (newh == NULL) {
+		newh = zero;
+	}
+
+	if (s.len < 2 * hash_size) {
+		return -1;
+	}
+
+	memcpy(s.buf, oldh, hash_size);
+	memcpy(s.buf + hash_size, newh, hash_size);
+	s.buf += 2 * hash_size;
+	s.len -= 2 * hash_size;
+
+	n = encode_string(r->name ? r->name : "", s);
+	if (n < 0) {
+		return -1;
+	}
+	s.len -= n;
+	s.buf += n;
+
+	n = encode_string(r->email ? r->email : "", s);
+	if (n < 0) {
+		return -1;
+	}
+	s.len -= n;
+	s.buf += n;
+
+	n = put_var_int(s, r->time);
+	if (n < 0) {
+		return -1;
+	}
+	s.buf += n;
+	s.len -= n;
+
+	if (s.len < 2) {
+		return -1;
+	}
+
+	put_u16(s.buf, r->tz_offset);
+	s.buf += 2;
+	s.len -= 2;
+
+	n = encode_string(r->message ? r->message : "", s);
+	if (n < 0) {
+		return -1;
+	}
+	s.len -= n;
+	s.buf += n;
+
+	return start.len - s.len;
+}
+
+static int log_record_decode(void *rec, struct slice key, byte val_type,
+			     struct slice in, int hash_size)
+{
+	struct slice start = in;
+	struct log_record *r = (struct log_record *)rec;
+	uint64_t max = 0;
+	uint64_t ts = 0;
+	struct slice dest = {};
+	int n;
+
+	if (key.len <= 9 || key.buf[key.len - 9] != 0) {
+		return FORMAT_ERROR;
+	}
+
+	r->ref_name = realloc(r->ref_name, key.len - 8);
+	memcpy(r->ref_name, key.buf, key.len - 8);
+	ts = get_u64(key.buf + key.len - 8);
+
+	r->update_index = (~max) - ts;
+
+	if (in.len < 2 * hash_size) {
+		return FORMAT_ERROR;
+	}
+
+	r->old_hash = realloc(r->old_hash, hash_size);
+	r->new_hash = realloc(r->new_hash, hash_size);
+
+	memcpy(r->old_hash, in.buf, hash_size);
+	memcpy(r->new_hash, in.buf + hash_size, hash_size);
+
+	in.buf += 2 * hash_size;
+	in.len -= 2 * hash_size;
+
+	n = decode_string(&dest, in);
+	if (n < 0) {
+		goto error;
+	}
+	in.len -= n;
+	in.buf += n;
+
+	r->name = realloc(r->name, dest.len + 1);
+	memcpy(r->name, dest.buf, dest.len);
+	r->name[dest.len] = 0;
+
+	slice_resize(&dest, 0);
+	n = decode_string(&dest, in);
+	if (n < 0) {
+		goto error;
+	}
+	in.len -= n;
+	in.buf += n;
+
+	r->email = realloc(r->email, dest.len + 1);
+	memcpy(r->email, dest.buf, dest.len);
+	r->email[dest.len] = 0;
+
+	ts = 0;
+	n = get_var_int(&ts, in);
+	if (n < 0) {
+		goto error;
+	}
+	in.len -= n;
+	in.buf += n;
+	r->time = ts;
+	if (in.len < 2) {
+		goto error;
+	}
+
+	r->tz_offset = get_u16(in.buf);
+	in.buf += 2;
+	in.len -= 2;
+
+	slice_resize(&dest, 0);
+	n = decode_string(&dest, in);
+	if (n < 0) {
+		goto error;
+	}
+	in.len -= n;
+	in.buf += n;
+
+	r->message = realloc(r->message, dest.len + 1);
+	memcpy(r->message, dest.buf, dest.len);
+	r->message[dest.len] = 0;
+
+	return start.len - in.len;
+
+error:
+	free(slice_yield(&dest));
+	return FORMAT_ERROR;
+}
+
+static bool null_streq(char *a, char *b)
+{
+	char *empty = "";
+	if (a == NULL) {
+		a = empty;
+	}
+	if (b == NULL) {
+		b = empty;
+	}
+	return 0 == strcmp(a, b);
+}
+
+static bool zero_hash_eq(byte *a, byte *b, int sz)
+{
+	if (a == NULL) {
+		a = zero;
+	}
+	if (b == NULL) {
+		b = zero;
+	}
+	return !memcmp(a, b, sz);
+}
+
+bool log_record_equal(struct log_record *a, struct log_record *b, int hash_size)
+{
+	return null_streq(a->name, b->name) && null_streq(a->email, b->email) &&
+	       null_streq(a->message, b->message) &&
+	       zero_hash_eq(a->old_hash, b->old_hash, hash_size) &&
+	       zero_hash_eq(a->new_hash, b->new_hash, hash_size) &&
+	       a->time == b->time && a->tz_offset == b->tz_offset &&
+	       a->update_index == b->update_index;
+}
+
+struct record_vtable log_record_vtable = {
+	.key = &log_record_key,
+	.type = &log_record_type,
+	.copy_from = &log_record_copy_from,
+	.val_type = &log_record_val_type,
+	.encode = &log_record_encode,
+	.decode = &log_record_decode,
+	.clear = &log_record_clear_void,
+};
+
+struct record new_record(byte typ)
+{
+	struct record rec;
+	switch (typ) {
+	case BLOCK_TYPE_REF: {
+		struct ref_record *r = calloc(1, sizeof(struct ref_record));
+		record_from_ref(&rec, r);
+		return rec;
+	}
+
+	case BLOCK_TYPE_OBJ: {
+		struct obj_record *r = calloc(1, sizeof(struct obj_record));
+		record_from_obj(&rec, r);
+		return rec;
+	}
+	case BLOCK_TYPE_LOG: {
+		struct log_record *r = calloc(1, sizeof(struct log_record));
+		record_from_log(&rec, r);
+		return rec;
+	}
+	case BLOCK_TYPE_INDEX: {
+		struct index_record *r = calloc(1, sizeof(struct index_record));
+		record_from_index(&rec, r);
+		return rec;
+	}
+	}
+	abort();
+	return rec;
+}
+
+static byte index_record_type(void)
+{
+	return BLOCK_TYPE_INDEX;
+}
+
+static void index_record_key(const void *r, struct slice *dest)
+{
+	struct index_record *rec = (struct index_record *)r;
+	slice_copy(dest, rec->last_key);
+}
+
+static void index_record_copy_from(void *rec, const void *src_rec,
+				   int hash_size)
+{
+	struct index_record *dst = (struct index_record *)rec;
+	struct index_record *src = (struct index_record *)src_rec;
+
+	slice_copy(&dst->last_key, src->last_key);
+	dst->offset = src->offset;
+}
+
+static void index_record_clear(void *rec)
+{
+	struct index_record *idx = (struct index_record *)rec;
+	free(slice_yield(&idx->last_key));
+}
+
+static byte index_record_val_type(const void *rec)
+{
+	return 0;
+}
+
+static int index_record_encode(const void *rec, struct slice out, int hash_size)
+{
+	const struct index_record *r = (const struct index_record *)rec;
+	struct slice start = out;
+
+	int n = put_var_int(out, r->offset);
+	if (n < 0) {
+		return n;
+	}
+
+	out.buf += n;
+	out.len -= n;
+
+	return start.len - out.len;
+}
+
+static int index_record_decode(void *rec, struct slice key, byte val_type,
+			       struct slice in, int hash_size)
+{
+	struct slice start = in;
+	struct index_record *r = (struct index_record *)rec;
+	int n = 0;
+
+	slice_copy(&r->last_key, key);
+
+	n = get_var_int(&r->offset, in);
+	if (n < 0) {
+		return n;
+	}
+
+	in.buf += n;
+	in.len -= n;
+	return start.len - in.len;
+}
+
+struct record_vtable index_record_vtable = {
+	.key = &index_record_key,
+	.type = &index_record_type,
+	.copy_from = &index_record_copy_from,
+	.val_type = &index_record_val_type,
+	.encode = &index_record_encode,
+	.decode = &index_record_decode,
+	.clear = &index_record_clear,
+};
+
+void record_key(struct record rec, struct slice *dest)
+{
+	rec.ops->key(rec.data, dest);
+}
+
+byte record_type(struct record rec)
+{
+	return rec.ops->type();
+}
+
+int record_encode(struct record rec, struct slice dest, int hash_size)
+{
+	return rec.ops->encode(rec.data, dest, hash_size);
+}
+
+void record_copy_from(struct record rec, struct record src, int hash_size)
+{
+	assert(src.ops->type() == rec.ops->type());
+
+	rec.ops->copy_from(rec.data, src.data, hash_size);
+}
+
+byte record_val_type(struct record rec)
+{
+	return rec.ops->val_type(rec.data);
+}
+
+int record_decode(struct record rec, struct slice key, byte extra,
+		  struct slice src, int hash_size)
+{
+	return rec.ops->decode(rec.data, key, extra, src, hash_size);
+}
+
+void record_clear(struct record rec)
+{
+	return rec.ops->clear(rec.data);
+}
+
+void record_from_ref(struct record *rec, struct ref_record *ref_rec)
+{
+	rec->data = ref_rec;
+	rec->ops = &ref_record_vtable;
+}
+
+void record_from_obj(struct record *rec, struct obj_record *obj_rec)
+{
+	rec->data = obj_rec;
+	rec->ops = &obj_record_vtable;
+}
+
+void record_from_index(struct record *rec, struct index_record *index_rec)
+{
+	rec->data = index_rec;
+	rec->ops = &index_record_vtable;
+}
+
+void record_from_log(struct record *rec, struct log_record *log_rec)
+{
+	rec->data = log_rec;
+	rec->ops = &log_record_vtable;
+}
+
+void *record_yield(struct record *rec)
+{
+	void *p = rec->data;
+	rec->data = NULL;
+	return p;
+}
+
+struct ref_record *record_as_ref(struct record rec)
+{
+	assert(record_type(rec) == BLOCK_TYPE_REF);
+	return (struct ref_record *)rec.data;
+}
+
+static bool hash_equal(byte *a, byte *b, int hash_size)
+{
+	if (a != NULL && b != NULL) {
+		return !memcmp(a, b, hash_size);
+	}
+
+	return a == b;
+}
+
+static bool str_equal(char *a, char *b)
+{
+	if (a != NULL && b != NULL) {
+		return 0 == strcmp(a, b);
+	}
+
+	return a == b;
+}
+
+bool ref_record_equal(struct ref_record *a, struct ref_record *b, int hash_size)
+{
+	assert(hash_size > 0);
+	return 0 == strcmp(a->ref_name, b->ref_name) &&
+	       a->update_index == b->update_index &&
+	       hash_equal(a->value, b->value, hash_size) &&
+	       hash_equal(a->target_value, b->target_value, hash_size) &&
+	       str_equal(a->target, b->target);
+}
+
+int ref_record_compare_name(const void *a, const void *b)
+{
+	return strcmp(((struct ref_record *)a)->ref_name,
+		      ((struct ref_record *)b)->ref_name);
+}
+
+bool ref_record_is_deletion(const struct ref_record *ref)
+{
+	return ref->value == NULL && ref->target == NULL &&
+	       ref->target_value == NULL;
+}
+
+int log_record_compare_key(const void *a, const void *b)
+{
+	struct log_record *la = (struct log_record *)a;
+	struct log_record *lb = (struct log_record *)b;
+
+	int cmp = strcmp(la->ref_name, lb->ref_name);
+	if (cmp) {
+		return cmp;
+	}
+	if (la->update_index > lb->update_index) {
+		return -1;
+	}
+	return (la->update_index < lb->update_index) ? 1 : 0;
+}
+
+bool log_record_is_deletion(const struct log_record *log)
+{
+	/* XXX */
+	return false;
+}
diff --git a/reftable/record.h b/reftable/record.h
new file mode 100644
index 0000000000..dffdd71fc2
--- /dev/null
+++ b/reftable/record.h
@@ -0,0 +1,79 @@
+/*
+Copyright 2020 Google LLC
+
+Use of this source code is governed by a BSD-style
+license that can be found in the LICENSE file or at
+https://developers.google.com/open-source/licenses/bsd
+*/
+
+#ifndef RECORD_H
+#define RECORD_H
+
+#include "reftable.h"
+#include "slice.h"
+
+struct record_vtable {
+	void (*key)(const void *rec, struct slice *dest);
+	byte (*type)(void);
+	void (*copy_from)(void *rec, const void *src, int hash_size);
+	byte (*val_type)(const void *rec);
+	int (*encode)(const void *rec, struct slice dest, int hash_size);
+	int (*decode)(void *rec, struct slice key, byte extra, struct slice src,
+		      int hash_size);
+	void (*clear)(void *rec);
+};
+
+/* record is a generic wrapper for differnt types of records. */
+struct record {
+	void *data;
+	struct record_vtable *ops;
+};
+
+int get_var_int(uint64_t *dest, struct slice in);
+int put_var_int(struct slice dest, uint64_t val);
+int common_prefix_size(struct slice a, struct slice b);
+
+int is_block_type(byte typ);
+struct record new_record(byte typ);
+
+extern struct record_vtable ref_record_vtable;
+
+int encode_key(bool *restart, struct slice dest, struct slice prev_key,
+	       struct slice key, byte extra);
+int decode_key(struct slice *key, byte *extra, struct slice last_key,
+	       struct slice in);
+
+struct index_record {
+	struct slice last_key;
+	uint64_t offset;
+};
+
+struct obj_record {
+	byte *hash_prefix;
+	int hash_prefix_len;
+	uint64_t *offsets;
+	int offset_len;
+};
+
+void record_key(struct record rec, struct slice *dest);
+byte record_type(struct record rec);
+void record_copy_from(struct record rec, struct record src, int hash_size);
+byte record_val_type(struct record rec);
+int record_encode(struct record rec, struct slice dest, int hash_size);
+int record_decode(struct record rec, struct slice key, byte extra,
+		  struct slice src, int hash_size);
+void record_clear(struct record rec);
+void *record_yield(struct record *rec);
+void record_from_obj(struct record *rec, struct obj_record *objrec);
+void record_from_index(struct record *rec, struct index_record *idxrec);
+void record_from_ref(struct record *rec, struct ref_record *refrec);
+void record_from_log(struct record *rec, struct log_record *logrec);
+struct ref_record *record_as_ref(struct record ref);
+
+/* for qsort. */
+int ref_record_compare_name(const void *a, const void *b);
+
+/* for qsort. */
+int log_record_compare_key(const void *a, const void *b);
+
+#endif
diff --git a/reftable/record_test.c b/reftable/record_test.c
new file mode 100644
index 0000000000..b95ac6aa1a
--- /dev/null
+++ b/reftable/record_test.c
@@ -0,0 +1,332 @@
+/*
+Copyright 2020 Google LLC
+
+Use of this source code is governed by a BSD-style
+license that can be found in the LICENSE file or at
+https://developers.google.com/open-source/licenses/bsd
+*/
+
+#include "record.h"
+
+#include "system.h"
+
+#include "basics.h"
+#include "constants.h"
+#include "reftable.h"
+#include "test_framework.h"
+
+void varint_roundtrip()
+{
+	uint64_t inputs[] = { 0,
+			      1,
+			      27,
+			      127,
+			      128,
+			      257,
+			      4096,
+			      ((uint64_t)1 << 63),
+			      ((uint64_t)1 << 63) + ((uint64_t)1 << 63) - 1 };
+	int i = 0;
+	for (i = 0; i < ARRAYSIZE(inputs); i++) {
+		byte dest[10];
+
+		struct slice out = { .buf = dest, .len = 10, .cap = 10 };
+
+		uint64_t in = inputs[i];
+		int n = put_var_int(out, in);
+		assert(n > 0);
+		out.len = n;
+
+		uint64_t got = 0;
+		n = get_var_int(&got, out);
+		assert(n > 0);
+
+		assert(got == in);
+	}
+}
+
+void test_common_prefix()
+{
+	struct {
+		const char *a, *b;
+		int want;
+	} cases[] = {
+		{ "abc", "ab", 2 },
+		{ "", "abc", 0 },
