# It's Difftastic!

Difftastic is an experimental structured diff tool that compares files
based on their syntax.

![screenshot](img/difftastic.png)

It currently supports the following languages:

* Clojure
* CSS
* Emacs Lisp
* Go
* JavaScript
* JSON
* OCaml
* Rust
* Scheme

The parsing logic is based on matched delimiters, so difftastic tends
to give best results on heavily parenthesised code (e.g. Lisps or
JSON).

If a file has an unrecognised extension, difftastic uses a
line-oriented diff.

## How It Works

(1) Parsing.

Difftastic treats source code as a sequence of atoms or (possibly
nested) lists.

Language syntax is defined in `src/regex_parser.rs`: you provide regular
expressions for atoms (including comments), open delimiters, and close
delimiters.

This is heavily inspired by
[Comby](https://github.com/comby-tools/comby), which handles a large
number of languages by using a similar approach.

(A [tree-sitter](https://tree-sitter.github.io/tree-sitter/) parsing
backend is also available by setting `DFT_TS=y`. It is not yet
recommended.)

(2) Diffing.

Difftastic treats diff calculations as a graph search problem. It
finds the minimal diff using Dijkstra's algorithm.

This is based on the excellent
[Autochrome](https://fazzone.github.io/autochrome.html) project.

(3) Printing.

Difftastic prints a side-by-side diff that fits the current
terminal. It will try to align unchanged nodes (see screenshot above).

## Known Problems

Crashes. The code is underdocumented, undertested, and
unfinished.

Performance. Difftastic scales relatively poorly on files with a large
number of changes, and can use a lot of memory. This might be solved
by A* search.

Comments. Small changes can show big diffs.

## Non-goals

Patch files. If you want to create a patch that you can later apply,
use `diff`. Difftastic ignores whitespace, so its output is
lossy. (AST patching is also a hard problem.)

## Installation

You can install the latest tag of difftastic with Cargo:

```
$ cargo install difftastic
```

Difftastic is still under heavy development, so there's usually major
bugfixes since the latest release. I currently recommend you check out
the repository and compile directly:

```
$ cargo build --release
```

This will give you a binary at `./target/release/difftastic` that you
can put in a directory on your `$PATH`.

### Adding a parser

Add the tree-sitter-FOO git repository as a subtree. 

```
$ git subtree add --prefix=vendor/tree-sitter-elisp git@github.com:Wilfred/tree-sitter-elisp.git main
```

Add a symlink to the C source directory (Cargo will not include the
parent directory when packaging, because the parent has a `Cargo.toml`).

```
$ cd vendor
$ ln -s tree-sitter-elisp/src tree-sitter-elisp-src
```
Update `build.rs` and `tree_sitter_parser.rs` to include the
definitions for the new parser.

## Git Usage

Once you've compiled `difftastic` and it's on `$PATH`, you can use it
with git commands. To see the changes to the current git repo in
difftastic, add the following to your `.gitconfig` and run
`git difftool`.

```
[diff]
        tool = difftastic

[difftool]
        prompt = false

[difftool "difftastic"]
        cmd = difftastic "$LOCAL" "$REMOTE"
```

Alternatively, to run difftastic as the default diff engine for a git
command:

```
$ GIT_EXTERNAL_DIFF=difftastic git diff
$ GIT_EXTERNAL_DIFF=difftastic git log -p --ext-diff
```

## License

Difftastic is open source under the MIT license, see LICENSE for more
details.

Files in `sample_files/` are also under the MIT license unless stated
otherwise in their header.

## Further Reading

The [wiki](https://github.com/Wilfred/difftastic/wiki) includes a
thorough overview of alternative diffing techniques and tools.