commit-graph: reuse existing Bloom filters during write

Add logic to
a) parse Bloom filter information from the commit graph file and,
b) re-use existing Bloom filters.

See Documentation/technical/commit-graph-format for the format in which
the Bloom filter information is written to the commit graph file.

To read Bloom filter for a given commit with lexicographic position
'i' we need to:
1. Read BIDX[i] which essentially gives us the starting index in BDAT for
   filter of commit i+1. It is essentially the index past the end
   of the filter of commit i. It is called end_index in the code.

2. For i>0, read BIDX[i-1] which will give us the starting index in BDAT
   for filter of commit i. It is called the start_index in the code.
   For the first commit, where i = 0, Bloom filter data starts at the
   beginning, just past the header in the BDAT chunk. Hence, start_index
   will be 0.

3. The length of the filter will be end_index - start_index, because
   BIDX[i] gives the cumulative 8-byte words including the ith
   commit's filter.

We toggle whether Bloom filters should be recomputed based on the
compute_if_not_present flag.

Helped-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Garima Singh <garima.singh@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

bloom.c

@@ -4,6 +4,8 @@
 #include "diffcore.h"
 #include "revision.h"
 #include "hashmap.h"
+#include "commit-graph.h"
+#include "commit.h"
 
 define_commit_slab(bloom_filter_slab, struct bloom_filter);
 
@@ -26,6 +28,36 @@ static inline unsigned char get_bitmask(uint32_t pos)
 	return ((unsigned char)1) << (pos & (BITS_PER_WORD - 1));
 }
 
+static int load_bloom_filter_from_graph(struct commit_graph *g,
+				   struct bloom_filter *filter,
+				   struct commit *c)
+{
+	uint32_t lex_pos, start_index, end_index;
+
+	while (c->graph_pos < g->num_commits_in_base)
+		g = g->base_graph;
+
+	/* The commit graph commit 'c' lives in doesn't carry bloom filters. */
+	if (!g->chunk_bloom_indexes)
+		return 0;
+
+	lex_pos = c->graph_pos - g->num_commits_in_base;
+
+	end_index = get_be32(g->chunk_bloom_indexes + 4 * lex_pos);
+
+	if (lex_pos > 0)
+		start_index = get_be32(g->chunk_bloom_indexes + 4 * (lex_pos - 1));
+	else
+		start_index = 0;
+
+	filter->len = end_index - start_index;
+	filter->data = (unsigned char *)(g->chunk_bloom_data +
+					sizeof(unsigned char) * start_index +
+					BLOOMDATA_CHUNK_HEADER_SIZE);
+
+	return 1;
+}
+
 /*
  * Calculate the murmur3 32-bit hash value for the given data
  * using the given seed.
@@ -127,7 +159,8 @@ void init_bloom_filters(void)
 }
 
 struct bloom_filter *get_bloom_filter(struct repository *r,
-				      struct commit *c)
+				      struct commit *c,
+					  int compute_if_not_present)
 {
 	struct bloom_filter *filter;
 	struct bloom_filter_settings settings = DEFAULT_BLOOM_FILTER_SETTINGS;
@@ -140,6 +173,20 @@ struct bloom_filter *get_bloom_filter(struct repository *r,
 
 	filter = bloom_filter_slab_at(&bloom_filters, c);
 
+	if (!filter->data) {
+		load_commit_graph_info(r, c);
+		if (c->graph_pos != COMMIT_NOT_FROM_GRAPH &&
+			r->objects->commit_graph->chunk_bloom_indexes) {
+			if (load_bloom_filter_from_graph(r->objects->commit_graph, filter, c))
+				return filter;
+			else
+				return NULL;
+		}
+	}
+
+	if (filter->data || !compute_if_not_present)
+		return filter;
+
 	repo_diff_setup(r, &diffopt);
 	diffopt.flags.recursive = 1;
 	diffopt.max_changes = max_changes;