storybook/src/position.rs

//! Position tracking utilities for converting byte offsets to line/column
//! positions
//!
//! This module provides efficient conversion between byte offsets and (line,
//! column) positions for LSP support.

use std::collections::HashMap;

/// Position tracker that can quickly convert byte offsets to line/column
/// positions
#[derive(Clone)]
pub struct PositionTracker {
    /// Map from line number to byte offset where that line starts
    line_starts: Vec<usize>,
    /// Cache of offset -> (line, col) lookups
    cache: HashMap<usize, (usize, usize)>,
}

impl PositionTracker {
    /// Create a new position tracker from source text
    pub fn new(source: &str) -> Self {
        let mut line_starts = vec![0];

        // Find all newline positions
        for (offset, ch) in source.char_indices() {
            if ch == '\n' {
                line_starts.push(offset + 1);
            }
        }

        Self {
            line_starts,
            cache: HashMap::new(),
        }
    }

    /// Convert a byte offset to (line, column) position
    /// Returns (line, col) where both are 0-indexed
    pub fn offset_to_position(&mut self, offset: usize) -> (usize, usize) {
        // Check cache first
        if let Some(&pos) = self.cache.get(&offset) {
            return pos;
        }

        // Binary search to find the line
        let line = match self.line_starts.binary_search(&offset) {
            | Ok(line) => line,      // Exact match - start of a line
            | Err(line) => line - 1, // Insert position tells us the line
        };

        let line_start = self.line_starts[line];
        let col = offset - line_start;

        self.cache.insert(offset, (line, col));
        (line, col)
    }

    /// Get the total number of lines
    pub fn line_count(&self) -> usize {
        self.line_starts.len()
    }

    /// Get the byte offset for the start of a line
    pub fn line_offset(&self, line: usize) -> Option<usize> {
        self.line_starts.get(line).copied()
    }
}

/// Create a Span with proper line/column information from byte offsets
pub fn create_span_with_position(
    tracker: &mut PositionTracker,
    start: usize,
    end: usize,
) -> crate::syntax::ast::Span {
    let (start_line, start_col) = tracker.offset_to_position(start);
    let (end_line, end_col) = tracker.offset_to_position(end);

    crate::syntax::ast::Span::with_position(start, end, start_line, start_col, end_line, end_col)
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn test_position_tracking() {
        let source = "line 1\nline 2\nline 3";
        let mut tracker = PositionTracker::new(source);

        // Start of first line
        assert_eq!(tracker.offset_to_position(0), (0, 0));

        // Middle of first line
        assert_eq!(tracker.offset_to_position(3), (0, 3));

        // Start of second line (after first \n)
        assert_eq!(tracker.offset_to_position(7), (1, 0));

        // Start of third line
        assert_eq!(tracker.offset_to_position(14), (2, 0));
    }

    #[test]
    fn test_multiline_unicode() {
        let source = "Hello 世界\nLine 2";
        let mut tracker = PositionTracker::new(source);

        // Start of file
        assert_eq!(tracker.offset_to_position(0), (0, 0));

        // After "Hello "
        assert_eq!(tracker.offset_to_position(6), (0, 6));

        // Start of line 2
        let newline_offset = "Hello 世界\n".len();
        assert_eq!(tracker.offset_to_position(newline_offset), (1, 0));
    }
}