diffcore-break: similarity estimator fix.

This is a companion patch to the previous fix to diffcore-rename.
The merging-back process should use a logic similar to what is used
there.

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

diffcore-break.c

@@ -45,8 +45,8 @@ static int should_break(struct diff_filespec *src,
 	 * The value we return is 1 if we want the pair to be broken,
 	 * or 0 if we do not.
 	 */
-	unsigned long delta_size, base_size, src_copied, literal_added;
-	int to_break = 0;
+	unsigned long delta_size, base_size, src_copied, literal_added,
+		src_removed;
 
 	*merge_score_p = 0; /* assume no deletion --- "do not break"
 			     * is the default.
@@ -72,33 +72,40 @@ static int should_break(struct diff_filespec *src,
 				   &src_copied, &literal_added))
 		return 0;
 
+	/* sanity */
+	if (src->size < src_copied)
+		src_copied = src->size;
+	if (dst->size < literal_added + src_copied) {
+		if (src_copied < dst->size)
+			literal_added = dst->size - src_copied;
+		else
+			literal_added = 0;
+	}
+	src_removed = src->size - src_copied;
+
 	/* Compute merge-score, which is "how much is removed
 	 * from the source material".  The clean-up stage will
 	 * merge the surviving pair together if the score is
 	 * less than the minimum, after rename/copy runs.
 	 */
-	if (src->size <= src_copied)
-		; /* all copied, nothing removed */
-	else {
-		delta_size = src->size - src_copied;
-		*merge_score_p = delta_size * MAX_SCORE / src->size;
-	}
-	
+	*merge_score_p = src_removed * MAX_SCORE / src->size;
+
 	/* Extent of damage, which counts both inserts and
 	 * deletes.
 	 */
-	if (src->size + literal_added <= src_copied)
-		delta_size = 0; /* avoid wrapping around */
-	else
-		delta_size = (src->size - src_copied) + literal_added;
-	
-	/* We break if the edit exceeds the minimum.
-	 * i.e. (break_score / MAX_SCORE < delta_size / base_size)
-	 */
-	if (break_score * base_size < delta_size * MAX_SCORE)
-		to_break = 1;
+	delta_size = src_removed + literal_added;
+	if (delta_size * MAX_SCORE / base_size < break_score)
+		return 0;
 
-	return to_break;
+	/* If you removed a lot without adding new material, that is
+	 * not really a rewrite.
+	 */
+	if ((src->size * break_score < src_removed * MAX_SCORE) &&
+	    (literal_added * 20 < src_removed) &&
+	    (literal_added * 20 < src_copied))
+		return 0;
+
+	return 1;
 }
 
 void diffcore_break(int break_score)

diffcore.h

@@ -17,8 +17,8 @@
  */
 #define MAX_SCORE 60000.0
 #define DEFAULT_RENAME_SCORE 30000 /* rename/copy similarity minimum (50%) */
-#define DEFAULT_BREAK_SCORE  30000 /* minimum for break to happen (50%)*/
-#define DEFAULT_MERGE_SCORE  48000 /* maximum for break-merge to happen (80%)*/
+#define DEFAULT_BREAK_SCORE  30000 /* minimum for break to happen (50%) */
+#define DEFAULT_MERGE_SCORE  36000 /* maximum for break-merge to happen 60%) */
 
 #define MINIMUM_BREAK_SIZE     400 /* do not break a file smaller than this */