thodg/got

diff --git a/lib/deltify.c b/lib/deltify.c
index ae0934b..8a0eff6 100644
--- a/lib/deltify.c
+++ b/lib/deltify.c
@@ -1,5 +1,6 @@
 /*
  * Copyright (c) 2020 Ori Bernstein
+ * Copyright (c) 2021 Stefan Sperling <stsp@openbsd.org>
  *
  * Permission to use, copy, modify, and distribute this software for any
  * purpose with or without fee is hereby granted, provided that the above
@@ -18,6 +19,7 @@
 #include <sys/queue.h>
 
 #include <assert.h>
+#include <errno.h>
 #include <limits.h>
 #include <stdint.h>
 #include <stdio.h>
@@ -26,265 +28,380 @@
 #include <sha1.h>
 
 #include "got_error.h"
-#include "got_object.h"
 
-#include "got_lib_delta.h"
-#include "got_lib_object.h"
+#include "got_lib_deltify.h"
 
-struct Dblock {
-	unsigned char	*buf;
-	int		len;
-	int		off;
-	uint64_t	hash;
-};
-
-struct Dtab {
-	unsigned char		*base;
-	int			nbase;
-	struct Dblock		*b;
-	int			nb;
-	int			sz;
-};
-
-struct Delta {
-	int	cpy;
-	int	off;
-	int	len;
-};
-
-enum {
-	Minchunk	= 128,
-	Maxchunk	= 8192,
-	Splitmask	= (1 << 8) - 1,
-	
-};
+#ifndef MIN
+#define	MIN(_a,_b) ((_a) < (_b) ? (_a) : (_b))
+#endif
 
