diff: support anchoring line(s)

Teach diff a new algorithm, one that attempts to prevent user-specified
lines from appearing as a deletion or addition in the end result. The
end user can use this by specifying "--anchored=<text>" one or more
times when using Git commands like "diff" and "show".

Signed-off-by: Jonathan Tan <jonathantanmy@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/diff-options.txt

@@ -80,6 +80,16 @@ endif::git-format-patch[]
 --histogram::
 	Generate a diff using the "histogram diff" algorithm.
 
+--anchored=<text>::
+	Generate a diff using the "anchored diff" algorithm.
++
+This option may be specified more than once.
++
+If a line exists in both the source and destination, exists only once,
+and starts with this text, this algorithm attempts to prevent it from
+appearing as a deletion or addition in the output. It uses the "patience
+diff" algorithm internally.
+
 --diff-algorithm={patience|minimal|histogram|myers}::
 	Choose a diff algorithm. The variants are as follows:
 +

diff.c

@@ -3210,6 +3210,8 @@ static void builtin_diff(const char *name_a,
 		ecbdata.opt = o;
 		ecbdata.header = header.len ? &header : NULL;
 		xpp.flags = o->xdl_opts;
+		xpp.anchors = o->anchors;
+		xpp.anchors_nr = o->anchors_nr;
 		xecfg.ctxlen = o->context;
 		xecfg.interhunkctxlen = o->interhunkcontext;
 		xecfg.flags = XDL_EMIT_FUNCNAMES;
@@ -3302,6 +3304,8 @@ static void builtin_diffstat(const char *name_a, const char *name_b,
 		memset(&xpp, 0, sizeof(xpp));
 		memset(&xecfg, 0, sizeof(xecfg));
 		xpp.flags = o->xdl_opts;
+		xpp.anchors = o->anchors;
+		xpp.anchors_nr = o->anchors_nr;
 		xecfg.ctxlen = o->context;
 		xecfg.interhunkctxlen = o->interhunkcontext;
 		if (xdi_diff_outf(&mf1, &mf2, diffstat_consume, diffstat,
@@ -4594,9 +4598,18 @@ int diff_opt_parse(struct diff_options *options,
 		DIFF_XDL_SET(options, INDENT_HEURISTIC);
 	else if (!strcmp(arg, "--no-indent-heuristic"))
 		DIFF_XDL_CLR(options, INDENT_HEURISTIC);
-	else if (!strcmp(arg, "--patience"))
+	else if (!strcmp(arg, "--patience")) {
+		int i;
 		options->xdl_opts = DIFF_WITH_ALG(options, PATIENCE_DIFF);
-	else if (!strcmp(arg, "--histogram"))
+		/*
+		 * Both --patience and --anchored use PATIENCE_DIFF
+		 * internally, so remove any anchors previously
+		 * specified.
+		 */
+		for (i = 0; i < options->anchors_nr; i++)
+			free(options->anchors[i]);
+		options->anchors_nr = 0;
+	} else if (!strcmp(arg, "--histogram"))
 		options->xdl_opts = DIFF_WITH_ALG(options, HISTOGRAM_DIFF);
 	else if ((argcount = parse_long_opt("diff-algorithm", av, &optarg))) {
 		long value = parse_algorithm_value(optarg);
@@ -4608,6 +4621,11 @@ int diff_opt_parse(struct diff_options *options,
 		options->xdl_opts &= ~XDF_DIFF_ALGORITHM_MASK;
 		options->xdl_opts |= value;
 		return argcount;
+	} else if (skip_prefix(arg, "--anchored=", &arg)) {
+		options->xdl_opts = DIFF_WITH_ALG(options, PATIENCE_DIFF);
+		ALLOC_GROW(options->anchors, options->anchors_nr + 1,
+			   options->anchors_alloc);
+		options->anchors[options->anchors_nr++] = xstrdup(arg);
 	}
 
 	/* flags options */

diff.h

@@ -166,6 +166,10 @@ struct diff_options {
 	const char *stat_sep;
 	long xdl_opts;
 
+	/* see Documentation/diff-options.txt */
+	char **anchors;
+	size_t anchors_nr, anchors_alloc;
+
 	int stat_width;
 	int stat_name_width;
 	int stat_graph_width;

