bloom.c: core Bloom filter implementation for changed paths.

Add the core implementation for computing Bloom filters for
the paths changed between a commit and it's first parent.

We fill the Bloom filters as (const char *data, int len) pairs
as `struct bloom_filters" within a commit slab.

Filters for commits with no changes and more than 512 changes,
is represented with a filter of length zero. There is no gain
in distinguishing between a computed filter of length zero for
a commit with no changes, and an uncomputed filter for new commits
or for commits with more than 512 changes. The effect on
`git log -- path` is the same in both cases. We will fall back to
the normal diffing algorithm when we can't benefit from the
existence of Bloom filters.

Helped-by: Jeff King <peff@peff.net>
Helped-by: Derrick Stolee <dstolee@microsoft.com>
Reviewed-by: Jakub Narębski <jnareb@gmail.com>
Signed-off-by: Garima Singh <garima.singh@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

bloom.c

@@ -1,5 +1,18 @@
 #include "git-compat-util.h"
 #include "bloom.h"
+#include "diff.h"
+#include "diffcore.h"
+#include "revision.h"
+#include "hashmap.h"
+
+define_commit_slab(bloom_filter_slab, struct bloom_filter);
+
+struct bloom_filter_slab bloom_filters;
+
+struct pathmap_hash_entry {
+    struct hashmap_entry entry;
+    const char path[FLEX_ARRAY];
+};
 
 static uint32_t rotate_left(uint32_t value, int32_t count)
 {
@@ -107,3 +120,87 @@ void add_key_to_filter(const struct bloom_key *key,
 		filter->data[block_pos] |= get_bitmask(hash_mod);
 	}
 }
+
+void init_bloom_filters(void)
+{
+	init_bloom_filter_slab(&bloom_filters);
+}
+
+struct bloom_filter *get_bloom_filter(struct repository *r,
+				      struct commit *c)
+{
+	struct bloom_filter *filter;
+	struct bloom_filter_settings settings = DEFAULT_BLOOM_FILTER_SETTINGS;
+	int i;
+	struct diff_options diffopt;
+
+	if (bloom_filters.slab_size == 0)
+		return NULL;
+
+	filter = bloom_filter_slab_at(&bloom_filters, c);
+
+	repo_diff_setup(r, &diffopt);
+	diffopt.flags.recursive = 1;
+	diff_setup_done(&diffopt);
+
+	if (c->parents)
+		diff_tree_oid(&c->parents->item->object.oid, &c->object.oid, "", &diffopt);
+	else
+		diff_tree_oid(NULL, &c->object.oid, "", &diffopt);
+	diffcore_std(&diffopt);
+
+	if (diff_queued_diff.nr <= 512) {
+		struct hashmap pathmap;
+		struct pathmap_hash_entry *e;
+		struct hashmap_iter iter;
+		hashmap_init(&pathmap, NULL, NULL, 0);
+
+		for (i = 0; i < diff_queued_diff.nr; i++) {
+			const char *path = diff_queued_diff.queue[i]->two->path;
+
+			/*
+			* Add each leading directory of the changed file, i.e. for
+			* 'dir/subdir/file' add 'dir' and 'dir/subdir' as well, so
+			* the Bloom filter could be used to speed up commands like
+			* 'git log dir/subdir', too.
+			*
+			* Note that directories are added without the trailing '/'.
+			*/
+			do {
+				char *last_slash = strrchr(path, '/');
+
+				FLEX_ALLOC_STR(e, path, path);
+				hashmap_entry_init(&e->entry, strhash(path));
+				hashmap_add(&pathmap, &e->entry);
+
+				if (!last_slash)
+					last_slash = (char*)path;
+				*last_slash = '\0';
+
+			} while (*path);
+
+			diff_free_filepair(diff_queued_diff.queue[i]);
+		}
+
+		filter->len = (hashmap_get_size(&pathmap) * settings.bits_per_entry + BITS_PER_WORD - 1) / BITS_PER_WORD;
+		filter->data = xcalloc(filter->len, sizeof(unsigned char));
+
+		hashmap_for_each_entry(&pathmap, &iter, e, entry) {
+			struct bloom_key key;
+			fill_bloom_key(e->path, strlen(e->path), &key, &settings);
+			add_key_to_filter(&key, filter, &settings);
+		}
+
+		hashmap_free_entries(&pathmap, struct pathmap_hash_entry, entry);
+	} else {
+		for (i = 0; i < diff_queued_diff.nr; i++)
+			diff_free_filepair(diff_queued_diff.queue[i]);
+		filter->data = NULL;
+		filter->len = 0;
+	}
+
+	free(diff_queued_diff.queue);
+	DIFF_QUEUE_CLEAR(&diff_queued_diff);
+
+	return filter;
+}