 static uint32_t geartab[] = {
-    0x67ed26b7, 0x32da500c, 0x53d0fee0, 0xce387dc7, 0xcd406d90, 0x2e83a4d4, 0x9fc9a38d, 0xb67259dc,
-    0xca6b1722, 0x6d2ea08c, 0x235cea2e, 0x3149bb5f, 0x1beda787, 0x2a6b77d5, 0x2f22d9ac, 0x91fc0544,
-    0xe413acfa, 0x5a30ff7a, 0xad6fdde0, 0x444fd0f5, 0x7ad87864, 0x58c5ff05, 0x8d2ec336, 0x2371f853,
-    0x550f8572, 0x6aa448dd, 0x7c9ddbcf, 0x95221e14, 0x2a82ec33, 0xcbec5a78, 0xc6795a0d, 0x243995b7,
-    0x1c909a2f, 0x4fded51c, 0x635d334b, 0x0e2b9999, 0x2702968d, 0x856de1d5, 0x3325d60e, 0xeb6a7502,
-    0xec2a9844, 0x0905835a, 0xa1820375, 0xa4be5cab, 0x96a6c058, 0x2c2ccd70, 0xba40fce3, 0xd794c46b,
-    0x8fbae83e, 0xc3aa7899, 0x3d3ff8ed, 0xa0d42b5b, 0x571c0c97, 0xd2811516, 0xf7e7b96c, 0x4fd2fcbd,
-    0xe2fdec94, 0x282cc436, 0x78e8e95c, 0x80a3b613, 0xcfbee20c, 0xd4a32d1c, 0x2a12ff13, 0x6af82936,
-    0xe5630258, 0x8efa6a98, 0x294fb2d1, 0xdeb57086, 0x5f0fddb3, 0xeceda7ce, 0x4c87305f, 0x3a6d3307,
-    0xe22d2942, 0x9d060217, 0x1e42ed02, 0xb6f63b52, 0x4367f39f, 0x055cf262, 0x03a461b2, 0x5ef9e382,
-    0x386bc03a, 0x2a1e79c7, 0xf1a0058b, 0xd4d2dea9, 0x56baf37d, 0x5daff6cc, 0xf03a951d, 0xaef7de45,
-    0xa8f4581e, 0x3960b555, 0xffbfff6d, 0xbe702a23, 0x8f5b6d6f, 0x061739fb, 0x98696f47, 0x3fd596d4,
-    0x151eac6b, 0xa9fcc4f5, 0x69181a12, 0x3ac5a107, 0xb5198fe7, 0x96bcb1da, 0x1b5ddf8e, 0xc757d650,
-    0x65865c3a, 0x8fc0a41a, 0x87435536, 0x99eda6f2, 0x41874794, 0x29cff4e8, 0xb70efd9a, 0x3103f6e7,
-    0x84d2453b, 0x15a450bd, 0x74f49af1, 0x60f664b1, 0xa1c86935, 0xfdafbce1, 0xe36353e3, 0x5d9ba739,
-    0xbc0559ba, 0x708b0054, 0xd41d808c, 0xb2f31723, 0x9027c41f, 0xf136d165, 0xb5374b12, 0x9420a6ac,
-    0x273958b6, 0xe6c2fad0, 0xebdc1f21, 0xfb33af8b, 0xc71c25cd, 0xe9a2d8e5, 0xbeb38a50, 0xbceb7cc2,
-    0x4e4e73f0, 0xcd6c251d, 0xde4c032c, 0x4b04ac30, 0x725b8b21, 0x4eb8c33b, 0x20d07b75, 0x0567aa63,
-    0xb56b2bb7, 0xc1f5fd3a, 0xcafd35ca, 0x470dd4da, 0xfe4f94cd, 0xfb8de424, 0xe8dbcf40, 0xfe50a37a,
-    0x62db5b5d, 0xf32f4ab6, 0x2c4a8a51, 0x18473dc0, 0xfe0cbb6e, 0xfe399efd, 0xdf34ecc9, 0x6ccd5055,
-    0x46097073, 0x139135c2, 0x721c76f6, 0x1c6a94b4, 0x6eee014d, 0x8a508e02, 0x3da538f5, 0x280d394f,
-    0x5248a0c4, 0x3ce94c6c, 0x9a71ad3a, 0x8493dd05, 0xe43f0ab6, 0x18e4ed42, 0x6c5c0e09, 0x42b06ec9,
-    0x8d330343, 0xa45b6f59, 0x2a573c0c, 0xd7fd3de6, 0xeedeab68, 0x5c84dafc, 0xbbd1b1a8, 0xa3ce1ad1,
-    0x85b70bed, 0xb6add07f, 0xa531309c, 0x8f8ab852, 0x564de332, 0xeac9ed0c, 0x73da402c, 0x3ec52761,
-    0x43af2f4d, 0xd6ff45c8, 0x4c367462, 0xd553bd6a, 0x44724855, 0x3b2aa728, 0x56e5eb65, 0xeaf16173,
-    0x33fa42ff, 0xd714bb5d, 0xfbd0a3b9, 0xaf517134, 0x9416c8cd, 0x534cf94f, 0x548947c2, 0x34193569,
-    0x32f4389a, 0xfe7028bc, 0xed73b1ed, 0x9db95770, 0x468e3922, 0x0440c3cd, 0x60059a62, 0x33504562,
-    0x2b229fbd, 0x5174dca5, 0xf7028752, 0xd63c6aa8, 0x31276f38, 0x0646721c, 0xb0191da8, 0xe00e6de0,
-    0x9eac1a6e, 0x9f7628a5, 0xed6c06ea, 0x0bb8af15, 0xf119fb12, 0x38693c1c, 0x732bc0fe, 0x84953275,
-    0xb82ec888, 0x33a4f1b3, 0x3099835e, 0x028a8782, 0x5fdd51d7, 0xc6c717b3, 0xb06caf71, 0x17c8c111,
-    0x61bad754, 0x9fd03061, 0xe09df1af, 0x3bc9eb73, 0x85878413, 0x9889aaf2, 0x3f5a9e46, 0x42c9f01f,
-    0x9984a4f4, 0xd5de43cc, 0xd294daed, 0xbecba2d2, 0xf1f6e72c, 0x5551128a, 0x83af87e2, 0x6f0342ba,
