diff-delta: allow reusing of the reference buffer index

When a reference buffer is used multiple times then its index can be
computed only once and reused multiple times.  This patch adds an extra
pointer to a pointer argument (from_index) to diff_delta() for this.

If from_index is NULL then everything is like before.

If from_index is non NULL and *from_index is NULL then the index is
created and its location stored to *from_index.  In this case the caller
has the responsibility to free the memory pointed to by *from_index.

If from_index and *from_index are non NULL then the index is reused as
is.

This currently saves about 10% of CPU time to repack the git archive.

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

delta.h

@@ -4,7 +4,8 @@
 /* handling of delta buffers */
 extern void *diff_delta(void *from_buf, unsigned long from_size,
 			void *to_buf, unsigned long to_size,
-		        unsigned long *delta_size, unsigned long max_size);
+		        unsigned long *delta_size, unsigned long max_size,
+			void **from_index);
 extern void *patch_delta(void *src_buf, unsigned long src_size,
 			 void *delta_buf, unsigned long delta_size,
 			 unsigned long *dst_size);

diff-delta.c

@@ -30,8 +30,7 @@ struct index {
 
 static struct index ** delta_index(const unsigned char *buf,
 				   unsigned long bufsize,
-				   unsigned long trg_bufsize,
-				   unsigned int *hash_shift)
+				   unsigned long trg_bufsize)
 {
 	unsigned long hsize;
 	unsigned int i, hshift, hlimit, *hash_count;
@@ -44,14 +43,17 @@ static struct index ** delta_index(const unsigned char *buf,
 	for (i = 8; (1 << i) < hsize && i < 24; i += 2);
 	hsize = 1 << i;
 	hshift = (i - 8) / 2;
-	*hash_shift = hshift;
 
-	/* allocate lookup index */
-	mem = malloc(hsize * sizeof(*hash) + bufsize * sizeof(*entry));
+	/*
+	 * Allocate lookup index.  Note the first hash pointer
+	 * is used to store the hash shift value.
+	 */
+	mem = malloc((1 + hsize) * sizeof(*hash) + bufsize * sizeof(*entry));
 	if (!mem)
 		return NULL;
 	hash = mem;
-	entry = mem + hsize * sizeof(*hash);
+	*hash++ = (void *)hshift;
+	entry = mem + (1 + hsize) * sizeof(*hash);
 	memset(hash, 0, hsize * sizeof(*hash));
 
 	/* allocate an array to count hash entries */
@@ -107,7 +109,7 @@ static struct index ** delta_index(const unsigned char *buf,
 	}
 	free(hash_count);
 
-	return hash;
+	return hash-1;
 }
 
 /* provide the size of the copy opcode given the block offset and size */
@@ -121,7 +123,8 @@ static struct index ** delta_index(const unsigned char *buf,
 void *diff_delta(void *from_buf, unsigned long from_size,
 		 void *to_buf, unsigned long to_size,
 		 unsigned long *delta_size,
-		 unsigned long max_size)
+		 unsigned long max_size,
+		 void **from_index)
 {
 	unsigned int i, outpos, outsize, inscnt, hash_shift;
 	const unsigned char *ref_data, *ref_top, *data, *top;
@@ -130,17 +133,25 @@ void *diff_delta(void *from_buf, unsigned long from_size,
 
 	if (!from_size || !to_size)
 		return NULL;
-	hash = delta_index(from_buf, from_size, to_size, &hash_shift);
-	if (!hash)
-		return NULL;
+	if (from_index && *from_index) {
+		hash = *from_index;
+	} else {
+		hash = delta_index(from_buf, from_size, to_size);
+		if (!hash)
+			return NULL;
+		if (from_index)
+			*from_index = hash;
+	}
+	hash_shift = (unsigned int)(*hash++);
 
 	outpos = 0;
 	outsize = 8192;
 	if (max_size && outsize >= max_size)
 		outsize = max_size + MAX_OP_SIZE + 1;
 	out = malloc(outsize);
 	if (!out) {
-		free(hash);
+		if (!from_index)
+			free(hash-1);
 		return NULL;
 	}
 