+		{ "abc", "abd", 2 },
+		{ "abc", "pqr", 0 },
+	};
+
+	int i = 0;
+	for (i = 0; i < ARRAYSIZE(cases); i++) {
+		struct slice a = {};
+		struct slice b = {};
+		slice_set_string(&a, cases[i].a);
+		slice_set_string(&b, cases[i].b);
+
+		int got = common_prefix_size(a, b);
+		assert(got == cases[i].want);
+
+		free(slice_yield(&a));
+		free(slice_yield(&b));
+	}
+}
+
+void set_hash(byte *h, int j)
+{
+	int i = 0;
+	for (i = 0; i < SHA1_SIZE; i++) {
+		h[i] = (j >> i) & 0xff;
+	}
+}
+
+void test_ref_record_roundtrip()
+{
+	int i = 0;
+	for (i = 0; i <= 3; i++) {
+		printf("subtest %d\n", i);
+		struct ref_record in = {};
+		switch (i) {
+		case 0:
+			break;
+		case 1:
+			in.value = malloc(SHA1_SIZE);
+			set_hash(in.value, 1);
+			break;
+		case 2:
+			in.value = malloc(SHA1_SIZE);
+			set_hash(in.value, 1);
+			in.target_value = malloc(SHA1_SIZE);
+			set_hash(in.target_value, 2);
+			break;
+		case 3:
+			in.target = strdup("target");
+			break;
+		}
+		in.ref_name = strdup("refs/heads/master");
+
+		struct record rec = {};
+		record_from_ref(&rec, &in);
+		assert(record_val_type(rec) == i);
+		byte buf[1024];
+		struct slice key = {};
+		record_key(rec, &key);
+		struct slice dest = {
+			.buf = buf,
+			.len = sizeof(buf),
+		};
+		int n = record_encode(rec, dest, SHA1_SIZE);
+		assert(n > 0);
+
+		struct ref_record out = {};
+		struct record rec_out = {};
+		record_from_ref(&rec_out, &out);
+		int m = record_decode(rec_out, key, i, dest, SHA1_SIZE);
+		assert(n == m);
+
+		assert((out.value != NULL) == (in.value != NULL));
+		assert((out.target_value != NULL) == (in.target_value != NULL));
+		assert((out.target != NULL) == (in.target != NULL));
+		free(slice_yield(&key));
+		record_clear(rec_out);
+		ref_record_clear(&in);
+	}
+}
+
+void test_log_record_roundtrip()
+{
+	struct log_record in = {
+		.ref_name = strdup("refs/heads/master"),
+		.old_hash = malloc(SHA1_SIZE),
+		.new_hash = malloc(SHA1_SIZE),
+		.name = strdup("han-wen"),
+		.email = strdup("hanwen@google.com"),
+		.message = strdup("test"),
+		.update_index = 42,
+		.time = 1577123507,
+		.tz_offset = 100,
+	};
+
+	struct record rec = {};
+	record_from_log(&rec, &in);
+
+	struct slice key = {};
+	record_key(rec, &key);
+
+	byte buf[1024];
+	struct slice dest = {
+		.buf = buf,
+		.len = sizeof(buf),
+	};
+
+	int n = record_encode(rec, dest, SHA1_SIZE);
+	assert(n > 0);
+
+	struct log_record out = {};
+	struct record rec_out = {};
+	record_from_log(&rec_out, &out);
+	int valtype = record_val_type(rec);
+	int m = record_decode(rec_out, key, valtype, dest, SHA1_SIZE);
+	assert(n == m);
+
+	assert(log_record_equal(&in, &out, SHA1_SIZE));
+	log_record_clear(&in);
+	free(slice_yield(&key));
+	record_clear(rec_out);
+}
+
+void test_u24_roundtrip()
+{
+	uint32_t in = 0x112233;
+	byte dest[3];
+
+	put_u24(dest, in);
+	uint32_t out = get_u24(dest);
+	assert(in == out);
+}
+
+void test_key_roundtrip()
+{
+	struct slice dest = {}, last_key = {}, key = {}, roundtrip = {};
+
+	slice_resize(&dest, 1024);
+	slice_set_string(&last_key, "refs/heads/master");
+	slice_set_string(&key, "refs/tags/bla");
+
+	bool restart;
+	byte extra = 6;
+	int n = encode_key(&restart, dest, last_key, key, extra);
+	assert(!restart);
+	assert(n > 0);
+
+	byte rt_extra;
+	int m = decode_key(&roundtrip, &rt_extra, last_key, dest);
+	assert(n == m);
+	assert(slice_equal(key, roundtrip));
+	assert(rt_extra == extra);
+
+	free(slice_yield(&last_key));
+	free(slice_yield(&key));
+	free(slice_yield(&dest));
+	free(slice_yield(&roundtrip));
+}
+
+void print_bytes(byte *p, int l)
+{
+	int i = 0;
+	for (i = 0; i < l; i++) {
+		byte c = *p;
+		if (c < 32) {
+			c = '.';
+		}
+		printf("%02x[%c] ", p[i], c);
+	}
+	printf("(%d)\n", l);
+}
+
+void test_obj_record_roundtrip()
+{
+	byte testHash1[SHA1_SIZE] = {};
+	set_hash(testHash1, 1);
+	uint64_t till9[] = { 1, 2, 3, 4, 500, 600, 700, 800, 9000 };
+
+	struct obj_record recs[3] = { {
+					      .hash_prefix = testHash1,
+					      .hash_prefix_len = 5,
+					      .offsets = till9,
+					      .offset_len = 3,
+				      },
+				      {
+					      .hash_prefix = testHash1,
+					      .hash_prefix_len = 5,
+					      .offsets = till9,
+					      .offset_len = 9,
+				      },
+				      {
+					      .hash_prefix = testHash1,
+					      .hash_prefix_len = 5,
+				      }
+
+	};
+	int i = 0;
+	for (i = 0; i < ARRAYSIZE(recs); i++) {
+		printf("subtest %d\n", i);
+		struct obj_record in = recs[i];
+		byte buf[1024];
+		struct record rec = {};
+		record_from_obj(&rec, &in);
+		struct slice key = {};
+		record_key(rec, &key);
+		struct slice dest = {
+			.buf = buf,
+			.len = sizeof(buf),
+		};
+		int n = record_encode(rec, dest, SHA1_SIZE);
+		assert(n > 0);
+		byte extra = record_val_type(rec);
+		struct obj_record out = {};
+		struct record rec_out = {};
+		record_from_obj(&rec_out, &out);
+		int m = record_decode(rec_out, key, extra, dest, SHA1_SIZE);
+		assert(n == m);
+
+		assert(in.hash_prefix_len == out.hash_prefix_len);
+		assert(in.offset_len == out.offset_len);
+
+		assert(!memcmp(in.hash_prefix, out.hash_prefix,
+			       in.hash_prefix_len));
+		assert(0 == memcmp(in.offsets, out.offsets,
+				   sizeof(uint64_t) * in.offset_len));
+		free(slice_yield(&key));
+		record_clear(rec_out);
+	}
+}
+
+void test_index_record_roundtrip()
+{
+	struct index_record in = { .offset = 42 };
+
+	slice_set_string(&in.last_key, "refs/heads/master");
+
+	struct slice key = {};
+	struct record rec = {};
+	record_from_index(&rec, &in);
+	record_key(rec, &key);
+
+	assert(0 == slice_compare(key, in.last_key));
+
+	byte buf[1024];
+	struct slice dest = {
+		.buf = buf,
+		.len = sizeof(buf),
+	};
+	int n = record_encode(rec, dest, SHA1_SIZE);
+	assert(n > 0);
+
+	byte extra = record_val_type(rec);
+	struct index_record out = {};
+	struct record out_rec;
+	record_from_index(&out_rec, &out);
+	int m = record_decode(out_rec, key, extra, dest, SHA1_SIZE);
+	assert(m == n);
+
+	assert(in.offset == out.offset);
+
+	record_clear(out_rec);
+	free(slice_yield(&key));
+	free(slice_yield(&in.last_key));
+}
+
+int main()
+{
+	add_test_case("test_log_record_roundtrip", &test_log_record_roundtrip);
+	add_test_case("test_ref_record_roundtrip", &test_ref_record_roundtrip);
+	add_test_case("varint_roundtrip", &varint_roundtrip);
+	add_test_case("test_key_roundtrip", &test_key_roundtrip);
+	add_test_case("test_common_prefix", &test_common_prefix);
+	add_test_case("test_obj_record_roundtrip", &test_obj_record_roundtrip);
+	add_test_case("test_index_record_roundtrip",
+		      &test_index_record_roundtrip);
+	add_test_case("test_u24_roundtrip", &test_u24_roundtrip);
+	test_main();
+}
diff --git a/reftable/reftable.h b/reftable/reftable.h
new file mode 100644
index 0000000000..d84cc2004a
--- /dev/null
+++ b/reftable/reftable.h
@@ -0,0 +1,399 @@
+/*
+Copyright 2020 Google LLC
+
+Use of this source code is governed by a BSD-style
+license that can be found in the LICENSE file or at
+https://developers.google.com/open-source/licenses/bsd
+*/
+
+#ifndef REFTABLE_H
+#define REFTABLE_H
+
+#include "system.h"
+
+typedef uint8_t byte;
+typedef byte bool;
+
+/* block_source is a generic wrapper for a seekable readable file.
+   It is generally passed around by value.
+ */
+struct block_source {
+	struct block_source_vtable *ops;
+	void *arg;
+};
+
+/* a contiguous segment of bytes. It keeps track of its generating block_source
+   so it can return itself into the pool.
+*/
+struct block {
+	byte *data;
+	int len;
+	struct block_source source;
+};
+
+/* block_source_vtable are the operations that make up block_source */
+struct block_source_vtable {
+	/* returns the size of a block source */
+	uint64_t (*size)(void *source);
+
+	/* reads a segment from the block source. It is an error to read
+	   beyond the end of the block */
+	int (*read_block)(void *source, struct block *dest, uint64_t off,
+			  uint32_t size);
+	/* mark the block as read; may return the data back to malloc */
+	void (*return_block)(void *source, struct block *blockp);
+
+	/* release all resources associated with the block source */
+	void (*close)(void *source);
+};
+
+/* opens a file on the file system as a block_source */
+int block_source_from_file(struct block_source *block_src, const char *name);
+
+/* write_options sets options for writing a single reftable. */
+struct write_options {
+	/* do not pad out blocks to block size. */
+	bool unpadded;
+
+	/* the blocksize. Should be less than 2^24. */
+	uint32_t block_size;
+
+	/* do not generate a SHA1 => ref index. */
+	bool skip_index_objects;
+
+	/* how often to write complete keys in each block. */
+	int restart_interval;
+};
+
+/* ref_record holds a ref database entry target_value */
+struct ref_record {
+	char *ref_name; /* Name of the ref, malloced. */
+	uint64_t update_index; /* Logical timestamp at which this value is
+				  written */
+	byte *value; /* SHA1, or NULL. malloced. */
+	byte *target_value; /* peeled annotated tag, or NULL. malloced. */
+	char *target; /* symref, or NULL. malloced. */
+};
+
+/* returns whether 'ref' represents a deletion */
+bool ref_record_is_deletion(const struct ref_record *ref);
+
+/* prints a ref_record onto stdout */
+void ref_record_print(struct ref_record *ref, int hash_size);
+
+/* frees and nulls all pointer values. */
+void ref_record_clear(struct ref_record *ref);
+
+/* returns whether two ref_records are the same */
+bool ref_record_equal(struct ref_record *a, struct ref_record *b,
+		      int hash_size);
+
+/* log_record holds a reflog entry */
+struct log_record {
+	char *ref_name;
+	uint64_t update_index;
+	byte *new_hash;
+	byte *old_hash;
+	char *name;
+	char *email;
+	uint64_t time;
+	int16_t tz_offset;
+	char *message;
+};
+
+/* returns whether 'ref' represents the deletion of a log record. */
+bool log_record_is_deletion(const struct log_record *log);
+
+/* frees and nulls all pointer values. */
+void log_record_clear(struct log_record *log);
+
+/* returns whether two records are equal. */
+bool log_record_equal(struct log_record *a, struct log_record *b,
+		      int hash_size);
+
+void log_record_print(struct log_record *log, int hash_size);
+
+/* iterator is the generic interface for walking over data stored in a
+   reftable. It is generally passed around by value.
+*/
+struct iterator {
+	struct iterator_vtable *ops;
+	void *iter_arg;
+};
+
+/* reads the next ref_record. Returns < 0 for error, 0 for OK and > 0:
+   end of iteration.
+*/
+int iterator_next_ref(struct iterator it, struct ref_record *ref);
+
+/* reads the next log_record. Returns < 0 for error, 0 for OK and > 0:
+   end of iteration.
+*/
+int iterator_next_log(struct iterator it, struct log_record *log);
+
+/* releases resources associated with an iterator. */
+void iterator_destroy(struct iterator *it);
+
+/* block_stats holds statistics for a single block type */
+struct block_stats {
+	/* total number of entries written */
+	int entries;
+	/* total number of key restarts */
+	int restarts;
+	/* total number of blocks */
+	int blocks;
+	/* total number of index blocks */
+	int index_blocks;
+	/* depth of the index */
+	int max_index_level;
+
+	/* offset of the first block for this type */
+	uint64_t offset;
+	/* offset of the top level index block for this type, or 0 if not
+	 * present */
+	uint64_t index_offset;
+};
+
+/* stats holds overall statistics for a single reftable */
+struct stats {
+	/* total number of blocks written. */
+	int blocks;
+	/* stats for ref data */
+	struct block_stats ref_stats;
+	/* stats for the SHA1 to ref map. */
+	struct block_stats obj_stats;
+	/* stats for index blocks */
+	struct block_stats idx_stats;
+	/* stats for log blocks */
+	struct block_stats log_stats;
+
+	/* disambiguation length of shortened object IDs. */
+	int object_id_len;
+};
+
+/* different types of errors */
+
+/* Unexpected file system behavior */
+#define IO_ERROR -2
+
+/* Format inconsistency on reading data
+ */
+#define FORMAT_ERROR -3
+
+/* File does not exist. Returned from block_source_from_file(),  because it
+   needs special handling in stack.
+*/
+#define NOT_EXIST_ERROR -4
+
+/* Trying to write out-of-date data. */
+#define LOCK_ERROR -5
+
+/* Misuse of the API:
+   - on writing a record with NULL ref_name.
+   - on writing a ref_record outside the table limits
+   - on writing a ref or log record before the stack's next_update_index
+   - on reading a ref_record from log iterator, or vice versa.
+ */
+#define API_ERROR -6
+
+/* Decompression error */
+#define ZLIB_ERROR -7
+
+const char *error_str(int err);
+
+/* new_writer creates a new writer */
+struct writer *new_writer(int (*writer_func)(void *, byte *, int),
+			  void *writer_arg, struct write_options *opts);
+
+/* write to a file descriptor. fdp should be an int* pointing to the fd. */
+int fd_writer(void *fdp, byte *data, int size);
+
+/* Set the range of update indices for the records we will add.  When
+   writing a table into a stack, the min should be at least
+   stack_next_update_index(), or API_ERROR is returned.
