path: root/Documentation/technical/bitmap-format.txt
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authorVicent Marti <>2013-11-14 12:44:02 (GMT)
committerJunio C Hamano <>2013-12-30 20:19:22 (GMT)
commit0d4455a3ab070e0477ab80ae641ef19146b7a736 (patch)
tree5441575e7a28e7d6898e10976153a92ba8274c2e /Documentation/technical/bitmap-format.txt
parente1273106f62927e3efdb1cfa107cb1a9f913274c (diff)
documentation: add documentation for the bitmap format
This is the technical documentation for the JGit-compatible Bitmap v1 on-disk format. Signed-off-by: Vicent Marti <> Signed-off-by: Jeff King <> Signed-off-by: Junio C Hamano <>
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+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:
+ 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.
+ 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
+ - 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
+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.