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Show HN: OmniDiff – Opensource, super fast, content aware Git difftool, in Rust (github.com)

1 points by FabCH ↗ HN
I have been researching what the ultimate diff tool would look like, based on actual diffs in open source repositories.

OmniDiff can diff: code (25 languages), config, images, PDFs, archives, cursors, fonts...

It's fast. It can diff C++ files of tens of thousands of lines in under 100ms, on a 2023 ThinkPad T14s.

It's open source. AGPLv3.

It's based on real diffs. I looked at the 7400 repositories that are listed in Gentoo packages, sampled 2000 diffs and solved them manually to get a good understanding of what is needed, before putting together the diff pipelines for each content type. Incidentally, this also forms the basis of the CI pipeline, so the robustness of the codebase is quite high.

I'm still developing it. It's v0.3.0 at this time. But I have been using it daily as my git difftool for months now, and it works well.

AI Disclosure: Yes, I do use AI. No, I do not vibecode.

87 comments

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anilgulecha
You have a good core idea here. You're pretty much killing your project in infancy with the AGPL+donation license model. Meanwhile, some other projects will take the core idea of "language aware differ" and release BSD, and one of those will win, including perhaps making it into git/future version controls.

IMO, you should seek to make this a standard that wins, and hopefully other opportunities that come will be where you make commercial hay. My unsolicited 2c.

FabCH
I really do care about copyleft and AGPL. It’s a political choice. I specifically chose it because I know from direct experience that big tech is allergic to it.

I actually don’t care about the commercial license. It’s more of an experiment for my own amusement than an actual attempt at building a commercial project.

If it inspires someone to build another one and we still end up with a better differ, I’m ok with that world. As long as the open source community has a better differ.

Note that there already are at least 3 that I am aware of. But they do perform worse on speed and accuracy.

shock
I support the choice of AGPL. I wish more software used it. Whenever I use AI to write code, in my experience it invariably chooses the MIT license. For example, for all the ansible roles that I've written using AI it put the MIT license in the meta file, unless otherwise prompted. It makes me wonder if it comes from RLHF.
IshKebab
> Meanwhile, some other projects will take the core idea of "language aware differ" and release BSD, and one of those will win, including perhaps making it into git/future version controls.

Diffsitter and Difftastic have been around for a while, both MIT, so I doubt it.

hebetude
Something nobody asked for with licenses nobody wants to use. Project is dead before it started
alexpadula
You know hebetube, my mother once said "if you have nothing nice to say: don't say it". You saying those things probably aren't going to make them turn over the license. "Project dead before it started" like who writes those kinds of things over someones work they spent time on. I'd love for you to explain your logic.
cocoto
How does it compare with diffstatic?
FabCH
Comparing is slightly complicated but roughly speaking:

Difftastic only diffs code, which is only 2/3 of all diffs.

When only measured on stuff difftastic supports:

Difftastic is perfect on 64% of diffs and near-perfect on 17% more.

Omnidiff is perfect on 90% of diffs and near-perfect on 5% more.

Speed is

p50 - 35ms difftastic vs 4ms OmniDiff p99 - 637ms vs 420ms

Difftastic wins on license. MIT vs AGPL.

LVB
[delayed]
FabCH
It was a tongue-in-cheek joke :)

But it was worth pointing out the difference in license, it is one of the ways in which the projects differ.

folmar
The more permissive MIT is often an advantage if you want to build it into something bigger, which is reasonable for diff tool.

Discrete users would arguably win more with integrity of AGPL, but developers have reasons to prefer MIT.

fishgoesblub
> but developers have reasons to prefer MIT

Hi! as a developer, I personally prefer it when the libraries/tools used in my projects are GPL or another Copyleft license. The drove of MIT projects from the past decade has been depressing.

lejalv
Servitude to their masters
Surac
obviously it's in rust. anything the world does not need is in rust
arkenflame
Really well done, I like this a lot. It produces much more minimal diffs than other tools I've tried.

One feature I miss from difftastic is showing diffs side-by-side on wide terminals, and unified when narrow.

nickjj
Delta has a really nice way to show diffs, it's more similar to GitHub's UI where not only the line is colored up differently but there's a different shade of color to highlight the characters.

There's a screenshot and video of it here that I wrote a while back: https://nickjanetakis.com/blog/awesome-git-diffs-with-delta-...

In OmniDiff's readme and web version I didn't see a way to do that.

Also Delta has a package for Arch in its official repos. For folks who just want a "it works" experience, that's a pretty big win. With Delta the same package can be installed in the same way on Debian, Arch and macOS with Brew using standard package managers. With OmniDiff, there's 3 different ways to install it (manual deb package on Debian, manual package compile on Arch and brew install on a Mac).

