diff: retire "compaction" heuristics

When a patch inserts a block of lines, whose last lines are the
same as the existing lines that appear before the inserted block,
"git diff" can choose any place between these existing lines as the
boundary between the pre-context and the added lines (adjusting the
end of the inserted block as appropriate) to come up with variants
of the same patch, and some variants are easier to read than others.

We have been trying to improve the choice of this boundary, and Git
2.11 shipped with an experimental "compaction-heuristic".  Since
then another attempt to improve the logic further resulted in a new
"indent-heuristic" logic.  It is agreed that the latter gives better
result overall, and the former outlived its usefulness.

Retire "compaction", and keep "indent" as an experimental feature.
The latter hopefully will be turned on by default in a future
release, but that should be done as a separate step.

Suggested-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/diff-config.txt

@@ -172,10 +172,8 @@ diff.tool::
 include::mergetools-diff.txt[]
 
 diff.indentHeuristic::
-diff.compactionHeuristic::
-	Set one of these options to `true` to enable one of two
-	experimental heuristics that shift diff hunk boundaries to
-	make patches easier to read.
+	Set this option to `true` to enable experimental heuristics
+	that shift diff hunk boundaries to make patches easier to read.
 
 diff.algorithm::
 	Choose a diff algorithm.  The variants are as follows:

Documentation/diff-heuristic-options.txt

@@ -1,7 +1,5 @@
 --indent-heuristic::
 --no-indent-heuristic::
---compaction-heuristic::
---no-compaction-heuristic::
 	These are to help debugging and tuning experimental heuristics
 	(which are off by default) that shift diff hunk boundaries to
 	make patches easier to read.

builtin/blame.c

@@ -2596,8 +2596,7 @@ int cmd_blame(int argc, const char **argv, const char *prefix)
 		 * and are only included here to get included in the "-h"
 		 * output:
 		 */
-		{ OPTION_LOWLEVEL_CALLBACK, 0, "indent-heuristic", NULL, NULL, N_("Use an experimental indent-based heuristic to improve diffs"), PARSE_OPT_NOARG, parse_opt_unknown_cb },
-		{ OPTION_LOWLEVEL_CALLBACK, 0, "compaction-heuristic", NULL, NULL, N_("Use an experimental blank-line-based heuristic to improve diffs"), PARSE_OPT_NOARG, parse_opt_unknown_cb },
+		{ OPTION_LOWLEVEL_CALLBACK, 0, "indent-heuristic", NULL, NULL, N_("Use an experimental heuristic to improve diffs"), PARSE_OPT_NOARG, parse_opt_unknown_cb },
 
 		OPT_BIT(0, "minimal", &xdl_opts, N_("Spend extra cycles to find better match"), XDF_NEED_MINIMAL),
 		OPT_STRING('S', NULL, &revs_file, N_("file"), N_("Use revisions from <file> instead of calling git-rev-list")),
@@ -2645,7 +2644,7 @@ int cmd_blame(int argc, const char **argv, const char *prefix)
 	}
 parse_done:
 	no_whole_file_rename = !DIFF_OPT_TST(&revs.diffopt, FOLLOW_RENAMES);
-	xdl_opts |= revs.diffopt.xdl_opts & (XDF_COMPACTION_HEURISTIC | XDF_INDENT_HEURISTIC);
+	xdl_opts |= revs.diffopt.xdl_opts & XDF_INDENT_HEURISTIC;
 	DIFF_OPT_CLR(&revs.diffopt, FOLLOW_RENAMES);
 	argc = parse_options_end(&ctx);
 

diff.c

@@ -28,7 +28,6 @@
 
 static int diff_detect_rename_default;
 static int diff_indent_heuristic; /* experimental */
-static int diff_compaction_heuristic; /* experimental */
 static int diff_rename_limit_default = 400;
 static int diff_suppress_blank_empty;
 static int diff_use_color_default = -1;
@@ -223,16 +222,8 @@ void init_diff_ui_defaults(void)
 
 int git_diff_heuristic_config(const char *var, const char *value, void *cb)
 {
-	if (!strcmp(var, "diff.indentheuristic")) {
+	if (!strcmp(var, "diff.indentheuristic"))
 		diff_indent_heuristic = git_config_bool(var, value);
-		if (diff_indent_heuristic)
-			diff_compaction_heuristic = 0;
-	}
-	if (!strcmp(var, "diff.compactionheuristic")) {
-		diff_compaction_heuristic = git_config_bool(var, value);
-		if (diff_compaction_heuristic)
-			diff_indent_heuristic = 0;
-	}
 	return 0;
 }
 
@@ -3380,8 +3371,6 @@ void diff_setup(struct diff_options *options)
 	options->xdl_opts |= diff_algorithm;
 	if (diff_indent_heuristic)
 		DIFF_XDL_SET(options, INDENT_HEURISTIC);
-	else if (diff_compaction_heuristic)
-		DIFF_XDL_SET(options, COMPACTION_HEURISTIC);
 
 	options->orderfile = diff_order_file_cfg;
 
