Implementation of PACK file in OCaml






ocaml-git -- Git format and protocol in pure OCaml

Support for on-disk and in-memory Git stores. Can read and write all the Git
objects: blobs, trees, commits and tags. It can also handle pack files, pack
indexes and index files (where the staging area lives - only for git-unix

All the objects share a consistent API, and convenience functions are provided
to manipulate the different objects. For instance, it is possible to make a pack
file position independent (as the Zlib compression might change the relative
offsets between the packed objects), to generate pack indexes from pack files,
or to expand the filesystem of a given commit.

The library comes with some command-line tools called ogit-* as a
Proof-of-concept of the core library which shares a similar interface with
git, but where all operations are mapped to the API exposed by ocaml-git (and
hence using only OCaml code). However, these tools are not meant to be used. They
are just examples of how to use ocaml-git.

ocaml-git wants to be a low-level library for irmin. By this fact,
high-level commands such as a (patience) diff, git status,
etc. are not implemented.

As a MirageOS project, ocaml-git is system agnostic. However, it provides a
git-unix package which uses UNIX syscall and is able to introspect a usual Git
repository in a filesystem. However, ocaml-git handles only Git objects
and does not populate your filesystem as git does. For example, Git_unix.Sync.fetch
does not give you files fetched from the repository but only synchronizes .git with
that repository.

The API documentation is available online.

Build, Install Instructions and Packages

To build and install the project, simply run:

$ opam install git
$ opam install git-unix
$ opam install git-mirage


ocaml-git uses 2 libraries with the linking-trick:

These libraries provide a C implementation and an OCaml implementation (mostly
to be compatible with js_of_ocaml). However, utop or any a build-system such
as ocamlbuild are not able to choose between these implementations. So, you
must explicitely choose one.

These libraries use virtual-library available with dune. If
your build-system is dune, you should not have any problem about that where
dune is able to take the default implementation of these libraries.

What is supported

  • The loose object files can be read and written;

  • The PACK files (collections of compressed loose objects using a
    binary-diff representation) and PACK indexes (indexes of pack
    files) can be read and written). The binary diff hunks are exposed using a
    high-level position-independent representation so that they can be manipulated
    more easily. Pack file can be created and is compressed.

  • The INDEX file (used as for managing the staging area) are fully
    supported, which means that git diff and git status will work as expected
    on a repository created by the library. This feature is only available for
    git-unix when it needs to introspect a file-system.

  • Cloning and fetching (using various options) are fully supported
    for the Git protocol, the smart-HTTP protocol and git+ssh. A subset of the
    protocol capabilities are implemented (mainly thin-pack, ofs-delta,
    side-band-64k and allow-reachable-sha1-in-want).

  • Pushing is still experimental and needs more testing.

  • An abstraction for Git Store
    Is available. Various store implementations are available:

What is not supported

  • No server-side operations are currently supported.

  • No GC.

  • Updates, merge and rebase are not supported. Use irmin instead.


Performance is comparable to the Git tool.


This utop example must run into the ocaml-git repository when the given path
is ..

# ;; load necessary modules
# #require "checkseum.c" ;;
# #require "digestif.c" ;;
# #require "git-unix" ;;

# ;; we are going to use this project's local repository
# module Store = Git_unix.Store ;; 
module Store = Git_unix.Store

# ;; this module is useful for finding git objects in a git store
# module Search = Git.Search.Make (Digestif.SHA1) (Store) ;;
module Search :
    type hash = Store.hash
    type store = Store.t
    type pred =
        [ `Commit of hash
        | `Tag of string * hash
        | `Tree of string * hash
        | `Tree_root of hash ]
    val pred : store -> ?full:bool -> hash -> pred list Lwt.t
    type path =
        [ `Commit of path | `Path of string list | `Tag of string * path ]
    val mem : store -> hash -> path -> bool Lwt.t
    val find : store -> hash -> path -> hash option Lwt.t

# ;; we want to read the contents of a blob under name [filename]
# let read filename =
  let open Lwt_result.Syntax in
  (* get store located in current root's .git folder *)
  let* store = Store.v (Fpath.v (Sys.getcwd ())) in
  (* find obj-id pointed at by master branch (reference) *)
  let* commit_id = Store.Ref.resolve store Git.Reference.master in
  let open Lwt.Syntax in
  (* find obj-id of of [filename] as a git blob *)
  let* blob_id = Search.find store commit_id (`Commit (`Path [ filename ])) in
  match blob_id with
  | None -> Lwt.return (Error (`Not_found commit_id))
  | Some hash ->
      (* read contents of the blob *) store hash
val read : string -> (Store.Value.t, Store.error) Lwt_result.t = <fun>

# let pp =
  let ok ppf = function
    | Git.Value.Blob b -> Fmt.string ppf (Git.Blob.to_string b)
    | _ -> Fmt.string ppf "#git-object"
  Fmt.result ~ok ~error:Store.pp_error;;
val pp : ('_weak1 Git.Value.t, Store.error) result Fmt.t = <fun>

# Lwt.Infix.(read "" >|= pp Fmt.stdout) ;;

ocaml-git -- Git format and protocol in pure OCaml

Support for on-disk and in-memory Git stores. Can read and write all
the Git objects: the usual blobs, trees, commits and tags but also
the pack files, pack indexes and the index file (where the staging area




MIT, see file for its text.