xdiff: disable cleanup_records heuristic with --minimal

The cleanup_records function marks some lines as changed before running
the actual diff algorithm. For most lines, this is a good performance
optimization, but it also marks lines that are surrounded by many
changed lines as changed as well. This can cause redundant changes and
longer-than-necessary diffs.

Whether this results in better-looking diffs is subjective. However, the
--minimal flag explicitly requests the shortest possible diff.

The change results in shorter diffs in about 1.3% of all diffs in Git's
history. Performance wise, I have measured the impact on
"git log -p -3000 --minimal > /dev/null". With this change, I get
  Time (mean ± σ): 2.363 s ±  0.023 s (25 runs)
and without this patch I measured
  Time (mean ± σ): 2.362 s ±  0.035 s (25 runs).
As the difference is well within the margin of error, this does not seem
to have an impact on performance.

Signed-off-by: Niels Glodny <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: yndolg

t/meson.build

@@ -501,6 +501,7 @@ integration_tests = [
   't4068-diff-symmetric-merge-base.sh',
   't4069-remerge-diff.sh',
   't4070-diff-pairs.sh',
+  't4071-diff-minimal.sh',
   't4100-apply-stat.sh',
   't4101-apply-nonl.sh',
   't4102-apply-rename.sh',

t/t4071-diff-minimal.sh

@@ -0,0 +1,14 @@
+#!/bin/sh
+
+test_description='minimal diff algorithm'
+
+. ./test-lib.sh
+
+test_expect_success 'minimal diff should not mark changes between changed lines' '
+	test_write_lines x x x x >pre &&
+	test_write_lines x x x A B C D x E F G >post &&
+	test_expect_code 1 git diff --no-index --minimal pre post >diff &&
+	test_grep ! ^[+-]x diff 
+'
+
+test_done

xdiff/xprepare.c

@@ -368,6 +368,7 @@ static int xdl_cleanup_records(xdlclassifier_t *cf, xdfile_t *xdf1, xdfile_t *xd
 	xrecord_t **recs;
 	xdlclass_t *rcrec;
 	char *dis, *dis1, *dis2;
+	int need_min = !!(cf->flags & XDF_NEED_MINIMAL);
 
 	if (!XDL_CALLOC_ARRAY(dis, xdf1->nrec + xdf2->nrec + 2))
 		return -1;
@@ -379,15 +380,15 @@ static int xdl_cleanup_records(xdlclassifier_t *cf, xdfile_t *xdf1, xdfile_t *xd
 	for (i = xdf1->dstart, recs = &xdf1->recs[xdf1->dstart]; i <= xdf1->dend; i++, recs++) {
 		rcrec = cf->rcrecs[(*recs)->ha];
 		nm = rcrec ? rcrec->len2 : 0;
-		dis1[i] = (nm == 0) ? 0: (nm >= mlim) ? 2: 1;
+		dis1[i] = (nm == 0) ? 0: (nm >= mlim && !need_min) ? 2: 1;
 	}
 
 	if ((mlim = xdl_bogosqrt(xdf2->nrec)) > XDL_MAX_EQLIMIT)
 		mlim = XDL_MAX_EQLIMIT;
 	for (i = xdf2->dstart, recs = &xdf2->recs[xdf2->dstart]; i <= xdf2->dend; i++, recs++) {
 		rcrec = cf->rcrecs[(*recs)->ha];
 		nm = rcrec ? rcrec->len1 : 0;
-		dis2[i] = (nm == 0) ? 0: (nm >= mlim) ? 2: 1;
+		dis2[i] = (nm == 0) ? 0: (nm >= mlim && !need_min) ? 2: 1;
 	}
 
 	for (nreff = 0, i = xdf1->dstart, recs = &xdf1->recs[xdf1->dstart];