diff --anchored: avoid checking unmatched lines

For a line to be an anchor it has to appear in each of the files being
diffed exactly once. With that in mind lets delay checking whether
a line is an anchor until we know there is exactly one instance of
the line in each file. As each line is checked at most once, there
is no need to cache the result of is_anchor() and we can drop that
field from the hashmap entries. When diffing 5000 recent commits in
git.git this gives a modest speedup of ~2%. In the (rather extreme)
example below that consists largely of deletions the speedup is ~16%.

    seq 0 10000000 >old
    printf '%s\n' 300000 100000 200000 >new
    git diff --no-index --anchored=300000 old new

Signed-off-by: Phillip Wood <phillip.wood@dunelm.org.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: phillipwood

xdiff/xpatience.c

@@ -61,12 +61,6 @@ 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.adoc for more information.
-		 */
-		unsigned anchor : 1;
 	} *entries, *first, *last;
 	/* were common records found? */
 	unsigned long has_matches;
@@ -85,8 +79,7 @@ static int is_anchor(xpparam_t const *xpp, const char *line)
 }
 
 /* The argument "pass" is 1 for the first file, 2 for the second. */
-static void insert_record(xpparam_t const *xpp, int line, struct hashmap *map,
-			  int pass)
+static void insert_record(int line, struct hashmap *map, int pass)
 {
 	xrecord_t *records = pass == 1 ?
 		map->env->xdf1.recs : map->env->xdf2.recs;
@@ -121,7 +114,6 @@ static void insert_record(xpparam_t const *xpp, int line, struct hashmap *map,
 		return;
 	map->entries[index].line1 = line;
 	map->entries[index].minimal_perfect_hash = record->minimal_perfect_hash;
-	map->entries[index].anchor = is_anchor(xpp, (const char *)map->env->xdf1.recs[line - 1].ptr);
 	if (!map->first)
 		map->first = map->entries + index;
 	if (map->last) {
@@ -153,11 +145,11 @@ static int fill_hashmap(xpparam_t const *xpp, xdfenv_t *env,
 
 	/* First, fill with entries from the first file */
 	while (count1--)
-		insert_record(xpp, line1++, result, 1);
+		insert_record(line1++, result, 1);
 
 	/* Then search for matches in the second file */
 	while (count2--)
-		insert_record(xpp, line2++, result, 2);
+		insert_record(line2++, result, 2);
 
 	return 0;
 }
@@ -194,6 +186,8 @@ static int binary_search(struct entry **sequence, int longest,
  */
 static int find_longest_common_sequence(struct hashmap *map, struct entry **res)
 {
+	xpparam_t const *xpp = map->xpp;
+	xrecord_t const *recs = map->env->xdf2.recs;
 	struct entry **sequence;
 	int longest = 0, i;
 	struct entry *entry;
@@ -220,7 +214,7 @@ static int find_longest_common_sequence(struct hashmap *map, struct entry **res)
 		if (i <= anchor_i)
 			continue;
 		sequence[i] = entry;
-		if (entry->anchor) {
+		if (is_anchor(xpp, (const char*)recs[entry->line2 - 1].ptr)) {
 			anchor_i = i;
 			longest = anchor_i + 1;
 		} else if (i == longest) {