+    0x67ed26b7, 0x32da500c, 0x53d0fee0, 0xce387dc7, 0xcd406d90, 0x2e83a4d4,
+    0x9fc9a38d, 0xb67259dc, 0xca6b1722, 0x6d2ea08c, 0x235cea2e, 0x3149bb5f,
+    0x1beda787, 0x2a6b77d5, 0x2f22d9ac, 0x91fc0544, 0xe413acfa, 0x5a30ff7a,
+    0xad6fdde0, 0x444fd0f5, 0x7ad87864, 0x58c5ff05, 0x8d2ec336, 0x2371f853,
+    0x550f8572, 0x6aa448dd, 0x7c9ddbcf, 0x95221e14, 0x2a82ec33, 0xcbec5a78,
+    0xc6795a0d, 0x243995b7, 0x1c909a2f, 0x4fded51c, 0x635d334b, 0x0e2b9999,
+    0x2702968d, 0x856de1d5, 0x3325d60e, 0xeb6a7502, 0xec2a9844, 0x0905835a,
+    0xa1820375, 0xa4be5cab, 0x96a6c058, 0x2c2ccd70, 0xba40fce3, 0xd794c46b,
+    0x8fbae83e, 0xc3aa7899, 0x3d3ff8ed, 0xa0d42b5b, 0x571c0c97, 0xd2811516,
+    0xf7e7b96c, 0x4fd2fcbd, 0xe2fdec94, 0x282cc436, 0x78e8e95c, 0x80a3b613,
+    0xcfbee20c, 0xd4a32d1c, 0x2a12ff13, 0x6af82936, 0xe5630258, 0x8efa6a98,
+    0x294fb2d1, 0xdeb57086, 0x5f0fddb3, 0xeceda7ce, 0x4c87305f, 0x3a6d3307,
+    0xe22d2942, 0x9d060217, 0x1e42ed02, 0xb6f63b52, 0x4367f39f, 0x055cf262,
+    0x03a461b2, 0x5ef9e382, 0x386bc03a, 0x2a1e79c7, 0xf1a0058b, 0xd4d2dea9,
+    0x56baf37d, 0x5daff6cc, 0xf03a951d, 0xaef7de45, 0xa8f4581e, 0x3960b555,
+    0xffbfff6d, 0xbe702a23, 0x8f5b6d6f, 0x061739fb, 0x98696f47, 0x3fd596d4,
+    0x151eac6b, 0xa9fcc4f5, 0x69181a12, 0x3ac5a107, 0xb5198fe7, 0x96bcb1da,
+    0x1b5ddf8e, 0xc757d650, 0x65865c3a, 0x8fc0a41a, 0x87435536, 0x99eda6f2,
+    0x41874794, 0x29cff4e8, 0xb70efd9a, 0x3103f6e7, 0x84d2453b, 0x15a450bd,
+    0x74f49af1, 0x60f664b1, 0xa1c86935, 0xfdafbce1, 0xe36353e3, 0x5d9ba739,
+    0xbc0559ba, 0x708b0054, 0xd41d808c, 0xb2f31723, 0x9027c41f, 0xf136d165,
+    0xb5374b12, 0x9420a6ac, 0x273958b6, 0xe6c2fad0, 0xebdc1f21, 0xfb33af8b,
+    0xc71c25cd, 0xe9a2d8e5, 0xbeb38a50, 0xbceb7cc2, 0x4e4e73f0, 0xcd6c251d,
+    0xde4c032c, 0x4b04ac30, 0x725b8b21, 0x4eb8c33b, 0x20d07b75, 0x0567aa63,
+    0xb56b2bb7, 0xc1f5fd3a, 0xcafd35ca, 0x470dd4da, 0xfe4f94cd, 0xfb8de424,
+    0xe8dbcf40, 0xfe50a37a, 0x62db5b5d, 0xf32f4ab6, 0x2c4a8a51, 0x18473dc0,
+    0xfe0cbb6e, 0xfe399efd, 0xdf34ecc9, 0x6ccd5055, 0x46097073, 0x139135c2,
+    0x721c76f6, 0x1c6a94b4, 0x6eee014d, 0x8a508e02, 0x3da538f5, 0x280d394f,
+    0x5248a0c4, 0x3ce94c6c, 0x9a71ad3a, 0x8493dd05, 0xe43f0ab6, 0x18e4ed42,
+    0x6c5c0e09, 0x42b06ec9, 0x8d330343, 0xa45b6f59, 0x2a573c0c, 0xd7fd3de6,
+    0xeedeab68, 0x5c84dafc, 0xbbd1b1a8, 0xa3ce1ad1, 0x85b70bed, 0xb6add07f,
+    0xa531309c, 0x8f8ab852, 0x564de332, 0xeac9ed0c, 0x73da402c, 0x3ec52761,
+    0x43af2f4d, 0xd6ff45c8, 0x4c367462, 0xd553bd6a, 0x44724855, 0x3b2aa728,
+    0x56e5eb65, 0xeaf16173, 0x33fa42ff, 0xd714bb5d, 0xfbd0a3b9, 0xaf517134,
+    0x9416c8cd, 0x534cf94f, 0x548947c2, 0x34193569, 0x32f4389a, 0xfe7028bc,
+    0xed73b1ed, 0x9db95770, 0x468e3922, 0x0440c3cd, 0x60059a62, 0x33504562,
+    0x2b229fbd, 0x5174dca5, 0xf7028752, 0xd63c6aa8, 0x31276f38, 0x0646721c,
+    0xb0191da8, 0xe00e6de0, 0x9eac1a6e, 0x9f7628a5, 0xed6c06ea, 0x0bb8af15,
+    0xf119fb12, 0x38693c1c, 0x732bc0fe, 0x84953275, 0xb82ec888, 0x33a4f1b3,
+    0x3099835e, 0x028a8782, 0x5fdd51d7, 0xc6c717b3, 0xb06caf71, 0x17c8c111,
+    0x61bad754, 0x9fd03061, 0xe09df1af, 0x3bc9eb73, 0x85878413, 0x9889aaf2,
+    0x3f5a9e46, 0x42c9f01f, 0x9984a4f4, 0xd5de43cc, 0xd294daed, 0xbecba2d2,
+    0xf1f6e72c, 0x5551128a, 0x83af87e2, 0x6f0342ba,
 };
 
