combine-diff: extend --cc logic to Octopus.

Santi Bejar points out that a hunk that changes from all the
same common parents except one is uninteresting.  The earlier
round marked changes from only one parent uninteresting, but
this also marks hunks that have the same change from all but one
parent uninteresting, which is a natural extension of the
original idea to Octopus merges.

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

combine-diff.c

@@ -271,21 +271,56 @@ static int interesting(struct sline *sline, unsigned long all_mask)
 	return ((sline->flag & all_mask) != all_mask || sline->lost_head);
 }
 
-static unsigned long line_diff_parents(struct sline *sline, unsigned long all_mask)
+static unsigned long line_common_diff(struct sline *sline, unsigned long all_mask)
 {
 	/*
-	 * Look at the line and see from which parents we have difference.
-	 * Lower bits of sline->flag records if the parent had this line,
-	 * so XOR with all_mask gives us on-bits for parents we have
-	 * differences with.
+	 * Look at the line and see from which parents we have the
+	 * same difference.
 	 */
-	unsigned long parents = (sline->flag ^ all_mask);
+
+	/* Lower bits of sline->flag records if the parent had this
+	 * line, so XOR with all_mask gives us on-bits for parents we
+	 * have differences with.
+	 */
+	unsigned long common_adds = (sline->flag ^ all_mask) & all_mask;
+	unsigned long common_removes = all_mask;
+
+	/* If all the parents have this line, that also counts as
+	 * having the same difference.
+	 */
+	if (!common_adds)
+		common_adds = all_mask;
+
+	if (sline->lost_head) {
+		/* Lost head list records the lines removed from
+		 * the parents, and parent_map records from which
+		 * parent the line was removed.
+		 */
+		struct lline *ll;
+		for (ll = sline->lost_head; ll; ll = ll->next) {
+			common_removes &= ll->parent_map;
+		}
+	}
+	return common_adds & common_removes;
+}
+
+static unsigned long line_all_diff(struct sline *sline, unsigned long all_mask)
+{
+	/*
+	 * Look at the line and see from which parents we have some difference.
+	 */
+	unsigned long different = (sline->flag ^ all_mask) & all_mask;
 	if (sline->lost_head) {
+		/* Lost head list records the lines removed from
+		 * the parents, and parent_map records from which
+		 * parent the line was removed.
+		 */
 		struct lline *ll;
-		for (ll = sline->lost_head; ll; ll = ll->next)
-			parents |= ll->parent_map;
+		for (ll = sline->lost_head; ll; ll = ll->next) {
+			different |= ll->parent_map;
+		}
 	}
-	return parents & all_mask;
+	return different;
 }
 
 static void make_hunks(struct sline *sline, unsigned long cnt,
@@ -316,36 +351,38 @@ static void make_hunks(struct sline *sline, unsigned long cnt,
 	if (!dense)
 		return;
 
-	/* Look at each hunk, and if it contains changes from only
-	 * one parent, mark that uninteresting.
+	/* Look at each hunk, and if we have changes from only one
+	 * parent, or the changes are the same from all but one
+	 * parent, mark that uninteresting.
 	 */
 	i = 0;
 	while (i < cnt) {
-		int j, hunk_end, diffs;
-		unsigned long parents;
+		int j, hunk_end, same, diff;
+		unsigned long same_diff, all_diff, this_diff;
 		while (i < cnt && !(sline[i].flag & mark))
 			i++;
 		if (cnt <= i)
 			break; /* No more interesting hunks */
 		for (hunk_end = i + 1; hunk_end < cnt; hunk_end++)
 			if (!(sline[hunk_end].flag & mark))
 				break;
-		/* [i..hunk_end) are interesting.  Now is it from
-		 * only one parent?
-		 * If lost lines are only from one parent and
-		 * remaining lines existed in parents other than
-		 * that parent, then the hunk is not that interesting.
+		/* [i..hunk_end) are interesting.  Now does it have
+		 * the same change with all but one parent?
 		 */
-		parents = 0;
-		diffs = 0;
-		for (j = i; j < hunk_end; j++)
-			parents |= line_diff_parents(sline + j, all_mask);
-		/* Now, how many bits from [0..num_parent) are on? */
+		same_diff = all_mask;
+		all_diff = 0;
+		for (j = i; j < hunk_end; j++) {
+			same_diff &= line_common_diff(sline + j, all_mask);
+			all_diff |= line_all_diff(sline + j, all_mask);
+		}
+		diff = same = 0;
 		for (j = 0; j < num_parent; j++) {
-			if (parents & (1UL<<j))
-				diffs++;
+			if (same_diff & (1UL<<j))
+				same++;
+			if (all_diff & (1UL<<j))
+				diff++;
 		}
-		if (diffs < 2) {
+		if ((num_parent - 1 <= same) || (diff == 1)) {
 			/* This hunk is not that interesting after all */
 			for (j = i; j < hunk_end; j++)
 				sline[j].flag &= ~mark;