# Git index format

## The Git index file has the following format

    All binary numbers are in network byte order. Version 2 is described
    here unless stated otherwise.

- A 12-byte header consisting of

      4-byte signature:
        The signature is { 'D', 'I', 'R', 'C' } (stands for "dircache")

      4-byte version number:
        The current supported versions are 2, 3 and 4.

      32-bit number of index entries.

- A number of sorted index entries (see below).

- Extensions

      Extensions are identified by signature. Optional extensions can
      be ignored if Git does not understand them.

      Git currently supports cached tree and resolve undo extensions.

      4-byte extension signature. If the first byte is 'A'..'Z' the
      extension is optional and can be ignored.

      32-bit size of the extension

      Extension data

- 160-bit SHA-1 over the content of the index file before this checksum.

## Index entry

    Index entries are sorted in ascending order on the name field,
    interpreted as a string of unsigned bytes (i.e. memcmp() order, no
    localization, no special casing of directory separator '/'). Entries
    with the same name are sorted by their stage field.

    32-bit ctime seconds, the last time a file's metadata changed
      this is stat(2) data

    32-bit ctime nanosecond fractions
      this is stat(2) data

    32-bit mtime seconds, the last time a file's data changed
      this is stat(2) data

    32-bit mtime nanosecond fractions
      this is stat(2) data

    32-bit dev
      this is stat(2) data

    32-bit ino
      this is stat(2) data

    32-bit mode, split into (high to low bits)

    4-bit object type
      valid values in binary are 1000 (regular file), 1010 (symbolic link)
      and 1110 (gitlink)

    3-bit unused

    9-bit unix permission. Only 0755 and 0644 are valid for regular files.
    Symbolic links and gitlinks have value 0 in this field.

    32-bit uid
      this is stat(2) data

    32-bit gid
      this is stat(2) data

    32-bit file size
      This is the on-disk size from stat(2), truncated to 32-bit.

    160-bit SHA-1 for the represented object

    A 16-bit 'flags' field split into (high to low bits)

    1-bit assume-valid flag

    1-bit extended flag (must be zero in version 2)

    2-bit stage (during merge)

    12-bit name length if the length is less than 0xFFF; otherwise 0xFFF
    is stored in this field.

    (Version 3 or later) A 16-bit field, only applicable if the
    "extended flag" above is 1, split into (high to low bits).

    1-bit reserved for future

    1-bit skip-worktree flag (used by sparse checkout)

    1-bit intent-to-add flag (used by "git add -N")

    13-bit unused, must be zero

    Entry path name (variable length) relative to top level directory
      (without leading slash). '/' is used as path separator. The special
      path components ".", ".." and ".git" (without quotes) are disallowed.
      Trailing slash is also disallowed.

    The exact encoding is undefined, but the '.' and '/' characters
    are encoded in 7-bit ASCII and the encoding cannot contain a NUL
    byte (iow, this is a UNIX pathname).

    (Version 4) In version 4, the entry path name is prefix-compressed
      relative to the path name for the previous entry (the very first
      entry is encoded as if the path name for the previous entry is an
      empty string).  At the beginning of an entry, an integer N in the
      variable width encoding (the same encoding as the offset is encoded
      for OFS_DELTA pack entries; see pack-format.txt) is stored, followed
      by a NUL-terminated string S.  Removing N bytes from the end of the
      path name for the previous entry, and replacing it with the string S
      yields the path name for this entry.

    1-8 nul bytes as necessary to pad the entry to a multiple of eight bytes
    while keeping the name NUL-terminated.

    (Version 4) In version 4, the padding after the pathname does not
    exist.

    Interpretation of index entries in split index mode is completely
    different. See below for details.

## Extensions

### Cached tree

    Cached tree extension contains pre-computed hashes for trees that can
    be derived from the index. It helps speed up tree object generation
    from index for a new commit.

    When a path is updated in index, the path must be invalidated and
    removed from tree cache.

    The signature for this extension is { 'T', 'R', 'E', 'E' }.

    A series of entries fill the entire extension; each of which
    consists of:

- NUL-terminated path component (relative to its parent directory);

- ASCII decimal number of entries in the index that is covered by the tree this entry represents (entry_count);

- A space (ASCII 32);

- ASCII decimal number that represents the number of subtrees this tree has;

- A newline (ASCII 10); and

- 160-bit object name for the object that would result from writing this span of index as a tree.

      An entry can be in an invalidated state and is represented by having
      a negative number in the entry_count field. In this case, there is no
      object name and the next entry starts immediately after the newline.
      When writing an invalid entry, -1 should always be used as entry_count.

      The entries are written out in the top-down, depth-first order.  The
      first entry represents the root level of the repository, followed by the
      first subtree--let's call this A--of the root level (with its name
      relative to the root level), followed by the first subtree of A (with
      its name relative to A), ...

### Resolve undo

    A conflict is represented in the index as a set of higher stage entries.
    When a conflict is resolved (e.g. with "git add path"), these higher
    stage entries will be removed and a stage-0 entry with proper resolution
    is added.

    When these higher stage entries are removed, they are saved in the
    resolve undo extension, so that conflicts can be recreated (e.g. with
    "git checkout -m"), in case users want to redo a conflict resolution
    from scratch.