 static uint64_t
-hash(unsigned char *p, int n)
+hashblk(const unsigned char *p, off_t n)
 {
 	unsigned char buf[SHA1_DIGEST_LENGTH];
 	uint64_t h;
-	SHA1Data(p, n, buf);
+	SHA1_CTX ctx;
+	SHA1Init(&ctx);
+	SHA1Update(&ctx, p, n);
+	SHA1Final(buf, &ctx);
 	memcpy(&h, buf, sizeof(h));
 	return be64toh(h);
 }
 
 static const struct got_error *
-addblk(struct Dtab *dt, void *buf, int len, int off, uint64_t h)
+addblk(struct got_delta_table *dt, FILE *f, off_t len, off_t offset, uint64_t h)
 {
-	int i, sz, probe;
-	struct Dblock *db;
-
-	probe = h % dt->sz;
-	while (dt->b[probe].buf != NULL) {
-		if (len == dt->b[probe].len &&
-		    memcmp(buf, dt->b[probe].buf, len) == 0)
-			return NULL;
-		probe = (probe + 1) % dt->sz;
+	const struct got_error *err = NULL;
+	int i, nalloc;
+	uint8_t buf[GOT_DELTIFY_MAXCHUNK];
+	size_t r = 0;
+
+	if (len == 0)
+		return NULL;
+
+	i = h % dt->nalloc;
+	while (dt->blocks[i].len != 0) {
+		if (r == 0) {
+			if (fseeko(f, offset, SEEK_SET) == -1)
+				return got_error_from_errno("fseeko");
+			r = fread(buf, 1, len, f);
+			if (r != len) {
+				if (ferror(f))
+					return got_ferror(f, GOT_ERR_IO);
+				return NULL;
+			}
+		}
+		/*
+		 * Avoid adding duplicate blocks.
+		 * NB: A matching hash is insufficient for detecting equality.
+		 * The hash can only detect inequality, so only check 'len'.
+		 */
+		if (len == dt->blocks[i].len) {
+			uint8_t buf2[GOT_DELTIFY_MAXCHUNK];
+			if (fseeko(f, dt->blocks[i].offset, SEEK_SET) == -1)
+				return got_error_from_errno("fseeko");
+			if (fread(buf2, 1, len, f) != len)
+				return got_ferror(f, GOT_ERR_IO);
+			if (memcmp(buf, buf2, len) == 0)
+				return NULL;
+		}
+
+		i = (i + 1) % dt->nalloc;
 	}
-	assert(dt->b[probe].buf == NULL);
-	dt->b[probe].buf = buf;
-	dt->b[probe].len = len;
-	dt->b[probe].off = off;
-	dt->b[probe].hash = h;
-	dt->nb++;
-	if (dt->sz < 2 * dt->nb) {
-		sz = dt->sz;
-		db = dt->b;
-		dt->sz *= 2;
-		dt->nb = 0;
-		dt->b = calloc(dt->sz, sizeof(struct Dblock));
-		if (dt->b == NULL)
-			return got_error_from_errno("calloc");
-		for (i = 0; i < sz; i++) {
-			if (db[i].buf != NULL)
-				return addblk(dt, db[i].buf, db[i].len,
-				    db[i].off, db[i].hash);
+	assert(dt->blocks[i].len == 0);
+	dt->blocks[i].len = len;
+	dt->blocks[i].offset = offset;
+	dt->blocks[i].hash = h;
+	dt->nblocks++;
+	if (dt->nalloc < 2 * dt->nblocks) {
+		struct got_delta_block *db;
+		nalloc = dt->nalloc * 2;
+		db = dt->blocks;
+		dt->blocks = calloc(nalloc, sizeof(struct got_delta_block));
+		if (dt->blocks == NULL) {
+			err = got_error_from_errno("calloc");
+			dt->blocks = db;
+			return err;
+		}
+		dt->nalloc = nalloc;
+		/*
+		 * Recompute all block positions. Hash-based indices of blocks
+		 * in the array depend on the allocated length of the array.
+		 */
+		dt->nblocks = 0;
+		for (i = 0; i < nalloc; i++) {
+			if (db[i].len == 0)
+				continue;
+			err = addblk(dt, f, db[i].len, db[i].offset,
+			    db[i].hash);
+			if (err)
+				break;
 		}
 		free(db);
 	}
 