FabCH
OmniDiff is character based by default. It will only highlight the changed characters. I suppose I could add an option to additionally highlight the line as well. Thanks for the feedback!

As for packaging. Unfortunately AUR is closed at the moment due to spam. I can add brew commands for Linux, happily.

In terms of “better”. It’s actually somewhat complex, but a simple way to think about it is that if you move a function up or down in the same class, text-based diffs paint that as “Deleted here, Inserted there” where ideally you would want it to say “Moved”.

davvid
Nicely done OP. Does it do 3-way diffs for merges as well? The instructions only mention the two-way `git difftool` configuration so maybe that'll be a future feature?

Any interest in getting the setup upstreamed[1] so that users only need to `git config --global diff.tool omnidiff` to enable it by default? Disclosure: I created `git difftool` a long time ago, and am always happy when builtin support for useful tools are added to it. Feel free to reach out if you'd like help making that happen.

[1] https://github.com/git/git/tree/master/mergetools

FabCH
At the moment the algorithm for code diffing is a particularly efficient implementation of APTED which is a tree diffing algorithm that doesn’t natively construct anything useful for a three way diff. I have a notebook with a bunch of failed attempts at a good algorithm for a three way merge, but so far nothing particularly impressive.

It’s a long term goal to solve, but it’s the hardest one.

I’ll definitely reach out to you to upstream. Thank you for the kind offer!

catlifeonmars
[delayed]
FabCH
It’s a 5 step pipeline.

In practice, tree hashing is doing a huge amount of work because most diffs are tiny. Half of all diffs are less than 10 lines, so a hash can extremely quickly match large subtrees.

In a profoundly degenerate case of a C++ header file with only a 20MB array of bytes, it detects a degenerate tree and uses Myers O(N) sequence diffing algorithm instead of treating it as a tree.

catlifeonmars
How would one craft an input that maximizes the runtime of your diff while staying below some (reasonable) bound of edit distance apart? Basically suppose I wanted to provide a malicious diff designed to hang or timeout the diff program, how would I construct it?
FabCH
You need to maximize the size of the residual tree sent to the final APTED step.

I actually don’t know how to do that and stay under a bounded edit distance.

Alternatively, you can crash it if you just encode the entire “Star Trek - In the pale moonlight” episode as a sequence of bytes in a const int array in C++.

catlifeonmars
> I actually don’t know how to do that and stay under a bounded edit distance.

I’m sure there’s an interesting result there, but I am not the person to find it :D

Good episode btw

smallnix
Very cool! I'd love to see support for figma or Photoshop, perhaps it's already possible via export to an image format, but native would be cool.
burner-account
I am sorry, I looked through the examples and I am not sure I prefer the output of omnidiff... Seeing how everyone else over here is instead overly enthusiastic makes me feel extremely dumb :D
FabCH
Don't feel dumb. You are just an outlier on the visual preferences.

I actually do TWO solutions for each test case: a minimal diff and a maximal diff. Pressing 'm' in the TUI pops up a settings page where you can choose how the diff works.

I am actively researching how many ways there are to "paint" the same diff and user preferences and non-uniqueness of diffs is something I am super interested in.

If you would like to, it would be super useful for me to know what diff you don't like and why. It would help make the diff better for users with different visual preferences.

andunie
This looks really good. It does look like you put a ton of effort into it, I'll give it a try.

I feel like the moved code cases would look great in https://github.com/andunieee/twain, I was looking into how I could slurp your code and make it work there (although for some reason I have the impression that I had already seen this repository in the past, I can't remember where).

andunie
I don't know what I did wrong, but using this as a git difftool (following the config) I didn't get any of the cool things from the demos, I got a weird single-column diff output that only showed the new side of the changes, not a diff.
FabCH
Perhaps the terminal is too small? What happens when you hit tab or “v”?
erutuon
I had the same problem. v switched to the side-by-side view, but it wasn't listed in the help screen so I didn't know to use it till I read your comment.
FabCH
Ok I had a deeper look at twain now.

If I understand your goal correctly, you are looking to especially focus on the visual part of the diff and provide a nice UI for it, and you would like to use OmniDiff as a library?

If so, then let's talk. I have been delaying the design of the API because nobody has a use for it yet, so it is mostly driven by the needs of the CLI/TUI at the moment.

Ping me on my email if you want.

enriquto
cool to see fresh Rust software with copyleft license !

I feel safer, as a user, enjoying the freedoms that the AGPL provides me. (No evil middleman will be able to give me non-free versions of this program.)

lejalv
So much this. Thanks OP!
eviks
But an evil midllman will relatively easily be able to give you non-free version, so that feeling should be fleeting
0c3ca83
You didn't even write the readme fully. Of course this is slop.
gslepak
Is there a way to use it with git without having to change your global git config? I.e. `git diff | omnidiff` or something?