+ */
+void writer_set_limits(struct writer *w, uint64_t min, uint64_t max);
+
+/* adds a ref_record. Must be called in ascending
+   order. The update_index must be within the limits set by
+   writer_set_limits(), or API_ERROR is returned.
+ */
+int writer_add_ref(struct writer *w, struct ref_record *ref);
+
+/* Convenience function to add multiple refs. Will sort the refs by
+   name before adding. */
+int writer_add_refs(struct writer *w, struct ref_record *refs, int n);
+
+/* adds a log_record. Must be called in ascending order (with more
+   recent log entries first.)
+ */
+int writer_add_log(struct writer *w, struct log_record *log);
+
+/* Convenience function to add multiple logs. Will sort the records by
+   key before adding. */
+int writer_add_logs(struct writer *w, struct log_record *logs, int n);
+
+/* writer_close finalizes the reftable. The writer is retained so statistics can
+ * be inspected. */
+int writer_close(struct writer *w);
+
+/* writer_stats returns the statistics on the reftable being written. */
+struct stats *writer_stats(struct writer *w);
+
+/* writer_free deallocates memory for the writer */
+void writer_free(struct writer *w);
+
+struct reader;
+
+/* new_reader opens a reftable for reading. If successful, returns 0
+ * code and sets pp.  The name is used for creating a
+ * stack. Typically, it is the basename of the file.
+ */
+int new_reader(struct reader **pp, struct block_source, const char *name);
+
+/* reader_seek_ref returns an iterator where 'name' would be inserted in the
+   table.
+
+   example:
+
+   struct reader *r = NULL;
+   int err = new_reader(&r, src, "filename");
+   if (err < 0) { ... }
+   struct iterator it = {};
+   err = reader_seek_ref(r, &it, "refs/heads/master");
+   if (err < 0) { ... }
+   struct ref_record ref = {};
+   while (1) {
+     err = iterator_next_ref(it, &ref);
+     if (err > 0) {
+       break;
+     }
+     if (err < 0) {
+       ..error handling..
+     }
+     ..found..
+   }
+   iterator_destroy(&it);
+   ref_record_clear(&ref);
+ */
+int reader_seek_ref(struct reader *r, struct iterator *it, const char *name);
+
+/* seek to logs for the given name, older than update_index. */
+int reader_seek_log_at(struct reader *r, struct iterator *it, const char *name,
+		       uint64_t update_index);
+
+/* seek to newest log entry for given name. */
+int reader_seek_log(struct reader *r, struct iterator *it, const char *name);
+
+/* closes and deallocates a reader. */
+void reader_free(struct reader *);
+
+/* return an iterator for the refs pointing to oid */
+int reader_refs_for(struct reader *r, struct iterator *it, byte *oid,
+		    int oid_len);
+
+/* return the max_update_index for a table */
+uint64_t reader_max_update_index(struct reader *r);
+
+/* return the min_update_index for a table */
+uint64_t reader_min_update_index(struct reader *r);
+
+/* a merged table is implements seeking/iterating over a stack of tables. */
+struct merged_table;
+
+/* new_merged_table creates a new merged table. It takes ownership of the stack
+   array.
+*/
+int new_merged_table(struct merged_table **dest, struct reader **stack, int n);
+
+/* returns an iterator positioned just before 'name' */
+int merged_table_seek_ref(struct merged_table *mt, struct iterator *it,
+			  const char *name);
+
+/* returns an iterator for log entry, at given update_index */
+int merged_table_seek_log_at(struct merged_table *mt, struct iterator *it,
+			     const char *name, uint64_t update_index);
+
+/* like merged_table_seek_log_at but look for the newest entry. */
+int merged_table_seek_log(struct merged_table *mt, struct iterator *it,
+			  const char *name);
+
+/* returns the max update_index covered by this merged table. */
+uint64_t merged_max_update_index(struct merged_table *mt);
+
+/* returns the min update_index covered by this merged table. */
+uint64_t merged_min_update_index(struct merged_table *mt);
+
+/* closes readers for the merged tables */
+void merged_table_close(struct merged_table *mt);
+
+/* releases memory for the merged_table */
+void merged_table_free(struct merged_table *m);
+
+/* a stack is a stack of reftables, which can be mutated by pushing a table to
+ * the top of the stack */
+struct stack;
+
+/* open a new reftable stack. The tables will be stored in 'dir', while the list
+   of tables is in 'list_file'. Typically, this should be .git/reftables and
+   .git/refs respectively.
+*/
+int new_stack(struct stack **dest, const char *dir, const char *list_file,
+	      struct write_options config);
+
+/* returns the update_index at which a next table should be written. */
+uint64_t stack_next_update_index(struct stack *st);
+
+/* add a new table to the stack. The write_table function must call
+   writer_set_limits, add refs and return an error value. */
+int stack_add(struct stack *st,
+	      int (*write_table)(struct writer *wr, void *write_arg),
+	      void *write_arg);
+
+/* returns the merged_table for seeking. This table is valid until the
+   next write or reload, and should not be closed or deleted.
+*/
+struct merged_table *stack_merged_table(struct stack *st);
+
+/* frees all resources associated with the stack. */
+void stack_destroy(struct stack *st);
+
+/* reloads the stack if necessary. */
+int stack_reload(struct stack *st);
+
+/* Policy for expiring reflog entries. */
+struct log_expiry_config {
+	/* Drop entries older than this timestamp */
+	uint64_t time;
+
+	/* Drop older entries */
+	uint64_t min_update_index;
+};
+
+/* compacts all reftables into a giant table. Expire reflog entries if config is
+ * non-NULL */
+int stack_compact_all(struct stack *st, struct log_expiry_config *config);
+
+/* heuristically compact unbalanced table stack. */
+int stack_auto_compact(struct stack *st);
+
+/* convenience function to read a single ref. Returns < 0 for error, 0
+   for success, and 1 if ref not found. */
+int stack_read_ref(struct stack *st, const char *refname,
+		   struct ref_record *ref);
+
+/* convenience function to read a single log. Returns < 0 for error, 0
+   for success, and 1 if ref not found. */
+int stack_read_log(struct stack *st, const char *refname,
+		   struct log_record *log);
+
+/* statistics on past compactions. */
+struct compaction_stats {
+	uint64_t bytes;
+	int attempts;
+	int failures;
+};
+
+struct compaction_stats *stack_compaction_stats(struct stack *st);
+
+#endif
diff --git a/reftable/reftable_test.c b/reftable/reftable_test.c
new file mode 100644
index 0000000000..1e89d37d9d
--- /dev/null
+++ b/reftable/reftable_test.c
@@ -0,0 +1,481 @@
+/*
+Copyright 2020 Google LLC
+
+Use of this source code is governed by a BSD-style
+license that can be found in the LICENSE file or at
+https://developers.google.com/open-source/licenses/bsd
+*/
+
+#include "reftable.h"
+
+#include "system.h"
+
+#include "basics.h"
+#include "block.h"
+#include "constants.h"
+#include "reader.h"
+#include "record.h"
+#include "test_framework.h"
+
+static const int update_index = 5;
+
+void test_buffer(void)
+{
+	struct slice buf = {};
+
+	byte in[] = "hello";
+	slice_write(&buf, in, sizeof(in));
+	struct block_source source;
+	block_source_from_slice(&source, &buf);
+	assert(block_source_size(source) == 6);
+	struct block out = {};
+	int n = block_source_read_block(source, &out, 0, sizeof(in));
+	assert(n == sizeof(in));
+	assert(!memcmp(in, out.data, n));
+	block_source_return_block(source, &out);
+
+	n = block_source_read_block(source, &out, 1, 2);
+	assert(n == 2);
+	assert(!memcmp(out.data, "el", 2));
+
+	block_source_return_block(source, &out);
+	block_source_close(&source);
+	free(slice_yield(&buf));
+}
+
+void write_table(char ***names, struct slice *buf, int N, int block_size)
+{
+	*names = calloc(sizeof(char *), N + 1);
+
+	struct write_options opts = {
+		.block_size = block_size,
+	};
+
+	struct writer *w = new_writer(&slice_write_void, buf, &opts);
+
+	writer_set_limits(w, update_index, update_index);
+	{
+		struct ref_record ref = {};
+		int i = 0;
+		for (i = 0; i < N; i++) {
+			byte hash[SHA1_SIZE];
+			set_test_hash(hash, i);
+
+			char name[100];
+			snprintf(name, sizeof(name), "refs/heads/branch%02d",
+				 i);
+
+			ref.ref_name = name;
+			ref.value = hash;
+			ref.update_index = update_index;
+			(*names)[i] = strdup(name);
+
+			int n = writer_add_ref(w, &ref);
+			assert(n == 0);
+		}
+	}
+	{
+		struct log_record log = {};
+		int i = 0;
+		for (i = 0; i < N; i++) {
+			byte hash[SHA1_SIZE];
+			set_test_hash(hash, i);
+
+			char name[100];
+			snprintf(name, sizeof(name), "refs/heads/branch%02d",
+				 i);
+
+			log.ref_name = name;
+			log.new_hash = hash;
+			log.update_index = update_index;
+			log.message = "message";
+
+			int n = writer_add_log(w, &log);
+			assert(n == 0);
+		}
+	}
+
+	int n = writer_close(w);
+	assert(n == 0);
+
+	struct stats *stats = writer_stats(w);
+	int i = 0;
+	for (i = 0; i < stats->ref_stats.blocks; i++) {
+		int off = i * opts.block_size;
+		if (off == 0) {
+			off = HEADER_SIZE;
+		}
+		assert(buf->buf[off] == 'r');
+	}
+
+	writer_free(w);
+	w = NULL;
+}
+
+void test_log_write_read(void)
+{
+	int N = 2;
+	char **names = calloc(sizeof(char *), N + 1);
+
+	struct write_options opts = {
+		.block_size = 256,
+	};
+
+	struct slice buf = {};
+	struct writer *w = new_writer(&slice_write_void, &buf, &opts);
+
+	writer_set_limits(w, 0, N);
+	{
+		struct ref_record ref = {};
+		int i = 0;
+		for (i = 0; i < N; i++) {
+			char name[256];
+			snprintf(name, sizeof(name), "b%02d%0*d", i, 130, 7);
+			names[i] = strdup(name);
+			puts(name);
+			ref.ref_name = name;
+			ref.update_index = i;
+
+			int err = writer_add_ref(w, &ref);
+			assert_err(err);
+		}
+	}
+
+	{
+		struct log_record log = {};
+		int i = 0;
+		for (i = 0; i < N; i++) {
+			byte hash1[SHA1_SIZE], hash2[SHA1_SIZE];
+			set_test_hash(hash1, i);
+			set_test_hash(hash2, i + 1);
+
+			log.ref_name = names[i];
+			log.update_index = i;
+			log.old_hash = hash1;
+			log.new_hash = hash2;
+
+			int err = writer_add_log(w, &log);
+			assert_err(err);
+		}
+	}
+
+	int n = writer_close(w);
+	assert(n == 0);
+
+	struct stats *stats = writer_stats(w);
+	assert(stats->log_stats.blocks > 0);
+	writer_free(w);
+	w = NULL;
+
+	struct block_source source = {};
+	block_source_from_slice(&source, &buf);
+
+	struct reader rd = {};
+	int err = init_reader(&rd, source, "file.log");
+	assert(err == 0);
+
+	{
+		struct iterator it = {};
+		err = reader_seek_ref(&rd, &it, names[N - 1]);
+		assert(err == 0);
+
+		struct ref_record ref = {};
+		err = iterator_next_ref(it, &ref);
+		assert_err(err);
+
+		/* end of iteration. */
+		err = iterator_next_ref(it, &ref);
+		assert(0 < err);
+
+		iterator_destroy(&it);
+		ref_record_clear(&ref);
+	}
+
+	{
+		struct iterator it = {};
+		err = reader_seek_log(&rd, &it, "");
+		assert(err == 0);
+
+		struct log_record log = {};
+		int i = 0;
+		while (true) {
+			int err = iterator_next_log(it, &log);
+			if (err > 0) {
+				break;
+			}
+
+			assert_err(err);
+			assert_streq(names[i], log.ref_name);
+			assert(i == log.update_index);
+			i++;
+		}
+
+		assert(i == N);
+		iterator_destroy(&it);
+	}
+
+	/* cleanup. */
+	free(slice_yield(&buf));
+	free_names(names);
+	reader_close(&rd);
+}
+
+void test_table_read_write_sequential(void)
+{
+	char **names;
+	struct slice buf = {};
+	int N = 50;
+	write_table(&names, &buf, N, 256);
+
+	struct block_source source = {};
+	block_source_from_slice(&source, &buf);
+
+	struct reader rd = {};
+	int err = init_reader(&rd, source, "file.ref");
+	assert(err == 0);
+
+	struct iterator it = {};
+	err = reader_seek_ref(&rd, &it, "");
+	assert(err == 0);
+
+	int j = 0;
+	while (true) {
+		struct ref_record ref = {};
+		int r = iterator_next_ref(it, &ref);
+		assert(r >= 0);
+		if (r > 0) {
+			break;
+		}
+		assert(0 == strcmp(names[j], ref.ref_name));
+		assert(update_index == ref.update_index);
+
+		j++;
+		ref_record_clear(&ref);
+	}
+	assert(j == N);
+	iterator_destroy(&it);
+	free(slice_yield(&buf));
+	free_names(names);
+
+	reader_close(&rd);
+}
+
+void test_table_write_small_table(void)
+{
+	char **names;
+	struct slice buf = {};
+	int N = 1;
+	write_table(&names, &buf, N, 4096);
+	assert(buf.len < 200);
+	free(slice_yield(&buf));
+	free_names(names);
+}
+
+void test_table_read_api(void)
+{
+	char **names;
+	struct slice buf = {};
+	int N = 50;
+	write_table(&names, &buf, N, 256);
+
+	struct reader rd = {};
+	struct block_source source = {};
+	block_source_from_slice(&source, &buf);
+
+	int err = init_reader(&rd, source, "file.ref");
+	assert(err == 0);
+
+	struct iterator it = {};
+	err = reader_seek_ref(&rd, &it, names[0]);
+	assert(err == 0);
