replace adler32 with Rabin's polynomial in diff-delta

This brings another small repacking speedup for sensibly the same pack
size.  On the Linux kernel repo, git-repack -a -f is 3.7% faster for a
0.4% larger pack.

Credits to Geert Bosch who brought the Rabin's polynomial idea to my
attention.

This also eliminate the issue of adler32() reading past the data buffer,
as noticed by Johannes Schindelin.

Signed-off-by: Nicolas Pitre <nico@cam.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>

Makefile

@@ -609,7 +609,7 @@ test-date$X: test-date.c date.o ctype.o
 	$(CC) $(ALL_CFLAGS) -o $@ $(ALL_LDFLAGS) test-date.c date.o ctype.o
 
 test-delta$X: test-delta.c diff-delta.o patch-delta.o
-	$(CC) $(ALL_CFLAGS) -o $@ $(ALL_LDFLAGS) $^ -lz
+	$(CC) $(ALL_CFLAGS) -o $@ $(ALL_LDFLAGS) $^
 
 check:
 	for i in *.c; do sparse $(ALL_CFLAGS) $(SPARSE_FLAGS) $$i || exit; done

diff-delta.c

@@ -20,18 +20,106 @@
 
 #include <stdlib.h>
 #include <string.h>
-#include <zlib.h>
 #include "delta.h"
 
 
-/* block size: min = 16, max = 64k, power of 2 */
-#define BLK_SIZE 16
-
 /* maximum hash entry list for the same hash bucket */
 #define HASH_LIMIT 64
 
