X-Git-Url: https://git.octo.it/?a=blobdiff_plain;f=diff-delta.c;h=1188b31cd0f1e2f3a1fc2096a10243a03b439021;hb=50ac7408018209a2829b7948119270fec7e14ec8;hp=27f83a085821eb6f187ff45e6e0a071d0807c337;hpb=207a1f3ce9894ad6fab59077b9cec1adc70ed6c5;p=git.git

diff --git a/diff-delta.c b/diff-delta.c
index 27f83a08..1188b31c 100644
--- a/diff-delta.c
+++ b/diff-delta.c
@@ -20,48 +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
+
+#define MIN(a, b) ((a) < (b) ? (a) : (b))
+
+#define GR_PRIME 0x9e370001
+#define HASH(v, shift) (((unsigned int)(v) * GR_PRIME) >> (shift))
+
 struct index {
 	const unsigned char *ptr;
+	unsigned int val;
 	struct index *next;
 };
 
 static struct index ** delta_index(const unsigned char *buf,
 				   unsigned long bufsize,
+				   unsigned long trg_bufsize,
 				   unsigned int *hash_shift)
 {
-	unsigned long hsize;
-	unsigned int hshift, i;
+	unsigned int i, hsize, hshift, hlimit, entries, *hash_count;
 	const unsigned char *data;
 	struct index *entry, **hash;
 	void *mem;
 
 	/* determine index hash size */
-	hsize = bufsize / 4;
-	for (i = 8; (1 << i) < hsize && i < 16; i++);
+	entries = bufsize  / BLK_SIZE;
+	hsize = entries / 4;
+	for (i = 4; (1 << i) < hsize && i < 31; i++);
 	hsize = 1 << i;
-	hshift = i - 8;
+	hshift = 32 - i;
 	*hash_shift = hshift;
 
 	/* allocate lookup index */
-	mem = malloc(hsize * sizeof(*hash) + bufsize * sizeof(*entry));
+	mem = malloc(hsize * sizeof(*hash) + entries * sizeof(*entry));
 	if (!mem)
 		return NULL;
 	hash = mem;
 	entry = mem + hsize * sizeof(*hash);
 	memset(hash, 0, hsize * sizeof(*hash));
 
-	/* then populate it */
-	data = buf + bufsize - 2;
-	while (data > buf) {
-		entry->ptr = --data;
-		i = data[0] ^ data[1] ^ (data[2] << hshift);
+	/* allocate an array to count hash entries */
+	hash_count = calloc(hsize, sizeof(*hash_count));
+	if (!hash_count) {
+		free(hash);
+		return NULL;
+	}
+
+	/* then populate the index */
+	data = buf + entries * BLK_SIZE - BLK_SIZE;
+	while (data >= buf) {
+		unsigned int val = adler32(0, data, BLK_SIZE);
+		i = HASH(val, hshift);
+		entry->ptr = data;
+		entry->val = val;
 		entry->next = hash[i];
 		hash[i] = entry++;
+		hash_count[i]++;
+		data -= BLK_SIZE;
  	}
 
+	/*
+	 * Determine a limit on the number of entries in the same hash
+	 * bucket.  This guard us against patological data sets causing
+	 * really bad hash distribution with most entries in the same hash
+	 * bucket that would bring us to O(m*n) computing costs (m and n
+	 * corresponding to reference and target buffer sizes).
+	 *
+	 * The more the target buffer is large, the more it is important to
+	 * have small entry lists for each hash buckets.  With such a limit
+	 * the cost is bounded to something more like O(m+n).
+	 */
+	hlimit = (1 << 26) / trg_bufsize;
+	if (hlimit < 4*BLK_SIZE)
+		hlimit = 4*BLK_SIZE;
+
+	/*
+	 * Now make sure none of the hash buckets has more entries than
+	 * we're willing to test.  Otherwise we cull the entry list
+	 * uniformly to still preserve a good repartition across
+	 * the reference buffer.
+	 */
+	for (i = 0; i < hsize; i++) {
+		if (hash_count[i] < hlimit)
+			continue;
+		entry = hash[i];
+		do {
+			struct index *keep = entry;
+			int skip = hash_count[i] / hlimit / 2;
+			do {
+				entry = entry->next;
+			} while(--skip && entry);
+			keep->next = entry;
+		} while(entry);
+	}
+	free(hash_count);
+
 	return hash;
 }
 
@@ -78,14 +136,15 @@ void *diff_delta(void *from_buf, unsigned long from_size,
 		 unsigned long *delta_size,
 		 unsigned long max_size)
 {
-	unsigned int i, outpos, outsize, inscnt, hash_shift;
+	unsigned int i, outpos, outsize, hash_shift;
+	int inscnt;
 	const unsigned char *ref_data, *ref_top, *data, *top;
 	unsigned char *out;
 	struct index *entry, **hash;
 
 	if (!from_size || !to_size)
 		return NULL;
-	hash = delta_index(from_buf, from_size, &hash_shift);
+	hash = delta_index(from_buf, from_size, to_size, &hash_shift);
 	if (!hash)
 		return NULL;
 
@@ -126,23 +185,23 @@ void *diff_delta(void *from_buf, unsigned long from_size,
 
 	while (data < top) {
 		unsigned int moff = 0, msize = 0;
-		if (data + 2 < top) {
-			i = data[0] ^ data[1] ^ (data[2] << hash_shift);
+		if (data + BLK_SIZE <= top) {
+			unsigned int val = adler32(0, data, BLK_SIZE);
+			i = HASH(val, hash_shift);
 			for (entry = hash[i]; entry; entry = entry->next) {
 				const unsigned char *ref = entry->ptr;
 				const unsigned char *src = data;
 				unsigned int ref_size = ref_top - ref;
+				if (entry->val != val)
+					continue;
 				if (ref_size > top - src)
 					ref_size = top - src;
 				if (ref_size > 0x10000)
 					ref_size = 0x10000;
 				if (ref_size <= msize)
 					break;
-				while (ref_size && *src++ == *ref) {
+				while (ref_size-- && *src++ == *ref)
 					ref++;
-					ref_size--;
-				}
-				ref_size = ref - entry->ptr;
 				if (msize < ref - entry->ptr) {
 					/* this is our best match so far */
 					msize = ref - entry->ptr;
@@ -164,6 +223,20 @@ void *diff_delta(void *from_buf, unsigned long from_size,
 			unsigned char *op;
 
 			if (inscnt) {
+				while (moff && ref_data[moff-1] == data[-1]) {
+					if (msize == 0x10000)
+						break;
+					/* we can match one byte back */
+					msize++;
+					moff--;
+					data--;
+					outpos--;
+					if (--inscnt)
+						continue;
+					outpos--;  /* remove count slot */
+					inscnt--;  /* make it -1 */
+					break;
+				}
 				out[outpos - inscnt - 1] = inscnt;
 				inscnt = 0;
 			}