+
+	struct log_record log = {};
+	err = iterator_next_log(it, &log);
+	assert(err == API_ERROR);
+
+	free(slice_yield(&buf));
+	int i = 0;
+	for (i = 0; i < N; i++) {
+		free(names[i]);
+	}
+	free(names);
+	reader_close(&rd);
+}
+
+void test_table_read_write_seek(bool index)
+{
+	char **names;
+	struct slice buf = {};
+	int N = 50;
+	write_table(&names, &buf, N, 256);
+
+	struct reader rd = {};
+	struct block_source source = {};
+	block_source_from_slice(&source, &buf);
+
+	int err = init_reader(&rd, source, "file.ref");
+	assert(err == 0);
+
+	if (!index) {
+		rd.ref_offsets.index_offset = 0;
+	}
+
+	int i = 0;
+	for (i = 1; i < N; i++) {
+		struct iterator it = {};
+		int err = reader_seek_ref(&rd, &it, names[i]);
+		assert(err == 0);
+		struct ref_record ref = {};
+		err = iterator_next_ref(it, &ref);
+		assert(err == 0);
+		assert(0 == strcmp(names[i], ref.ref_name));
+		assert(i == ref.value[0]);
+
+		ref_record_clear(&ref);
+		iterator_destroy(&it);
+	}
+
+	free(slice_yield(&buf));
+	for (i = 0; i < N; i++) {
+		free(names[i]);
+	}
+	free(names);
+	reader_close(&rd);
+}
+
+void test_table_read_write_seek_linear(void)
+{
+	test_table_read_write_seek(false);
+}
+
+void test_table_read_write_seek_index(void)
+{
+	test_table_read_write_seek(true);
+}
+
+void test_table_refs_for(bool indexed)
+{
+	int N = 50;
+
+	char **want_names = calloc(sizeof(char *), N + 1);
+
+	int want_names_len = 0;
+	byte want_hash[SHA1_SIZE];
+	set_test_hash(want_hash, 4);
+
+	struct write_options opts = {
+		.block_size = 256,
+	};
+
+	struct slice buf = {};
+	struct writer *w = new_writer(&slice_write_void, &buf, &opts);
+	{
+		struct ref_record ref = {};
+		int i = 0;
+		for (i = 0; i < N; i++) {
+			byte hash[SHA1_SIZE];
+			memset(hash, i, sizeof(hash));
+			char fill[51] = {};
+			memset(fill, 'x', 50);
+			char name[100];
+			/* Put the variable part in the start */
+			snprintf(name, sizeof(name), "br%02d%s", i, fill);
+			name[40] = 0;
+			ref.ref_name = name;
+
+			byte hash1[SHA1_SIZE];
+			byte hash2[SHA1_SIZE];
+
+			set_test_hash(hash1, i / 4);
+			set_test_hash(hash2, 3 + i / 4);
+			ref.value = hash1;
+			ref.target_value = hash2;
+
+			/* 80 bytes / entry, so 3 entries per block. Yields 17 */
+			/* blocks. */
+			int n = writer_add_ref(w, &ref);
+			assert(n == 0);
+
+			if (!memcmp(hash1, want_hash, SHA1_SIZE) ||
+			    !memcmp(hash2, want_hash, SHA1_SIZE)) {
+				want_names[want_names_len++] = strdup(name);
+			}
+		}
+	}
+
+	int n = writer_close(w);
+	assert(n == 0);
+
+	writer_free(w);
+	w = NULL;
+
+	struct reader rd;
+	struct block_source source = {};
+	block_source_from_slice(&source, &buf);
+
+	int err = init_reader(&rd, source, "file.ref");
+	assert(err == 0);
+	if (!indexed) {
+		rd.obj_offsets.present = 0;
+	}
+
+	struct iterator it = {};
+	err = reader_seek_ref(&rd, &it, "");
+	assert(err == 0);
+	iterator_destroy(&it);
+
+	err = reader_refs_for(&rd, &it, want_hash, SHA1_SIZE);
+	assert(err == 0);
+
+	struct ref_record ref = {};
+
+	int j = 0;
+	while (true) {
+		int err = iterator_next_ref(it, &ref);
+		assert(err >= 0);
+		if (err > 0) {
+			break;
+		}
+
+		assert(j < want_names_len);
+		assert(0 == strcmp(ref.ref_name, want_names[j]));
+		j++;
+		ref_record_clear(&ref);
+	}
+	assert(j == want_names_len);
+
+	free(slice_yield(&buf));
+	free_names(want_names);
+	iterator_destroy(&it);
+	reader_close(&rd);
+}
+
+void test_table_refs_for_no_index(void)
+{
+	test_table_refs_for(false);
+}
+
+void test_table_refs_for_obj_index(void)
+{
+	test_table_refs_for(true);
+}
+
+int main()
+{
+	add_test_case("test_log_write_read", test_log_write_read);
+	add_test_case("test_table_write_small_table",
+		      &test_table_write_small_table);
+	add_test_case("test_buffer", &test_buffer);
+	add_test_case("test_table_read_api", &test_table_read_api);
+	add_test_case("test_table_read_write_sequential",
+		      &test_table_read_write_sequential);
+	add_test_case("test_table_read_write_seek_linear",
+		      &test_table_read_write_seek_linear);
+	add_test_case("test_table_read_write_seek_index",
+		      &test_table_read_write_seek_index);
+	add_test_case("test_table_read_write_refs_for_no_index",
+		      &test_table_refs_for_no_index);
+	add_test_case("test_table_read_write_refs_for_obj_index",
+		      &test_table_refs_for_obj_index);
+	test_main();
+}
diff --git a/reftable/slice.c b/reftable/slice.c
new file mode 100644
index 0000000000..efbe625253
--- /dev/null
+++ b/reftable/slice.c
@@ -0,0 +1,199 @@
+/*
+Copyright 2020 Google LLC
+
+Use of this source code is governed by a BSD-style
+license that can be found in the LICENSE file or at
+https://developers.google.com/open-source/licenses/bsd
+*/
+
+#include "slice.h"
+
+#include "system.h"
+
+#include "reftable.h"
+
+void slice_set_string(struct slice *s, const char *str)
+{
+	if (str == NULL) {
+		s->len = 0;
+		return;
+	}
+
+	{
+		int l = strlen(str);
+		l++; /* \0 */
+		slice_resize(s, l);
+		memcpy(s->buf, str, l);
+		s->len = l - 1;
+	}
+}
+
+void slice_resize(struct slice *s, int l)
+{
+	if (s->cap < l) {
+		int c = s->cap * 2;
+		if (c < l) {
+			c = l;
+		}
+		s->cap = c;
+		s->buf = realloc(s->buf, s->cap);
+	}
+	s->len = l;
+}
+
+void slice_append_string(struct slice *d, const char *s)
+{
+	int l1 = d->len;
+	int l2 = strlen(s);
+
+	slice_resize(d, l2 + l1);
+	memcpy(d->buf + l1, s, l2);
+}
+
+void slice_append(struct slice *s, struct slice a)
+{
+	int end = s->len;
+	slice_resize(s, s->len + a.len);
+	memcpy(s->buf + end, a.buf, a.len);
+}
+
+byte *slice_yield(struct slice *s)
+{
+	byte *p = s->buf;
+	s->buf = NULL;
+	s->cap = 0;
+	s->len = 0;
+	return p;
+}
+
+void slice_copy(struct slice *dest, struct slice src)
+{
+	slice_resize(dest, src.len);
+	memcpy(dest->buf, src.buf, src.len);
+}
+
+/* return the underlying data as char*. len is left unchanged, but
+   a \0 is added at the end. */
+const char *slice_as_string(struct slice *s)
+{
+	if (s->cap == s->len) {
+		int l = s->len;
+		slice_resize(s, l + 1);
+		s->len = l;
+	}
+	s->buf[s->len] = 0;
+	return (const char *)s->buf;
+}
+
+/* return a newly malloced string for this slice */
+char *slice_to_string(struct slice in)
+{
+	struct slice s = {};
+	slice_resize(&s, in.len + 1);
+	s.buf[in.len] = 0;
+	memcpy(s.buf, in.buf, in.len);
+	return (char *)slice_yield(&s);
+}
+
+bool slice_equal(struct slice a, struct slice b)
+{
+	if (a.len != b.len) {
+		return 0;
+	}
+	return memcmp(a.buf, b.buf, a.len) == 0;
+}
+
+int slice_compare(struct slice a, struct slice b)
+{
+	int min = a.len < b.len ? a.len : b.len;
+	int res = memcmp(a.buf, b.buf, min);
+	if (res != 0) {
+		return res;
+	}
+	if (a.len < b.len) {
+		return -1;
+	} else if (a.len > b.len) {
+		return 1;
+	} else {
+		return 0;
+	}
+}
+
+int slice_write(struct slice *b, byte *data, int sz)
+{
+	if (b->len + sz > b->cap) {
+		int newcap = 2 * b->cap + 1;
+		if (newcap < b->len + sz) {
+			newcap = (b->len + sz);
+		}
+		b->buf = realloc(b->buf, newcap);
+		b->cap = newcap;
+	}
+
+	memcpy(b->buf + b->len, data, sz);
+	b->len += sz;
+	return sz;
+}
+
+int slice_write_void(void *b, byte *data, int sz)
+{
+	return slice_write((struct slice *)b, data, sz);
+}
+
+static uint64_t slice_size(void *b)
+{
+	return ((struct slice *)b)->len;
+}
+
+static void slice_return_block(void *b, struct block *dest)
+{
+	memset(dest->data, 0xff, dest->len);
+	free(dest->data);
+}
+
+static void slice_close(void *b)
+{
+}
+
+static int slice_read_block(void *v, struct block *dest, uint64_t off,
+			    uint32_t size)
+{
+	struct slice *b = (struct slice *)v;
+	assert(off + size <= b->len);
+	dest->data = calloc(size, 1);
+	memcpy(dest->data, b->buf + off, size);
+	dest->len = size;
+	return size;
+}
+
+struct block_source_vtable slice_vtable = {
+	.size = &slice_size,
+	.read_block = &slice_read_block,
+	.return_block = &slice_return_block,
+	.close = &slice_close,
+};
+
+void block_source_from_slice(struct block_source *bs, struct slice *buf)
+{
+	bs->ops = &slice_vtable;
+	bs->arg = buf;
+}
+
+static void malloc_return_block(void *b, struct block *dest)
+{
+	memset(dest->data, 0xff, dest->len);
+	free(dest->data);
+}
+
+struct block_source_vtable malloc_vtable = {
+	.return_block = &malloc_return_block,
+};
+
+struct block_source malloc_block_source_instance = {
+	.ops = &malloc_vtable,
+};
+
+struct block_source malloc_block_source(void)
+{
+	return malloc_block_source_instance;
+}
diff --git a/reftable/slice.h b/reftable/slice.h
new file mode 100644
index 0000000000..f12a6db228
--- /dev/null
+++ b/reftable/slice.h
@@ -0,0 +1,39 @@
+/*
+Copyright 2020 Google LLC
+
+Use of this source code is governed by a BSD-style
+license that can be found in the LICENSE file or at
+https://developers.google.com/open-source/licenses/bsd
+*/
+
+#ifndef SLICE_H
+#define SLICE_H
+
+#include "basics.h"
+#include "reftable.h"
+
+struct slice {
+	byte *buf;
+	int len;
+	int cap;
+};
+
+void slice_set_string(struct slice *dest, const char *);
+void slice_append_string(struct slice *dest, const char *);
+char *slice_to_string(struct slice src);
+const char *slice_as_string(struct slice *src);
+bool slice_equal(struct slice a, struct slice b);
+byte *slice_yield(struct slice *s);
+void slice_copy(struct slice *dest, struct slice src);
+void slice_resize(struct slice *s, int l);
+int slice_compare(struct slice a, struct slice b);
+int slice_write(struct slice *b, byte *data, int sz);
+int slice_write_void(void *b, byte *data, int sz);
+void slice_append(struct slice *dest, struct slice add);
+
+struct block_source;
+void block_source_from_slice(struct block_source *bs, struct slice *buf);
+
+struct block_source malloc_block_source(void);
+
+#endif
diff --git a/reftable/slice_test.c b/reftable/slice_test.c
new file mode 100644
index 0000000000..40a391383c
--- /dev/null
+++ b/reftable/slice_test.c
@@ -0,0 +1,38 @@
+/*
+Copyright 2020 Google LLC
+
+Use of this source code is governed by a BSD-style
+license that can be found in the LICENSE file or at
+https://developers.google.com/open-source/licenses/bsd
+*/
+
+#include "slice.h"
+
+#include "system.h"
+
+#include "basics.h"
+#include "record.h"
+#include "reftable.h"
+#include "test_framework.h"
+
+void test_slice(void)
+{
+	struct slice s = {};
+	slice_set_string(&s, "abc");
+	assert(0 == strcmp("abc", slice_as_string(&s)));
+
+	struct slice t = {};
+	slice_set_string(&t, "pqr");
+
+	slice_append(&s, t);
+	assert(0 == strcmp("abcpqr", slice_as_string(&s)));
+
+	free(slice_yield(&s));
+	free(slice_yield(&t));
+}
+
+int main()
+{
+	add_test_case("test_slice", &test_slice);
+	test_main();
+}
diff --git a/reftable/stack.c b/reftable/stack.c
new file mode 100644
index 0000000000..58b1656364
--- /dev/null
+++ b/reftable/stack.c
@@ -0,0 +1,985 @@
+/*
+Copyright 2020 Google LLC
+
+Use of this source code is governed by a BSD-style
+license that can be found in the LICENSE file or at
+https://developers.google.com/open-source/licenses/bsd
+*/
+
+#include "stack.h"
+
+#include "system.h"
+#include "merged.h"
+#include "reader.h"
+#include "reftable.h"
+#include "writer.h"
+
+int new_stack(struct stack **dest, const char *dir, const char *list_file,
+	      struct write_options config)
+{
+	struct stack *p = calloc(sizeof(struct stack), 1);
+	int err = 0;
+	*dest = NULL;
+	p->list_file = strdup(list_file);
+	p->reftable_dir = strdup(dir);
+	p->config = config;
+
+	err = stack_reload(p);
+	if (err < 0) {
+		stack_destroy(p);
+	} else {
+		*dest = p;
+	}
+	return err;
+}
+
+static int fread_lines(FILE *f, char ***namesp)
+{
+	long size = 0;
+	int err = fseek(f, 0, SEEK_END);
+	char *buf = NULL;
+	if (err < 0) {
+		err = IO_ERROR;
+		goto exit;
+	}
+	size = ftell(f);
+	if (size < 0) {
+		err = IO_ERROR;
+		goto exit;
+	}
+	err = fseek(f, 0, SEEK_SET);
+	if (err < 0) {
+		err = IO_ERROR;
+		goto exit;
+	}
+
+	buf = malloc(size + 1);
+	if (fread(buf, 1, size, f) != size) {
+		err = IO_ERROR;
+		goto exit;
+	}
+	buf[size] = 0;
+
+	parse_names(buf, size, namesp);
+exit:
+	free(buf);
+	return err;
+}
+
+int read_lines(const char *filename, char ***namesp)
+{
+	FILE *f = fopen(filename, "r");
+	int err = 0;
+	if (f == NULL) {
+		if (errno == ENOENT) {
+			*namesp = calloc(sizeof(char *), 1);
+			return 0;
+		}
+
+		return IO_ERROR;
+	}
+	err = fread_lines(f, namesp);
+	fclose(f);
+	return err;
+}
+
+struct merged_table *stack_merged_table(struct stack *st)
+{
+	return st->merged;
+}
+
+/* Close and free the stack */
+void stack_destroy(struct stack *st)
+{
+	if (st->merged == NULL) {
+		return;
+	}
+
+	merged_table_close(st->merged);
+	merged_table_free(st->merged);
+	st->merged = NULL;
+
+	free(st->list_file);
+	st->list_file = NULL;