-#define GR_PRIME 0x9e370001
-#define HASH(v, shift) (((unsigned int)(v) * GR_PRIME) >> (shift))
+#define RABIN_SHIFT 23
+#define RABIN_WINDOW 16
+
+static const unsigned int T[256] = {
+	0x00000000, 0xab59b4d1, 0x56b369a2, 0xfdeadd73, 0x063f6795, 0xad66d344,
+	0x508c0e37, 0xfbd5bae6, 0x0c7ecf2a, 0xa7277bfb, 0x5acda688, 0xf1941259,
+	0x0a41a8bf, 0xa1181c6e, 0x5cf2c11d, 0xf7ab75cc, 0x18fd9e54, 0xb3a42a85,
+	0x4e4ef7f6, 0xe5174327, 0x1ec2f9c1, 0xb59b4d10, 0x48719063, 0xe32824b2,
+	0x1483517e, 0xbfdae5af, 0x423038dc, 0xe9698c0d, 0x12bc36eb, 0xb9e5823a,
+	0x440f5f49, 0xef56eb98, 0x31fb3ca8, 0x9aa28879, 0x6748550a, 0xcc11e1db,
+	0x37c45b3d, 0x9c9defec, 0x6177329f, 0xca2e864e, 0x3d85f382, 0x96dc4753,
+	0x6b369a20, 0xc06f2ef1, 0x3bba9417, 0x90e320c6, 0x6d09fdb5, 0xc6504964,
+	0x2906a2fc, 0x825f162d, 0x7fb5cb5e, 0xd4ec7f8f, 0x2f39c569, 0x846071b8,
+	0x798aaccb, 0xd2d3181a, 0x25786dd6, 0x8e21d907, 0x73cb0474, 0xd892b0a5,
+	0x23470a43, 0x881ebe92, 0x75f463e1, 0xdeadd730, 0x63f67950, 0xc8afcd81,
+	0x354510f2, 0x9e1ca423, 0x65c91ec5, 0xce90aa14, 0x337a7767, 0x9823c3b6,
+	0x6f88b67a, 0xc4d102ab, 0x393bdfd8, 0x92626b09, 0x69b7d1ef, 0xc2ee653e,
+	0x3f04b84d, 0x945d0c9c, 0x7b0be704, 0xd05253d5, 0x2db88ea6, 0x86e13a77,
+	0x7d348091, 0xd66d3440, 0x2b87e933, 0x80de5de2, 0x7775282e, 0xdc2c9cff,
+	0x21c6418c, 0x8a9ff55d, 0x714a4fbb, 0xda13fb6a, 0x27f92619, 0x8ca092c8,
+	0x520d45f8, 0xf954f129, 0x04be2c5a, 0xafe7988b, 0x5432226d, 0xff6b96bc,
+	0x02814bcf, 0xa9d8ff1e, 0x5e738ad2, 0xf52a3e03, 0x08c0e370, 0xa39957a1,
+	0x584ced47, 0xf3155996, 0x0eff84e5, 0xa5a63034, 0x4af0dbac, 0xe1a96f7d,
+	0x1c43b20e, 0xb71a06df, 0x4ccfbc39, 0xe79608e8, 0x1a7cd59b, 0xb125614a,
+	0x468e1486, 0xedd7a057, 0x103d7d24, 0xbb64c9f5, 0x40b17313, 0xebe8c7c2,
+	0x16021ab1, 0xbd5bae60, 0x6cb54671, 0xc7ecf2a0, 0x3a062fd3, 0x915f9b02,
+	0x6a8a21e4, 0xc1d39535, 0x3c394846, 0x9760fc97, 0x60cb895b, 0xcb923d8a,
+	0x3678e0f9, 0x9d215428, 0x66f4eece, 0xcdad5a1f, 0x3047876c, 0x9b1e33bd,
+	0x7448d825, 0xdf116cf4, 0x22fbb187, 0x89a20556, 0x7277bfb0, 0xd92e0b61,
+	0x24c4d612, 0x8f9d62c3, 0x7836170f, 0xd36fa3de, 0x2e857ead, 0x85dcca7c,
+	0x7e09709a, 0xd550c44b, 0x28ba1938, 0x83e3ade9, 0x5d4e7ad9, 0xf617ce08,
+	0x0bfd137b, 0xa0a4a7aa, 0x5b711d4c, 0xf028a99d, 0x0dc274ee, 0xa69bc03f,
+	0x5130b5f3, 0xfa690122, 0x0783dc51, 0xacda6880, 0x570fd266, 0xfc5666b7,
+	0x01bcbbc4, 0xaae50f15, 0x45b3e48d, 0xeeea505c, 0x13008d2f, 0xb85939fe,
+	0x438c8318, 0xe8d537c9, 0x153feaba, 0xbe665e6b, 0x49cd2ba7, 0xe2949f76,
+	0x1f7e4205, 0xb427f6d4, 0x4ff24c32, 0xe4abf8e3, 0x19412590, 0xb2189141,
+	0x0f433f21, 0xa41a8bf0, 0x59f05683, 0xf2a9e252, 0x097c58b4, 0xa225ec65,
+	0x5fcf3116, 0xf49685c7, 0x033df00b, 0xa86444da, 0x558e99a9, 0xfed72d78,
+	0x0502979e, 0xae5b234f, 0x53b1fe3c, 0xf8e84aed, 0x17bea175, 0xbce715a4,
+	0x410dc8d7, 0xea547c06, 0x1181c6e0, 0xbad87231, 0x4732af42, 0xec6b1b93,
+	0x1bc06e5f, 0xb099da8e, 0x4d7307fd, 0xe62ab32c, 0x1dff09ca, 0xb6a6bd1b,
+	0x4b4c6068, 0xe015d4b9, 0x3eb80389, 0x95e1b758, 0x680b6a2b, 0xc352defa,
+	0x3887641c, 0x93ded0cd, 0x6e340dbe, 0xc56db96f, 0x32c6cca3, 0x999f7872,
+	0x6475a501, 0xcf2c11d0, 0x34f9ab36, 0x9fa01fe7, 0x624ac294, 0xc9137645,
+	0x26459ddd, 0x8d1c290c, 0x70f6f47f, 0xdbaf40ae, 0x207afa48, 0x8b234e99,
+	0x76c993ea, 0xdd90273b, 0x2a3b52f7, 0x8162e626, 0x7c883b55, 0xd7d18f84,
