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diff --git a/Documentation/technical/bitmap-format.txt b/Documentation/technical/bitmap-format.txt new file mode 100644 index 0000000000..f8c18a0f7a --- /dev/null +++ b/Documentation/technical/bitmap-format.txt @@ -0,0 +1,164 @@ +GIT bitmap v1 format +==================== + + - A header appears at the beginning: + + 4-byte signature: {'B', 'I', 'T', 'M'} + + 2-byte version number (network byte order) + The current implementation only supports version 1 + of the bitmap index (the same one as JGit). + + 2-byte flags (network byte order) + + The following flags are supported: + + - BITMAP_OPT_FULL_DAG (0x1) REQUIRED + This flag must always be present. It implies that the bitmap + index has been generated for a packfile with full closure + (i.e. where every single object in the packfile can find + its parent links inside the same packfile). This is a + requirement for the bitmap index format, also present in JGit, + that greatly reduces the complexity of the implementation. + + - BITMAP_OPT_HASH_CACHE (0x4) + If present, the end of the bitmap file contains + `N` 32-bit name-hash values, one per object in the + pack. The format and meaning of the name-hash is + described below. + + 4-byte entry count (network byte order) + + The total count of entries (bitmapped commits) in this bitmap index. + + 20-byte checksum + + The SHA1 checksum of the pack this bitmap index belongs to. + + - 4 EWAH bitmaps that act as type indexes + + Type indexes are serialized after the hash cache in the shape + of four EWAH bitmaps stored consecutively (see Appendix A for + the serialization format of an EWAH bitmap). + + There is a bitmap for each Git object type, stored in the following + order: + + - Commits + - Trees + - Blobs + - Tags + + In each bitmap, the `n`th bit is set to true if the `n`th object + in the packfile is of that type. + + The obvious consequence is that the OR of all 4 bitmaps will result + in a full set (all bits set), and the AND of all 4 bitmaps will + result in an empty bitmap (no bits set). + + - N entries with compressed bitmaps, one for each indexed commit + + Where `N` is the total amount of entries in this bitmap index. + Each entry contains the following: + + - 4-byte object position (network byte order) + The position **in the index for the packfile** where the + bitmap for this commit is found. + + - 1-byte XOR-offset + The xor offset used to compress this bitmap. For an entry + in position `x`, a XOR offset of `y` means that the actual + bitmap representing this commit is composed by XORing the + bitmap for this entry with the bitmap in entry `x-y` (i.e. + the bitmap `y` entries before this one). + + Note that this compression can be recursive. In order to + XOR this entry with a previous one, the previous entry needs + to be decompressed first, and so on. + + The hard-limit for this offset is 160 (an entry can only be + xor'ed against one of the 160 entries preceding it). This + number is always positive, and hence entries are always xor'ed + with **previous** bitmaps, not bitmaps that will come afterwards + in the index. + + - 1-byte flags for this bitmap + At the moment the only available flag is `0x1`, which hints + that this bitmap can be re-used when rebuilding bitmap indexes + for the repository. + + - The compressed bitmap itself, see Appendix A. + +== Appendix A: Serialization format for an EWAH bitmap + +Ewah bitmaps are serialized in the same protocol as the JAVAEWAH +library, making them backwards compatible with the JGit +implementation: + + - 4-byte number of bits of the resulting UNCOMPRESSED bitmap + + - 4-byte number of words of the COMPRESSED bitmap, when stored + + - N x 8-byte words, as specified by the previous field + + This is the actual content of the compressed bitmap. + + - 4-byte position of the current RLW for the compressed + bitmap + +All words are stored in network byte order for their corresponding +sizes. + +The compressed bitmap is stored in a form of run-length encoding, as +follows. It consists of a concatenation of an arbitrary number of +chunks. Each chunk consists of one or more 64-bit words + + H L_1 L_2 L_3 .... L_M + +H is called RLW (run length word). It consists of (from lower to higher +order bits): + + - 1 bit: the repeated bit B + + - 32 bits: repetition count K (unsigned) + + - 31 bits: literal word count M (unsigned) + +The bitstream represented by the above chunk is then: + + - K repetitions of B + + - The bits stored in `L_1` through `L_M`. Within a word, bits at + lower order come earlier in the stream than those at higher + order. + +The next word after `L_M` (if any) must again be a RLW, for the next +chunk. For efficient appending to the bitstream, the EWAH stores a +pointer to the last RLW in the stream. + + +== Appendix B: Optional Bitmap Sections + +These sections may or may not be present in the `.bitmap` file; their +presence is indicated by the header flags section described above. + +Name-hash cache +--------------- + +If the BITMAP_OPT_HASH_CACHE flag is set, the end of the bitmap contains +a cache of 32-bit values, one per object in the pack. The value at +position `i` is the hash of the pathname at which the `i`th object +(counting in index order) in the pack can be found. This can be fed +into the delta heuristics to compare objects with similar pathnames. + +The hash algorithm used is: + + hash = 0; + while ((c = *name++)) + if (!isspace(c)) + hash = (hash >> 2) + (c << 24); + +Note that this hashing scheme is tied to the BITMAP_OPT_HASH_CACHE flag. +If implementations want to choose a different hashing scheme, they are +free to do so, but MUST allocate a new header flag (because comparing +hashes made under two different schemes would be pointless). |