+	free(st->reftable_dir);
+	st->reftable_dir = NULL;
+	free(st);
+}
+
+static struct reader **stack_copy_readers(struct stack *st, int cur_len)
+{
+	struct reader **cur = calloc(sizeof(struct reader *), cur_len);
+	int i = 0;
+	for (i = 0; i < cur_len; i++) {
+		cur[i] = st->merged->stack[i];
+	}
+	return cur;
+}
+
+static int stack_reload_once(struct stack *st, char **names, bool reuse_open)
+{
+	int cur_len = st->merged == NULL ? 0 : st->merged->stack_len;
+	struct reader **cur = stack_copy_readers(st, cur_len);
+	int err = 0;
+	int names_len = names_length(names);
+	struct reader **new_tables =
+		malloc(sizeof(struct reader *) * names_len);
+	int new_tables_len = 0;
+	struct merged_table *new_merged = NULL;
+
+	struct slice table_path = {};
+
+	while (*names) {
+		struct reader *rd = NULL;
+		char *name = *names++;
+
+		/* this is linear; we assume compaction keeps the number of
+		   tables under control so this is not quadratic. */
+		int j = 0;
+		for (j = 0; reuse_open && j < cur_len; j++) {
+			if (cur[j] != NULL && 0 == strcmp(cur[j]->name, name)) {
+				rd = cur[j];
+				cur[j] = NULL;
+				break;
+			}
+		}
+
+		if (rd == NULL) {
+			struct block_source src = {};
+			slice_set_string(&table_path, st->reftable_dir);
+			slice_append_string(&table_path, "/");
+			slice_append_string(&table_path, name);
+
+			err = block_source_from_file(
+				&src, slice_as_string(&table_path));
+			if (err < 0) {
+				goto exit;
+			}
+
+			err = new_reader(&rd, src, name);
+			if (err < 0) {
+				goto exit;
+			}
+		}
+
+		new_tables[new_tables_len++] = rd;
+	}
+
+	/* success! */
+	err = new_merged_table(&new_merged, new_tables, new_tables_len);
+	if (err < 0) {
+		goto exit;
+	}
+
+	new_tables = NULL;
+	new_tables_len = 0;
+	if (st->merged != NULL) {
+		merged_table_clear(st->merged);
+		merged_table_free(st->merged);
+	}
+	st->merged = new_merged;
+
+	{
+		int i = 0;
+		for (i = 0; i < cur_len; i++) {
+			if (cur[i] != NULL) {
+				reader_close(cur[i]);
+				reader_free(cur[i]);
+			}
+		}
+	}
+exit:
+	free(slice_yield(&table_path));
+	{
+		int i = 0;
+		for (i = 0; i < new_tables_len; i++) {
+			reader_close(new_tables[i]);
+		}
+	}
+	free(new_tables);
+	free(cur);
+	return err;
+}
+
+/* return negative if a before b. */
+static int tv_cmp(struct timeval *a, struct timeval *b)
+{
+	time_t diff = a->tv_sec - b->tv_sec;
+	int udiff = a->tv_usec - b->tv_usec;
+
+	if (diff != 0) {
+		return diff;
+	}
+
+	return udiff;
+}
+
+static int stack_reload_maybe_reuse(struct stack *st, bool reuse_open)
+{
+	struct timeval deadline = {};
+	int err = gettimeofday(&deadline, NULL);
+	int64_t delay = 0;
+	int tries = 0;
+	if (err < 0) {
+		return err;
+	}
+
+	deadline.tv_sec += 3;
+	while (true) {
+		char **names = NULL;
+		char **names_after = NULL;
+		struct timeval now = {};
+		int err = gettimeofday(&now, NULL);
+		if (err < 0) {
+			return err;
+		}
+
+		/* Only look at deadlines after the first few times. This
+		   simplifies debugging in GDB */
+		tries++;
+		if (tries > 3 && tv_cmp(&now, &deadline) >= 0) {
+			break;
+		}
+
+		err = read_lines(st->list_file, &names);
+		if (err < 0) {
+			free_names(names);
+			return err;
+		}
+		err = stack_reload_once(st, names, reuse_open);
+		if (err == 0) {
+			free_names(names);
+			break;
+		}
+		if (err != NOT_EXIST_ERROR) {
+			free_names(names);
+			return err;
+		}
+
+		err = read_lines(st->list_file, &names_after);
+		if (err < 0) {
+			free_names(names);
+			return err;
+		}
+
+		if (names_equal(names_after, names)) {
+			free_names(names);
+			free_names(names_after);
+			return -1;
+		}
+		free_names(names);
+		free_names(names_after);
+
+		delay = delay + (delay * rand()) / RAND_MAX + 100;
+		usleep(delay);
+	}
+
+	return 0;
+}
+
+int stack_reload(struct stack *st)
+{
+	return stack_reload_maybe_reuse(st, true);
+}
+
+/* -1 = error
+ 0 = up to date
+ 1 = changed. */
+static int stack_uptodate(struct stack *st)
+{
+	char **names = NULL;
+	int err = read_lines(st->list_file, &names);
+	int i = 0;
+	if (err < 0) {
+		return err;
+	}
+
+	for (i = 0; i < st->merged->stack_len; i++) {
+		if (names[i] == NULL) {
+			err = 1;
+			goto exit;
+		}
+
+		if (strcmp(st->merged->stack[i]->name, names[i])) {
+			err = 1;
+			goto exit;
+		}
+	}
+
+	if (names[st->merged->stack_len] != NULL) {
+		err = 1;
+		goto exit;
+	}
+
+exit:
+	free_names(names);
+	return err;
+}
+
+int stack_add(struct stack *st, int (*write)(struct writer *wr, void *arg),
+	      void *arg)
+{
+	int err = stack_try_add(st, write, arg);
+	if (err < 0) {
+		if (err == LOCK_ERROR) {
+			err = stack_reload(st);
+		}
+		return err;
+	}
+
+	return stack_auto_compact(st);
+}
+
+static void format_name(struct slice *dest, uint64_t min, uint64_t max)
+{
+	char buf[100];
+	snprintf(buf, sizeof(buf), "%012lx-%012lx", min, max);
+	slice_set_string(dest, buf);
+}
+
+int stack_try_add(struct stack *st,
+		  int (*write_table)(struct writer *wr, void *arg), void *arg)
+{
+	struct slice lock_name = {};
+	struct slice temp_tab_name = {};
+	struct slice tab_name = {};
+	struct slice next_name = {};
+	struct slice table_list = {};
+	struct writer *wr = NULL;
+	int err = 0;
+	int tab_fd = 0;
+	int lock_fd = 0;
+	uint64_t next_update_index = 0;
+
+	slice_set_string(&lock_name, st->list_file);
+	slice_append_string(&lock_name, ".lock");
+
+	lock_fd = open(slice_as_string(&lock_name), O_EXCL | O_CREAT | O_WRONLY,
+		       0644);
+	if (lock_fd < 0) {
+		if (errno == EEXIST) {
+			err = LOCK_ERROR;
+			goto exit;
+		}
+		err = IO_ERROR;
+		goto exit;
+	}
+
+	err = stack_uptodate(st);
+	if (err < 0) {
+		goto exit;
+	}
+
+	if (err > 1) {
+		err = LOCK_ERROR;
+		goto exit;
+	}
+
+	next_update_index = stack_next_update_index(st);
+
+	slice_resize(&next_name, 0);
+	format_name(&next_name, next_update_index, next_update_index);
+
+	slice_set_string(&temp_tab_name, st->reftable_dir);
+	slice_append_string(&temp_tab_name, "/");
+	slice_append(&temp_tab_name, next_name);
+	slice_append_string(&temp_tab_name, ".temp.XXXXXX");
+
+	tab_fd = mkstemp((char *)slice_as_string(&temp_tab_name));
+	if (tab_fd < 0) {
+		err = IO_ERROR;
+		goto exit;
+	}
+
+	wr = new_writer(fd_writer, &tab_fd, &st->config);
+	err = write_table(wr, arg);
+	if (err < 0) {
+		goto exit;
+	}
+
+	err = writer_close(wr);
+	if (err < 0) {
+		goto exit;
+	}
+
+	err = close(tab_fd);
+	tab_fd = 0;
+	if (err < 0) {
+		err = IO_ERROR;
+		goto exit;
+	}
+
+	if (wr->min_update_index < next_update_index) {
+		err = API_ERROR;
+		goto exit;
+	}
+
+	{
+		int i = 0;
+		for (i = 0; i < st->merged->stack_len; i++) {
+			slice_append_string(&table_list,
+					    st->merged->stack[i]->name);
+			slice_append_string(&table_list, "\n");
+		}
+	}
+
+	format_name(&next_name, wr->min_update_index, wr->max_update_index);
+	slice_append_string(&next_name, ".ref");
+	slice_append(&table_list, next_name);
+	slice_append_string(&table_list, "\n");
+
+	slice_set_string(&tab_name, st->reftable_dir);
+	slice_append_string(&tab_name, "/");
+	slice_append(&tab_name, next_name);
+
+	err = rename(slice_as_string(&temp_tab_name),
+		     slice_as_string(&tab_name));
+	if (err < 0) {
+		err = IO_ERROR;
+		goto exit;
+	}
+	free(slice_yield(&temp_tab_name));
+
+	err = write(lock_fd, table_list.buf, table_list.len);
+	if (err < 0) {
+		err = IO_ERROR;
+		goto exit;
+	}
+	err = close(lock_fd);
+	lock_fd = 0;
+	if (err < 0) {
+		unlink(slice_as_string(&tab_name));
+		err = IO_ERROR;
+		goto exit;
+	}
+
+	err = rename(slice_as_string(&lock_name), st->list_file);
+	if (err < 0) {
+		unlink(slice_as_string(&tab_name));
+		err = IO_ERROR;
+		goto exit;
+	}
+
+	err = stack_reload(st);
+exit:
+	if (tab_fd > 0) {
+		close(tab_fd);
+		tab_fd = 0;
+	}
+	if (temp_tab_name.len > 0) {
+		unlink(slice_as_string(&temp_tab_name));
+	}
+	unlink(slice_as_string(&lock_name));
+
+	if (lock_fd > 0) {
+		close(lock_fd);
+		lock_fd = 0;
+	}
+
+	free(slice_yield(&lock_name));
+	free(slice_yield(&temp_tab_name));
+	free(slice_yield(&tab_name));
+	free(slice_yield(&next_name));
+	free(slice_yield(&table_list));
+	writer_free(wr);
+	return err;
+}
+
+uint64_t stack_next_update_index(struct stack *st)
+{
+	int sz = st->merged->stack_len;
+	if (sz > 0) {
+		return reader_max_update_index(st->merged->stack[sz - 1]) + 1;
+	}
+	return 1;
+}
+
+static int stack_compact_locked(struct stack *st, int first, int last,
+				struct slice *temp_tab,
+				struct log_expiry_config *config)
+{
+	struct slice next_name = {};
+	int tab_fd = -1;
+	struct writer *wr = NULL;
+	int err = 0;
+
+	format_name(&next_name,
+		    reader_min_update_index(st->merged->stack[first]),
+		    reader_max_update_index(st->merged->stack[first]));
+
+	slice_set_string(temp_tab, st->reftable_dir);
+	slice_append_string(temp_tab, "/");
+	slice_append(temp_tab, next_name);
+	slice_append_string(temp_tab, ".temp.XXXXXX");
+
+	tab_fd = mkstemp((char *)slice_as_string(temp_tab));
+	wr = new_writer(fd_writer, &tab_fd, &st->config);
+
+	err = stack_write_compact(st, wr, first, last, config);
+	if (err < 0) {
+		goto exit;
+	}
+	err = writer_close(wr);
+	if (err < 0) {
+		goto exit;
+	}
+	writer_free(wr);
+
+	err = close(tab_fd);
+	tab_fd = 0;
+
+exit:
+	if (tab_fd > 0) {
+		close(tab_fd);
+		tab_fd = 0;
+	}
+	if (err != 0 && temp_tab->len > 0) {
+		unlink(slice_as_string(temp_tab));
+		free(slice_yield(temp_tab));
+	}
+	free(slice_yield(&next_name));
+	return err;
+}
+
+int stack_write_compact(struct stack *st, struct writer *wr, int first,
+			int last, struct log_expiry_config *config)
+{
+	int subtabs_len = last - first + 1;
+	struct reader **subtabs =
+		calloc(sizeof(struct reader *), last - first + 1);
+	struct merged_table *mt = NULL;
+	int err = 0;
+	struct iterator it = {};
+	struct ref_record ref = {};
+	struct log_record log = {};
+
+	int i = 0, j = 0;
+	for (i = first, j = 0; i <= last; i++) {
+		struct reader *t = st->merged->stack[i];
+		subtabs[j++] = t;
+		st->stats.bytes += t->size;
+	}
+	writer_set_limits(wr, st->merged->stack[first]->min_update_index,
+			  st->merged->stack[last]->max_update_index);
+
+	err = new_merged_table(&mt, subtabs, subtabs_len);
+	if (err < 0) {
+		free(subtabs);
+		goto exit;
+	}
+
+	err = merged_table_seek_ref(mt, &it, "");
+	if (err < 0) {
+		goto exit;
+	}
+
+	while (true) {
+		err = iterator_next_ref(it, &ref);
+		if (err > 0) {
+			err = 0;
+			break;
+		}
+		if (err < 0) {
+			break;
+		}
+		if (first == 0 && ref_record_is_deletion(&ref)) {
+			continue;
+		}
+
+		err = writer_add_ref(wr, &ref);
+		if (err < 0) {
+			break;
+		}
+	}
+
+	err = merged_table_seek_log(mt, &it, "");
+	if (err < 0) {
+		goto exit;
+	}
+
+	while (true) {
+		err = iterator_next_log(it, &log);
+		if (err > 0) {
+			err = 0;
+			break;
+		}
+		if (err < 0) {
+			break;
+		}
+		if (first == 0 && log_record_is_deletion(&log)) {
+			continue;
+		}
+
+		/* XXX collect stats? */
+
+		if (config != NULL && config->time > 0 &&
+		    log.time < config->time) {
+			continue;
+		}
+
+		if (config != NULL && config->min_update_index > 0 &&
+		    log.update_index < config->min_update_index) {
+			continue;
+		}
+
+		err = writer_add_log(wr, &log);
+		if (err < 0) {
+			break;
+		}
+	}
+
+exit:
+	iterator_destroy(&it);
+	if (mt != NULL) {
+		merged_table_clear(mt);
+		merged_table_free(mt);
+	}
+	ref_record_clear(&ref);
+
+	return err;
+}
+
+/* <  0: error. 0 == OK, > 0 attempt failed; could retry. */
+static int stack_compact_range(struct stack *st, int first, int last,
+			       struct log_expiry_config *expiry)
+{
+	struct slice temp_tab_name = {};
+	struct slice new_table_name = {};
+	struct slice lock_file_name = {};
+	struct slice ref_list_contents = {};
+	struct slice new_table_path = {};
+	int err = 0;
+	bool have_lock = false;
+	int lock_file_fd = 0;
+	int compact_count = last - first + 1;
+	char **delete_on_success = calloc(sizeof(char *), compact_count + 1);
+	char **subtable_locks = calloc(sizeof(char *), compact_count + 1);
+	int i = 0;
+	int j = 0;
+
+	if (first > last || (expiry == NULL && first == last)) {
+		err = 0;
+		goto exit;
+	}
+
+	st->stats.attempts++;
+
+	slice_set_string(&lock_file_name, st->list_file);
+	slice_append_string(&lock_file_name, ".lock");
+
+	lock_file_fd = open(slice_as_string(&lock_file_name),
+			    O_EXCL | O_CREAT | O_WRONLY, 0644);
+	if (lock_file_fd < 0) {
+		if (errno == EEXIST) {
+			err = 1;
+		} else {
+			err = IO_ERROR;
+		}
+		goto exit;
+	}
+	have_lock = true;
+	err = stack_uptodate(st);
+	if (err != 0) {
+		goto exit;
+	}
+
+	for (i = first, j = 0; i <= last; i++) {
+		struct slice subtab_name = {};
+		struct slice subtab_lock = {};
+		slice_set_string(&subtab_name, st->reftable_dir);