+	0x2c043562, 0x875d81b3, 0x7ab75cc0, 0xd1eee811
+};
+
+static const unsigned int U[256] = {
+	0x00000000, 0x7eb5200d, 0x5633f4cb, 0x2886d4c6, 0x073e5d47, 0x798b7d4a,
+	0x510da98c, 0x2fb88981, 0x0e7cba8e, 0x70c99a83, 0x584f4e45, 0x26fa6e48,
+	0x0942e7c9, 0x77f7c7c4, 0x5f711302, 0x21c4330f, 0x1cf9751c, 0x624c5511,
+	0x4aca81d7, 0x347fa1da, 0x1bc7285b, 0x65720856, 0x4df4dc90, 0x3341fc9d,
+	0x1285cf92, 0x6c30ef9f, 0x44b63b59, 0x3a031b54, 0x15bb92d5, 0x6b0eb2d8,
+	0x4388661e, 0x3d3d4613, 0x39f2ea38, 0x4747ca35, 0x6fc11ef3, 0x11743efe,
+	0x3eccb77f, 0x40799772, 0x68ff43b4, 0x164a63b9, 0x378e50b6, 0x493b70bb,
+	0x61bda47d, 0x1f088470, 0x30b00df1, 0x4e052dfc, 0x6683f93a, 0x1836d937,
+	0x250b9f24, 0x5bbebf29, 0x73386bef, 0x0d8d4be2, 0x2235c263, 0x5c80e26e,
+	0x740636a8, 0x0ab316a5, 0x2b7725aa, 0x55c205a7, 0x7d44d161, 0x03f1f16c,
+	0x2c4978ed, 0x52fc58e0, 0x7a7a8c26, 0x04cfac2b, 0x73e5d470, 0x0d50f47d,
+	0x25d620bb, 0x5b6300b6, 0x74db8937, 0x0a6ea93a, 0x22e87dfc, 0x5c5d5df1,
+	0x7d996efe, 0x032c4ef3, 0x2baa9a35, 0x551fba38, 0x7aa733b9, 0x041213b4,
+	0x2c94c772, 0x5221e77f, 0x6f1ca16c, 0x11a98161, 0x392f55a7, 0x479a75aa,
+	0x6822fc2b, 0x1697dc26, 0x3e1108e0, 0x40a428ed, 0x61601be2, 0x1fd53bef,
+	0x3753ef29, 0x49e6cf24, 0x665e46a5, 0x18eb66a8, 0x306db26e, 0x4ed89263,
+	0x4a173e48, 0x34a21e45, 0x1c24ca83, 0x6291ea8e, 0x4d29630f, 0x339c4302,
+	0x1b1a97c4, 0x65afb7c9, 0x446b84c6, 0x3adea4cb, 0x1258700d, 0x6ced5000,
+	0x4355d981, 0x3de0f98c, 0x15662d4a, 0x6bd30d47, 0x56ee4b54, 0x285b6b59,
+	0x00ddbf9f, 0x7e689f92, 0x51d01613, 0x2f65361e, 0x07e3e2d8, 0x7956c2d5,
+	0x5892f1da, 0x2627d1d7, 0x0ea10511, 0x7014251c, 0x5facac9d, 0x21198c90,
+	0x099f5856, 0x772a785b, 0x4c921c31, 0x32273c3c, 0x1aa1e8fa, 0x6414c8f7,
+	0x4bac4176, 0x3519617b, 0x1d9fb5bd, 0x632a95b0, 0x42eea6bf, 0x3c5b86b2,
+	0x14dd5274, 0x6a687279, 0x45d0fbf8, 0x3b65dbf5, 0x13e30f33, 0x6d562f3e,
+	0x506b692d, 0x2ede4920, 0x06589de6, 0x78edbdeb, 0x5755346a, 0x29e01467,
+	0x0166c0a1, 0x7fd3e0ac, 0x5e17d3a3, 0x20a2f3ae, 0x08242768, 0x76910765,
+	0x59298ee4, 0x279caee9, 0x0f1a7a2f, 0x71af5a22, 0x7560f609, 0x0bd5d604,
+	0x235302c2, 0x5de622cf, 0x725eab4e, 0x0ceb8b43, 0x246d5f85, 0x5ad87f88,
+	0x7b1c4c87, 0x05a96c8a, 0x2d2fb84c, 0x539a9841, 0x7c2211c0, 0x029731cd,
+	0x2a11e50b, 0x54a4c506, 0x69998315, 0x172ca318, 0x3faa77de, 0x411f57d3,
+	0x6ea7de52, 0x1012fe5f, 0x38942a99, 0x46210a94, 0x67e5399b, 0x19501996,
+	0x31d6cd50, 0x4f63ed5d, 0x60db64dc, 0x1e6e44d1, 0x36e89017, 0x485db01a,
+	0x3f77c841, 0x41c2e84c, 0x69443c8a, 0x17f11c87, 0x38499506, 0x46fcb50b,
+	0x6e7a61cd, 0x10cf41c0, 0x310b72cf, 0x4fbe52c2, 0x67388604, 0x198da609,
+	0x36352f88, 0x48800f85, 0x6006db43, 0x1eb3fb4e, 0x238ebd5d, 0x5d3b9d50,
+	0x75bd4996, 0x0b08699b, 0x24b0e01a, 0x5a05c017, 0x728314d1, 0x0c3634dc,
+	0x2df207d3, 0x534727de, 0x7bc1f318, 0x0574d315, 0x2acc5a94, 0x54797a99,
+	0x7cffae5f, 0x024a8e52, 0x06852279, 0x78300274, 0x50b6d6b2, 0x2e03f6bf,
+	0x01bb7f3e, 0x7f0e5f33, 0x57888bf5, 0x293dabf8, 0x08f998f7, 0x764cb8fa,
+	0x5eca6c3c, 0x207f4c31, 0x0fc7c5b0, 0x7172e5bd, 0x59f4317b, 0x27411176,
+	0x1a7c5765, 0x64c97768, 0x4c4fa3ae, 0x32fa83a3, 0x1d420a22, 0x63f72a2f,
+	0x4b71fee9, 0x35c4dee4, 0x1400edeb, 0x6ab5cde6, 0x42331920, 0x3c86392d,
+	0x133eb0ac, 0x6d8b90a1, 0x450d4467, 0x3bb8646a
+};
 
