copy vs rename detection: avoid unnecessary O(n*m) loops

The core rename detection had some rather stupid code to check if a
pathname was used by a later modification or rename, which basically
walked the whole pathname space for all renames for each rename, in
order to tell whether it was a pure rename (no remaining users) or
should be considered a copy (other users of the source file remaining).

That's really silly, since we can just keep a count of users around, and
replace all those complex and expensive loops with just testing that
simple counter (but this all depends on the previous commit that shared
the diff_filespec data structure by using a separate reference count).

Note that the reference count is not the same as the rename count: they
behave otherwise rather similarly, but the reference count is tied to
the allocation (and decremented at de-allocation, so that when it turns
zero we can get rid of the memory), while the rename count is tied to
the renames and is decremented when we find a rename (so that when it
turns zero we know that it was a rename, not a copy).

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 17 insertions, 23 deletions

diff --git a/diff.c b/diff.c
index 0b320f6b96..af85b94d1b 100644
--- a/diff.c
+++ b/diff.c
@@ -2597,9 +2597,9 @@ void diff_debug_filepair(const struct diff_filepair *p, int i)
 {
 	diff_debug_filespec(p->one, i, "one");
 	diff_debug_filespec(p->two, i, "two");
-	fprintf(stderr, "score %d, status %c stays %d broken %d\n",
+	fprintf(stderr, "score %d, status %c rename_used %d broken %d\n",
 		p->score, p->status ? p->status : '?',
-		p->source_stays, p->broken_pair);
+		p->one->rename_used, p->broken_pair);
 }
 
 void diff_debug_queue(const char *msg, struct diff_queue_struct *q)
@@ -2617,8 +2617,8 @@ void diff_debug_queue(const char *msg, struct diff_queue_struct *q)
 
 static void diff_resolve_rename_copy(void)
 {
-	int i, j;
-	struct diff_filepair *p, *pp;
+	int i;
+	struct diff_filepair *p;
 	struct diff_queue_struct *q = &diff_queued_diff;
 
 	diff_debug_queue("resolve-rename-copy", q);
@@ -2640,27 +2640,21 @@ static void diff_resolve_rename_copy(void)
 		 * either in-place edit or rename/copy edit.
 		 */
 		else if (DIFF_PAIR_RENAME(p)) {
-			if (p->source_stays) {
-				p->status = DIFF_STATUS_COPIED;
-				continue;
-			}
-			/* See if there is some other filepair that
-			 * copies from the same source as us.  If so
-			 * we are a copy.  Otherwise we are either a
-			 * copy if the path stays, or a rename if it
-			 * does not, but we already handled "stays" case.
+			/*
+			 * A rename might have re-connected a broken
+			 * pair up, causing the pathnames to be the
+			 * same again. If so, that's not a rename at
+			 * all, just a modification..
+			 *
+			 * Otherwise, see if this source was used for
+			 * multiple renames, in which case we decrement
+			 * the count, and call it a copy.
 			 */
-			for (j = i + 1; j < q->nr; j++) {
-				pp = q->queue[j];
-				if (strcmp(pp->one->path, p->one->path))
-					continue; /* not us */
-				if (!DIFF_PAIR_RENAME(pp))
-					continue; /* not a rename/copy */
-				/* pp is a rename/copy from the same source */
+			if (!strcmp(p->one->path, p->two->path))
+				p->status = DIFF_STATUS_MODIFIED;
+			else if (--p->one->rename_used > 0)
 				p->status = DIFF_STATUS_COPIED;
-				break;
-			}
-			if (!p->status)
+			else
 				p->status = DIFF_STATUS_RENAMED;
 		}
 		else if (hashcmp(p->one->sha1, p->two->sha1) ||