Crate ropey

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Ropey is a utf8 text rope for Rust. It is fast, robust, and can handle huge texts and memory-incoherent edits with ease.

Ropey’s atomic unit of text is Unicode scalar values (or chars in Rust) encoded as utf8. All of Ropey’s editing and slicing operations are done in terms of char indices, which prevents accidental creation of invalid utf8 data.

The library is made up of four main components:

  • Rope: the main rope type.
  • RopeSlice: an immutable view into part of a Rope.
  • iter: iterators over Rope/RopeSlice data.
  • RopeBuilder: an efficient incremental Rope builder.

A Basic Example

Let’s say we want to open up a text file, replace the 516th line (the writing was terrible!), and save it back to disk. It’s contrived, but will give a good sampling of the APIs and how they work together.

use std::fs::File;
use std::io::{BufReader, BufWriter};
use ropey::Rope;

// Load a text file.
let mut text = Rope::from_reader(

// Print the 516th line (zero-indexed) to see the terrible
// writing.
println!("{}", text.line(515));

// Get the start/end char indices of the line.
let start_idx = text.line_to_char(515);
let end_idx = text.line_to_char(516);

// Remove the line...

// ...and replace it with something better.
text.insert(start_idx, "The flowers are... so... dunno.\n");

// Print the changes, along with the previous few lines for context.
let start_idx = text.line_to_char(511);
let end_idx = text.line_to_char(516);
println!("{}", text.slice(start_idx..end_idx));

// Write the file back out to disk.

More examples can be found in the examples directory of the git repository. Many of those examples demonstrate doing non-trivial things with Ropey such as grapheme handling, search-and-replace, and streaming loading of non-utf8 text files.

Low-level APIs

Ropey also provides access to some of its low-level APIs, enabling client code to efficiently work with a Rope’s data and implement new functionality. The most important of those API’s are:

  • The chunk_at_*() chunk-fetching methods of Rope and RopeSlice.
  • The Chunks iterator.
  • The functions in str_utils for operating on &str slices.

Internally, each Rope stores text as a segemented collection of utf8 strings. The chunk-fetching methods and Chunks iterator provide direct access to those strings (or “chunks”) as &str slices, allowing client code to work directly with the underlying utf8 data.

The chunk-fetching methods and str_utils functions are the basic building blocks that Ropey itself uses to build much of its functionality. For example, the Rope::byte_to_char() method can be reimplemented as a free function like this:

use ropey::{

fn byte_to_char(rope: &Rope, byte_idx: usize) -> usize {
    let (chunk, b, c, _) = rope.chunk_at_byte(byte_idx);
    c + byte_to_char_idx(chunk, byte_idx - b)

And this will be just as efficient as Ropey’s implementation.

The chunk-fetching methods in particular are among the fastest functions that Ropey provides, generally operating in the sub-hundred nanosecond range for medium-sized (~200kB) documents on recent-ish computer systems.

A Note About Line Breaks

Some of Ropey’s APIs use the concept of line breaks or lines of text.

Ropey considers the start of the rope and positions immediately after line breaks to be the start of new lines. And it treats line breaks as being a part of the lines they mark the end of.

For example, the rope "Hello" has a single line: "Hello". The rope "Hello\nworld" has two lines: "Hello\n" and "world". And the rope "Hello\nworld\n" has three lines: "Hello\n", "world\n", and "".

Ropey can be configured at build time via feature flags to recognize different line breaks. Ropey always recognizes:

  • U+000A — LF (Line Feed)
  • U+000D U+000A — CRLF (Carriage Return + Line Feed)

With the cr_lines feature, the following are also recognized:

  • U+000D — CR (Carriage Return)

With the unicode_lines feature, in addition to all of the above, the following are also recognized (bringing Ropey into conformance with Unicode Annex #14):

  • U+000B — VT (Vertical Tab)
  • U+000C — FF (Form Feed)
  • U+0085 — NEL (Next Line)
  • U+2028 — Line Separator
  • U+2029 — Paragraph Separator

(Note: unicode_lines is enabled by default, and always implies cr_lines.)

CRLF pairs are always treated as a single line break, and are never split across chunks. Note, however, that slicing can still split them.

A Note About SIMD Acceleration

Ropey has a simd feature flag (enabled by default) that enables explicit SIMD on supported platforms to improve performance.

There is a bit of a footgun here: if you disable default features to configure line break behavior (as per the section above) then SIMD will also get disabled, and performance will suffer. So be careful to explicitly re-enable the simd feature flag (if desired) when doing that.


  • Iterators over a Rope’s data.
  • Utility functions for utf8 string slices.



  • Ropey’s error type.

Type Aliases