t/t4065-diff-anchored.sh

@@ -0,0 +1,94 @@
+#!/bin/sh
+
+test_description='anchored diff algorithm'
+
+. ./test-lib.sh
+
+test_expect_success '--anchored' '
+	printf "a\nb\nc\n" >pre &&
+	printf "c\na\nb\n" >post &&
+
+	# normally, c is moved to produce the smallest diff
+	test_expect_code 1 git diff --no-index pre post >diff &&
+	grep "^+c" diff &&
+
+	# with anchor, a is moved
+	test_expect_code 1 git diff --no-index --anchored=c pre post >diff &&
+	grep "^+a" diff
+'
+
+test_expect_success '--anchored multiple' '
+	printf "a\nb\nc\nd\ne\nf\n" >pre &&
+	printf "c\na\nb\nf\nd\ne\n" >post &&
+
+	# with 1 anchor, c is not moved, but f is moved
+	test_expect_code 1 git diff --no-index --anchored=c pre post >diff &&
+	grep "^+a" diff && # a is moved instead of c
+	grep "^+f" diff &&
+
+	# with 2 anchors, c and f are not moved
+	test_expect_code 1 git diff --no-index --anchored=c --anchored=f pre post >diff &&
+	grep "^+a" diff &&
+	grep "^+d" diff # d is moved instead of f
+'
+
+test_expect_success '--anchored with nonexistent line has no effect' '
+	printf "a\nb\nc\n" >pre &&
+	printf "c\na\nb\n" >post &&
+
+	test_expect_code 1 git diff --no-index --anchored=x pre post >diff &&
+	grep "^+c" diff
+'
+
+test_expect_success '--anchored with non-unique line has no effect' '
+	printf "a\nb\nc\nd\ne\nc\n" >pre &&
+	printf "c\na\nb\nc\nd\ne\n" >post &&
+
+	test_expect_code 1 git diff --no-index --anchored=c pre post >diff &&
+	grep "^+c" diff
+'
+
+test_expect_success 'diff still produced with impossible multiple --anchored' '
+	printf "a\nb\nc\n" >pre &&
+	printf "c\na\nb\n" >post &&
+
+	test_expect_code 1 git diff --no-index --anchored=a --anchored=c pre post >diff &&
+	mv post expected_post &&
+
+	# Ensure that the diff is correct by applying it and then
+	# comparing the result with the original
+	git apply diff &&
+	diff expected_post post
+'
+
+test_expect_success 'later algorithm arguments override earlier ones' '
+	printf "a\nb\nc\n" >pre &&
+	printf "c\na\nb\n" >post &&
+
+	test_expect_code 1 git diff --no-index --patience --anchored=c pre post >diff &&
+	grep "^+a" diff &&
+
+	test_expect_code 1 git diff --no-index --anchored=c --patience pre post >diff &&
+	grep "^+c" diff &&
+
+	test_expect_code 1 git diff --no-index --histogram --anchored=c pre post >diff &&
+	grep "^+a" diff &&
+
+	test_expect_code 1 git diff --no-index --anchored=c --histogram pre post >diff &&
+	grep "^+c" diff
+'
+
+test_expect_success '--anchored works with other commands like "git show"' '
+	printf "a\nb\nc\n" >file &&
+	git add file &&
+	git commit -m foo &&
+	printf "c\na\nb\n" >file &&
+	git add file &&
+	git commit -m foo &&
+
+	# with anchor, a is moved
+	git show --patience --anchored=c >diff &&
+	grep "^+a" diff
+'
+
+test_done

xdiff/xdiff.h

@@ -86,6 +86,10 @@ typedef struct s_mmbuffer {
 
 typedef struct s_xpparam {
 	unsigned long flags;
+
+	/* See Documentation/diff-options.txt. */
+	char **anchors;
+	size_t anchors_nr;
 } xpparam_t;
 
 typedef struct s_xdemitcb {

xdiff/xpatience.c

@@ -62,6 +62,12 @@ struct hashmap {
 		 * initially, "next" reflects only the order in file1.
 		 */
 		struct entry *next, *previous;
+
+		/*
+		 * If 1, this entry can serve as an anchor. See
+		 * Documentation/diff-options.txt for more information.
+		 */
+		unsigned anchor : 1;
 	} *entries, *first, *last;
 	/* were common records found? */
 	unsigned long has_matches;
@@ -70,8 +76,19 @@ struct hashmap {
 	xpparam_t const *xpp;
 };
 
+static int is_anchor(xpparam_t const *xpp, const char *line)
+{
+	int i;
+	for (i = 0; i < xpp->anchors_nr; i++) {
+		if (!strncmp(line, xpp->anchors[i], strlen(xpp->anchors[i])))
+			return 1;
+	}
+	return 0;
+}
+
 /* The argument "pass" is 1 for the first file, 2 for the second. */
-static void insert_record(int line, struct hashmap *map, int pass)
+static void insert_record(xpparam_t const *xpp, int line, struct hashmap *map,
+			  int pass)
 {
 	xrecord_t **records = pass == 1 ?
 		map->env->xdf1.recs : map->env->xdf2.recs;
@@ -110,6 +127,7 @@ static void insert_record(int line, struct hashmap *map, int pass)
 		return;
 	map->entries[index].line1 = line;
 	map->entries[index].hash = record->ha;
+	map->entries[index].anchor = is_anchor(xpp, map->env->xdf1.recs[line - 1]->ptr);
 	if (!map->first)
 		map->first = map->entries + index;
 	if (map->last) {
@@ -147,11 +165,11 @@ static int fill_hashmap(mmfile_t *file1, mmfile_t *file2,
 
 	/* First, fill with entries from the first file */
 	while (count1--)
-		insert_record(line1++, result, 1);
+		insert_record(xpp, line1++, result, 1);
 
 	/* Then search for matches in the second file */
 	while (count2--)
-		insert_record(line2++, result, 2);
+		insert_record(xpp, line2++, result, 2);
 
 	return 0;
 }
@@ -192,15 +210,29 @@ static struct entry *find_longest_common_sequence(struct hashmap *map)
 	int longest = 0, i;
 	struct entry *entry;
 
+	/*
+	 * If not -1, this entry in sequence must never be overridden.
+	 * Therefore, overriding entries before this has no effect, so
+	 * do not do that either.
+	 */
+	int anchor_i = -1;
+
 	for (entry = map->first; entry; entry = entry->next) {
 		if (!entry->line2 || entry->line2 == NON_UNIQUE)
 			continue;
 		i = binary_search(sequence, longest, entry);
 		entry->previous = i < 0 ? NULL : sequence[i];
-		sequence[++i] = entry;
+		++i;
-		if (i == longest)
+		if (i <= anchor_i)
+			continue;
+		sequence[i] = entry;
+		if (entry->anchor) {
+			anchor_i = i;
+			longest = anchor_i + 1;
+		} else if (i == longest) {
 			longest++;
 		}
+	}
 
 	/* No common unique lines were found */
 	if (!longest) {