Duff – libXdiff implementation in OCaml

Duff is a little library to implement libXdiff in OCaml. This
library is a part of the 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:

$ ./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, 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).