Documentation/technical/bitmap-format.txt

   1 GIT bitmap v1 format
   2 ====================
   3
   4         - A header appears at the beginning:
   5
   6                 4-byte signature: {'B', 'I', 'T', 'M'}
   7
   8                 2-byte version number (network byte order)
   9                         The current implementation only supports version 1
  10                         of the bitmap index (the same one as JGit).
  11
  12                 2-byte flags (network byte order)
  13
  14                         The following flags are supported:
  15
  16                         - BITMAP_OPT_FULL_DAG (0x1) REQUIRED
  17                         This flag must always be present. It implies that the bitmap
  18                         index has been generated for a packfile with full closure
  19                         (i.e. where every single object in the packfile can find
  20                          its parent links inside the same packfile). This is a
  21                         requirement for the bitmap index format, also present in JGit,
  22                         that greatly reduces the complexity of the implementation.
  23
  24                         - BITMAP_OPT_HASH_CACHE (0x4)
  25                         If present, the end of the bitmap file contains
  26                         `N` 32-bit name-hash values, one per object in the
  27                         pack. The format and meaning of the name-hash is
  28                         described below.
  29
  30                 4-byte entry count (network byte order)
  31
  32                         The total count of entries (bitmapped commits) in this bitmap index.
  33
  34                 20-byte checksum
  35
  36                         The SHA1 checksum of the pack this bitmap index belongs to.
  37
  38         - 4 EWAH bitmaps that act as type indexes
  39
  40                 Type indexes are serialized after the hash cache in the shape
  41                 of four EWAH bitmaps stored consecutively (see Appendix A for
  42                 the serialization format of an EWAH bitmap).
  43
  44                 There is a bitmap for each Git object type, stored in the following
  45                 order:
  46
  47                         - Commits
  48                         - Trees
  49                         - Blobs
  50                         - Tags
  51
  52                 In each bitmap, the `n`th bit is set to true if the `n`th object
  53                 in the packfile is of that type.
  54
  55                 The obvious consequence is that the OR of all 4 bitmaps will result
  56                 in a full set (all bits set), and the AND of all 4 bitmaps will
  57                 result in an empty bitmap (no bits set).
  58
  59         - N entries with compressed bitmaps, one for each indexed commit
  60
  61                 Where `N` is the total amount of entries in this bitmap index.
  62                 Each entry contains the following:
  63
  64                 - 4-byte object position (network byte order)
  65                         The position **in the index for the packfile** where the
  66                         bitmap for this commit is found.
  67
  68                 - 1-byte XOR-offset
  69                         The xor offset used to compress this bitmap. For an entry
  70                         in position `x`, a XOR offset of `y` means that the actual
  71                         bitmap representing this commit is composed by XORing the
  72                         bitmap for this entry with the bitmap in entry `x-y` (i.e.
  73                         the bitmap `y` entries before this one).
  74
  75                         Note that this compression can be recursive. In order to
  76                         XOR this entry with a previous one, the previous entry needs
  77                         to be decompressed first, and so on.
  78
  79                         The hard-limit for this offset is 160 (an entry can only be
  80                         xor'ed against one of the 160 entries preceding it). This
  81                         number is always positive, and hence entries are always xor'ed
  82                         with **previous** bitmaps, not bitmaps that will come afterwards
  83                         in the index.
  84
  85                 - 1-byte flags for this bitmap
  86                         At the moment the only available flag is `0x1`, which hints
  87                         that this bitmap can be re-used when rebuilding bitmap indexes
  88                         for the repository.
  89
  90                 - The compressed bitmap itself, see Appendix A.
  91
  92 == Appendix A: Serialization format for an EWAH bitmap
  93
  94 Ewah bitmaps are serialized in the same protocol as the JAVAEWAH
  95 library, making them backwards compatible with the JGit
  96 implementation:
  97
  98         - 4-byte number of bits of the resulting UNCOMPRESSED bitmap
  99
 100         - 4-byte number of words of the COMPRESSED bitmap, when stored
 101
 102         - N x 8-byte words, as specified by the previous field
 103
 104                 This is the actual content of the compressed bitmap.
 105
 106         - 4-byte position of the current RLW for the compressed
 107                 bitmap
 108
 109 All words are stored in network byte order for their corresponding
 110 sizes.
 111
 112 The compressed bitmap is stored in a form of run-length encoding, as
 113 follows.  It consists of a concatenation of an arbitrary number of
 114 chunks.  Each chunk consists of one or more 64-bit words
 115
 116      H  L_1  L_2  L_3 .... L_M
 117
 118 H is called RLW (run length word).  It consists of (from lower to higher
 119 order bits):
 120
 121      - 1 bit: the repeated bit B
 122
 123      - 32 bits: repetition count K (unsigned)
 124
 125      - 31 bits: literal word count M (unsigned)
 126
 127 The bitstream represented by the above chunk is then:
 128
 129      - K repetitions of B
 130
 131      - The bits stored in `L_1` through `L_M`.  Within a word, bits at
 132        lower order come earlier in the stream than those at higher
 133        order.
 134
 135 The next word after `L_M` (if any) must again be a RLW, for the next
 136 chunk.  For efficient appending to the bitstream, the EWAH stores a
 137 pointer to the last RLW in the stream.
 138
 139
 140 == Appendix B: Optional Bitmap Sections
 141
 142 These sections may or may not be present in the `.bitmap` file; their
 143 presence is indicated by the header flags section described above.
 144
 145 Name-hash cache
 146 ---------------
 147
 148 If the BITMAP_OPT_HASH_CACHE flag is set, the end of the bitmap contains
 149 a cache of 32-bit values, one per object in the pack. The value at
 150 position `i` is the hash of the pathname at which the `i`th object
 151 (counting in index order) in the pack can be found.  This can be fed
 152 into the delta heuristics to compare objects with similar pathnames.
 153
 154 The hash algorithm used is:
 155
 156     hash = 0;
 157     while ((c = *name++))
 158             if (!isspace(c))
 159                     hash = (hash >> 2) + (c << 24);
 160
 161 Note that this hashing scheme is tied to the BITMAP_OPT_HASH_CACHE flag.
 162 If implementations want to choose a different hashing scheme, they are
 163 free to do so, but MUST allocate a new header flag (because comparing
 164 hashes made under two different schemes would be pointless).