+		slice_append_string(&subtab_name, "/");
+		slice_append_string(&subtab_name,
+				    reader_name(st->merged->stack[i]));
+
+		slice_copy(&subtab_lock, subtab_name);
+		slice_append_string(&subtab_lock, ".lock");
+
+		{
+			int sublock_file_fd =
+				open(slice_as_string(&subtab_lock),
+				     O_EXCL | O_CREAT | O_WRONLY, 0644);
+			if (sublock_file_fd > 0) {
+				close(sublock_file_fd);
+			} else if (sublock_file_fd < 0) {
+				if (errno == EEXIST) {
+					err = 1;
+				}
+				err = IO_ERROR;
+			}
+		}
+
+		subtable_locks[j] = (char *)slice_as_string(&subtab_lock);
+		delete_on_success[j] = (char *)slice_as_string(&subtab_name);
+		j++;
+
+		if (err != 0) {
+			goto exit;
+		}
+	}
+
+	err = unlink(slice_as_string(&lock_file_name));
+	if (err < 0) {
+		goto exit;
+	}
+	have_lock = false;
+
+	err = stack_compact_locked(st, first, last, &temp_tab_name, expiry);
+	if (err < 0) {
+		goto exit;
+	}
+
+	lock_file_fd = open(slice_as_string(&lock_file_name),
+			    O_EXCL | O_CREAT | O_WRONLY, 0644);
+	if (lock_file_fd < 0) {
+		if (errno == EEXIST) {
+			err = 1;
+		} else {
+			err = IO_ERROR;
+		}
+		goto exit;
+	}
+	have_lock = true;
+
+	format_name(&new_table_name, st->merged->stack[first]->min_update_index,
+		    st->merged->stack[last]->max_update_index);
+	slice_append_string(&new_table_name, ".ref");
+
+	slice_set_string(&new_table_path, st->reftable_dir);
+	slice_append_string(&new_table_path, "/");
+
+	slice_append(&new_table_path, new_table_name);
+
+	err = rename(slice_as_string(&temp_tab_name),
+		     slice_as_string(&new_table_path));
+	if (err < 0) {
+		goto exit;
+	}
+
+	for (i = 0; i < first; i++) {
+		slice_append_string(&ref_list_contents,
+				    st->merged->stack[i]->name);
+		slice_append_string(&ref_list_contents, "\n");
+	}
+	slice_append(&ref_list_contents, new_table_name);
+	slice_append_string(&ref_list_contents, "\n");
+	for (i = last + 1; i < st->merged->stack_len; i++) {
+		slice_append_string(&ref_list_contents,
+				    st->merged->stack[i]->name);
+		slice_append_string(&ref_list_contents, "\n");
+	}
+
+	err = write(lock_file_fd, ref_list_contents.buf, ref_list_contents.len);
+	if (err < 0) {
+		unlink(slice_as_string(&new_table_path));
+		goto exit;
+	}
+	err = close(lock_file_fd);
+	lock_file_fd = 0;
+	if (err < 0) {
+		unlink(slice_as_string(&new_table_path));
+		goto exit;
+	}
+
+	err = rename(slice_as_string(&lock_file_name), st->list_file);
+	if (err < 0) {
+		unlink(slice_as_string(&new_table_path));
+		goto exit;
+	}
+	have_lock = false;
+
+	for (char **p = delete_on_success; *p; p++) {
+		if (strcmp(*p, slice_as_string(&new_table_path))) {
+			unlink(*p);
+		}
+	}
+
+	err = stack_reload_maybe_reuse(st, first < last);
+exit:
+	for (char **p = subtable_locks; *p; p++) {
+		unlink(*p);
+	}
+	free_names(delete_on_success);
+	free_names(subtable_locks);
+	if (lock_file_fd > 0) {
+		close(lock_file_fd);
+		lock_file_fd = 0;
+	}
+	if (have_lock) {
+		unlink(slice_as_string(&lock_file_name));
+	}
+	free(slice_yield(&new_table_name));
+	free(slice_yield(&new_table_path));
+	free(slice_yield(&ref_list_contents));
+	free(slice_yield(&temp_tab_name));
+	free(slice_yield(&lock_file_name));
+	return err;
+}
+
+int stack_compact_all(struct stack *st, struct log_expiry_config *config)
+{
+	return stack_compact_range(st, 0, st->merged->stack_len - 1, config);
+}
+
+static int stack_compact_range_stats(struct stack *st, int first, int last,
+				     struct log_expiry_config *config)
+{
+	int err = stack_compact_range(st, first, last, config);
+	if (err > 0) {
+		st->stats.failures++;
+	}
+	return err;
+}
+
+static int segment_size(struct segment *s)
+{
+	return s->end - s->start;
+}
+
+int fastlog2(uint64_t sz)
+{
+	int l = 0;
+	assert(sz > 0);
+	for (; sz; sz /= 2) {
+		l++;
+	}
+	return l - 1;
+}
+
+struct segment *sizes_to_segments(int *seglen, uint64_t *sizes, int n)
+{
+	struct segment *segs = calloc(sizeof(struct segment), n);
+	int next = 0;
+	struct segment cur = {};
+	int i = 0;
+	for (i = 0; i < n; i++) {
+		int log = fastlog2(sizes[i]);
+		if (cur.log != log && cur.bytes > 0) {
+			struct segment fresh = {
+				.start = i,
+			};
+
+			segs[next++] = cur;
+			cur = fresh;
+		}
+
+		cur.log = log;
+		cur.end = i + 1;
+		cur.bytes += sizes[i];
+	}
+
+	segs[next++] = cur;
+	*seglen = next;
+	return segs;
+}
+
+struct segment suggest_compaction_segment(uint64_t *sizes, int n)
+{
+	int seglen = 0;
+	struct segment *segs = sizes_to_segments(&seglen, sizes, n);
+	struct segment min_seg = {
+		.log = 64,
+	};
+	int i = 0;
+	for (i = 0; i < seglen; i++) {
+		if (segment_size(&segs[i]) == 1) {
+			continue;
+		}
+
+		if (segs[i].log < min_seg.log) {
+			min_seg = segs[i];
+		}
+	}
+
+	while (min_seg.start > 0) {
+		int prev = min_seg.start - 1;
+		if (fastlog2(min_seg.bytes) < fastlog2(sizes[prev])) {
+			break;
+		}
+
+		min_seg.start = prev;
+		min_seg.bytes += sizes[prev];
+	}
+
+	free(segs);
+	return min_seg;
+}
+
+static uint64_t *stack_table_sizes_for_compaction(struct stack *st)
+{
+	uint64_t *sizes = calloc(sizeof(uint64_t), st->merged->stack_len);
+	int i = 0;
+	for (i = 0; i < st->merged->stack_len; i++) {
+		/* overhead is 24 + 68 = 92. */
+		sizes[i] = st->merged->stack[i]->size - 91;
+	}
+	return sizes;
+}
+
+int stack_auto_compact(struct stack *st)
+{
+	uint64_t *sizes = stack_table_sizes_for_compaction(st);
+	struct segment seg =
+		suggest_compaction_segment(sizes, st->merged->stack_len);
+	free(sizes);
+	if (segment_size(&seg) > 0) {
+		return stack_compact_range_stats(st, seg.start, seg.end - 1,
+						 NULL);
+	}
+
+	return 0;
+}
+
+struct compaction_stats *stack_compaction_stats(struct stack *st)
+{
+	return &st->stats;
+}
+
+int stack_read_ref(struct stack *st, const char *refname,
+		   struct ref_record *ref)
+{
+	struct iterator it = {};
+	struct merged_table *mt = stack_merged_table(st);
+	int err = merged_table_seek_ref(mt, &it, refname);
+	if (err) {
+		goto exit;
+	}
+
+	err = iterator_next_ref(it, ref);
+	if (err) {
+		goto exit;
+	}
+
+	if (strcmp(ref->ref_name, refname) || ref_record_is_deletion(ref)) {
+		err = 1;
+		goto exit;
+	}
+
+exit:
+	iterator_destroy(&it);
+	return err;
+}
+
+int stack_read_log(struct stack *st, const char *refname,
+		   struct log_record *log)
+{
+	struct iterator it = {};
+	struct merged_table *mt = stack_merged_table(st);
+	int err = merged_table_seek_log(mt, &it, refname);
+	if (err) {
+		goto exit;
+	}
+
+	err = iterator_next_log(it, log);
+	if (err) {
+		goto exit;
+	}
+
+	if (strcmp(log->ref_name, refname) || log_record_is_deletion(log)) {
+		err = 1;
+		goto exit;
+	}
+
+exit:
+	iterator_destroy(&it);
+	return err;
+}
diff --git a/reftable/stack.h b/reftable/stack.h
new file mode 100644
index 0000000000..d5e2c93c29
--- /dev/null
+++ b/reftable/stack.h
@@ -0,0 +1,40 @@
+/*
+Copyright 2020 Google LLC
+
+Use of this source code is governed by a BSD-style
+license that can be found in the LICENSE file or at
+https://developers.google.com/open-source/licenses/bsd
+*/
+
+#ifndef STACK_H
+#define STACK_H
+
+#include "reftable.h"
+
+struct stack {
+	char *list_file;
+	char *reftable_dir;
+
+	struct write_options config;
+
+	struct merged_table *merged;
+	struct compaction_stats stats;
+};
+
+int read_lines(const char *filename, char ***lines);
+int stack_try_add(struct stack *st,
+		  int (*write_table)(struct writer *wr, void *arg), void *arg);
+int stack_write_compact(struct stack *st, struct writer *wr, int first,
+			int last, struct log_expiry_config *config);
+int fastlog2(uint64_t sz);
+
+struct segment {
+	int start, end;
+	int log;
+	uint64_t bytes;
+};
+
+struct segment *sizes_to_segments(int *seglen, uint64_t *sizes, int n);
+struct segment suggest_compaction_segment(uint64_t *sizes, int n);
+
+#endif
diff --git a/reftable/stack_test.c b/reftable/stack_test.c
new file mode 100644
index 0000000000..d80cb1789a
--- /dev/null
+++ b/reftable/stack_test.c
@@ -0,0 +1,281 @@
+/*
+Copyright 2020 Google LLC
+
+Use of this source code is governed by a BSD-style
+license that can be found in the LICENSE file or at
+https://developers.google.com/open-source/licenses/bsd
+*/
+
+#include "stack.h"
+
+#include "system.h"
+
+#include "basics.h"
+#include "constants.h"
+#include "record.h"
+#include "reftable.h"
+#include "test_framework.h"
+
+void test_read_file(void)
+{
+	char fn[256] = "/tmp/stack.test_read_file.XXXXXX";
+	int fd = mkstemp(fn);
+	assert(fd > 0);
+
+	char out[1024] = "line1\n\nline2\nline3";
+
+	int n = write(fd, out, strlen(out));
+	assert(n == strlen(out));
+	int err = close(fd);
+	assert(err >= 0);
+
+	char **names = NULL;
+	err = read_lines(fn, &names);
+	assert_err(err);
+
+	char *want[] = { "line1", "line2", "line3" };
+	int i = 0;
+	for (i = 0; names[i] != NULL; i++) {
+		assert(0 == strcmp(want[i], names[i]));
+	}
+	free_names(names);
+	remove(fn);
+}
+
+void test_parse_names(void)
+{
+	char buf[] = "line\n";
+	char **names = NULL;
+	parse_names(buf, strlen(buf), &names);
+
+	assert(NULL != names[0]);
+	assert(0 == strcmp(names[0], "line"));
+	assert(NULL == names[1]);
+	free_names(names);
+}
+
+void test_names_equal(void)
+{
+	char *a[] = { "a", "b", "c", NULL };
+	char *b[] = { "a", "b", "d", NULL };
+	char *c[] = { "a", "b", NULL };
+
+	assert(names_equal(a, a));
+	assert(!names_equal(a, b));
+	assert(!names_equal(a, c));
+}
+
+int write_test_ref(struct writer *wr, void *arg)
+{
+	struct ref_record *ref = arg;
+
+	writer_set_limits(wr, ref->update_index, ref->update_index);
+	int err = writer_add_ref(wr, ref);
+
+	return err;
+}
+
+int write_test_log(struct writer *wr, void *arg)
+{
+	struct log_record *log = arg;
+
+	writer_set_limits(wr, log->update_index, log->update_index);
+	int err = writer_add_log(wr, log);
+
+	return err;
+}
+
+void test_stack_add(void)
+{
+	int i = 0;
+	char dir[256] = "/tmp/stack.test_stack_add.XXXXXX";
+	assert(mkdtemp(dir));
+	printf("%s\n", dir);
+	char fn[256] = "";
+	strcat(fn, dir);
+	strcat(fn, "/refs");
+
+	struct write_options cfg = {};
+	struct stack *st = NULL;
+	int err = new_stack(&st, dir, fn, cfg);
+	assert_err(err);
+
+	struct ref_record refs[2] = {};
+	struct log_record logs[2] = {};
+	int N = ARRAYSIZE(refs);
+	for (i = 0; i < N; i++) {
+		char buf[256];
+		snprintf(buf, sizeof(buf), "branch%02d", i);
+		refs[i].ref_name = strdup(buf);
+		refs[i].value = malloc(SHA1_SIZE);
+		refs[i].update_index = i + 1;
+		set_test_hash(refs[i].value, i);
+
+		logs[i].ref_name = strdup(buf);
+		logs[i].update_index = N + i + 1;
+		logs[i].new_hash = malloc(SHA1_SIZE);
+		logs[i].email = strdup("identity@invalid");
+		set_test_hash(logs[i].new_hash, i);
+	}
+
+	for (i = 0; i < N; i++) {
+		int err = stack_add(st, &write_test_ref, &refs[i]);
+		assert_err(err);
+	}
+
+	for (i = 0; i < N; i++) {
+		int err = stack_add(st, &write_test_log, &logs[i]);
+		assert_err(err);
+	}
+
+	err = stack_compact_all(st, NULL);
+	assert_err(err);
+
+	for (i = 0; i < N; i++) {
+		struct ref_record dest = {};
+		int err = stack_read_ref(st, refs[i].ref_name, &dest);
+		assert_err(err);
+		assert(ref_record_equal(&dest, refs + i, SHA1_SIZE));
+		ref_record_clear(&dest);
+	}
+
+	for (i = 0; i < N; i++) {
+		struct log_record dest = {};
+		int err = stack_read_log(st, refs[i].ref_name, &dest);
+		assert_err(err);
+		assert(log_record_equal(&dest, logs + i, SHA1_SIZE));
+		log_record_clear(&dest);
+	}
+
+	/* cleanup */
+	stack_destroy(st);
+	for (i = 0; i < N; i++) {
+		ref_record_clear(&refs[i]);
+		log_record_clear(&logs[i]);
+	}
+}
+
+void test_log2(void)
+{
+	assert(1 == fastlog2(3));
+	assert(2 == fastlog2(4));
+	assert(2 == fastlog2(5));
+}
+
+void test_sizes_to_segments(void)
+{
+	uint64_t sizes[] = { 2, 3, 4, 5, 7, 9 };
+	/* .................0  1  2  3  4  5 */
+
+	int seglen = 0;
+	struct segment *segs =
+		sizes_to_segments(&seglen, sizes, ARRAYSIZE(sizes));
+	assert(segs[2].log == 3);
+	assert(segs[2].start == 5);
+	assert(segs[2].end == 6);
+
+	assert(segs[1].log == 2);
+	assert(segs[1].start == 2);
+	assert(segs[1].end == 5);
+	free(segs);
+}
+
+void test_suggest_compaction_segment(void)
+{
+	{
+		uint64_t sizes[] = { 128, 64, 17, 16, 9, 9, 9, 16, 16 };
+		/* .................0    1    2  3   4  5  6 */
+		struct segment min =
+			suggest_compaction_segment(sizes, ARRAYSIZE(sizes));
+		assert(min.start == 2);
+		assert(min.end == 7);
+	}
+
+	{
+		uint64_t sizes[] = { 64, 32, 16, 8, 4, 2 };
+		struct segment result =
+			suggest_compaction_segment(sizes, ARRAYSIZE(sizes));
+		assert(result.start == result.end);
+	}
+}
+
+void test_reflog_expire(void)
+{
+	char dir[256] = "/tmp/stack.test_reflog_expire.XXXXXX";
+	assert(mkdtemp(dir));
+	printf("%s\n", dir);
+	char fn[256] = "";
