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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).
+