hjkl-buffer 0.34.1

//! Soft-wrap helpers shared between the renderer, viewport scroll,
//! and the buffer's vertical motion code.

use std::cell::RefCell;

use unicode_width::UnicodeWidthChar;

thread_local! {
    /// Reused `(char, width)` scratch for [`wrap_segments`]. `wrap_segments` is
    /// called once per visible row per frame; reusing this buffer avoids a
    /// per-call heap allocation (it grows to the widest line seen, then stays).
    static WRAP_SCRATCH: RefCell<Vec<(char, u16)>> = const { RefCell::new(Vec::new()) };
}

/// Soft-wrap mode controlling how doc rows wider than the text area
/// turn into multiple visual rows. Default is [`Wrap::None`] — every
/// doc row is exactly one screen row and `top_col` clips the left
/// side, mirroring vim's `set nowrap` default for sqeel today.
#[derive(Debug, Clone, Copy, PartialEq, Eq, Default)]
pub enum Wrap {
    /// Single screen row per doc row; clip with `top_col`.
    #[default]
    None,
    /// Break at the cell boundary regardless of word edges.
    Char,
    /// Break at the last whitespace inside the visible width when
    /// possible; falls back to a char break for runs longer than the
    /// width.
    Word,
}

/// Split `line` into char-index segments `[start, end)` such that
/// each segment's display width fits within `width` cells.
/// `Wrap::Word` rewinds to the last whitespace inside the candidate
/// segment when a break would otherwise split a word; falls through
/// to a char break for runs longer than `width`. `Wrap::None` is not
/// expected here — callers branch before calling — but is handled
/// for completeness as a single segment covering the full line.
pub fn wrap_segments(line: &str, width: u16, mode: Wrap) -> Vec<(usize, usize)> {
    if matches!(mode, Wrap::None) || width == 0 || line.is_empty() {
        return vec![(0, line.chars().count())];
    }
    WRAP_SCRATCH.with(|scratch| {
        let mut chars = scratch.borrow_mut();
        chars.clear();
        chars.extend(
            line.chars()
                .map(|c| (c, c.width().unwrap_or(1).max(1) as u16)),
        );
        let total = chars.len();
        let mut segs = Vec::new();
        let mut start = 0usize;
        while start < total {
            let mut cells: u16 = 0;
            let mut i = start;
            while i < total {
                let w = chars[i].1;
                if cells + w > width {
                    break;
                }
                cells += w;
                i += 1;
            }
            // A single char wider than `width` (e.g. a double-width CJK/emoji
            // char in a 1-cell text area) consumes zero cells above, leaving
            // `i == start`. Force progress by emitting it as its own
            // overflowing segment; without this `break_at` collapses to
            // `start`, `start` never advances, and the loop spins forever
            // pushing `(start, start)` until the process OOMs.
            if i == start {
                i = start + 1;
            }
            if i == total {
                segs.push((start, total));
                break;
            }
            let break_at = if matches!(mode, Wrap::Word) {
                // Look for the last whitespace inside [start, i] so the
                // segment ends *after* that whitespace. Falls back to a
                // hard char break when the segment has no whitespace.
                (start..i)
                    .rev()
                    .find(|&k| chars[k].0.is_whitespace())
                    .map(|k| k + 1)
                    .filter(|&end| end > start)
                    .unwrap_or(i)
            } else {
                i
            };
            segs.push((start, break_at));
            start = break_at;
        }
        if segs.is_empty() {
            segs.push((0, 0));
        }
        segs
    })
}

/// Returns the index into `segments` whose `[start, end)` covers
/// `col`. The past-end cursor (`col == last segment's end`) maps to
/// the last segment, matching vim's "EOL on the visual row that
/// holds the line's last char" behaviour.
pub fn segment_for_col(segments: &[(usize, usize)], col: usize) -> usize {
    if segments.is_empty() {
        return 0;
    }
    if let Some(idx) = segments.iter().position(|&(s, e)| col >= s && col < e) {
        return idx;
    }
    segments.len() - 1
}

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

    #[test]
    fn none_returns_full_line_segment() {
        let segs = wrap_segments("hello world", 4, Wrap::None);
        assert_eq!(segs, vec![(0, 11)]);
    }

    #[test]
    fn wide_char_wider_than_width_terminates() {
        // Regression: a double-width char in a 1-cell area used to spin forever
        // (i == start → break_at == start → start never advances → OOM). Each
        // wide char must become its own overflowing segment and progress.
        let segs = wrap_segments("你好", 1, Wrap::Char);
        assert_eq!(segs, vec![(0, 1), (1, 2)]);
        let segs = wrap_segments("你好", 1, Wrap::Word);
        assert_eq!(segs, vec![(0, 1), (1, 2)]);
        // Mixed narrow + wide with a 1-cell width still fully covers the line.
        let segs = wrap_segments("a你b", 1, Wrap::Char);
        assert_eq!(segs, vec![(0, 1), (1, 2), (2, 3)]);
    }

    #[test]
    fn segment_for_col_finds_containing_segment() {
        let segs = vec![(0, 4), (4, 8), (8, 10)];
        assert_eq!(segment_for_col(&segs, 0), 0);
        assert_eq!(segment_for_col(&segs, 3), 0);
        assert_eq!(segment_for_col(&segs, 4), 1);
        assert_eq!(segment_for_col(&segs, 7), 1);
        assert_eq!(segment_for_col(&segs, 9), 2);
        // Past-end col clamps to last segment.
        assert_eq!(segment_for_col(&segs, 10), 2);
        assert_eq!(segment_for_col(&segs, 99), 2);
    }
}