Git Index Extensions: Split Index and Untracked Cache

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.

This section describes two Git index extensions: the split index extension, which stores changes to be made on top of a shared index file, and the untracked cache extension, which saves the untracked file list and verification data, including environment information, stat data, and hash values.