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gitformat-commit-graph: describe version 2 of BDAT
The code change to Git to support version 2 will be done in subsequent commits. Signed-off-by: Jonathan Tan <jonathantanmy@google.com> Signed-off-by: Junio C Hamano <gitster@pobox.com> Signed-off-by: Taylor Blau <me@ttaylorr.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>
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@ -142,13 +142,16 @@ All multi-byte numbers are in network byte order.
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==== Bloom Filter Data (ID: {'B', 'D', 'A', 'T'}) [Optional]
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* It starts with header consisting of three unsigned 32-bit integers:
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- Version of the hash algorithm being used. We currently only support
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value 1 which corresponds to the 32-bit version of the murmur3 hash
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- Version of the hash algorithm being used. We currently support
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value 2 which corresponds to the 32-bit version of the murmur3 hash
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implemented exactly as described in
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https://en.wikipedia.org/wiki/MurmurHash#Algorithm and the double
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hashing technique using seed values 0x293ae76f and 0x7e646e2 as
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described in https://doi.org/10.1007/978-3-540-30494-4_26 "Bloom Filters
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in Probabilistic Verification"
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in Probabilistic Verification". Version 1 Bloom filters have a bug that appears
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when char is signed and the repository has path names that have characters >=
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0x80; Git supports reading and writing them, but this ability will be removed
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in a future version of Git.
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- The number of times a path is hashed and hence the number of bit positions
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that cumulatively determine whether a file is present in the commit.
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- The minimum number of bits 'b' per entry in the Bloom filter. If the filter
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