[PATCH] Add -B flag to diff-* brothers.

A new diffcore transformation, diffcore-break.c, is introduced.

When the -B flag is given, a patch that represents a complete
rewrite is broken into a deletion followed by a creation.  This
makes it easier to review such a complete rewrite patch.

The -B flag takes the same syntax as the -M and -C flags to
specify the minimum amount of non-source material the resulting
file needs to have to be considered a complete rewrite, and
defaults to 99% if not specified.

As the new test t4008-diff-break-rewrite.sh demonstrates, if a
file is a complete rewrite, it is broken into a delete/create
pair, which can further be subjected to the usual rename
detection if -M or -C is used.  For example, if file0 gets
completely rewritten to make it as if it were rather based on
file1 which itself disappeared, the following happens:

    The original change looks like this:

	file0     --> file0' (quite different from file0)
	file1     --> /dev/null

    After diffcore-break runs, it would become this:

	file0     --> /dev/null
	/dev/null --> file0'
	file1     --> /dev/null

    Then diffcore-rename matches them up:

	file1     --> file0'

The internal score values are finer grained now.  Earlier
maximum of 10000 has been raised to 60000; there is no user
visible changes but there is no reason to waste available bits.

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

diffcore-break.c

@@ -0,0 +1,127 @@
+/*
+ * Copyright (C) 2005 Junio C Hamano
+ */
+#include "cache.h"
+#include "diff.h"
+#include "diffcore.h"
+#include "delta.h"
+#include "count-delta.h"
+
+static int very_different(struct diff_filespec *src,
+			  struct diff_filespec *dst,
+			  int min_score)
+{
+	/* dst is recorded as a modification of src.  Are they so
+	 * different that we are better off recording this as a pair
+	 * of delete and create?  min_score is the minimum amount of
+	 * new material that must exist in the dst and not in src for
+	 * the pair to be considered a complete rewrite, and recommended
+	 * to be set to a very high value, 99% or so.
+	 *
+	 * The value we return represents the amount of new material
+	 * that is in dst and not in src.  We return 0 when we do not
+	 * want to get the filepair broken.
+	 */
+	void *delta;
+	unsigned long delta_size, base_size;
+
+	if (!S_ISREG(src->mode) || !S_ISREG(dst->mode))
+		return 0; /* leave symlink rename alone */
+
+	if (diff_populate_filespec(src, 1) || diff_populate_filespec(dst, 1))
+		return 0; /* error but caught downstream */
+
+	delta_size = ((src->size < dst->size) ?
+		      (dst->size - src->size) : (src->size - dst->size));
+
+	/* Notice that we use max of src and dst as the base size,
+	 * unlike rename similarity detection.  This is so that we do
+	 * not mistake a large addition as a complete rewrite.
+	 */
+	base_size = ((src->size < dst->size) ? dst->size : src->size);
+
+	/*
+	 * If file size difference is too big compared to the
+	 * base_size, we declare this a complete rewrite.
+	 */
+	if (base_size * min_score < delta_size * MAX_SCORE)
+		return MAX_SCORE;
+
+	if (diff_populate_filespec(src, 0) || diff_populate_filespec(dst, 0))
+		return 0; /* error but caught downstream */
+
+	delta = diff_delta(src->data, src->size,
+			   dst->data, dst->size,
+			   &delta_size);
+
+	/* A delta that has a lot of literal additions would have
+	 * big delta_size no matter what else it does.
+	 */
+	if (base_size * min_score < delta_size * MAX_SCORE)
+		return MAX_SCORE;
+
+	/* Estimate the edit size by interpreting delta. */
+	delta_size = count_delta(delta, delta_size);
+	free(delta);
+	if (delta_size == UINT_MAX)
+		return 0; /* error in delta computation */
+
+	if (base_size < delta_size)
+		return MAX_SCORE;
+
+	return delta_size * MAX_SCORE / base_size; 
+}
+
+void diffcore_break(int min_score)
+{
+	struct diff_queue_struct *q = &diff_queued_diff;
+	struct diff_queue_struct outq;
+	int i;
+
+	if (!min_score)
+		min_score = DEFAULT_BREAK_SCORE;
+
+	outq.nr = outq.alloc = 0;
+	outq.queue = NULL;
+
+	for (i = 0; i < q->nr; i++) {
+		struct diff_filepair *p = q->queue[i];
+		int score;
+
+		/* We deal only with in-place edit of non directory.
+		 * We do not break anything else.
+		 */
+		if (DIFF_FILE_VALID(p->one) && DIFF_FILE_VALID(p->two) &&
+		    !S_ISDIR(p->one->mode) && !S_ISDIR(p->two->mode) &&
+		    !strcmp(p->one->path, p->two->path)) {
+			score = very_different(p->one, p->two, min_score);
+			if (min_score <= score) {
+				/* Split this into delete and create */
+				struct diff_filespec *null_one, *null_two;
+				struct diff_filepair *dp;
+
+				/* deletion of one */
+				null_one = alloc_filespec(p->one->path);
+				dp = diff_queue(&outq, p->one, null_one);
+				dp->score = score;
+				dp->broken_pair = 1;
+
+				/* creation of two */
+				null_two = alloc_filespec(p->two->path);
+				dp = diff_queue(&outq, null_two, p->two);
+				dp->score = score;
+				dp->broken_pair = 1;
+
+				free(p); /* not diff_free_filepair(), we are
+					  * reusing one and two here.
+					  */
+				continue;
+			}
+		}
+		diff_q(&outq, p);
+	}
+	free(q->queue);
+	*q = outq;
+
+	return;
+}