@@ -241,7 +252,8 @@ void *diff_delta(void *from_buf, unsigned long from_size,
 				out = realloc(out, outsize);
 			if (!out) {
 				free(tmp);
-				free(hash);
+				if (!from_index)
+					free(hash-1);
 				return NULL;
 			}
 		}
@@ -250,7 +262,8 @@ void *diff_delta(void *from_buf, unsigned long from_size,
 	if (inscnt)
 		out[outpos - inscnt - 1] = inscnt;
 
-	free(hash);
+	if (!from_index)
+		free(hash-1);
 	*delta_size = outpos;
 	return out;
 }

diffcore-break.c

@@ -71,7 +71,7 @@ static int should_break(struct diff_filespec *src,
 
 	delta = diff_delta(src->data, src->size,
 			   dst->data, dst->size,
-			   &delta_size, 0);
+			   &delta_size, 0, NULL);
 	if (!delta)
 		return 0; /* error but caught downstream */
 

diffcore-rename.c

@@ -168,7 +168,7 @@ static int estimate_similarity(struct diff_filespec *src,
 	delta_limit = base_size * (MAX_SCORE-minimum_score) / MAX_SCORE;
 	delta = diff_delta(src->data, src->size,
 			   dst->data, dst->size,
-			   &delta_size, delta_limit);
+			   &delta_size, delta_limit, NULL);
 	if (!delta)
 		/* If delta_limit is exceeded, we have too much differences */
 		return 0;

pack-objects.c

@@ -204,7 +204,7 @@ static void *delta_against(void *buf, unsigned long size, struct object_entry *e
 	if (!otherbuf)
 		die("unable to read %s", sha1_to_hex(entry->delta->sha1));
         delta_buf = diff_delta(otherbuf, othersize,
-			       buf, size, &delta_size, 0);
+			       buf, size, &delta_size, 0, NULL);
         if (!delta_buf || delta_size != entry->delta_size)
         	die("delta size changed");
         free(buf);
@@ -810,6 +810,7 @@ static int type_size_sort(const struct object_entry *a, const struct object_entr
 struct unpacked {
 	struct object_entry *entry;
 	void *data;
+	void **delta_index;
 };
 
 /*
@@ -891,7 +892,8 @@ static int try_delta(struct unpacked *cur, struct unpacked *old, unsigned max_de
 	if (sizediff >= max_size)
 		return -1;
 	delta_buf = diff_delta(old->data, oldsize,
-			       cur->data, size, &delta_size, max_size);
+			       cur->data, size, &delta_size,
+			       max_size, old->delta_index);
 	if (!delta_buf)
 		return 0;
 	cur_entry->delta = old_entry;
@@ -948,6 +950,7 @@ static void find_deltas(struct object_entry **list, int window, int depth)
 			 */
 			continue;
 
+		free(n->delta_index);
 		free(n->data);
 		n->entry = entry;
 		n->data = read_sha1_file(entry->sha1, type, &size);
@@ -974,8 +977,10 @@ static void find_deltas(struct object_entry **list, int window, int depth)
 	if (progress)
 		fputc('\n', stderr);
 
-	for (i = 0; i < window; ++i)
+	for (i = 0; i < window; ++i) {
+		free(array[i].delta_index);
 		free(array[i].data);
+	}
 	free(array);
 }
 

test-delta.c

@@ -63,7 +63,7 @@ int main(int argc, char *argv[])
 	if (argv[1][1] == 'd')
 		out_buf = diff_delta(from_buf, from_size,
 				     data_buf, data_size,
-				     &out_size, 0);
+				     &out_size, 0, NULL);
 	else
 		out_buf = patch_delta(from_buf, from_size,
 				      data_buf, data_size,