Documentation/technical/bitmap-format.txt

   1 GIT bitmap v1 format
   2 ====================
   3
   4 == Pack and multi-pack bitmaps
   5
   6 Bitmaps store reachability information about the set of objects in a packfile,
   7 or a multi-pack index (MIDX). The former is defined obviously, and the latter is
   8 defined as the union of objects in packs contained in the MIDX.
   9
  10 A bitmap may belong to either one pack, or the repository's multi-pack index (if
  11 it exists). A repository may have at most one bitmap.
  12
  13 An object is uniquely described by its bit position within a bitmap:
  14
  15         - If the bitmap belongs to a packfile, the __n__th bit corresponds to
  16         the __n__th object in pack order. For a function `offset` which maps
  17         objects to their byte offset within a pack, pack order is defined as
  18         follows:
  19
  20                 o1 <= o2 <==> offset(o1) <= offset(o2)
  21
  22         - If the bitmap belongs to a MIDX, the __n__th bit corresponds to the
  23         __n__th object in MIDX order. With an additional function `pack` which
  24         maps objects to the pack they were selected from by the MIDX, MIDX order
  25         is defined as follows:
  26
  27                 o1 <= o2 <==> pack(o1) <= pack(o2) /\ offset(o1) <= offset(o2)
  28 +
  29 The ordering between packs is done according to the MIDX's .rev file.
  30 Notably, the preferred pack sorts ahead of all other packs.
  31
  32 The on-disk representation (described below) of a bitmap is the same regardless
  33 of whether or not that bitmap belongs to a packfile or a MIDX. The only
  34 difference is the interpretation of the bits, which is described above.
  35
  36 Certain bitmap extensions are supported (see: Appendix B). No extensions are
  37 required for bitmaps corresponding to packfiles. For bitmaps that correspond to
  38 MIDXs, both the bit-cache and rev-cache extensions are required.
  39
  40 == On-disk format
  41
  42     * A header appears at the beginning:
  43
  44         4-byte signature: :: {'B', 'I', 'T', 'M'}
  45
  46         2-byte version number (network byte order): ::
  47
  48             The current implementation only supports version 1
  49             of the bitmap index (the same one as JGit).
  50
  51         2-byte flags (network byte order): ::
  52
  53             The following flags are supported:
  54
  55             ** {empty}
  56             BITMAP_OPT_FULL_DAG (0x1) REQUIRED: :::
  57
  58             This flag must always be present. It implies that the
  59             bitmap index has been generated for a packfile or
  60             multi-pack index (MIDX) with full closure (i.e. where
  61             every single object in the packfile/MIDX can find its
  62             parent links inside the same packfile/MIDX). This is a
  63             requirement for the bitmap index format, also present in
  64             JGit, that greatly reduces the complexity of the
  65             implementation.
  66
  67             ** {empty}
  68             BITMAP_OPT_HASH_CACHE (0x4): :::
  69
  70             If present, the end of the bitmap file contains
  71             `N` 32-bit name-hash values, one per object in the
  72             pack/MIDX. The format and meaning of the name-hash is
  73             described below.
  74
  75                 ** {empty}
  76                 BITMAP_OPT_LOOKUP_TABLE (0x10): :::
  77                 If present, the end of the bitmap file contains a table
  78                 containing a list of `N` <commit_pos, offset, xor_row>
  79                 triplets. The format and meaning of the table is described
  80                 below.
  81 +
  82 NOTE: Unlike the xor_offset used to compress an individual bitmap,
  83 `xor_row` stores an *absolute* index into the lookup table, not a location
  84 relative to the current entry.
  85
  86         4-byte entry count (network byte order): ::
  87             The total count of entries (bitmapped commits) in this bitmap index.
  88
  89         20-byte checksum: ::
  90             The SHA1 checksum of the pack/MIDX this bitmap index
  91             belongs to.
  92
  93     * 4 EWAH bitmaps that act as type indexes
  94 +
  95 Type indexes are serialized after the hash cache in the shape
  96 of four EWAH bitmaps stored consecutively (see Appendix A for
  97 the serialization format of an EWAH bitmap).
  98 +
  99 There is a bitmap for each Git object type, stored in the following
 100 order:
 101 +
 102     - Commits
 103     - Trees
 104     - Blobs
 105     - Tags
 106
 107 +
 108 In each bitmap, the `n`th bit is set to true if the `n`th object
 109 in the packfile or multi-pack index is of that type.
 110 +
 111 The obvious consequence is that the OR of all 4 bitmaps will result
 112 in a full set (all bits set), and the AND of all 4 bitmaps will
 113 result in an empty bitmap (no bits set).
 114
 115     * N entries with compressed bitmaps, one for each indexed commit
 116 +
 117 Where `N` is the total amount of entries in this bitmap index.
 118 Each entry contains the following:
 119
 120         ** {empty}
 121         4-byte object position (network byte order): ::
 122             The position **in the index for the packfile or
 123             multi-pack index** where the bitmap for this commit is
 124             found.
