🌾 crop

crop is an implementation of a text rope, a data structure designed to be used in applications that need to handle frequent edits to arbitrarily large buffers, such as text editors.

crop's Rope is backed by a B-tree, ensuring that the time complexity of inserting, deleting or replacing a piece of text is always logarithmic in the size of the Rope.

crop places an extreme focus on performance: check out the benchmarks to see how it stacks up against similar projects.

Built with parallelism in mind

Ropes use thread-safe reference counting to share data between threads. Cloning a Rope takes up only 16 extra bytes of memory, and its copy-on-write semantics allow the actual text contents to be cloned incrementally as different clones diverge due to user edits.

This allows to cheaply snapshot a Rope and send it to a background thread to perform any IO or CPU-intensive computations, while the main thread is kept responsive and always ready for the next batch of edits.

Example usage

// A `Rope` can be created either directly from a string or incrementally
// using the `RopeBuilder`.

let mut builder = RopeBuilder::new();

builder
    .append("I am a 🦀\n")
    .append("Who walks the shore\n")
    .append("And pinches toes all day.\n")
    .append("\n")
    .append("If I were you\n")
    .append("I'd wear some 👟\n")
    .append("And not get in my way.\n");

let mut rope: Rope = builder.build();

// `Rope`s can be sliced to obtain `RopeSlice`s.
//
// A `RopeSlice` is to a `Rope` as a `&str` is to a `String`: the former in
// each pair is a borrowed reference of the latter.

// A `Rope` can be sliced using either byte offsets:

let byte_slice: RopeSlice = rope.byte_slice(..32);

assert_eq!(byte_slice, "I am a 🦀\nWho walks the shore\n");

// or line offsets:

let line_slice: RopeSlice = rope.line_slice(..2);

assert_eq!(line_slice, byte_slice);

// We can also get a `RopeSlice` by asking the `Rope` for a specific line
// index:

assert_eq!(rope.line(5), "I'd wear some 👟");

// We can modify that line by getting its start/end byte offsets:

let start: usize = rope.byte_of_line(5);

let end: usize = rope.byte_of_line(6);

// and replacing that byte range with some other text:

rope.replace(start..end, "I'd rock some 👠\n");

assert_eq!(rope.line(5), "I'd rock some 👠");

// `Rope`s use `Arc`s to share data between threads, so cloning them is
// extremely cheap.

let snapshot: Rope = rope.clone();

// This allows to save a `Rope` to disk in a background thread while
// keeping the main thread responsive.

thread::spawn(move || {
    let mut file =
        BufWriter::new(File::create("my_little_poem.txt").unwrap());

    // The text content is stored as separate chunks in the leaves of the
    // B-tree.
    //
    // We can iterate over them using the `Chunks` iterator which yields the
    // chunks of the `Rope` as string slices.

    for chunk in snapshot.chunks() {
        file.write_all(chunk.as_bytes()).unwrap();
    }
})
.join()
.unwrap();

Check out the docs for a more in-depth overview of the crate.

Comparison with other ropes

As of April 2023 there are (to my knowledge) 3 rope crates that are still actively maintained: crop, Jumprope and Ropey. The following is a quick (and incomplete) overview of their features and tradeoffs to help you decide which one is best suited for your specific use case.

Speed

The following results were obtained by running the real world, character-by-character editing traces provided by crdt-benchmarks on a 2018 MacBook Pro with an Intel Core i7.

Dataset	crop (ms)	Jumprope (ms)	Ropey (ms)	std::string::String (ms)
automerge-paper	12.39	12.52	44.14	108.57
rustcode	2.67	2.86	7.96	13.40
sveltecomponent	0.95	1.08	3.65	1.22
seph-blog1	6.47	6.94	23.46	22.26

Cheap clones

Both crop and Ropey allow their Ropes to be cloned in O(1) in time and space by sharing data between clones, whereas cloning a JumpRope involves re-allocating the actual text contents, just like it would with a regular String.

Indexing metric

Jumprope and Ropey both use Unicode codepoint offsets (chars in Rust) as their primary indexing metric. crop uses UTF-8 code unit (aka byte) offsets, just like Rust's Strings.

Line breaks

Both crop and Ropey track line breaks, allowing you to convert between line and byte offsets and to iterate over the lines of their Ropes and RopeSlices. Ropey can be configured to recognize all Unicode line breaks, while crop only recognizes LF and CRLF as line terminators.

Jumprope doesn't currently have any line-based APIs.

Acknowledgements

• A significant portion of crop's public API was inspired by the excellent Ropey crate (from which I also borrowed some test vectors). Unlike crop, Ropey uses code points (chars in Rust-speak) as its primary indexing metric. If you'd prefer to work with char offsets rather than byte offsets Ropey might be a great alternative;

• Even though the implementations are quite different, crop's Metric trait was inspired by the homonymous trait in xi_rope. Check out the second blog post in the "Rope science" series by Raph Levien for more infos.