Implement Myers Heuristics

[mitsuhiko]

GNU diff and others have some internal heuristics to bail if there are too many changes. There are basically two optimizations:

• Discard lines which are completely distinct between two files: this has the advantage that they are entirely ignored by the diffing algorithm which helps to reduce how much has to be diffed greatly
• Bail if the snake becomes too long

https://github.com/reviewboard/reviewboard/blob/master/reviewboard/diffviewer/myersdiff.py

To be more aligned with git it might make sense to implement the heuristics in the current Algorithm::Myers variant and have a secondary Algorithm::MyersMinimal which has these heuristics disabled (git calls the variants myers and minimal).

These heuristics are likely needed as currently lcs outperform myers greatly if used on completely distinct files.

[potocpav]

I found a way to easily implement the distinct-line heuristic. It doesn't modify the Myers algorithm at all, and rather builds on top of it. The algorithm works in three steps:

1. Encode the input lists of type Vec<T> into an "optimized" representation:

enum Elems<T> {
    UniqueRun(Vec<T>),
    NormalElem(T),
}

In this representation, consecutive unique lines are concatenated into UniqueRun, and non-unique lines are just inside NormalElem. The whole sequence is then Vec<Elems<T>>.

2. Perform Meyers diff on Vec<Elems<T>>, instead of Vec<T> directly.

3. Decode Vec<Elems<T>> back into Vec<T> to get the result.

This solves the super common pathological case of nearly-distinct files. However, if unique and non-unique lines are mixed together, it still fails.

I implemented this algorithm for the Pijul crate. I don't know how easy/hard it would be to adapt for this crate. Just leaving this here in case it's useful.

[mitsuhiko]

Thank you for that @potocpav. I will have a look and evaluate this. The underlying design is still somewhat similar to pijul so it should be easy enough to adapt.

[P-E-Meunier]

More specifically, this project started as a fork of "diffs", right?

[mitsuhiko]

Yep. See also #1