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-Git index format
-================
-
-== The Git index file has the following format
-
-  All binary numbers are in network byte order.
-  In a repository using the traditional SHA-1, checksums and object IDs
-  (object names) mentioned below are all computed using SHA-1.  Similarly,
-  in SHA-256 repositories, these values are computed using SHA-256.
-  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
-
-   - Hash checksum 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.
-
-  Object name 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
-
-  - 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 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:
-
-  - Hash of the shared index file. The shared index file path
-    is $GIT_DIR/sharedindex.<hash>. If all 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)
-
-  - Hash of $GIT_DIR/info/exclude. A null hash means the file
-    does not exist.
-
-  - Hash of core.excludesfile. A null hash 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 hash 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 hashes. The n-th hash 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
-
-  - Hash 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:
-
-	Hash("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