@@ -3876,16 +3865,10 @@ int diff_opt_parse(struct diff_options *options,
 		DIFF_XDL_SET(options, IGNORE_WHITESPACE_AT_EOL);
 	else if (!strcmp(arg, "--ignore-blank-lines"))
 		DIFF_XDL_SET(options, IGNORE_BLANK_LINES);
-	else if (!strcmp(arg, "--indent-heuristic")) {
+	else if (!strcmp(arg, "--indent-heuristic"))
 		DIFF_XDL_SET(options, INDENT_HEURISTIC);
-		DIFF_XDL_CLR(options, COMPACTION_HEURISTIC);
-	} else if (!strcmp(arg, "--no-indent-heuristic"))
+	else if (!strcmp(arg, "--no-indent-heuristic"))
 		DIFF_XDL_CLR(options, INDENT_HEURISTIC);
-	else if (!strcmp(arg, "--compaction-heuristic")) {
-		DIFF_XDL_SET(options, COMPACTION_HEURISTIC);
-		DIFF_XDL_CLR(options, INDENT_HEURISTIC);
-	} else if (!strcmp(arg, "--no-compaction-heuristic"))
-		DIFF_XDL_CLR(options, COMPACTION_HEURISTIC);
 	else if (!strcmp(arg, "--patience"))
 		options->xdl_opts = DIFF_WITH_ALG(options, PATIENCE_DIFF);
 	else if (!strcmp(arg, "--histogram"))

git-add--interactive.perl

@@ -46,7 +46,6 @@
 
 my $diff_algorithm = $repo->config('diff.algorithm');
 my $diff_indent_heuristic = $repo->config_bool('diff.indentheuristic');
-my $diff_compaction_heuristic = $repo->config_bool('diff.compactionheuristic');
 my $diff_filter = $repo->config('interactive.difffilter');
 
 my $use_readkey = 0;
@@ -753,8 +752,6 @@ sub parse_diff {
 	}
 	if ($diff_indent_heuristic) {
 		splice @diff_cmd, 1, 0, "--indent-heuristic";
-	} elsif ($diff_compaction_heuristic) {
-		splice @diff_cmd, 1, 0, "--compaction-heuristic";
 	}
 	if (defined $patch_mode_revision) {
 		push @diff_cmd, get_diff_reference($patch_mode_revision);

xdiff/xdiff.h

@@ -41,8 +41,7 @@ extern "C" {
 
 #define XDF_IGNORE_BLANK_LINES (1 << 7)
 
-#define XDF_COMPACTION_HEURISTIC (1 << 8)
-#define XDF_INDENT_HEURISTIC (1 << 9)
+#define XDF_INDENT_HEURISTIC (1 << 8)
 
 #define XDL_EMIT_FUNCNAMES (1 << 0)
 #define XDL_EMIT_FUNCCONTEXT (1 << 2)

xdiff/xdiffi.c

@@ -400,11 +400,6 @@ static xdchange_t *xdl_add_change(xdchange_t *xscr, long i1, long i2, long chg1,
 }
 
 
-static int is_blank_line(xrecord_t *rec, long flags)
-{
-	return xdl_blankline(rec->ptr, rec->size, flags);
-}
-
 static int recs_match(xrecord_t *rec1, xrecord_t *rec2, long flags)
 {
 	return (rec1->ha == rec2->ha &&
@@ -821,7 +816,6 @@ int xdl_change_compact(xdfile_t *xdf, xdfile_t *xdfo, long flags) {
 	struct xdlgroup g, go;
 	long earliest_end, end_matching_other;
 	long groupsize;
-	unsigned int blank_lines;
 
 	group_init(xdf, &g);
 	group_init(xdfo, &go);
@@ -846,13 +840,6 @@ int xdl_change_compact(xdfile_t *xdf, xdfile_t *xdfo, long flags) {
 			 */
 			end_matching_other = -1;
 
-			/*
-			 * Boolean value that records whether there are any blank
-			 * lines that could be made to be the last line of this
-			 * group.
-			 */
-			blank_lines = 0;
-
 			/* Shift the group backward as much as possible: */
 			while (!group_slide_up(xdf, &g, flags))
 				if (group_previous(xdfo, &go))
@@ -869,11 +856,6 @@ int xdl_change_compact(xdfile_t *xdf, xdfile_t *xdfo, long flags) {
 
 			/* Now shift the group forward as far as possible: */
 			while (1) {
-				if (!blank_lines)
-					blank_lines = is_blank_line(
-							xdf->recs[g.end - 1],
-							flags);
-
 				if (group_slide_down(xdf, &g, flags))
 					break;
 				if (group_next(xdfo, &go))
@@ -906,21 +888,6 @@ int xdl_change_compact(xdfile_t *xdf, xdfile_t *xdfo, long flags) {
 				if (group_previous(xdfo, &go))
 					xdl_bug("group sync broken sliding to match");
 			}
-		} else if ((flags & XDF_COMPACTION_HEURISTIC) && blank_lines) {
-			/*
-			 * Compaction heuristic: if it is possible to shift the
-			 * group to make its bottom line a blank line, do so.
-			 *
-			 * As we already shifted the group forward as far as
-			 * possible in the earlier loop, we only need to handle
-			 * backward shifts, not forward ones.
-			 */
-			while (!is_blank_line(xdf->recs[g.end - 1], flags)) {
-				if (group_slide_up(xdf, &g, flags))
-					xdl_bug("blank line disappeared");
-				if (group_previous(xdfo, &go))
-					xdl_bug("group sync broken sliding to blank line");
-			}
 		} else if (flags & XDF_INDENT_HEURISTIC) {
 			/*
 			 * Indent heuristic: a group of pure add/delete lines