cranpose_ui/

text_layout_result.rs

//! Text layout result with cached glyph positions.
//!
//! This module provides `TextLayoutResult` which caches glyph X positions
//! computed during text measurement, enabling O(1) cursor positioning and
//! selection rendering instead of O(n²) substring measurements.

use std::hash::{Hash, Hasher};

use cranpose_core::hash::default;

/// Layout information for a single line of text.
#[derive(Debug, Clone)]
pub struct LineLayout {
    /// Byte offset where line starts
    pub start_offset: usize,
    /// Byte offset where line ends (exclusive, before \n or at text end)
    pub end_offset: usize,
    /// Y position of line top
    pub y: f32,
    /// Height of line
    pub height: f32,
}

/// Visual glyph bounds emitted by the text shaper.
#[derive(Debug, Clone, Copy, PartialEq)]
pub struct GlyphLayout {
    /// Logical line index for this glyph box.
    pub line_index: usize,
    /// Byte offset where glyph coverage starts.
    pub start_offset: usize,
    /// Byte offset where glyph coverage ends (exclusive).
    pub end_offset: usize,
    /// X position from line origin.
    pub x: f32,
    /// Y position from paragraph top.
    pub y: f32,
    /// Glyph box width.
    pub width: f32,
    /// Glyph box height.
    pub height: f32,
}

/// Cached text layout result with pre-computed glyph positions.
///
/// Compute once during `measure()`, reuse for:
/// - Cursor X position rendering
/// - Selection highlight geometry
/// - Click-to-position cursor
#[derive(Debug, Clone)]
pub struct TextLayoutData {
    /// Total width of laid out text
    pub width: f32,
    /// Total height of laid out text
    pub height: f32,
    /// Height of a single line
    pub line_height: f32,
    /// X position at each character boundary (including end)
    pub glyph_x_positions: Vec<f32>,
    /// Byte offset for each character index
    pub char_to_byte: Vec<usize>,
    /// Line layout information
    pub lines: Vec<LineLayout>,
    /// Visual glyph boxes in shaped order.
    pub glyph_layouts: Vec<GlyphLayout>,
}

/// The measured layout of one piece of text. It maps byte offsets to x positions and back for
/// cursor placement, selection highlights and hit testing.
#[derive(Debug, Clone)]
pub struct TextLayoutResult {
    /// Total width of laid out text
    pub width: f32,
    /// Total height of laid out text
    pub height: f32,
    /// Height of a single line
    pub line_height: f32,
    glyph_x_positions: Vec<f32>,
    cursor_positions_monotonic: bool,
    char_to_byte: Vec<usize>,
    /// Line layout information
    pub lines: Vec<LineLayout>,
    glyph_layouts: Vec<GlyphLayout>,
    text_hash: u64,
}

impl TextLayoutResult {
    /// Creates a new layout result with the given glyph positions.
    pub fn new(text: &str, data: TextLayoutData) -> Self {
        Self {
            width: data.width,
            height: data.height,
            line_height: data.line_height,
            cursor_positions_monotonic: data
                .glyph_x_positions
                .windows(2)
                .all(|pair| pair[0] <= pair[1]),
            glyph_x_positions: data.glyph_x_positions,
            char_to_byte: data.char_to_byte,
            lines: data.lines,
            glyph_layouts: data.glyph_layouts,
            text_hash: Self::hash_text(text),
        }
    }

    /// Returns X position for cursor at given byte offset.
    /// Uses logical byte boundaries, including positions shared by a shaped cluster.
    pub fn get_cursor_x(&self, byte_offset: usize) -> f32 {
        let char_idx = self
            .char_to_byte
            .partition_point(|&byte| byte <= byte_offset)
            .saturating_sub(1);

        self.glyph_x_positions
            .get(char_idx)
            .copied()
            .unwrap_or(self.width)
    }