 struct index_entry {
 	const unsigned char *ptr;
@@ -42,13 +130,13 @@ struct index_entry {
 struct delta_index {
 	const void *src_buf;
 	unsigned long src_size;
-	unsigned int hash_shift;
+	unsigned int hash_mask;
 	struct index_entry *hash[0];
 };
 
 struct delta_index * create_delta_index(const void *buf, unsigned long bufsize)
 {
-	unsigned int i, hsize, hshift, entries, *hash_count;
+	unsigned int i, hsize, hmask, entries, *hash_count;
 	const unsigned char *data, *buffer = buf;
 	struct delta_index *index;
 	struct index_entry *entry, **hash;
@@ -57,12 +145,14 @@ struct delta_index * create_delta_index(const void *buf, unsigned long bufsize)
 	if (!buf || !bufsize)
 		return NULL;
 
-	/* determine index hash size */
-	entries = bufsize  / BLK_SIZE;
+	/* Determine index hash size.  Note that indexing skips the
+	   first byte to allow for optimizing the rabin polynomial
+	   initialization in create_delta(). */
+	entries = (bufsize - 1)  / RABIN_WINDOW;
 	hsize = entries / 4;
 	for (i = 4; (1 << i) < hsize && i < 31; i++);
 	hsize = 1 << i;
-	hshift = 32 - i;
+	hmask = hsize - 1;
 
 	/* allocate lookup index */
 	mem = malloc(sizeof(*index) +
@@ -78,7 +168,7 @@ struct delta_index * create_delta_index(const void *buf, unsigned long bufsize)
 
 	index->src_buf = buf;
 	index->src_size = bufsize;
-	index->hash_shift = hshift;
+	index->hash_mask = hmask;
 	memset(hash, 0, hsize * sizeof(*hash));
 
 	/* allocate an array to count hash entries */
@@ -89,17 +179,19 @@ struct delta_index * create_delta_index(const void *buf, unsigned long bufsize)
 	}
 
 	/* then populate the index */
-	data = buffer + entries * BLK_SIZE - BLK_SIZE;
+	data = buffer + entries * RABIN_WINDOW - RABIN_WINDOW;
 	while (data >= buffer) {
-		unsigned int val = adler32(0, data, BLK_SIZE);
-		i = HASH(val, hshift);
-		entry->ptr = data;
+		unsigned int val = 0;
+		for (i = 1; i <= RABIN_WINDOW; i++)
+			val = ((val << 8) | data[i]) ^ T[val >> RABIN_SHIFT];
+		i = val & hmask;
+		entry->ptr = data + RABIN_WINDOW;
 		entry->val = val;
 		entry->next = hash[i];
 		hash[i] = entry++;
 		hash_count[i]++;
-		data -= BLK_SIZE;
- 	}
+		data -= RABIN_WINDOW;
+	}
 
