164 lines
5.4 KiB
Text
164 lines
5.4 KiB
Text
GIT bitmap v1 format
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====================
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- A header appears at the beginning:
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4-byte signature: {'B', 'I', 'T', 'M'}
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2-byte version number (network byte order)
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The current implementation only supports version 1
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of the bitmap index (the same one as JGit).
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2-byte flags (network byte order)
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The following flags are supported:
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- BITMAP_OPT_FULL_DAG (0x1) REQUIRED
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This flag must always be present. It implies that the bitmap
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index has been generated for a packfile with full closure
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(i.e. where every single object in the packfile can find
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its parent links inside the same packfile). This is a
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requirement for the bitmap index format, also present in JGit,
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that greatly reduces the complexity of the implementation.
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- BITMAP_OPT_HASH_CACHE (0x4)
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If present, the end of the bitmap file contains
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`N` 32-bit name-hash values, one per object in the
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pack. The format and meaning of the name-hash is
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described below.
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4-byte entry count (network byte order)
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The total count of entries (bitmapped commits) in this bitmap index.
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20-byte checksum
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The SHA1 checksum of the pack this bitmap index belongs to.
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- 4 EWAH bitmaps that act as type indexes
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Type indexes are serialized after the hash cache in the shape
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of four EWAH bitmaps stored consecutively (see Appendix A for
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the serialization format of an EWAH bitmap).
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There is a bitmap for each Git object type, stored in the following
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order:
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- Commits
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- Trees
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- Blobs
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- Tags
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In each bitmap, the `n`th bit is set to true if the `n`th object
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in the packfile is of that type.
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The obvious consequence is that the OR of all 4 bitmaps will result
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in a full set (all bits set), and the AND of all 4 bitmaps will
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result in an empty bitmap (no bits set).
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- N entries with compressed bitmaps, one for each indexed commit
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Where `N` is the total amount of entries in this bitmap index.
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Each entry contains the following:
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- 4-byte object position (network byte order)
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The position **in the index for the packfile** where the
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bitmap for this commit is found.
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- 1-byte XOR-offset
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The xor offset used to compress this bitmap. For an entry
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in position `x`, a XOR offset of `y` means that the actual
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bitmap representing this commit is composed by XORing the
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bitmap for this entry with the bitmap in entry `x-y` (i.e.
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the bitmap `y` entries before this one).
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Note that this compression can be recursive. In order to
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XOR this entry with a previous one, the previous entry needs
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to be decompressed first, and so on.
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The hard-limit for this offset is 160 (an entry can only be
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xor'ed against one of the 160 entries preceding it). This
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number is always positive, and hence entries are always xor'ed
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with **previous** bitmaps, not bitmaps that will come afterwards
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in the index.
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- 1-byte flags for this bitmap
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At the moment the only available flag is `0x1`, which hints
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that this bitmap can be re-used when rebuilding bitmap indexes
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for the repository.
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- The compressed bitmap itself, see Appendix A.
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== Appendix A: Serialization format for an EWAH bitmap
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Ewah bitmaps are serialized in the same protocol as the JAVAEWAH
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library, making them backwards compatible with the JGit
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implementation:
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- 4-byte number of bits of the resulting UNCOMPRESSED bitmap
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- 4-byte number of words of the COMPRESSED bitmap, when stored
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- N x 8-byte words, as specified by the previous field
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This is the actual content of the compressed bitmap.
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- 4-byte position of the current RLW for the compressed
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bitmap
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All words are stored in network byte order for their corresponding
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sizes.
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The compressed bitmap is stored in a form of run-length encoding, as
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follows. It consists of a concatenation of an arbitrary number of
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chunks. Each chunk consists of one or more 64-bit words
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H L_1 L_2 L_3 .... L_M
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H is called RLW (run length word). It consists of (from lower to higher
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order bits):
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- 1 bit: the repeated bit B
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- 32 bits: repetition count K (unsigned)
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- 31 bits: literal word count M (unsigned)
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The bitstream represented by the above chunk is then:
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- K repetitions of B
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- The bits stored in `L_1` through `L_M`. Within a word, bits at
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lower order come earlier in the stream than those at higher
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order.
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The next word after `L_M` (if any) must again be a RLW, for the next
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chunk. For efficient appending to the bitstream, the EWAH stores a
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pointer to the last RLW in the stream.
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== Appendix B: Optional Bitmap Sections
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These sections may or may not be present in the `.bitmap` file; their
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presence is indicated by the header flags section described above.
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Name-hash cache
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---------------
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If the BITMAP_OPT_HASH_CACHE flag is set, the end of the bitmap contains
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a cache of 32-bit values, one per object in the pack. The value at
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position `i` is the hash of the pathname at which the `i`th object
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(counting in index order) in the pack can be found. This can be fed
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into the delta heuristics to compare objects with similar pathnames.
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The hash algorithm used is:
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hash = 0;
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while ((c = *name++))
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if (!isspace(c))
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hash = (hash >> 2) + (c << 24);
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Note that this hashing scheme is tied to the BITMAP_OPT_HASH_CACHE flag.
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If implementations want to choose a different hashing scheme, they are
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free to do so, but MUST allocate a new header flag (because comparing
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hashes made under two different schemes would be pointless).
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