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diff --git a/third_party/git/Documentation/technical/bitmap-format.txt b/third_party/git/Documentation/technical/bitmap-format.txt deleted file mode 100644 index f8c18a0f7aec..000000000000 --- a/third_party/git/Documentation/technical/bitmap-format.txt +++ /dev/null @@ -1,164 +0,0 @@ -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). |