-	return NULL;
+	return err;
 }
 
-static struct Dblock *
-lookup(struct Dtab *dt, unsigned char *p, int n)
+static const struct got_error *
+lookupblk(struct got_delta_block **block, struct got_delta_table *dt,
+    unsigned char *p, off_t len, FILE *basefile)
 {
-	int probe;
+	int i;
 	uint64_t h;
+	uint8_t buf[GOT_DELTIFY_MAXCHUNK];
+	size_t r;
 
-	h = hash(p, n);
-	for (probe = h % dt->sz; dt->b[probe].buf != NULL;
-	     probe = (probe + 1) % dt->sz) {
-		if (dt->b[probe].hash != h)
-			continue;
-		if (n != dt->b[probe].len)
-			continue;
-		if (memcmp(p, dt->b[probe].buf, n) != 0)
+	*block = NULL;
+
+	h = hashblk(p, len);
+	for (i = h % dt->nalloc; dt->blocks[i].len != 0;
+	     i = (i + 1) % dt->nalloc) {
+		if (dt->blocks[i].hash != h ||
+		    dt->blocks[i].len != len)
 			continue;
-		return &dt->b[probe];
+		if (fseeko(basefile, dt->blocks[i].offset, SEEK_SET) == -1)
+			return got_error_from_errno("fseeko");
+		r = fread(buf, 1, len, basefile);
+		if (r != len)
+			return got_ferror(basefile, GOT_ERR_IO);
+		if (memcmp(p, buf, len) == 0) {
+			*block = &dt->blocks[i];
+			break;
+		}
 	}
 	return NULL;
 }
 