+	strcat(fn, dir);
+	strcat(fn, "/refs");
+
+	struct write_options cfg = {};
+	struct stack *st = NULL;
+	int err = new_stack(&st, dir, fn, cfg);
+	assert_err(err);
+
+	struct log_record logs[20] = {};
+	int N = ARRAYSIZE(logs) - 1;
+	int i = 0;
+	for (i = 1; i <= N; i++) {
+		char buf[256];
+		snprintf(buf, sizeof(buf), "branch%02d", i);
+
+		logs[i].ref_name = strdup(buf);
+		logs[i].update_index = i;
+		logs[i].time = i;
+		logs[i].new_hash = malloc(SHA1_SIZE);
+		logs[i].email = strdup("identity@invalid");
+		set_test_hash(logs[i].new_hash, i);
+	}
+
+	for (i = 1; i <= N; i++) {
+		int err = stack_add(st, &write_test_log, &logs[i]);
+		assert_err(err);
+	}
+
+	err = stack_compact_all(st, NULL);
+	assert_err(err);
+
+	struct log_expiry_config expiry = {
+		.time = 10,
+	};
+	err = stack_compact_all(st, &expiry);
+	assert_err(err);
+
+	struct log_record log = {};
+	err = stack_read_log(st, logs[9].ref_name, &log);
+	assert(err == 1);
+
+	err = stack_read_log(st, logs[11].ref_name, &log);
+	assert_err(err);
+
+	expiry.min_update_index = 15;
+	err = stack_compact_all(st, &expiry);
+	assert_err(err);
+
+	err = stack_read_log(st, logs[14].ref_name, &log);
+	assert(err == 1);
+
+	err = stack_read_log(st, logs[16].ref_name, &log);
+	assert_err(err);
+
+	/* cleanup */
+	stack_destroy(st);
+	for (i = 0; i < N; i++) {
+		log_record_clear(&logs[i]);
+	}
+}
+
+int main()
+{
+	add_test_case("test_reflog_expire", test_reflog_expire);
+	add_test_case("test_suggest_compaction_segment",
+		      &test_suggest_compaction_segment);
+	add_test_case("test_sizes_to_segments", &test_sizes_to_segments);
+	add_test_case("test_log2", &test_log2);
+	add_test_case("test_parse_names", &test_parse_names);
+	add_test_case("test_read_file", &test_read_file);
+	add_test_case("test_names_equal", &test_names_equal);
+	add_test_case("test_stack_add", &test_stack_add);
+	test_main();
+}
diff --git a/reftable/system.h b/reftable/system.h
new file mode 100644
index 0000000000..97214ae210
--- /dev/null
+++ b/reftable/system.h
@@ -0,0 +1,36 @@
+/*
+Copyright 2020 Google LLC
+
+Use of this source code is governed by a BSD-style
+license that can be found in the LICENSE file or at
+https://developers.google.com/open-source/licenses/bsd
+*/
+
+#ifndef SYSTEM_H
+#define SYSTEM_H
+
+#include "config.h"
+
+#ifndef REFTABLE_STANDALONE
+#include "git-compat-util.h"
+#else
+#include <assert.h>
+#include <errno.h>
+#include <fcntl.h>
+#include <stdint.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <sys/stat.h>
+#include <sys/time.h>
+#include <sys/types.h>
+#include <unistd.h>
+#include <zlib.h>
+
+#define PRIuMAX "lu"
+#define PRIdMAX "ld"
+#define PRIxMAX "lx"
+
+#endif
+
+#endif
diff --git a/reftable/test_framework.c b/reftable/test_framework.c
new file mode 100644
index 0000000000..b9f633934a
--- /dev/null
+++ b/reftable/test_framework.c
@@ -0,0 +1,67 @@
+/*
+Copyright 2020 Google LLC
+
+Use of this source code is governed by a BSD-style
+license that can be found in the LICENSE file or at
+https://developers.google.com/open-source/licenses/bsd
+*/
+
+#include "test_framework.h"
+
+#include "system.h"
+
+#include "constants.h"
+
+struct test_case **test_cases;
+int test_case_len;
+int test_case_cap;
+
+struct test_case *new_test_case(const char *name, void (*testfunc)())
+{
+	struct test_case *tc = malloc(sizeof(struct test_case));
+	tc->name = name;
+	tc->testfunc = testfunc;
+	return tc;
+}
+
+struct test_case *add_test_case(const char *name, void (*testfunc)())
+{
+	struct test_case *tc = new_test_case(name, testfunc);
+	if (test_case_len == test_case_cap) {
+		test_case_cap = 2 * test_case_cap + 1;
+		test_cases = realloc(test_cases,
+				     sizeof(struct test_case) * test_case_cap);
+	}
+
+	test_cases[test_case_len++] = tc;
+	return tc;
+}
+
+void test_main()
+{
+	int i = 0;
+	for (i = 0; i < test_case_len; i++) {
+		printf("case %s\n", test_cases[i]->name);
+		test_cases[i]->testfunc();
+	}
+}
+
+void set_test_hash(byte *p, int i)
+{
+	memset(p, (byte)i, SHA1_SIZE);
+}
+
+void print_names(char **a)
+{
+	if (a == NULL || *a == NULL) {
+		puts("[]");
+		return;
+	}
+	puts("[");
+	char **p = a;
+	while (*p) {
+		puts(*p);
+		p++;
+	}
+	puts("]");
+}
diff --git a/reftable/test_framework.h b/reftable/test_framework.h
new file mode 100644
index 0000000000..d3e793c3a6
--- /dev/null
+++ b/reftable/test_framework.h
@@ -0,0 +1,64 @@
+/*
+Copyright 2020 Google LLC
+
+Use of this source code is governed by a BSD-style
+license that can be found in the LICENSE file or at
+https://developers.google.com/open-source/licenses/bsd
+*/
+
+#ifndef TEST_FRAMEWORK_H
+#define TEST_FRAMEWORK_H
+
+#include "system.h"
+
+#include "reftable.h"
+
+#ifdef NDEBUG
+#undef NDEBUG
+#endif
+
+#include "system.h"
+
+#ifdef assert
+#undef assert
+#endif
+
+#define assert_err(c)                                                 \
+	if (c != 0) {                                                 \
+		fflush(stderr);                                       \
+		fflush(stdout);                                       \
+		fprintf(stderr, "%s: %d: error == %d (%s), want 0\n", \
+			__FILE__, __LINE__, c, error_str(c));         \
+		abort();                                              \
+	}
+
+#define assert_streq(a, b)                                               \
+	if (strcmp(a, b)) {                                              \
+		fflush(stderr);                                          \
+		fflush(stdout);                                          \
+		fprintf(stderr, "%s:%d: %s (%s) != %s (%s)\n", __FILE__, \
+			__LINE__, #a, a, #b, b);                         \
+		abort();                                                 \
+	}
+
+#define assert(c)                                                          \
+	if (!(c)) {                                                        \
+		fflush(stderr);                                            \
+		fflush(stdout);                                            \
+		fprintf(stderr, "%s: %d: failed assertion %s\n", __FILE__, \
+			__LINE__, #c);                                     \
+		abort();                                                   \
+	}
+
+struct test_case {
+	const char *name;
+	void (*testfunc)();
+};
+
+struct test_case *new_test_case(const char *name, void (*testfunc)());
+struct test_case *add_test_case(const char *name, void (*testfunc)());
+void test_main();
+
+void set_test_hash(byte *p, int i);
+
+#endif
diff --git a/reftable/tree.c b/reftable/tree.c
new file mode 100644
index 0000000000..9bf7fe531f
--- /dev/null
+++ b/reftable/tree.c
@@ -0,0 +1,66 @@
+/*
+Copyright 2020 Google LLC
+
+Use of this source code is governed by a BSD-style
+license that can be found in the LICENSE file or at
+https://developers.google.com/open-source/licenses/bsd
+*/
+
+#include "tree.h"
+
+#include "system.h"
+
+struct tree_node *tree_search(void *key, struct tree_node **rootp,
+			      int (*compare)(const void *, const void *),
+			      int insert)
+{
+	if (*rootp == NULL) {
+		if (!insert) {
+			return NULL;
+		} else {
+			struct tree_node *n =
+				calloc(sizeof(struct tree_node), 1);
+			n->key = key;
+			*rootp = n;
+			return *rootp;
+		}
+	}
+
+	{
+		int res = compare(key, (*rootp)->key);
+		if (res < 0) {
+			return tree_search(key, &(*rootp)->left, compare,
+					   insert);
+		} else if (res > 0) {
+			return tree_search(key, &(*rootp)->right, compare,
+					   insert);
+		}
+	}
+	return *rootp;
+}
+
+void infix_walk(struct tree_node *t, void (*action)(void *arg, void *key),
+		void *arg)
+{
+	if (t->left != NULL) {
+		infix_walk(t->left, action, arg);
+	}
+	action(arg, t->key);
+	if (t->right != NULL) {
+		infix_walk(t->right, action, arg);
+	}
+}
+
+void tree_free(struct tree_node *t)
+{
+	if (t == NULL) {
+		return;
+	}
+	if (t->left != NULL) {
+		tree_free(t->left);
+	}
+	if (t->right != NULL) {
+		tree_free(t->right);
+	}
+	free(t);
+}
diff --git a/reftable/tree.h b/reftable/tree.h
new file mode 100644
index 0000000000..86a71715ae
--- /dev/null
+++ b/reftable/tree.h
@@ -0,0 +1,24 @@
+/*
+Copyright 2020 Google LLC
+
+Use of this source code is governed by a BSD-style
+license that can be found in the LICENSE file or at
+https://developers.google.com/open-source/licenses/bsd
+*/
+
+#ifndef TREE_H
+#define TREE_H
+
+struct tree_node {
+	void *key;
+	struct tree_node *left, *right;
+};
+
+struct tree_node *tree_search(void *key, struct tree_node **rootp,
+			      int (*compare)(const void *, const void *),
+			      int insert);
+void infix_walk(struct tree_node *t, void (*action)(void *arg, void *key),
+		void *arg);
+void tree_free(struct tree_node *t);
+
+#endif
diff --git a/reftable/tree_test.c b/reftable/tree_test.c
new file mode 100644
index 0000000000..842985376c
--- /dev/null
+++ b/reftable/tree_test.c
@@ -0,0 +1,61 @@
+/*
+Copyright 2020 Google LLC
+
+Use of this source code is governed by a BSD-style
+license that can be found in the LICENSE file or at
+https://developers.google.com/open-source/licenses/bsd
+*/
+
+#include "tree.h"
+
+#include "basics.h"
+#include "record.h"
+#include "reftable.h"
+#include "test_framework.h"
+
+static int test_compare(const void *a, const void *b)
+{
+	return a - b;
+}
+
+struct curry {
+	void *last;
+};
+
+void check_increasing(void *arg, void *key)
+{
+	struct curry *c = (struct curry *)arg;
+	if (c->last != NULL) {
+		assert(test_compare(c->last, key) < 0);
+	}
+	c->last = key;
+}
+
+void test_tree()
+{
+	struct tree_node *root = NULL;
+
+	void *values[11] = {};
+	struct tree_node *nodes[11] = {};
+	int i = 1;
+	do {
+		nodes[i] = tree_search(values + i, &root, &test_compare, 1);
+		i = (i * 7) % 11;
+	} while (i != 1);
+
+	for (i = 1; i < ARRAYSIZE(nodes); i++) {
+		assert(values + i == nodes[i]->key);
+		assert(nodes[i] ==
+		       tree_search(values + i, &root, &test_compare, 0));
+	}
+
+	struct curry c = {};
+	infix_walk(root, check_increasing, &c);
+	tree_free(root);
+}
+
+int main()
+{
+	add_test_case("test_tree", &test_tree);
+	test_main();
+}
diff --git a/reftable/writer.c b/reftable/writer.c
new file mode 100644
index 0000000000..cb8e4bbf89
--- /dev/null
+++ b/reftable/writer.c
@@ -0,0 +1,624 @@
+/*
+Copyright 2020 Google LLC
+
+Use of this source code is governed by a BSD-style
+license that can be found in the LICENSE file or at
+https://developers.google.com/open-source/licenses/bsd
+*/
+
+#include "writer.h"
+
+#include "system.h"
+
+#include "block.h"
+#include "constants.h"
+#include "record.h"
+#include "reftable.h"
+#include "tree.h"
+
+static struct block_stats *writer_block_stats(struct writer *w, byte typ)
+{
+	switch (typ) {
+	case 'r':
+		return &w->stats.ref_stats;
+	case 'o':
+		return &w->stats.obj_stats;
+	case 'i':
+		return &w->stats.idx_stats;
+	case 'g':
+		return &w->stats.log_stats;
+	}
+	assert(false);
+}
+
+/* write data, queuing the padding for the next write. Returns negative for
+ * error. */
+static int padded_write(struct writer *w, byte *data, size_t len, int padding)
+{
+	int n = 0;
+	if (w->pending_padding > 0) {
+		byte *zeroed = calloc(w->pending_padding, 1);
+		int n = w->write(w->write_arg, zeroed, w->pending_padding);
+		if (n < 0) {
+			return n;
+		}
+
+		w->pending_padding = 0;
+		free(zeroed);
+	}
+
+	w->pending_padding = padding;
+	n = w->write(w->write_arg, data, len);
+	if (n < 0) {
+		return n;
+	}
+	n += padding;
+	return 0;
+}
+
+static void options_set_defaults(struct write_options *opts)
+{
+	if (opts->restart_interval == 0) {
+		opts->restart_interval = 16;
+	}
+
+	if (opts->block_size == 0) {
+		opts->block_size = DEFAULT_BLOCK_SIZE;
+	}
+}
+
+static int writer_write_header(struct writer *w, byte *dest)
+{
+	memcpy((char *)dest, "REFT", 4);
+	dest[4] = 1; /* version */
+	put_u24(dest + 5, w->opts.block_size);
+	put_u64(dest + 8, w->min_update_index);
+	put_u64(dest + 16, w->max_update_index);
+	return 24;
+}
+
+static void writer_reinit_block_writer(struct writer *w, byte typ)
+{
+	int block_start = 0;
+	if (w->next == 0) {
+		block_start = HEADER_SIZE;
+	}
+
+	block_writer_init(&w->block_writer_data, typ, w->block,
+			  w->opts.block_size, block_start, w->hash_size);
+	w->block_writer = &w->block_writer_data;
+	w->block_writer->restart_interval = w->opts.restart_interval;
+}
+
+struct writer *new_writer(int (*writer_func)(void *, byte *, int),
+			  void *writer_arg, struct write_options *opts)
+{
+	struct writer *wp = calloc(sizeof(struct writer), 1);
+	options_set_defaults(opts);
+	if (opts->block_size >= (1 << 24)) {
+		/* TODO - error return? */
+		abort();
+	}
+	wp->hash_size = SHA1_SIZE;
+	wp->block = calloc(opts->block_size, 1);
+	wp->write = writer_func;
+	wp->write_arg = writer_arg;
