[PATCH] diff: Update -B heuristics.

As Linus pointed out on the mailing list discussion, -B should
break a files that has many inserts even if it still keeps
enough of the original contents, so that the broken pieces can
later be matched with other files by -M or -C.  However, if such
a broken pair does not get picked up by -M or -C, we would want
to apply different criteria; namely, regardless of the amount of
new material in the result, the determination of "rewrite"
should be done by looking at the amount of original material
still left in the result.  If you still have the original 97
lines from a 100-line document, it does not matter if you add
your own 13 lines to make a 110-line document, or if you add 903
lines to make a 1000-line document.  It is not a rewrite but an
in-place edit.  On the other hand, if you did lose 97 lines from
the original, it does not matter if you added 27 lines to make a
30-line document or if you added 997 lines to make a 1000-line
document.  You did a complete rewrite in either case.

This patch introduces a post-processing phase that runs after
diffcore-rename matches up broken pairs diffcore-break creates.
The purpose of this post-processing is to pick up these broken
pieces and merge them back into in-place modifications.  For
this, the score parameter -B option takes is changed into a pair
of numbers, and it takes "-B99/80" format when fully spelled
out.  The first number is the minimum amount of "edit" (same
definition as what diffcore-rename uses, which is "sum of
deletion and insertion") that a modification needs to have to be
broken, and the second number is the minimum amount of "delete"
a surviving broken pair must have to avoid being merged back
together.  It can be abbreviated to "-B" to use default for
both, "-B9" or "-B9/" to use 90% for "edit" but default (80%)
for merge avoidance, or "-B/75" to use default (99%) "edit" and
75% for merge avoidance.

Signed-off-by: Junio C Hamano <junkio@cox.net>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

1 files changed, 11 insertions, 0 deletions

diff --git a/diffcore.h b/diffcore.h
index 2e613ebe8..194d40cb4 100644
--- a/diffcore.h
+++ b/diffcore.h
@@ -8,9 +8,19 @@
  * (e.g. diffcore-rename, diffcore-pickaxe).  Never include this header
  * in anything else.
  */
+
+/* We internally use unsigned short as the score value,
+ * and rely on an int capable to hold 32-bits.  -B can take
+ * -Bmerge_score/break_score format and the two scores are
+ * passed around in one int (high 16-bit for merge and low 16-bit
+ * for break).
+ */
 #define MAX_SCORE 60000
 #define DEFAULT_RENAME_SCORE 30000 /* rename/copy similarity minimum (50%) */
 #define DEFAULT_BREAK_SCORE  59400 /* minimum for break to happen (99%)*/
+#define DEFAULT_MERGE_SCORE  48000 /* maximum for break-merge to happen (80%)*/
+
+#define MINIMUM_BREAK_SIZE     400 /* do not break a file smaller than this */
 
 struct diff_filespec {
 	unsigned char sha1[20];
@@ -76,6 +86,7 @@ extern void diff_q(struct diff_queue_struct *, struct diff_filepair *);
 extern void diffcore_pathspec(const char **pathspec);
 extern void diffcore_break(int);
 extern void diffcore_rename(int rename_copy, int);
+extern void diffcore_merge_broken(void);
 extern void diffcore_pickaxe(const char *needle, int opts);
 extern void diffcore_order(const char *orderfile);