[PATCH] improve diff-delta with sparse and/or repetitive data

From: Nicolas Pitre <nico@cam.org>
Date: May 3, 2006, 03:31 UTC
Message-ID: <Pine.LNX.4.64.0605022226070.28543@localhost.localdomain>

It is useless to preserve multiple hash entries for consecutive blocks with the same hash. Keeping only the first one will allow for matching the longest string of identical bytes while subsequent blocks will only allow for shorter matches. The backward matching code will match the end of it as necessary.

This improves both performances (no repeated string compare with long successions of identical bytes, or even small group of bytes), as well as compression (less likely to need random hash bucket entry culling), especially with sparse files.

With well behaved data sets this patch doesn't change much.

Signed-off-by: Nicolas Pitre <nico@cam.org>

---

diff --git a/diff-delta.c b/diff-delta.c
index 35e517d..c618875 100644
--- a/diff-delta.c
+++ b/diff-delta.c
@@ -136,11 +136,12 @@ struct delta_index {
 
 struct delta_index * create_delta_index(const void *buf, unsigned long bufsize)
 {
-	unsigned int i, hsize, hmask, entries, *hash_count;
+	unsigned int i, hsize, hmask, entries, prev_val, *hash_count;
 	const unsigned char *data, *buffer = buf;
 	struct delta_index *index;
 	struct index_entry *entry, **hash;
 	void *mem;
+	unsigned long memsize;
 
 	if (!buf || !bufsize)
 		return NULL;
@@ -155,9 +156,10 @@ struct delta_index * create_delta_index(
 	hmask = hsize - 1;
 
 	/* allocate lookup index */
-	mem = malloc(sizeof(*index) +
-		     sizeof(*hash) * hsize +
-		     sizeof(*entry) * entries);
+	memsize = sizeof(*index) +
+		  sizeof(*hash) * hsize +
+		  sizeof(*entry) * entries;
+	mem = malloc(memsize);
 	if (!mem)
 		return NULL;
 	index = mem;
@@ -179,18 +181,26 @@ struct delta_index * create_delta_index(
 	}
 
 	/* then populate the index */
-	data = buffer + entries * RABIN_WINDOW - RABIN_WINDOW;
-	while (data >= buffer) {
+	prev_val = ~0;
+	for (data = buffer + entries * RABIN_WINDOW - RABIN_WINDOW;
+	     data >= buffer;
+	     data -= RABIN_WINDOW) {
 		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 -= RABIN_WINDOW;
+		if (val == prev_val) {
+			/* keep the lowest of consecutive identical blocks */
+			entry[-1].ptr = data + RABIN_WINDOW;
+		} else {
+			prev_val = val;
+			i = val & hmask;
+			entry->ptr = data + RABIN_WINDOW;
+			entry->val = val;
+			entry->next = hash[i];
+			hash[i] = entry++;
+			hash_count[i]++;
+			entries--;
+		}
 	}
 
 	/*
@@ -220,6 +230,10 @@ struct delta_index * create_delta_index(
 	}
 	free(hash_count);
 
+	/* If we didn't use all hash entries, free the unused memory. */
+	if (entries)
+		index = realloc(index, memsize - entries * sizeof(*entry));
+
 	return index;
 }