diff-delta: bound hash list length to avoid O(m*n) behavior

The diff-delta code can exhibit O(m*n) behavior with some patological
data set where most hash entries end up in the same hash bucket.

The latest code rework reduced the block size making it particularly
vulnerable to this issue, but the issue was always there and can be
triggered regardless of the block size.

This patch does two things:

1) the hashing has been reworked to offer a better distribution to
   atenuate the problem a bit, and

2) a limit is imposed to the number of entries that can exist in the
   same hash bucket.

Because of the above the code is a bit more expensive on average, but
the problematic samples used to diagnoze the issue are now orders of
magnitude less expensive to process with only a slight loss in
compression.

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

diff-delta.c

@@ -30,19 +30,20 @@ struct index {
 
 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, hshift, hlimit, *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++);
+	for (i = 8; (1 << i) < hsize && i < 24; i += 2);
 	hsize = 1 << i;
-	hshift = i - 8;
+	hshift = (i - 8) / 2;
 	*hash_shift = hshift;
 
 	/* allocate lookup index */
@@ -53,15 +54,59 @@ static struct index ** delta_index(const unsigned char *buf,
 	entry = mem + hsize * sizeof(*hash);
 	memset(hash, 0, hsize * sizeof(*hash));
 
-	/* then populate it */
+	/* 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 + bufsize - 2;
 	while (data > buf) {
 		entry->ptr = --data;
-		i = data[0] ^ data[1] ^ (data[2] << hshift);
+		i = data[0] ^ ((data[1] ^ (data[2] << hshift)) << hshift);
 		entry->next = hash[i];
 		hash[i] = entry++;
+		hash_count[i]++;
  	}
 
+	/*
+	 * 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 < 16)
+		hlimit = 16;
+
+	/*
+	 * Now make sure none of the hash buckets has more entries than
+	 * we're willing to test.  Otherwise we short-circuit 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;
 }
 
@@ -85,7 +130,7 @@ void *diff_delta(void *from_buf, unsigned long from_size,
 
 	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;
 
@@ -126,8 +171,8 @@ 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 + 3 <= top) {
+			i = data[0] ^ ((data[1] ^ (data[2] << hash_shift)) << hash_shift);
 			for (entry = hash[i]; entry; entry = entry->next) {
 				const unsigned char *ref = entry->ptr;
 				const unsigned char *src = data;
@@ -138,11 +183,9 @@ void *diff_delta(void *from_buf, unsigned long from_size,
 					ref_size = 0x10000;
 				if (ref_size <= msize)
 					break;
-				while (ref_size && *src++ == *ref) {
-					ref++;
-					ref_size--;
-				}
-				ref_size = ref - entry->ptr;
+				if (*ref != *src)
+					continue;
+				while (ref_size-- && *++src == *++ref);
 				if (msize < ref - entry->ptr) {
 					/* this is our best match so far */
 					msize = ref - entry->ptr;