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Documentation: changed-path Bloom filters use byte words
In Documentation/technical/commit-graph-format.txt, the definition of the BIDX chunk specifies the length is a number of 8-byte words. During development we discovered that using 8-byte words in the Murmur3 hash algorithm causes issues with big-endian versus little- endian machines. Thus, the hash algorithm was adapted to work on a byte-by-byte basis. However, this caused a change in the definition of a "word" in bloom.h. Now, a "word" is a single byte, which allows filters to be as small as two bytes. These length-two filters are demonstrated in t0095-bloom.sh, and a larger filter of length 25 is demonstrated as well. The original point of using 8-byte words was for alignment reasons. It also presented opportunities for extremely sparse Bloom filters when there were a small number of changes at a commit, creating a very low false-positive rate. However, modifying the format at this point is unlikely to be a valuable exercise. Also, this use of single-byte granularity does present opportunities to save space. It is unclear if 8-byte alignment of the filters would present any meaningful performance benefits. Modify the format document to reflect reality. Signed-off-by: Derrick Stolee <dstolee@microsoft.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>
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@ -97,10 +97,10 @@ CHUNK DATA:
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bit on. The other bits correspond to the position of the last parent.
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Bloom Filter Index (ID: {'B', 'I', 'D', 'X'}) (N * 4 bytes) [Optional]
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* The ith entry, BIDX[i], stores the number of 8-byte word blocks in all
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Bloom filters from commit 0 to commit i (inclusive) in lexicographic
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order. The Bloom filter for the i-th commit spans from BIDX[i-1] to
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BIDX[i] (plus header length), where BIDX[-1] is 0.
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* The ith entry, BIDX[i], stores the number of bytes in all Bloom filters
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from commit 0 to commit i (inclusive) in lexicographic order. The Bloom
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filter for the i-th commit spans from BIDX[i-1] to BIDX[i] (plus header
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length), where BIDX[-1] is 0.
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* The BIDX chunk is ignored if the BDAT chunk is not present.
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Bloom Filter Data (ID: {'B', 'D', 'A', 'T'}) [Optional]
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