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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 and 3.
 
     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) 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).
 
  1-8 nul bytes as necessary to pad the entry to a multiple of eight bytes
  while keeping the name NUL-terminated.
 
== 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 -1
  in the entry_count field.
 
  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 resoluton
  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).