+static int path_matches(struct cache_entry *a, struct cache_entry *b)
+{
+ int len = ce_namelen(a);
+ return ce_namelen(b) == len &&
+ !memcmp(a->name, b->name, len);
+}
+
+static int same(struct cache_entry *a, struct cache_entry *b)
+{
+ return a->ce_mode == b->ce_mode &&
+ !memcmp(a->sha1, b->sha1, 20);
+}
+
+
+/*
+ * This removes all trivial merges that don't change the tree
+ * and collapses them to state 0.
+ *
+ * _Any_ other merge is left to user policy. That includes "both
+ * created the same file", and "both removed the same file" - which are
+ * trivial, but the user might still want to _note_ it.
+ */
+static struct cache_entry *merge_entries(struct cache_entry *a,
+ struct cache_entry *b,
+ struct cache_entry *c)
+{
+ int len = ce_namelen(a);
+
+ /*
+ * Are they all the same filename? We won't do
+ * any name merging
+ */
+ if (ce_namelen(b) != len ||
+ ce_namelen(c) != len ||
+ memcmp(a->name, b->name, len) ||
+ memcmp(a->name, c->name, len))
+ return NULL;
+
+ /*
+ * Ok, all three entries describe the same
+ * filename, but maybe the contents or file
+ * mode have changed?
+ *
+ * The trivial cases end up being the ones where two
+ * out of three files are the same:
+ * - both destinations the same, trivially take either
+ * - one of the destination versions hasn't changed,
+ * take the other.
+ *
+ * The "all entries exactly the same" case falls out as
+ * a special case of any of the "two same" cases.
+ *
+ * Here "a" is "original", and "b" and "c" are the two
+ * trees we are merging.
+ */
+ if (same(b,c))
+ return c;
+ if (same(a,b))
+ return c;
+ if (same(a,c))
+ return b;
+ return NULL;
+}
+
+static void trivially_merge_cache(struct cache_entry **src, int nr)
+{
+ static struct cache_entry null_entry;
+ struct cache_entry **dst = src;
+ struct cache_entry *old = &null_entry;
+
+ while (nr) {
+ struct cache_entry *ce, *result;
+
+ ce = src[0];
+
+ /* We throw away original cache entries except for the stat information */
+ if (!ce_stage(ce)) {
+ old = ce;
+ src++;
+ nr--;
+ active_nr--;
+ continue;
+ }
+ if (nr > 2 && (result = merge_entries(ce, src[1], src[2])) != NULL) {
+ /*
+ * See if we can re-use the old CE directly?
+ * That way we get the uptodate stat info.
+ */
+ if (path_matches(result, old) && same(result, old))
+ *result = *old;
+ ce = result;
+ ce->ce_flags &= ~htons(CE_STAGEMASK);
+ src += 2;
+ nr -= 2;
+ active_nr -= 2;
+ }
+ *dst = ce;
+ src++;
+ dst++;
+ nr--;
+ }
+}
+