https://github.com/mirage/duff

Pure OCaml implementation of libXdiff (Rabin's fingerprint)
https://github.com/mirage/duff

diff fingerprint git patch rabin

Last synced: about 2 months ago
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Pure OCaml implementation of libXdiff (Rabin's fingerprint)

• Host: GitHub
• URL: https://github.com/mirage/duff
• Owner: mirage
• License: mit
• Created: 2018-04-06T11:55:25.000Z (over 6 years ago)
• Default Branch: main
• Last Pushed: 2023-06-27T08:14:31.000Z (about 1 year ago)
• Last Synced: 2024-05-09T23:26:04.987Z (4 months ago)
• Topics: diff, fingerprint, git, patch, rabin
• Language: OCaml
• Size: 442 KB
• Stars: 21
• Watchers: 20
• Forks: 3
• Open Issues: 0
• Metadata Files:
  • Readme: README.md
  • Changelog: CHANGES.md
  • License: LICENSE.md

Awesome Lists containing this project

• awesome-list - duff
• awesome-ocaml - Duff - Implementation of Rabin's fingerprint and delta compression by P. MacDonald in OCaml (same as [libXdiff](http://www.xmailserver.org/xdiff-lib.html) (Algorithms and Data Structures)

README

        
Duff – libXdiff implementation in OCaml

=======================================

Duff is a little library to implement [libXdiff][libXdiff] in OCaml. This

library is a part of the [ocaml-git][ocaml-git] project. This code is a

translation of `diff-delta.c` available on the git project in OCaml. So, it

respects some git's constraints unlike libXdiff.

## Examples

This library let the user to calculate an `index` from a source (a hash-table)

which can be computed with a blob. Then, from `index` (which represents your

source) and a blob, we generate a list of `Copy` and `Insert` elements.

- `Copy (off, len)` means to take a slice of `len` bytes from your source at

  `off` (absolute offset) and copy it.

- `Insert (off, len)` means to store a slice of `len` bytes from your __blob__

  at `off` (absolute offset) and copy it.

  

From this information, we can have a tiny representation of your blob which can

be reconstruct with your source. The goal is to store `Copy` *opcode* with `off`

and `len`, and `Insert` *opcode* which contains a slice of your blob.

Finally, to produce a PACK file in git or ocaml-git, we use this algorithm and

this representation to optimize storage of your blobs (cf. `git gc`).

### Binary

You can see an example of `duff` in `bin` directory. It's an executable to

represent a _thin_ representation of your file. Then, you can reconstruct it

with `patch` sub-command.

This is an example to use `duff`:

```sh

$ ./duff.exe diff source target > target.xduff

$ ./duff.exe patch source < target.xduff > target.new

$ diff target target.new

$ echo $?

0

```

The internal format used is close to what `git` does internally (without `zlib`

layer). However, it does not correspond to an _official_ format. The binary is

not optimized to be used in a production environment but feedback and

improvement on it are welcome.

## Limitations

Because this project is used by [ocaml-git][ocaml-git], we have some

limitations:

- We compute at most `0xFFFFFFFE` bytes from source

- An `insert` block can not be bigger than `0x10000` bytes

For example, libXdiff computes a bigger source than this implementation. Then,

limitation about `insert` block depends on the PACK (git) file format. So, don't

ask me to compute bigger source or merge and produce bigger `insert` block -

these constraints is outside the scope of this library.

From this limitation, `Copy` *opcode* have an offset between 0x0 and 0xFFFFFFE

and `off + len` is lower than 0xFFFFFFFE.

## Fuzzer

We provide a fuzzer to randomly test this library. Currently (4/9/2018),

`afl-fuzz` did not find any bugs and it computed 67.7k cycles (117 paths).

[libXdiff]: http://www.xmailserver.org/xdiff-lib.html

[ocaml-git]: https://github.com/mirage/ocaml-git