You can do this with delta: `git diff | delta --side-by-side`

nh2
Piping the output of `git diff` might withhold information from the tool that it needs for its feature set, while `delta` is just a different way to display any diff output.
BobbyTables2
For me the biggest issue is “git rebase” merge conflicts because the surrounding context was modified (especially near beginning/end of functions where the context includes unrelated stuff), or similar in a markdown document.

(If commit A adds a line to the end of a bullet list in markdown and commit B does the same, they will conflict even though resolution is trivial for a human)

zem
the resolution depends on a deeper understanding of the contents of the list and whether they are ordered according to some scheme (in which case you will need to look at the two lines and see which one goes first)
eviks
That would also be trivial for a syntax-tree-based tool, right?
anentropic
For me the answer to painful merge conflicts has been... Claude
jbellis
Not sure why you're getting downvoted. I haven't resolved conflicts by hand in almost a year.
kspacewalk2
Resolution is ambiguous without context. The most trivial example: does order matter? If so, which goes first?
blahgeek
> can diff 99.95% of code changes

I’m not sure if I’m understanding this correctly : so for the remaining 0.05% of code changes, this tool would just… miss it? That seems horrible for a diff tool :O

FabCH
It will crash.

But so will every other tool, because it is things like a C++ header file with 128MB of PNG data hardcoded as an array.

Editors don’t even open those files.

xeyownt
Fair enough.

But then I would rephrase that because it's confusing, and not doing justice to your tool.

FabCH
Good point.

Thanks!

grey-area
Perhaps it could just check the length of files and not crash?!? Don’t you think that’s sensible?
FabCH
It's a tradeoff. The length itself is not a problem. It will happily diff normal multi-MB files.

The problem is the degenerative syntax tree that TreeSitter produces in case of, for example a multi-million array of bytes. This causes a stack overflow in the TreeSitter C code which takes down the Rust process with it.

The correct solution would be to prevent TS from crashing, or isolate the C<->Rust boundary better.

Or just, not support that for v0.3. Because if you are diffing the collected works of Shakespeare as a binary blob in a C++ header, maybe diffing isn't your biggest problem.

dmitrygr
> Editors don’t even open those files.

vi opens multi-GB files just fine.

And for multi-TB files, "less -n" works.

You need to broaden your horizons about editors. Not all of then are javascript-based slopfests

FabCH
The crash will happen in TreeSitter which is a widely used AST library that all modern editors, including neovim, depend on for syntax highlighting and which is also used by many LSPs.

Additionally, here's an appropriate XKCD: https://xkcd.com/378/

dmitrygr
Please demonstrate that crash on “less -n” or vim with no plugins. I just happily opened a multi gb file in both. No issue
FabCH
As soon as you demonstrate editing a file with less :)
z0al
How does it compare to Difftastic[1]? Both built on top of TreeSitter and solve similar problems.

[1] https://difftastic.wilfred.me.uk/

FabCH
Comparing is slightly complicated but roughly speaking:

Difftastic only diffs code, which is only 2/3 of all diffs.

When only measured on stuff difftastic supports:

Difftastic is perfect on 64% of diffs and near-perfect on 17% more.

Omnidiff is perfect on 90% of diffs and near-perfect on 5% more.

Speed is

p50 - 35ms difftastic vs 4ms OmniDiff p99 - 637ms vs 420ms

Difftastic is MIT vs AGPL.

z0al
> Difftastic only diffs code, which is only 2/3 of all diffs.

Care to elaborate? What does Omnidiff offer for non-code files?

Tooster
> Difftastic is perfect on 64% of diffs and near-perfect on 17% more.

> Omnidiff is perfect on 90% of diffs and near-perfect on 5% more.

I did not find examples in the repo or any way to test what "perfect" or "near perfect" in those cases mean. On the playground you compare against unix diff only. Based on my quick comparisons with difft, I do not see much difference. Could you add a link to comparisons or show how it differs and what are those numbers based on??? I'm not dismissive, just looking for a proof or showcase.

FabCH
It’s in the research/ directory.

Fair warning: the true comparison is fairly complex and doesn’t really lend itself to “quick comparison”.

gabagoolagonist
I tried using this as configured with `omnidiff git configure`, but when I use `git difftool` it shows me the entire file in the TUI and I have to scroll down to where diff is. Is this intended behavior? Setting `--context` seems to have no effect.
FabCH
Press “n“ or “p” to jump to next/previous difference.

Press ? for the help menu.