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>

1 files changed, 1 insertions, 1 deletions

diff --git a/t/helper/test-bloom.c b/t/helper/test-bloom.c
index f18d1b722e..ce412664ba 100644
--- a/t/helper/test-bloom.c
+++ b/t/helper/test-bloom.c
@@ -39,7 +39,7 @@ static void get_bloom_filter_for_commit(const struct object_id *commit_oid)
 	struct bloom_filter *filter;
 	setup_git_directory();
 	c = lookup_commit(the_repository, commit_oid);
-	filter = get_bloom_filter(the_repository, c);
+	filter = get_bloom_filter(the_repository, c, 1);
 	print_bloom_filter(filter);
 }