diffcore-delta: make change counter to byte oriented again.

Junio C Hamano · Junio C Hamano · commit ba23bbc8efff · 2006-03-04T13:34:50.000-08:00
The textual line oriented change counter was fun but was not
very effective.  It tended to overcount the changes.  This one
changes it to a simple N-letter substring based implementation.

Signed-off-by: Junio C Hamano &lt;junkio@cox.net&gt;
diff --git a/diffcore-delta.c b/diffcore-delta.c
@@ -1,32 +1,53 @@
 #include "cache.h"
 #include "diff.h"
 #include "diffcore.h"
-#include "delta.h"
-#include "count-delta.h"
-
-static int diffcore_count_changes_1(void *src, unsigned long src_size,
-				    void *dst, unsigned long dst_size,
-				    unsigned long delta_limit,
-				    unsigned long *src_copied,
-				    unsigned long *literal_added)
+
+/*
+ * Idea here is very simple.
+ *
+ * We have total of (sz-N+1) N-byte overlapping sequences in buf whose
+ * size is sz.  If the same N-byte sequence appears in both source and
+ * destination, we say the byte that starts that sequence is shared
+ * between them (i.e. copied from source to destination).
+ *
+ * For each possible N-byte sequence, if the source buffer has more
+ * instances of it than the destination buffer, that means the
+ * difference are the number of bytes not copied from source to
+ * destination.  If the counts are the same, everything was copied
+ * from source to destination.  If the destination has more,
+ * everything was copied, and destination added more.
+ *
+ * We are doing an approximation so we do not really have to waste
+ * memory by actually storing the sequence.  We just hash them into
+ * somewhere around 2^16 hashbuckets and count the occurrences.
+ *
+ * The length of the sequence is arbitrarily set to 8 for now.
+ */
+
+#define HASHBASE 65537 /* next_prime(2^16) */
+
+static void hash_chars(unsigned char *buf, unsigned long sz, int *count)
 {
-	void *delta;
-	unsigned long delta_size;
-
-	delta = diff_delta(src, src_size,
-			   dst, dst_size,
-			   &delta_size, delta_limit);
-	if (!delta)
-		/* If delta_limit is exceeded, we have too much differences */
-		return -1;
+	unsigned int accum1, accum2, i;
 
-	/* Estimate the edit size by interpreting delta. */
-	if (count_delta(delta, delta_size, src_copied, literal_added)) {
-		free(delta);
-		return -1;
+	/* an 8-byte shift register made of accum1 and accum2.  New
+	 * bytes come at LSB of accum2, and shifted up to accum1
+	 */
+	for (i = accum1 = accum2 = 0; i < 7; i++, sz--) {
+		accum1 = (accum1 << 8) | (accum2 >> 24);
+		accum2 = (accum2 << 8) | *buf++;
+	}
+	while (sz) {
+		accum1 = (accum1 << 8) | (accum2 >> 24);
+		accum2 = (accum2 << 8) | *buf++;
+		/* We want something that hashes permuted byte
+		 * sequences nicely; simpler hash like (accum1 ^
+		 * accum2) does not perform as well.
+		 */
+		i = (accum1 + accum2 * 0x61) % HASHBASE;
+		count[i]++;
+		sz--;
 	}
-	free(delta);
-	return 0;
 }
 
 int diffcore_count_changes(void *src, unsigned long src_size,
@@ -35,9 +56,28 @@ int diffcore_count_changes(void *src, unsigned long src_size,
 			   unsigned long *src_copied,
 			   unsigned long *literal_added)
 {
-	return diffcore_count_changes_1(src, src_size,
-					dst, dst_size,
-					delta_limit,
-					src_copied,
-					literal_added);
+	int *src_count, *dst_count, i;
+	unsigned long sc, la;
+
+	if (src_size < 8 || dst_size < 8)
+		return -1;
+
+	src_count = xcalloc(HASHBASE * 2, sizeof(int));
+	dst_count = src_count + HASHBASE;
+	hash_chars(src, src_size, src_count);
+	hash_chars(dst, dst_size, dst_count);
+
+	sc = la = 0;
+	for (i = 0; i < HASHBASE; i++) {
+		if (src_count[i] < dst_count[i]) {
+			la += dst_count[i] - src_count[i];
+			sc += src_count[i];
+		}
+		else /* i.e. if (dst_count[i] <= src_count[i]) */
+			sc += dst_count[i];
+	}
+	*src_copied = sc;
+	*literal_added = la;
+	free(src_count);
+	return 0;
 }
