bloom: de-duplicate directory entries

When computing a changed-path Bloom filter, we need to take the
files that changed from the diff computation and extract the parent
directories. That way, a directory pathspec such as "Documentation"
could match commits that change "Documentation/git.txt".

However, the current code does a poor job of this process. The paths
are added to a hashmap, but we do not check if an entry already
exists with that path. This can create many duplicate entries and
cause the filter to have a much larger length than it should. This
means that the filter is more sparse than intended, which helps the
false positive rate, but wastes a lot of space.

Properly use hashmap_get() before hashmap_add(). Also be sure to
include a comparison function so these can be matched correctly.

This has an effect on a test in t0095-bloom.sh. This makes sense,
there are ten changes inside "smallDir" so the total number of
paths in the filter should be 11. This would result in 11 * 10 bits
required, and with 8 bits per byte, this results in 14 bytes.

Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

bloom.c

@@ -158,6 +158,19 @@ void init_bloom_filters(void)
 	init_bloom_filter_slab(&bloom_filters);
 }
 
+static int pathmap_cmp(const void *hashmap_cmp_fn_data,
+		       const struct hashmap_entry *eptr,
+		       const struct hashmap_entry *entry_or_key,
+		       const void *keydata)
+{
+	const struct pathmap_hash_entry *e1, *e2;
+
+	e1 = container_of(eptr, const struct pathmap_hash_entry, entry);
+	e2 = container_of(entry_or_key, const struct pathmap_hash_entry, entry);
+
+	return strcmp(e1->path, e2->path);
+}
+
 struct bloom_filter *get_bloom_filter(struct repository *r,
 				      struct commit *c,
 				      int compute_if_not_present)
@@ -206,7 +219,7 @@ struct bloom_filter *get_bloom_filter(struct repository *r,
 		struct hashmap pathmap;
 		struct pathmap_hash_entry *e;
 		struct hashmap_iter iter;
-		hashmap_init(&pathmap, NULL, NULL, 0);
+		hashmap_init(&pathmap, pathmap_cmp, NULL, 0);
 
 		for (i = 0; i < diff_queued_diff.nr; i++) {
 			const char *path = diff_queued_diff.queue[i]->two->path;
@@ -224,7 +237,11 @@ struct bloom_filter *get_bloom_filter(struct repository *r,
 
 				FLEX_ALLOC_STR(e, path, path);
 				hashmap_entry_init(&e->entry, strhash(path));
+
+				if (!hashmap_get(&pathmap, &e->entry, NULL))
 					hashmap_add(&pathmap, &e->entry);
+				else
+					free(e);
 
 				if (!last_slash)
 					last_slash = (char*)path;

t/t0095-bloom.sh

@@ -89,8 +89,8 @@ test_expect_success 'get bloom filter for commit with 10 changes' '
 	git add smallDir &&
 	git commit -m "commit with 10 changes" &&
 	cat >expect <<-\EOF &&
-	Filter_Length:25
-	Filter_Data:82|a0|65|47|0c|92|90|c0|a1|40|02|a0|e2|40|e0|04|0a|9a|66|cf|80|19|85|42|23|
+	Filter_Length:14
+	Filter_Data:02|b3|c4|a0|34|e7|fe|eb|cb|47|fe|a0|e8|72|
 	EOF
 	test-tool bloom get_filter_for_commit "$(git rev-parse HEAD)" >actual &&
 	test_cmp expect actual