    The signature for this extension is { 'R', 'E', 'U', 'C' }.

    A series of entries fill the entire extension; each of which
    consists of:

- NUL-terminated pathname the entry describes (relative to the root of the repository, i.e. full pathname);

- Three NUL-terminated ASCII octal numbers, entry mode of entries in stage 1 to 3 (a missing stage is represented by "0" in this field); and

- At most three 160-bit object names of the entry in stages from 1 to 3 (nothing is written for a missing stage).

### Split index

    In split index mode, the majority of index entries could be stored
    in a separate file. This extension records the changes to be made on
    top of that to produce the final index.

    The signature for this extension is { 'l', 'i', 'n', 'k' }.

    The extension consists of:

- 160-bit SHA-1 of the shared index file. The shared index file path is $GIT_DIR/sharedindex.<SHA-1>. If all 160 bits are zero, the index does not require a shared index file.

- An ewah-encoded delete bitmap, each bit represents an entry in the shared index. If a bit is set, its corresponding entry in the shared index will be removed from the final index. Note, because a delete operation changes index entry positions, but we do need original positions in replace phase, it’s best to just mark entries for removal, then do a mass deletion after replacement.

- An ewah-encoded replace bitmap, each bit represents an entry in the shared index. If a bit is set, its corresponding entry in the shared index will be replaced with an entry in this index file. All replaced entries are stored in sorted order in this index. The first "1" bit in the replace bitmap corresponds to the first index entry, the second "1" bit to the second entry and so on. Replaced entries may have empty path names to save space.

      The remaining index entries after replaced ones will be added to the
      final index. These added entries are also sorted by entry name then
      stage.

## Untracked cache

    Untracked cache saves the untracked file list and necessary data to
    verify the cache. The signature for this extension is { 'U', 'N',
    'T', 'R' }.

    The extension starts with

- A sequence of NUL-terminated strings, preceded by the size of the sequence in variable width encoding. Each string describes the environment where the cache can be used.

- Stat data of $GIT_DIR/info/exclude. See "Index entry" section from ctime field until "file size".

- Stat data of core.excludesfile

- 32-bit dir_flags (see struct dir_struct)

- 160-bit SHA-1 of $GIT_DIR/info/exclude. Null SHA-1 means the file does not exist.

- 160-bit SHA-1 of core.excludesfile. Null SHA-1 means the file does not exist.

- NUL-terminated string of per-dir exclude file name. This usually is ".gitignore".

- The number of following directory blocks, variable width encoding. If this number is zero, the extension ends here with a following NUL.

- A number of directory blocks in depth-first-search order, each consists of

- The number of untracked entries, variable width encoding.

- The number of sub-directory blocks, variable width encoding.

- The directory name terminated by NUL.

- A number of untracked file/dir names terminated by NUL.

The remaining data of each directory block is grouped by type:

- An ewah bitmap, the n-th bit marks whether the n-th directory has valid untracked cache entries.

- An ewah bitmap, the n-th bit records "check-only" bit of read_directory_recursive() for the n-th directory.

- An ewah bitmap, the n-th bit indicates whether SHA-1 and stat data is valid for the n-th directory and exists in the next data.

- An array of stat data. The n-th data corresponds with the n-th "one" bit in the previous ewah bitmap.

- An array of SHA-1. The n-th SHA-1 corresponds with the n-th "one" bit in the previous ewah bitmap.

- One NUL.

## File System Monitor cache

    The file system monitor cache tracks files for which the core.fsmonitor
    hook has told us about changes.  The signature for this extension is
    { 'F', 'S', 'M', 'N' }.

    The extension starts with

- 32-bit version number: the current supported version is 1.

- 64-bit time: the extension data reflects all changes through the given time which is stored as the nanoseconds elapsed since midnight, January 1, 1970.

- 32-bit bitmap size: the size of the CE_FSMONITOR_VALID bitmap.

- An ewah bitmap, the n-th bit indicates whether the n-th index entry is not CE_FSMONITOR_VALID.

## End of Index Entry

    The End of Index Entry (EOIE) is used to locate the end of the variable
    length index entries and the beginning of the extensions. Code can take
    advantage of this to quickly locate the index extensions without having
    to parse through all of the index entries.

    Because it must be able to be loaded before the variable length cache
    entries and other index extensions, this extension must be written last.
    The signature for this extension is { 'E', 'O', 'I', 'E' }.

    The extension consists of:

- 32-bit offset to the end of the index entries

- 160-bit SHA-1 over the extension types and their sizes (but not their contents). E.g. if we have "TREE" extension that is N-bytes long, "REUC" extension that is M-bytes long, followed by "EOIE", then the hash would be:

      SHA-1("TREE" + <binary representation of N> +
      	"REUC" + <binary representation of M>)

## Index Entry Offset Table

    The Index Entry Offset Table (IEOT) is used to help address the CPU
    cost of loading the index by enabling multi-threading the process of
    converting cache entries from the on-disk format to the in-memory format.
    The signature for this extension is { 'I', 'E', 'O', 'T' }.

    The extension consists of:

- 32-bit version (currently 1)

- A number of index offset entries each consisting of:

- 32-bit offset from the beginning of the file to the first cache entry in this block of entries.

- 32-bit count of cache entries in this block