Agent skill

ropey

Efficient UTF-8 text rope data structure for editors

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SKILL.md

ropey

Ropey is a UTF-8 text rope library for Rust, optimized for efficient text manipulation in editors. A rope is a tree-based data structure that stores text as a sequence of smaller chunks, enabling O(log N) insertions and deletions at arbitrary positions - far more efficient than String's O(N) for large documents.

Why Ropes?

Traditional strings require shifting all characters after an edit point, making insertions/deletions O(N). Ropes split text into chunks organized in a tree, so edits only affect local nodes:

  • Insert/Delete: O(M + log N) where M is the edited text length
  • Index Conversion: O(log N)
  • Length Queries: O(1)
  • Clone: O(1) with copy-on-write semantics

This makes ropey ideal for text editors handling documents of any size.

Key Types

Rope

The main ownable text container. Cloning is O(1) via data sharing.

rust
use ropey::Rope;

let mut rope = Rope::from_str("Hello world!");
rope.insert(6, "beautiful ");
rope.remove(0..6);
assert_eq!(rope, "beautiful world!");

RopeSlice<'a>

An immutable view into part of a Rope. Behaves like &str but for ropes.

rust
let rope = Rope::from_str("Hello\nWorld\n");
let slice = rope.slice(0..5);  // "Hello"
let line = rope.line(1);       // "World\n" as RopeSlice

RopeBuilder

Efficient incremental rope construction for streaming/large files.

rust
use ropey::RopeBuilder;

let mut builder = RopeBuilder::new();
builder.append("Hello ");
builder.append("world!");
let rope = builder.finish();

Iterators (ropey::iter)

  • Bytes - iterate over raw bytes
  • Chars - iterate over Unicode chars
  • Lines - iterate over lines (includes trailing newline)
  • Chunks - iterate over internal &str chunks (for low-level access)

All iterators support bidirectional traversal via next() and prev().

Common Operations

Creating Ropes

rust
// From string
let rope = Rope::from_str("content");

// From file
let rope = Rope::from_reader(BufReader::new(File::open("file.txt")?))?;

// Empty
let rope = Rope::new();

Editing

rust
let mut rope = Rope::from_str("Hello world!");

// Insert at char index
rope.insert(6, "beautiful ");

// Insert single char
rope.insert_char(0, '!');

// Remove range (char indices)
rope.remove(0..6);

// Append another rope
rope.append(other_rope);

// Split at position
let right = rope.split_off(5);

Querying

rust
let rope = Rope::from_str("Hello\nWorld\n");

// Lengths
rope.len_bytes();  // Total bytes
rope.len_chars();  // Total chars
rope.len_lines();  // Total lines (3 for above)

// Access by index
rope.byte(0);      // First byte
rope.char(0);      // First char: 'H'
rope.line(1);      // Second line as RopeSlice: "World\n"

Slicing

rust
// By char range
let slice = rope.slice(0..5);
let slice = rope.slice(5..);
let slice = rope.slice(..5);

// By byte range (must align with char boundaries!)
let slice = rope.byte_slice(0..5);

Writing to File

rust
rope.write_to(BufWriter::new(File::create("output.txt")?))?;

Index Conversions

Critical: Ropey uses three distinct indexing systems. Mixing them causes bugs.

Method From To
byte_to_char(byte_idx) byte char
byte_to_line(byte_idx) byte line
char_to_byte(char_idx) char byte
char_to_line(char_idx) char line
line_to_byte(line_idx) line byte (start)
line_to_char(line_idx) line char (start)
rust
let rope = Rope::from_str("Hello\nWorld\n");

// Get char index where line 1 starts
let start = rope.line_to_char(1);  // 6

// Get line containing char index 8
let line = rope.char_to_line(8);   // 1

// Convert byte to char (handles multi-byte UTF-8)
let char_idx = rope.byte_to_char(7);

UTF-16 Support (for JS/LSP interop)

rust
rope.len_utf16_cu();           // Total UTF-16 code units
rope.char_to_utf16_cu(idx);    // char -> UTF-16 index
rope.utf16_cu_to_char(idx);    // UTF-16 -> char index