    /// Returns byte offset for X position.
    /// Uses binary search for monotonic positions and nearest-edge lookup for bidirectional text.
    pub fn get_offset_for_x(&self, x: f32) -> usize {
        if self.glyph_x_positions.is_empty() {
            return 0;
        }
        if !self.cursor_positions_monotonic {
            return self
                .glyph_x_positions
                .iter()
                .enumerate()
                .min_by(|(_, left), (_, right)| (**left - x).abs().total_cmp(&(**right - x).abs()))
                .and_then(|(index, _)| self.char_to_byte.get(index))
                .copied()
                .unwrap_or(0);
        }

        let char_idx = match self
            .glyph_x_positions
            .binary_search_by(|pos| pos.partial_cmp(&x).unwrap_or(std::cmp::Ordering::Equal))
        {
            Ok(i) => i,
            Err(i) => {
                if i == 0 {
                    0
                } else if i >= self.glyph_x_positions.len() {
                    self.glyph_x_positions.len() - 1
                } else {
                    let before = self.glyph_x_positions[i - 1];
                    let after = self.glyph_x_positions[i];
                    if (x - before) < (after - x) { i - 1 } else { i }
                }
            }
        };

        self.char_to_byte.get(char_idx).copied().unwrap_or(0)
    }

    /// Checks if this layout result is valid for the given text.
    pub fn is_valid_for(&self, text: &str) -> bool {
        self.text_hash == Self::hash_text(text)
    }

    /// Returns visual glyph boxes emitted by shaping/layout.
    pub fn glyph_layouts(&self) -> &[GlyphLayout] {
        &self.glyph_layouts
    }

    fn hash_text(text: &str) -> u64 {
        let mut hasher = default::new();
        text.hash(&mut hasher);
        hasher.finish()
    }

    /// Creates a simple layout for monospaced text (for fallback).
    pub fn monospaced(text: &str, char_width: f32, line_height: f32) -> Self {
        let mut glyph_x_positions = Vec::new();
        let mut char_to_byte = Vec::new();
        let mut glyph_layouts = Vec::new();
        let mut cursor_x = 0.0;

        for (byte_offset, _c) in text.char_indices() {
            glyph_x_positions.push(cursor_x);
            char_to_byte.push(byte_offset);
            cursor_x += char_width;
        }
        glyph_x_positions.push(cursor_x);
        char_to_byte.push(text.len());

        let mut line_x = 0.0;
        let mut line_y = 0.0;
        let mut line_index = 0usize;
        for (byte_offset, c) in text.char_indices() {
            if c == '\n' {
                line_index = line_index.saturating_add(1);
                line_y += line_height;
                line_x = 0.0;
                continue;
            }
            let glyph_start = byte_offset;
            let glyph_end = glyph_start + c.len_utf8();
            glyph_layouts.push(GlyphLayout {
                line_index,
                start_offset: glyph_start,
                end_offset: glyph_end,
                x: line_x,
                y: line_y,
                width: char_width,
                height: line_height,
            });
            line_x += char_width;
        }

        let line_texts: Vec<&str> = text.split('\n').collect();
        let line_count = line_texts.len();
        let mut lines = Vec::with_capacity(line_count);
        let mut line_start = 0;
        let mut y = 0.0;
        let mut max_width: f32 = 0.0;

        for (i, line_text) in line_texts.iter().enumerate() {
            let line_end = if i == line_count - 1 {
                text.len()
            } else {
                line_start + line_text.len()
            };

            let line_width = line_text.chars().count() as f32 * char_width;
            max_width = max_width.max(line_width);

            lines.push(LineLayout {
                start_offset: line_start,
                end_offset: line_end,
                y,
                height: line_height,
            });

            line_start = line_end + 1;
            y += line_height;
        }

        if lines.is_empty() {
            lines.push(LineLayout {
                start_offset: 0,
                end_offset: 0,
                y: 0.0,
                height: line_height,
            });
        }

        Self::new(
            text,
            TextLayoutData {
                width: max_width,
                height: lines.len() as f32 * line_height,
                line_height,
                glyph_x_positions,
                char_to_byte,
                lines,
                glyph_layouts,
            },
        )
    }
}

#[cfg(test)]
#[path = "tests/text_layout_result_tests.rs"]
mod tests;