-static int
-nextblk(unsigned char *s, unsigned char *e)
+static const struct got_error *
+nextblk(uint8_t *buf, off_t *blocklen, FILE *f)
 {
 	uint32_t gh;
-	unsigned char *p;
+	const unsigned char *p;
+	size_t r;
+	off_t pos = ftello(f);
 
-	if ((e - s) < Minchunk)
-		return e - s;
-	p = s + Minchunk;
-	if ((e - s) > Maxchunk)
-		e = s + Maxchunk;
+	*blocklen = 0;
+
+	r = fread(buf, 1, GOT_DELTIFY_MAXCHUNK, f);
+	if (r == 0 && ferror(f))
+		return got_ferror(f, GOT_ERR_IO);
+	if (r < GOT_DELTIFY_MINCHUNK)
+		return NULL; /* no more delta-worthy blocks left */
+
+	/* Got a deltifiable block. Find the split-point where it ends. */
+	p = buf + GOT_DELTIFY_MINCHUNK;
 	gh = 0;
-	while (p != e) {
+	while (p != buf + r) {
 		gh = (gh << 1) + geartab[*p++];
-		if ((gh & Splitmask) == 0)
+		if ((gh & GOT_DELTIFY_SPLITMASK) == 0)
 			break;
 	}
-	return p - s;
+
+	*blocklen = (p - buf);
+	if (fseeko(f, pos + *blocklen, SEEK_SET) == -1)
+		return got_error_from_errno("fseeko");
+
+	return NULL;
 }
 
 const struct got_error *
-dtinit(struct Dtab *dt, struct got_object *obj)
+got_deltify_init(struct got_delta_table **dt, FILE *f, off_t fileoffset,
+    off_t filesize)
 {
-	unsigned char *s = NULL, *e;
+	const struct got_error *err = NULL;
 	uint64_t h;
-	long n, o;
-	
-	o = 0;
-#ifdef notyet
-	s = (unsigned char *)obj->data;
-#endif
-	e = s + obj->size;
-	dt->nb = 0;
-	dt->sz = 128;
-	dt->b = calloc(dt->sz, sizeof(struct Dblock));
-	if (dt->b == NULL)
+
+	*dt = calloc(1, sizeof(**dt));
+	if (*dt == NULL)
 		return got_error_from_errno("calloc");
-#ifdef notyet
-	dt->base = (unsigned char *)obj->data;
-#endif
-	dt->nbase = obj->size;
-	while (s != e) {
-		n = nextblk(s, e);
-		h = hash(s, n);
-		addblk(dt, s, n, o, h);
-		s += n;
-		o += n;
+
+	(*dt)->nblocks = 0;
+	(*dt)->nalloc = 128;
+	(*dt)->blocks = calloc((*dt)->nalloc, sizeof(struct got_delta_block));
+	if ((*dt)->blocks == NULL) {
+		err = got_error_from_errno("calloc");
+		goto done;
 	}
 