Indexing Caveats

1. Char vs Byte Indices

Ropey's primary indexing is by char (Unicode scalar value), not byte. This matters for non-ASCII text:

rust
let rope = Rope::from_str("cafe");  // 5 bytes, 4 chars
rope.char(3);           // '' (1 char, 2 bytes)
rope.len_chars();       // 4
rope.len_bytes();       // 5

2. Line Counting

  • Lines are zero-indexed
  • len_lines() counts logical lines (text after last newline = extra line)
  • Line iterators include the trailing newline in each line
rust
let rope = Rope::from_str("a\nb");   // 3 lines: "a\n", "b", ""
let rope = Rope::from_str("a\nb\n"); // 3 lines: "a\n", "b\n", ""

3. One-Past-End Indexing

Many methods accept one-past-the-end indices:

rust
let rope = Rope::from_str("abc");  // len_chars() = 3
rope.byte_to_char(3);  // Valid, returns 3
rope.char(3);          // PANICS! char() is strict

4. CRLF Handling

CRLF (\r\n) is treated as a single line break. By default, ropey recognizes:

  • \n (LF)
  • \r\n (CRLF)

With unicode_lines feature (default): also \r, \x0B, \x0C, \u{0085}, \u{2028}, \u{2029}.

Low-Level Chunk Access

For performance-critical code, access raw chunks directly:

rust
// Get chunk containing char index
let (chunk, chunk_byte_idx, chunk_char_idx, chunk_line_idx) = 
    rope.chunk_at_char(char_idx);

// Iterate chunks
for chunk in rope.chunks() {
    // Process &str directly
}

// Use str_utils for operations on chunks
use ropey::str_utils::byte_to_char_idx;
let local_char = byte_to_char_idx(chunk, byte_offset_in_chunk);

Usage in script-kit-gpui

In script-kit-gpui, ropey is used indirectly through gpui-component's Input component for the editor buffer. The crate handles:

  • Editor buffer storage with efficient editing
  • Char/byte offset conversion (critical for cursor positioning)
  • Line-based operations (go to line, line numbers)

The editor.rs file shows patterns for converting between char and byte offsets when dealing with gpui-component's selection API.

Anti-patterns

Using byte indices with char methods

rust
// WRONG - byte index used with char method
let byte_pos = some_api_returning_bytes();
rope.char(byte_pos);  // BUG if byte_pos is a byte index!

// CORRECT
let char_pos = rope.byte_to_char(byte_pos);
rope.char(char_pos);

Assuming 1:1 char:byte mapping

rust
// WRONG - assumes ASCII
let byte_idx = char_idx;

// CORRECT
let byte_idx = rope.char_to_byte(char_idx);

Forgetting line semantics

rust
// WRONG - off by one on last line
let line_count = rope.to_string().split('\n').count();

// CORRECT
let line_count = rope.len_lines();

Mutating while iterating

rust
// WRONG - can't mutate during iteration
for line in rope.lines() {
    rope.insert(0, "prefix");  // BUG!
}

// CORRECT - collect first
let line_count = rope.len_lines();
for i in (0..line_count).rev() {
    let start = rope.line_to_char(i);
    rope.insert(start, "prefix");
}

Ignoring panics on out-of-bounds

rust
// PANICS if idx >= len_chars()
rope.char(idx);

// SAFE - returns Option
rope.get_char(idx);

// SAFE - returns Result
rope.try_char_to_byte(idx);

Performance Tips

  1. Clone freely - O(1) due to structural sharing
  2. Use chunks() for bulk processing - avoid per-char overhead
  3. Prefer RopeSlice over to_string() - zero-copy views
  4. Use RopeBuilder for construction - more efficient than repeated inserts
  5. Batch edits at end - avoid shifting for sequential appends

Feature Flags

  • simd (default): SIMD acceleration for counting/searching
  • unicode_lines (default): Full Unicode line break support
  • cr_lines: Treat standalone \r as line break

Reference

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