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 begining 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 begining of the file to the first cache entry
        in this block of entries.

    - 32-bit count of cache entries in this block