merge-recursive: optimize time complexity for process_renames

Avoid O(n^2) complexity in `process_renames()` when building a sorted
`string_list` by constructing it unsorted and sorting it afterward,
reducing the complexity to O(n log n).

Signed-off-by: Meet Soni <meetsoni3017@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

diff --git a/merge-recursive.c b/merge-recursive.c
index 5dfaf32b2c18809209da4a0fa306b23b7e08c98e..884ccf99a58d3b41a5d592ddb1167bc011a3d965 100644 (file)
--- a/merge-recursive.c
+++ b/merge-recursive.c
@@ -2758,23 +2758,22 @@ static int process_renames(struct merge_options *opt,
        const struct rename *sre;
 
        /*
-        * FIXME: As string-list.h notes, it's O(n^2) to build a sorted
-        * string_list one-by-one, but O(n log n) to build it unsorted and
-        * then sort it.  Note that as we build the list, we do not need to
-        * check if the existing destination path is already in the list,
-        * because the structure of diffcore_rename guarantees we won't
-        * have duplicates.
+        * Note that as we build the list, we do not need to check if the
+        * existing destination path is already in the list, because the
+        * structure of diffcore_rename guarantees we won't have duplicates.
         */
        for (i = 0; i < a_renames->nr; i++) {
                sre = a_renames->items[i].util;
-               string_list_insert(&a_by_dst, sre->pair->two->path)->util
+               string_list_append(&a_by_dst, sre->pair->two->path)->util
                        = (void *)sre;
        }
        for (i = 0; i < b_renames->nr; i++) {
                sre = b_renames->items[i].util;
-               string_list_insert(&b_by_dst, sre->pair->two->path)->util
+               string_list_append(&b_by_dst, sre->pair->two->path)->util
                        = (void *)sre;
        }
+       string_list_sort(&a_by_dst);
+       string_list_sort(&b_by_dst);
 
        for (i = 0, j = 0; i < a_renames->nr || j < b_renames->nr;) {
                struct string_list *renames1, *renames2Dst;