+	wp->opts = *opts;
+	writer_reinit_block_writer(wp, BLOCK_TYPE_REF);
+
+	return wp;
+}
+
+void writer_set_limits(struct writer *w, uint64_t min, uint64_t max)
+{
+	w->min_update_index = min;
+	w->max_update_index = max;
+}
+
+void writer_free(struct writer *w)
+{
+	free(w->block);
+	free(w);
+}
+
+struct obj_index_tree_node {
+	struct slice hash;
+	uint64_t *offsets;
+	int offset_len;
+	int offset_cap;
+};
+
+static int obj_index_tree_node_compare(const void *a, const void *b)
+{
+	return slice_compare(((const struct obj_index_tree_node *)a)->hash,
+			     ((const struct obj_index_tree_node *)b)->hash);
+}
+
+static void writer_index_hash(struct writer *w, struct slice hash)
+{
+	uint64_t off = w->next;
+
+	struct obj_index_tree_node want = { .hash = hash };
+
+	struct tree_node *node = tree_search(&want, &w->obj_index_tree,
+					     &obj_index_tree_node_compare, 0);
+	struct obj_index_tree_node *key = NULL;
+	if (node == NULL) {
+		key = calloc(sizeof(struct obj_index_tree_node), 1);
+		slice_copy(&key->hash, hash);
+		tree_search((void *)key, &w->obj_index_tree,
+			    &obj_index_tree_node_compare, 1);
+	} else {
+		key = node->key;
+	}
+
+	if (key->offset_len > 0 && key->offsets[key->offset_len - 1] == off) {
+		return;
+	}
+
+	if (key->offset_len == key->offset_cap) {
+		key->offset_cap = 2 * key->offset_cap + 1;
+		key->offsets = realloc(key->offsets,
+				       sizeof(uint64_t) * key->offset_cap);
+	}
+
+	key->offsets[key->offset_len++] = off;
+}
+
+static int writer_add_record(struct writer *w, struct record rec)
+{
+	int result = -1;
+	struct slice key = {};
+	int err = 0;
+	record_key(rec, &key);
+	if (slice_compare(w->last_key, key) >= 0) {
+		goto exit;
+	}
+
+	slice_copy(&w->last_key, key);
+	if (w->block_writer == NULL) {
+		writer_reinit_block_writer(w, record_type(rec));
+	}
+
+	assert(block_writer_type(w->block_writer) == record_type(rec));
+
+	if (block_writer_add(w->block_writer, rec) == 0) {
+		result = 0;
+		goto exit;
+	}
+
+	err = writer_flush_block(w);
+	if (err < 0) {
+		result = err;
+		goto exit;
+	}
+
+	writer_reinit_block_writer(w, record_type(rec));
+	err = block_writer_add(w->block_writer, rec);
+	if (err < 0) {
+		result = err;
+		goto exit;
+	}
+
+	result = 0;
+exit:
+	free(slice_yield(&key));
+	return result;
+}
+
+int writer_add_ref(struct writer *w, struct ref_record *ref)
+{
+	struct record rec = {};
+	struct ref_record copy = *ref;
+	int err = 0;
+
+	if (ref->ref_name == NULL) {
+		return API_ERROR;
+	}
+	if (ref->update_index < w->min_update_index ||
+	    ref->update_index > w->max_update_index) {
+		return API_ERROR;
+	}
+
+	record_from_ref(&rec, &copy);
+	copy.update_index -= w->min_update_index;
+	err = writer_add_record(w, rec);
+	if (err < 0) {
+		return err;
+	}
+
+	if (!w->opts.skip_index_objects && ref->value != NULL) {
+		struct slice h = {
+			.buf = ref->value,
+			.len = w->hash_size,
+		};
+
+		writer_index_hash(w, h);
+	}
+	if (!w->opts.skip_index_objects && ref->target_value != NULL) {
+		struct slice h = {
+			.buf = ref->target_value,
+			.len = w->hash_size,
+		};
+		writer_index_hash(w, h);
+	}
+	return 0;
+}
+
+int writer_add_refs(struct writer *w, struct ref_record *refs, int n)
+{
+	int err = 0;
+	int i = 0;
+	qsort(refs, n, sizeof(struct ref_record), ref_record_compare_name);
+	for (i = 0; err == 0 && i < n; i++) {
+		err = writer_add_ref(w, &refs[i]);
+	}
+	return err;
+}
+
+int writer_add_log(struct writer *w, struct log_record *log)
+{
+	if (log->ref_name == NULL) {
+		return API_ERROR;
+	}
+
+	if (w->block_writer != NULL &&
+	    block_writer_type(w->block_writer) == BLOCK_TYPE_REF) {
+		int err = writer_finish_public_section(w);
+		if (err < 0) {
+			return err;
+		}
+	}
+
+	w->next -= w->pending_padding;
+	w->pending_padding = 0;
+
+	{
+		struct record rec = {};
+		int err;
+		record_from_log(&rec, log);
+		err = writer_add_record(w, rec);
+		return err;
+	}
+}
+
+int writer_add_logs(struct writer *w, struct log_record *logs, int n)
+{
+	int err = 0;
+	int i = 0;
+	qsort(logs, n, sizeof(struct log_record), log_record_compare_key);
+	for (i = 0; err == 0 && i < n; i++) {
+		err = writer_add_log(w, &logs[i]);
+	}
+	return err;
+}
+
+static int writer_finish_section(struct writer *w)
+{
+	byte typ = block_writer_type(w->block_writer);
+	uint64_t index_start = 0;
+	int max_level = 0;
+	int threshold = w->opts.unpadded ? 1 : 3;
+	int before_blocks = w->stats.idx_stats.blocks;
+	int err = writer_flush_block(w);
+	int i = 0;
+	if (err < 0) {
+		return err;
+	}
+
+	while (w->index_len > threshold) {
+		struct index_record *idx = NULL;
+		int idx_len = 0;
+
+		max_level++;
+		index_start = w->next;
+		writer_reinit_block_writer(w, BLOCK_TYPE_INDEX);
+
+		idx = w->index;
+		idx_len = w->index_len;
+
+		w->index = NULL;
+		w->index_len = 0;
+		w->index_cap = 0;
+		for (i = 0; i < idx_len; i++) {
+			struct record rec = {};
+			record_from_index(&rec, idx + i);
+			if (block_writer_add(w->block_writer, rec) == 0) {
+				continue;
+			}
+
+			{
+				int err = writer_flush_block(w);
+				if (err < 0) {
+					return err;
+				}
+			}
+
+			writer_reinit_block_writer(w, BLOCK_TYPE_INDEX);
+
+			err = block_writer_add(w->block_writer, rec);
+			assert(err == 0);
+		}
+		for (i = 0; i < idx_len; i++) {
+			free(slice_yield(&idx[i].last_key));
+		}
+		free(idx);
+	}
+
+	writer_clear_index(w);
+
+	err = writer_flush_block(w);
+	if (err < 0) {
+		return err;
+	}
+
+	{
+		struct block_stats *bstats = writer_block_stats(w, typ);
+		bstats->index_blocks =
+			w->stats.idx_stats.blocks - before_blocks;
+		bstats->index_offset = index_start;
+		bstats->max_index_level = max_level;
+	}
+
+	/* Reinit lastKey, as the next section can start with any key. */
+	w->last_key.len = 0;
+
+	return 0;
+}
+
+struct common_prefix_arg {
+	struct slice *last;
+	int max;
+};
+
+static void update_common(void *void_arg, void *key)
+{
+	struct common_prefix_arg *arg = (struct common_prefix_arg *)void_arg;
+	struct obj_index_tree_node *entry = (struct obj_index_tree_node *)key;
+	if (arg->last != NULL) {
+		int n = common_prefix_size(entry->hash, *arg->last);
+		if (n > arg->max) {
+			arg->max = n;
+		}
+	}
+	arg->last = &entry->hash;
+}
+
+struct write_record_arg {
+	struct writer *w;
+	int err;
+};
+
+static void write_object_record(void *void_arg, void *key)
+{
+	struct write_record_arg *arg = (struct write_record_arg *)void_arg;
+	struct obj_index_tree_node *entry = (struct obj_index_tree_node *)key;
+	struct obj_record obj_rec = {
+		.hash_prefix = entry->hash.buf,
+		.hash_prefix_len = arg->w->stats.object_id_len,
+		.offsets = entry->offsets,
+		.offset_len = entry->offset_len,
+	};
+	struct record rec = {};
+	if (arg->err < 0) {
+		goto exit;
+	}
+
+	record_from_obj(&rec, &obj_rec);
+	arg->err = block_writer_add(arg->w->block_writer, rec);
+	if (arg->err == 0) {
+		goto exit;
+	}
+
+	arg->err = writer_flush_block(arg->w);
+	if (arg->err < 0) {
+		goto exit;
+	}
+
+	writer_reinit_block_writer(arg->w, BLOCK_TYPE_OBJ);
+	arg->err = block_writer_add(arg->w->block_writer, rec);
+	if (arg->err == 0) {
+		goto exit;
+	}
+	obj_rec.offset_len = 0;
+	arg->err = block_writer_add(arg->w->block_writer, rec);
+
+	/* Should be able to write into a fresh block. */
+	assert(arg->err == 0);
+
+exit:;
+}
+
+static void object_record_free(void *void_arg, void *key)
+{
+	struct obj_index_tree_node *entry = (struct obj_index_tree_node *)key;
+
+	free(entry->offsets);
+	entry->offsets = NULL;
+	free(slice_yield(&entry->hash));
+	free(entry);
+}
+
+static int writer_dump_object_index(struct writer *w)
+{
+	struct write_record_arg closure = { .w = w };
+	struct common_prefix_arg common = {};
+	if (w->obj_index_tree != NULL) {
+		infix_walk(w->obj_index_tree, &update_common, &common);
+	}
+	w->stats.object_id_len = common.max + 1;
+
+	writer_reinit_block_writer(w, BLOCK_TYPE_OBJ);
+
+	if (w->obj_index_tree != NULL) {
+		infix_walk(w->obj_index_tree, &write_object_record, &closure);
+	}
+
+	if (closure.err < 0) {
+		return closure.err;
+	}
+	return writer_finish_section(w);
+}
+
+int writer_finish_public_section(struct writer *w)
+{
+	byte typ = 0;
+	int err = 0;
+
+	if (w->block_writer == NULL) {
+		return 0;
+	}
+
+	typ = block_writer_type(w->block_writer);
+	err = writer_finish_section(w);
+	if (err < 0) {
+		return err;
+	}
+	if (typ == BLOCK_TYPE_REF && !w->opts.skip_index_objects &&
+	    w->stats.ref_stats.index_blocks > 0) {
+		err = writer_dump_object_index(w);
+		if (err < 0) {
+			return err;
+		}
+	}
+
+	if (w->obj_index_tree != NULL) {
+		infix_walk(w->obj_index_tree, &object_record_free, NULL);
+		tree_free(w->obj_index_tree);
+		w->obj_index_tree = NULL;
+	}
+
+	w->block_writer = NULL;
+	return 0;
+}
+
+int writer_close(struct writer *w)
+{
+	byte footer[68];
+	byte *p = footer;
+
+	writer_finish_public_section(w);
+
+	writer_write_header(w, footer);
+	p += 24;
+	put_u64(p, w->stats.ref_stats.index_offset);
+	p += 8;
+	put_u64(p, (w->stats.obj_stats.offset) << 5 | w->stats.object_id_len);
+	p += 8;
+	put_u64(p, w->stats.obj_stats.index_offset);
+	p += 8;
+
+	put_u64(p, w->stats.log_stats.offset);
+	p += 8;
+	put_u64(p, w->stats.log_stats.index_offset);
+	p += 8;
+
+	put_u32(p, crc32(0, footer, p - footer));
+	p += 4;
+	w->pending_padding = 0;
+
+	{
+		int n = padded_write(w, footer, sizeof(footer), 0);
+		if (n < 0) {
+			return n;
+		}
+	}
+
+	/* free up memory. */
+	block_writer_clear(&w->block_writer_data);
+	writer_clear_index(w);
+	free(slice_yield(&w->last_key));
+	return 0;
+}
+
+void writer_clear_index(struct writer *w)
+{
+	int i = 0;
+	for (i = 0; i < w->index_len; i++) {
+		free(slice_yield(&w->index[i].last_key));
+	}
+
+	free(w->index);
+	w->index = NULL;
+	w->index_len = 0;
+	w->index_cap = 0;
+}
+
+const int debug = 0;
+
+static int writer_flush_nonempty_block(struct writer *w)
+{
+	byte typ = block_writer_type(w->block_writer);
+	struct block_stats *bstats = writer_block_stats(w, typ);
+	uint64_t block_typ_off = (bstats->blocks == 0) ? w->next : 0;
+	int raw_bytes = block_writer_finish(w->block_writer);
+	int padding = 0;
+	int err = 0;
+	if (raw_bytes < 0) {
+		return raw_bytes;
+	}
+
+	if (!w->opts.unpadded && typ != BLOCK_TYPE_LOG) {
+		padding = w->opts.block_size - raw_bytes;
+	}
+
+	if (block_typ_off > 0) {
+		bstats->offset = block_typ_off;
+	}
+
+	bstats->entries += w->block_writer->entries;
+	bstats->restarts += w->block_writer->restart_len;
+	bstats->blocks++;
+	w->stats.blocks++;
+
+	if (debug) {
+		fprintf(stderr, "block %c off %" PRIuMAX " sz %d (%d)\n", typ,
+			w->next, raw_bytes,
+			get_u24(w->block + w->block_writer->header_off + 1));
+	}
+
+	if (w->next == 0) {
+		writer_write_header(w, w->block);
+	}
+
+	err = padded_write(w, w->block, raw_bytes, padding);
+	if (err < 0) {
+		return err;
+	}
+
+	if (w->index_cap == w->index_len) {
+		w->index_cap = 2 * w->index_cap + 1;
+		w->index = realloc(w->index,
+				   sizeof(struct index_record) * w->index_cap);
+	}
+
+	{
+		struct index_record ir = {
+			.offset = w->next,
+		};
+		slice_copy(&ir.last_key, w->block_writer->last_key);
+		w->index[w->index_len] = ir;
+	}
+
+	w->index_len++;
+	w->next += padding + raw_bytes;
+	block_writer_reset(&w->block_writer_data);
+	w->block_writer = NULL;
+	return 0;
+}
+
+int writer_flush_block(struct writer *w)
+{
+	if (w->block_writer == NULL) {
+		return 0;
+	}
+	if (w->block_writer->entries == 0) {
+		return 0;
+	}
+	return writer_flush_nonempty_block(w);
+}
+
+struct stats *writer_stats(struct writer *w)
+{
+	return &w->stats;
+}
diff --git a/reftable/writer.h b/reftable/writer.h
new file mode 100644
index 0000000000..bd2386474b
--- /dev/null
+++ b/reftable/writer.h
@@ -0,0 +1,46 @@
+/*
+Copyright 2020 Google LLC
+
+Use of this source code is governed by a BSD-style
+license that can be found in the LICENSE file or at
+https://developers.google.com/open-source/licenses/bsd
+*/
+
+#ifndef WRITER_H
+#define WRITER_H
+
+#include "basics.h"
+#include "block.h"
+#include "reftable.h"
+#include "slice.h"
+#include "tree.h"
+
+struct writer {
+	int (*write)(void *, byte *, int);
+	void *write_arg;
+	int pending_padding;
+	int hash_size;
+	struct slice last_key;
+
+	uint64_t next;
+	uint64_t min_update_index, max_update_index;
+	struct write_options opts;
+
+	byte *block;
+	struct block_writer *block_writer;
+	struct block_writer block_writer_data;
+	struct index_record *index;
+	int index_len;
+	int index_cap;
+
+	/* tree for use with tsearch */
+	struct tree_node *obj_index_tree;
+
+	struct stats stats;
+};
+
+int writer_flush_block(struct writer *w);
+void writer_clear_index(struct writer *w);
+int writer_finish_public_section(struct writer *w);
+
+#endif