 125
 126         ** {empty}
 127         1-byte XOR-offset: ::
 128             The xor offset used to compress this bitmap. For an entry
 129             in position `x`, a XOR offset of `y` means that the actual
 130             bitmap representing this commit is composed by XORing the
 131             bitmap for this entry with the bitmap in entry `x-y` (i.e.
 132             the bitmap `y` entries before this one).
 133 +
 134 NOTE: This compression can be recursive. In order to
 135 XOR this entry with a previous one, the previous entry needs
 136 to be decompressed first, and so on.
 137 +
 138 The hard-limit for this offset is 160 (an entry can only be
 139 xor'ed against one of the 160 entries preceding it). This
 140 number is always positive, and hence entries are always xor'ed
 141 with **previous** bitmaps, not bitmaps that will come afterwards
 142 in the index.
 143
 144         ** {empty}
 145         1-byte flags for this bitmap: ::
 146             At the moment the only available flag is `0x1`, which hints
 147             that this bitmap can be re-used when rebuilding bitmap indexes
 148             for the repository.
 149
 150         ** The compressed bitmap itself, see Appendix A.
 151
 152         * {empty}
 153         TRAILER: ::
 154                 Trailing checksum of the preceding contents.
 155
 156 == Appendix A: Serialization format for an EWAH bitmap
 157
 158 Ewah bitmaps are serialized in the same protocol as the JAVAEWAH
 159 library, making them backwards compatible with the JGit
 160 implementation:
 161
 162         - 4-byte number of bits of the resulting UNCOMPRESSED bitmap
 163
 164         - 4-byte number of words of the COMPRESSED bitmap, when stored
 165
 166         - N x 8-byte words, as specified by the previous field
 167 +
 168 This is the actual content of the compressed bitmap.
 169
 170         - 4-byte position of the current RLW for the compressed
 171                 bitmap
 172
 173 All words are stored in network byte order for their corresponding
 174 sizes.
 175
 176 The compressed bitmap is stored in a form of run-length encoding, as
 177 follows.  It consists of a concatenation of an arbitrary number of
 178 chunks.  Each chunk consists of one or more 64-bit words
 179
 180      H  L_1  L_2  L_3 .... L_M
 181
 182 H is called RLW (run length word).  It consists of (from lower to higher
 183 order bits):
 184
 185      - 1 bit: the repeated bit B
 186
 187      - 32 bits: repetition count K (unsigned)
 188
 189      - 31 bits: literal word count M (unsigned)
 190
 191 The bitstream represented by the above chunk is then:
 192
 193      - K repetitions of B
 194
 195      - The bits stored in `L_1` through `L_M`.  Within a word, bits at
 196        lower order come earlier in the stream than those at higher
 197        order.
 198
 199 The next word after `L_M` (if any) must again be a RLW, for the next
 200 chunk.  For efficient appending to the bitstream, the EWAH stores a
 201 pointer to the last RLW in the stream.
 202
 203
 204 == Appendix B: Optional Bitmap Sections
 205
 206 These sections may or may not be present in the `.bitmap` file; their
 207 presence is indicated by the header flags section described above.
 208
 209 Name-hash cache
 210 ---------------
 211
 212 If the BITMAP_OPT_HASH_CACHE flag is set, the end of the bitmap contains
 213 a cache of 32-bit values, one per object in the pack/MIDX. The value at
 214 position `i` is the hash of the pathname at which the `i`th object
 215 (counting in index or multi-pack index order) in the pack/MIDX can be found.
 216 This can be fed into the delta heuristics to compare objects with similar
 217 pathnames.
 218
 219 The hash algorithm used is:
 220
 221     hash = 0;
 222     while ((c = *name++))
 223             if (!isspace(c))
 224                     hash = (hash >> 2) + (c << 24);
 225
 226 Note that this hashing scheme is tied to the BITMAP_OPT_HASH_CACHE flag.
 227 If implementations want to choose a different hashing scheme, they are
 228 free to do so, but MUST allocate a new header flag (because comparing
 229 hashes made under two different schemes would be pointless).
 230
 231 Commit lookup table
 232 -------------------
 233
 234 If the BITMAP_OPT_LOOKUP_TABLE flag is set, the last `N * (4 + 8 + 4)`
 235 bytes (preceding the name-hash cache and trailing hash) of the `.bitmap`
 236 file contains a lookup table specifying the information needed to get
 237 the desired bitmap from the entries without parsing previous unnecessary
 238 bitmaps.
 239
 240 For a `.bitmap` containing `nr_entries` reachability bitmaps, the table
 241 contains a list of `nr_entries` <commit_pos, offset, xor_row> triplets
 242 (sorted in the ascending order of `commit_pos`). The content of i'th
 243 triplet is -
 244
 245         * {empty}
 246         commit_pos (4 byte integer, network byte order): ::
 247         It stores the object position of a commit (in the midx or pack
 248         index).
 249
 250         * {empty}
 251         offset (8 byte integer, network byte order): ::
 252         The offset from which that commit's bitmap can be read.
 253
 254         * {empty}
 255         xor_row (4 byte integer, network byte order): ::
 256         The position of the triplet whose bitmap is used to compress
 257         this one, or `0xffffffff` if no such bitmap exists.