combine-diff: reuse diff from the same blob.

When dealing with an insanely large Octopus, it is possible to
optimize by noticing that more than one parents have the same
blob and avoid running diff between a parent and the merge
result by reusing an earlier result.

Signed-off-by: Junio C Hamano <junkio@cox.net>

combine-diff.c

@@ -523,6 +523,30 @@ static void dump_sline(struct sline *sline, int cnt, int num_parent)
 	}
 }
 
+static void reuse_combine_diff(struct sline *sline, unsigned long cnt,
+			       int i, int j)
+{
+	/* We have already examined parent j and we know parent i
+	 * and parent j are the same, so reuse the combined result
+	 * of parent j for parent i.
+	 */
+	unsigned long lno, imask, jmask;
+	imask = (1UL<<i);
+	jmask = (1UL<<j);
+
+	for (lno = 0; lno < cnt; lno++) {
+		struct lline *ll = sline->lost_head;
+		while (ll) {
+			if (ll->parent_map & jmask)
+				ll->parent_map |= imask;
+			ll = ll->next;
+		}
+		if (!(sline->flag & jmask))
+			sline->flag &= ~imask;
+		sline++;
+	}
+}
+
 int show_combined_diff(struct combine_diff_path *elem, int num_parent,
 		       int dense, const char *header, int show_empty)
 {
@@ -596,8 +620,19 @@ int show_combined_diff(struct combine_diff_path *elem, int num_parent,
 		sline[cnt-1].flag = (1UL<<num_parent) - 1;
 	}
 
-	for (i = 0; i < num_parent; i++)
-		combine_diff(elem->parent_sha1[i], ourtmp, sline, cnt, i);
+	for (i = 0; i < num_parent; i++) {
+		int j;
+		for (j = 0; j < i; j++) {
+			if (!memcmp(elem->parent_sha1[i],
+				    elem->parent_sha1[j], 20)) {
+				reuse_combine_diff(sline, cnt, i, j);
+				break;
+			}
+		}
+		if (i <= j)
+			combine_diff(elem->parent_sha1[i], ourtmp, sline,
+				     cnt, i);
+	}
 
 	show_hunks = make_hunks(sline, cnt, num_parent, dense);