From: Vicent Marti <>

This is the technical documentation for the JGit-compatible Bitmap v1
on-disk format.

Signed-off-by: Vicent Marti <>
Signed-off-by: Jeff King <>
 Documentation/technical/bitmap-format.txt | 131 ++++++++++++++++++++++++++++++
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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:
+                       - BITMAP_OPT_FULL_DAG (0x1) REQUIRED
+                       This flag must always be present. It implies that the 
+                       index has been generated for a packfile with full 
+                       (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 
+               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 
+       - 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 
+               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 
+               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 
+                       in position `x`, a XOR offset of `y` means that the 
+                       bitmap representing this commit is composed by XORing 
+                       bitmap for this entry with the bitmap in entry `x-y` 
+                       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 
+                       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). 
+                       number is always positive, and hence entries are always 
+                       with **previous** bitmaps, not bitmaps that will come 
+                       in the index.
+               - 1-byte flags for this bitmap
+                       At the moment the only available flag is `0x1`, which 
+                       that this bitmap can be re-used when rebuilding bitmap 
+                       for the repository.
+               - The compressed bitmap itself, see Appendix A.
+== Appendix A: Serialization format for an EWAH bitmap
+Ewah bitmaps are serialized in the 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.

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