-	return NULL;
+	if (fseeko(f, fileoffset, SEEK_SET) == -1)
+		return got_error_from_errno("fseeko");
+
+	while (fileoffset < filesize) {
+		uint8_t buf[GOT_DELTIFY_MAXCHUNK];
+		off_t blocklen;
+		err = nextblk(buf, &blocklen, f);
+		if (err)
+			goto done;
+		if (blocklen == 0)
+			break;
+		h = hashblk(buf, blocklen);
+		err = addblk(*dt, f, blocklen, fileoffset, h);
+		if (err)
+			goto done;
+		fileoffset += blocklen;
+	}
+done:
+	if (err) {
+		free((*dt)->blocks);
+		free(*dt);
+		*dt = NULL;
+	}
+
+	return err;
 }
 
 void
-dtclear(struct Dtab *dt)
+got_deltify_free(struct got_delta_table *dt)
 {
-	free(dt->b);
+	free(dt->blocks);
+	free(dt);
 }
 
 static const struct got_error *
-emitdelta(struct Delta **pd, int *nd, int cpy, int off, int len)
+emitdelta(struct got_delta_instruction **deltas, int *ndeltas, int copy,
+    off_t offset, off_t len)
 {
-	struct Delta *d, *p;
+	struct got_delta_instruction *d, *p;
 
-	*nd += 1;
-	p = reallocarray(*pd, *nd, sizeof(struct Delta));
+	*ndeltas += 1;
+	p = reallocarray(*deltas, *ndeltas,
+	    sizeof(struct got_delta_instruction));
 	if (p == NULL)
 		return got_error_from_errno("realloc");
-	*pd = p;
-	d = &(*pd)[*nd - 1];
-	d->cpy = cpy;
-	d->off = off;
+	*deltas = p;
+	d = &(*deltas)[*ndeltas - 1];
+	d->copy = copy;
+	d->offset = offset;
 	d->len = len;
 	return NULL;
 }
 
-static int
-stretch(struct Dtab *dt, struct Dblock *b, unsigned char *s,
-    unsigned char *e, int n)
+static const struct got_error *
+stretchblk(FILE *basefile, struct got_delta_block *block, FILE *f,
+    off_t filesize, off_t *blocklen)
 {
-	unsigned char *p, *q, *eb;
-
-	if (b == NULL)
-		return n;
-	p = s + n;
-	q = dt->base + b->off + n;
-	eb = dt->base + dt->nbase;
-	while (n < (1 << 24) - 1) {
-		if (p == e || q == eb)
+	uint8_t basebuf[GOT_DELTIFY_MAXCHUNK], buf[GOT_DELTIFY_MAXCHUNK];
+	size_t base_r, r, i;
+	off_t orig_blocklen = *blocklen;
+	off_t pos = ftello(f);
+	int buf_equal = 1;
+
+	if (fseeko(basefile, block->offset, SEEK_SET) == -1)
+		return got_error_from_errno("fseeko");
+
+	while (buf_equal && *blocklen < (1 << 24) - 1) {
+		base_r = fread(basebuf, 1, sizeof(basebuf), basefile);
+		if (base_r == 0) {
+			if (ferror(basefile))
+				return got_ferror(basefile, GOT_ERR_IO);
 			break;
-		if (*p != *q)
+		}
+		r = fread(buf, 1, sizeof(buf), f);
+		if (r == 0) {
+			if (ferror(f))
+				return got_ferror(f, GOT_ERR_IO);
 			break;
-		p++;
-		q++;
-		n++;
+		}
+		for (i = 0; i < MIN(base_r, r); i++) {
+			if (buf[i] != basebuf[i]) {
+				buf_equal = 0;
+				break;
+			}
+			(*blocklen)++;
+		}
 	}
-	return n;
+
+	if (fseeko(f, pos + *blocklen - orig_blocklen, SEEK_SET) == -1)
+		return got_error_from_errno("fseeko");
+
+	return NULL;
 }
 
