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#102 in Data structures

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JumpRope

Because inserting into a string should be fast.

A rope is a data structure for efficiently editing large strings, or for processing editing traces.

As far as I know, JumpRope is the world's fastest rope implementation.

Unlike traditional strings, JumpRope allows you to:

  • Efficiently insert or delete arbitrary keystrokes from anywhere in the document. Using real world editing traces, jumprope can process about 35-40 million edits per second.
  • Index using unicode character offsets or wchar offsets (like you find in JS and other languages). Jumprope can efficiently convert between these formats.

JumpRope is optimized for large strings like source code files and text documents. If your strings are very small (less than 100 bytes), you should probably just use Rust's built in std String or a small-string-optimized string library like SmartString.

JumpRope is similar to ropey. Ropey supports a few more features (like converting line/column positions). However, jumprope is about 3x faster than ropey when processing real editing operations (see below) and jumprope compiles to a smaller wasm bundle. (Ropey is 30kb brotli compressed, vs 18kb for jumprope).

API documentation

Jumprope on crates.io

Add this to Cargo.toml to use:

jumprope = "1.0.0"

Usage

JumpRope isn't a drop-in replacement for string, but it supports many similar methods. The most important additions are the insert, remove and replace methods - which let you edit strings in-place in (typically) log(n) time relative to the size of the existing document.

use jumprope::JumpRope;

fn main() {
    let mut rope = JumpRope::from("Some large text document");
    rope.insert(5, "really "); // "Some really large text document"
    rope.replace(0..4, "My rad");  // "My rad really large text document"
    assert_eq!(rope, "My rad really large text document");

    // Extract to a string
    let s: String = rope.to_string();
    assert_eq!(s, "My rad really large text document");
}

You can read content back out of a rope by:

  • Converting the rope to a string using rope.to_string() (requires allocations)
  • Iterating over characters using rope.chars()
  • (Fastest) iterating over &str chunks with rope.substrings(). This returns an iterator over contained &str items in the document.

If you want to read a subsection of the rope, you can use rope.slice_substrings(10..20) to read all the content within a given range in the rope. Eg:

fn main() {
    let rope = JumpRope::from("xxxGreetings!xxx");

    let string = rope.slice_substrings(3..13).collect::<String>();
    assert_eq!(string, "Greetings!");
}

For more details, see JumpRope API documentation

Wchar conversion

In some languages (notably Javascript, Java and C#) strings are measured by the number of 2-byte "characters" needed when encoding the string using UTF16.

This is awkward because its difficult to efficiently convert between unicode character offsets (used by jumprope, diamond types and other editors) and these editing locations. The naive approach is an O(n) operation.

Jumprope supports doing this conversion in O(log n) time, by adding extra indexing information to the skip list. This feature is disabled by default, because the extra bookkeeping slows down jumprope by about 15%.

To use this feature, enable the wchar_conversion feature flag:

jumprope = { version = "1.0.0", features = ["wchar_conversion"] }

This feature flag enables a bunch of extra wchar-related methods for interacting with a document:

  • rope.len_wchars() -> usize: Return the length of the string in wchars.
  • rope.chars_to_wchars(chars: usize) -> usize: Convert a char offset to a wchar offset
  • rope.wchars_to_chars(wchars: usize) -> usize: Convert a wchar index back to a unicode character count
  • rope.insert_at_wchar(pos_wchar: usize, content: &str): Insert content at the specified wchar offset
  • rope.remove_at_wchar(range: Range<usize>): Remove the specified range, specified using wchar offsets
  • rope.replace_at_wchar(range: Range<usize>, content: &str): Replace the specified range with content

See documentation on docs.rs for more information about these methods.

Buffered strings

JumpRope also has an API for buffered edits. Usually when humans edit a string, they insert or delete runs of characters. If you merge these editing runs together before applying them, jumprope is about 10x faster again.

Jumprope provides a wrapper API to do this transparently in the form of JumpRopeBuf. JumpRopeBuf does a best-effort attempt to merge incoming writes together before flushing (writing) them to the contained jumprope object.

This API may be missing some methods found on JumpRope. You can usually work around any missing methods by calling rope.borrow() or rope.as_mut() to flush pending changes and access a pointer to the underlying rope. But please file issues if you find any missing functions, because adding direct implementations will usually result in better performance.

See JumpRopeBuf module documentation for usage.

History / motivation

This code is based on an older skiplist based C rope library I wrote several years ago as an excuse to play with skip lists. It has a few notable differences:

  • Instead of simply being implemented as a skiplist, jumprope is a skiplist where each leaf node contains a Gap Buffer.
  • Jumprope is faster. (See table below)

Benchmarks

Running the editing traces from crdt-benchmarks, jumprope is faster than any other library in cargo that I know of:

Running on a single core of a Ryzen 5800X:

Dataset Raw string XiRope Ropey librope (C) Jumprope
automerge-paper 3908.13 ms 518.75 ms 25.16 ms 16.28 ms 6.66 ms
rustcode 569.44 ms DNF 4.71 ms 3.93 ms 1.66 ms
sveltecomponent 41.05 ms 24.83 ms 2.31 ms 1.59 ms 0.59 ms
seph-blog1 1238.44 ms DNF 13.04 ms 10.01 ms 3.81 ms

Full criterion report is here.

I tried AnRope as well, but it crashed while processing these datasets.

LICENSE

Licensed under the ISC license:

Copyright 2018 Joseph Gentle

Permission to use, copy, modify, and/or distribute this software for any purpose with or without fee is hereby granted, provided that the above copyright notice and this permission notice appear in all copies.

THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.

Dependencies

~390KB