StringZipper

A MoonBit port of the OCaml LSP string zipper data structure. This library provides efficient text editing operations with cursor position tracking, specifically designed for LSP (Language Server Protocol) implementations.

Overview

This is a port of the OCaml LSP string zipper, adapted for MoonBit's native UTF-16 string handling. A StringZipper represents a position (cursor) in a text document and provides operations to:

• Navigate to specific positions using Position information
• Insert text at the current position
• Delete text between positions
• Apply text changes
• Track line and column information

The zipper maintains efficient operations by using a gap buffer-like approach with string views directly.

Simplified MoonBit API

This MoonBit implementation provides a simplified API that takes advantage of MoonBit's native UTF-16 string encoding:

Creating a StringZipper

test "create zipper from string" {
  let zipper = @string_zipper.StringZipper::of_string("Hello World")
  inspect(zipper.to_string(), content="Hello World")
}

Position-based Navigation

test "navigate to position" {
  let zipper = @string_zipper.StringZipper::of_string("Hello World")
  let position = @string_zipper.Position::new(0, 6) // Line 0, character 6
  let new_zipper = zipper.goto_position(position)
  inspect(new_zipper.offset(), content="6")
}

Text Editing

test "insert text" {
  let zipper = @string_zipper.StringZipper::of_string("Hello World")
  let zipper2 = zipper.insert("Beautiful ")
  inspect(zipper2.to_string(), content="Beautiful Hello World")
}

test "apply text change - replace World with MoonBit" {
  let zipper = @string_zipper.StringZipper::of_string("Hello World")
  let start_pos = @string_zipper.Position::new(0, 6)
  let end_pos = @string_zipper.Position::new(0, 11)
  let range = @string_zipper.Range::new(start_pos, end_pos)
  let zipper3 = zipper.apply_change(range, replacement="MoonBit")
  inspect(zipper3.to_string(), content="Hello MoonBit")
}

Line Operations

test "line navigation" {
  let zipper = @string_zipper.StringZipper::of_string("line1\nline2\nline3")

  // Go to line 1 (0-based)
  let zipper2 = zipper.goto_line(1)
  inspect(zipper2.offset(), content="6")

  // Go to end of document
  let zipper3 = zipper.goto_end()
  let final_zipper = zipper3.insert("!")
  inspect(final_zipper.to_string(), content="line1\nline2\nline3!")
}

Features

• Simplified API: No encoding parameters needed - MoonBit strings are natively UTF-16
• Direct String Views: Uses MoonBit's @string.View directly for efficiency
• Unicode Support: Full Unicode support through MoonBit's native string handling
• Production Ready: Uses MoonBit standard library functions
• LSP Compatible: Character positions correspond to UTF-16 code units (standard in LSP)
• Efficient Navigation: O(1) character advance, O(n) for line operations
• Memory Efficient: Uses string views to avoid copying
• Line Tracking: Maintains line numbers automatically
• Immutable: All operations return new zipper instances

API Reference

Core Functions

Function	Description	Example
StringZipper::of_string(text)	Create zipper from string	StringZipper::of_string("hello")
zipper.to_string()	Convert zipper to string	zipper.to_string()
zipper.offset()	Get current position offset	zipper.offset()
zipper.insert(text)	Insert text at position	zipper.insert(" world")
zipper.goto_position(position)	Navigate to position	zipper.goto_position(Position::new(0, 5))
zipper.goto_line(line)	Navigate to line	zipper.goto_line(1)
zipper.goto_end()	Navigate to end	zipper.goto_end()
zipper.apply_change(range, replacement)	Apply text change	zipper.apply_change(range, replacement="new text")
drop_until(start, end)	Delete text between positions	drop_until(zipper1, zipper2)
zipper.squash()	Squash and return (zipper, string)	let (z, s) = zipper.squash()

Utility Functions

Function	Description
zipper.to_string_debug()	Debug representation
add_buffer_between(buffer, start, end)	Add text between positions to buffer

Types

The library defines the following types for position and range operations:

• Position: Represents a position in a text document with line (0-based) and character (0-based, UTF-16 code units) fields
• Range: Represents a range with start and end Position values
• StringZipper: The main data structure for efficient text editing operations

Use Cases

This library is ideal for:

• Language Server Protocol (LSP) implementations
• Text editors and IDEs
• Code transformation tools
• Any application requiring efficient text manipulation with cursor tracking

Advantages of the MoonBit Version

1. Simplified API: No encoding parameters needed
2. Native Performance: Leverages MoonBit's native UTF-16 string handling
3. Type Safety: Strong type system prevents encoding-related bugs
4. Modern Design: Uses MoonBit's string views for zero-copy operations
5. LSP Ready: Character positions are UTF-16 code units by default