 	/*
 	 * Determine a limit on the number of entries in the same hash
@@ -136,20 +228,18 @@ void free_delta_index(struct delta_index *index)
 	free(index);
 }
 
-/* provide the size of the copy opcode given the block offset and size */
-#define COPYOP_SIZE(o, s) \
-    (!!(o & 0xff) + !!(o & 0xff00) + !!(o & 0xff0000) + !!(o & 0xff000000) + \
-     !!(s & 0xff) + !!(s & 0xff00) + 1)
-
-/* the maximum size for any opcode */
-#define MAX_OP_SIZE COPYOP_SIZE(0xffffffff, 0xffffffff)
+/*
+ * The maximum size for any opcode sequence, including the initial header
+ * plus rabin window plus biggest copy.
+ */
+#define MAX_OP_SIZE	(5 + 5 + 1 + RABIN_WINDOW + 7)
 
 void *
 create_delta(const struct delta_index *index,
 	     const void *trg_buf, unsigned long trg_size,
 	     unsigned long *delta_size, unsigned long max_size)
 {
-	unsigned int i, outpos, outsize, hash_shift;
+	unsigned int i, outpos, outsize, hash_mask, val;
 	int inscnt;
 	const unsigned char *ref_data, *ref_top, *data, *top;
 	unsigned char *out;
@@ -185,14 +275,22 @@ create_delta(const struct delta_index *index,
 	ref_top = ref_data + index->src_size;
 	data = trg_buf;
 	top = trg_buf + trg_size;
-	hash_shift = index->hash_shift;
-	inscnt = 0;
+	hash_mask = index->hash_mask;
+
+	outpos++;
+	val = 0;
+	for (i = 0; i < RABIN_WINDOW && data < top; i++, data++) {
+		out[outpos++] = *data;
+		val = ((val << 8) | *data) ^ T[val >> RABIN_SHIFT];
+	}
+	inscnt = i;
 
 	while (data < top) {
 		unsigned int moff = 0, msize = 0;
 		struct index_entry *entry;
-		unsigned int val = adler32(0, data, BLK_SIZE);
-		i = HASH(val, hash_shift);
+		val ^= U[data[-RABIN_WINDOW]];
+		val = ((val << 8) | *data) ^ T[val >> RABIN_SHIFT];
+		i = val & hash_mask;
 		for (entry = index->hash[i]; entry; entry = entry->next) {
 			const unsigned char *ref = entry->ptr;
 			const unsigned char *src = data;
@@ -214,7 +312,7 @@ create_delta(const struct delta_index *index,
 			}
 		}
 
-		if (!msize || msize < COPYOP_SIZE(moff, msize)) {
+		if (msize < 4) {
 			if (!inscnt)
 				outpos++;
 			out[outpos++] = *data++;
@@ -226,6 +324,20 @@ create_delta(const struct delta_index *index,
 		} else {
 			unsigned char *op;
 
+			if (msize >= RABIN_WINDOW) {
+				const unsigned char *sk;
+				sk = data + msize - RABIN_WINDOW;
+				val = 0;
+				for (i = 0; i < RABIN_WINDOW; i++)
+					val = ((val << 8) | *sk++) ^ T[val >> RABIN_SHIFT];
+			} else {
+				const unsigned char *sk = data + 1;
+				for (i = 1; i < msize; i++) {
+					val ^= U[sk[-RABIN_WINDOW]];
+					val = ((val << 8) | *sk++) ^ T[val >> RABIN_SHIFT];
+				}
+			}
+
 			if (inscnt) {
 				while (moff && ref_data[moff-1] == data[-1]) {
 					if (msize == 0x10000)
@@ -270,9 +382,8 @@ create_delta(const struct delta_index *index,
 			if (max_size && outsize >= max_size)
 				outsize = max_size + MAX_OP_SIZE + 1;
 			if (max_size && outpos > max_size)
-				out = NULL;
-			else
-				out = realloc(out, outsize);
+				break;
+			out = realloc(out, outsize);
 			if (!out) {
 				free(tmp);
 				return NULL;
@@ -283,6 +394,11 @@ create_delta(const struct delta_index *index,
 	if (inscnt)
 		out[outpos - inscnt - 1] = inscnt;
 
+	if (max_size && outpos > max_size) {
+		free(out);
+		return NULL;
+	}
+
 	*delta_size = outpos;
 	return out;
 }