[PATCH v2 07/20] merge-ort: avoid repeating fill_tree_descriptor() on the same tree

[Elijah Newren <newren@gmail.com>]

Three-way merges, by their nature, are going to often have two or more
trees match at a given subdirectory.  We can avoid calling
fill_tree_descriptor() on the same tree by checking when these trees
match.  Noting when various oids match will also be useful in other
calculations and optimizations as well.

Signed-off-by: Elijah Newren <newren@gmail.com>
---
 merge-ort.c | 26 ++++++++++++++++++++++----
 1 file changed, 22 insertions(+), 4 deletions(-)

diff --git a/merge-ort.c b/merge-ort.c
index 626eb9713e..d3c1d00fc6 100644
--- a/merge-ort.c
+++ b/merge-ort.c
@@ -99,6 +99,15 @@ static int collect_merge_info_callback(int n,
 	unsigned mbase_null = !(mask & 1);
 	unsigned side1_null = !(mask & 2);
 	unsigned side2_null = !(mask & 4);
+	unsigned side1_matches_mbase = (!side1_null && !mbase_null &&
+					names[0].mode == names[1].mode &&
+					oideq(&names[0].oid, &names[1].oid));
+	unsigned side2_matches_mbase = (!side2_null && !mbase_null &&
+					names[0].mode == names[2].mode &&
+					oideq(&names[0].oid, &names[2].oid));
+	unsigned sides_match = (!side1_null && !side2_null &&
+				names[1].mode == names[2].mode &&
+				oideq(&names[1].oid, &names[2].oid));
 
 	/* n = 3 is a fundamental assumption. */
 	if (n != 3)
@@ -154,10 +163,19 @@ static int collect_merge_info_callback(int n,
 		newinfo.pathlen = st_add3(newinfo.pathlen, p->pathlen, 1);
 
 		for (i = 0; i < 3; i++, dirmask >>= 1) {
-			const struct object_id *oid = NULL;
-			if (dirmask & 1)
-				oid = &names[i].oid;
-			buf[i] = fill_tree_descriptor(opt->repo, t + i, oid);
+			if (i == 1 && side1_matches_mbase)
+				t[1] = t[0];
+			else if (i == 2 && side2_matches_mbase)
+				t[2] = t[0];
+			else if (i == 2 && sides_match)
+				t[2] = t[1];
+			else {
+				const struct object_id *oid = NULL;
+				if (dirmask & 1)
+					oid = &names[i].oid;
+				buf[i] = fill_tree_descriptor(opt->repo,
+							      t + i, oid);
+			}
 		}
 
 		original_dir_name = opti->current_dir_name;
-- 
2.29.0.471.ga4f56089c0