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

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 <junkio@cox.net>

diffcore-delta.c

@@ -1,32 +1,53 @@
 #include "cache.h"
 #include "diff.h"
 #include "diffcore.h"
-#include "delta.h"
+
-#include "count-delta.h"
+/*
-
+ * Idea here is very simple.
-static int diffcore_count_changes_1(void *src, unsigned long src_size,
+ *
-				    void *dst, unsigned long dst_size,
+ * We have total of (sz-N+1) N-byte overlapping sequences in buf whose
-				    unsigned long delta_limit,
+ * size is sz.  If the same N-byte sequence appears in both source and
-				    unsigned long *src_copied,
+ * destination, we say the byte that starts that sequence is shared
-				    unsigned long *literal_added)
+ * 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 int accum1, accum2, i;
-	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;
 
-	/* Estimate the edit size by interpreting delta. */
+	/* an 8-byte shift register made of accum1 and accum2.  New
-	if (count_delta(delta, delta_size, src_copied, literal_added)) {
+	 * bytes come at LSB of accum2, and shifted up to accum1
-		free(delta);
+	 */
-		return -1;
+	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,
+	int *src_count, *dst_count, i;
-					dst, dst_size,
+	unsigned long sc, la;
-					delta_limit,
+
-					src_copied,
+	if (src_size < 8 || dst_size < 8)
-					literal_added);
+		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;
 }