-struct Delta *
-deltify(struct got_object *obj, struct Dtab *dt, int *pnd)
+const struct got_error *
+got_deltify(struct got_delta_instruction **deltas, int *ndeltas,
+    FILE *f, off_t fileoffset, off_t filesize,
+    struct got_delta_table *dt, FILE *basefile,
+    off_t basefile_size)
 {
-	struct Delta *d;
-	struct Dblock *b;
-	unsigned char *s = NULL, *e;
-	long n, o;
-	
-	o = 0;
-	d = NULL;
-#ifdef notyet
-	s = (unsigned char *)obj->data;
-#endif
-	e = s + obj->size;
-	*pnd = 0;
-	while (s != e) {
-		n = nextblk(s, e);
-		b = lookup(dt, s, n);
-		n = stretch(dt, b, s, e, n);
-		if (b != NULL)
-			emitdelta(&d, pnd, 1, b->off, n);
-		else
-			emitdelta(&d, pnd, 0, o, n);
-		s += n;
-		o += n;
+	const struct got_error *err = NULL;
+	const off_t offset0 = fileoffset;
+
+	*deltas = NULL;
+	*ndeltas = 0;
+
+	/*
+	 * offset0 indicates where data to be deltified begins.
+	 * For example, we want to avoid deltifying a Git object header at
+	 * the beginning of the file.
+	 */
+	if (fseeko(f, offset0, SEEK_SET) == -1)
+		return got_error_from_errno("fseeko");
+
+	while (fileoffset < filesize) {
+		uint8_t buf[GOT_DELTIFY_MAXCHUNK];
+		off_t blocklen;
+		struct got_delta_block *block;
+		err = nextblk(buf, &blocklen, f);
+		if (err)
+			break;
+		if (blocklen == 0)
+			break;
+		err = lookupblk(&block, dt, buf, blocklen, basefile);
+		if (err)
+			break;
+		if (block != NULL) {
+			/*
+			 * We have found a matching block in the delta base.
+			 * Attempt to stretch the block as far as possible and
+			 * generate a copy instruction.
+			 */
+			err = stretchblk(basefile, block, f, filesize,
+			    &blocklen);
+			if (err)
+				break;
+			emitdelta(deltas, ndeltas, 1, block->offset, blocklen);
+		} else {
+			/*
+			 * No match.
+			 * This block needs to be sourced from the file itself.
+			 */
+			emitdelta(deltas, ndeltas, 0, fileoffset - offset0,
+			    blocklen);
+		}
+		fileoffset += blocklen;
+	}
+
+	if (err) {
+		free(*deltas);
+		*deltas = NULL;
+		*ndeltas = 0;
 	}
-	return d;
+	return err;
 }
diff --git a/lib/got_lib_deltify.h b/lib/got_lib_deltify.h
new file mode 100644
index 0000000..39069a6
--- /dev/null
+++ b/lib/got_lib_deltify.h
@@ -0,0 +1,48 @@
+/*
+ * Copyright (c) 2020 Ori Bernstein
+ * Copyright (c) 2021 Stefan Sperling <stsp@openbsd.org>
+ *
+ * Permission to use, copy, modify, and distribute this software for any
+ * purpose with or without fee is hereby granted, provided that the above
+ * copyright notice and this permission notice appear in all copies.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
+ * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
+ * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
+ * ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
+ * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
+ * ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
+ * OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
+ */
+
+struct got_delta_block {
+	off_t		len;
+	off_t		offset;
+	uint64_t	hash;
+};
+
+struct got_delta_table {
+	struct got_delta_block	*blocks;
+	int			nblocks;
+	int			nalloc;
+};
+
+struct got_delta_instruction {
+	int	copy;
+	off_t	offset;
+	off_t	len;
+};
+
+enum {
+	GOT_DELTIFY_MINCHUNK	= 128,
+	GOT_DELTIFY_MAXCHUNK	= 8192,
+	GOT_DELTIFY_SPLITMASK	= (1 << 8) - 1,
+	
+};
+
+const struct got_error *got_deltify_init(struct got_delta_table **dt, FILE *f,
+    off_t fileoffset, off_t filesize);
+const struct got_error *got_deltify(struct got_delta_instruction **deltas,
+    int *ndeltas, FILE *f, off_t fileoffset, off_t filesize,
+    struct got_delta_table *dt, FILE *basefile, off_t basefile_size);
+void got_deltify_free(struct got_delta_table *dt);