text_typeset/layout/

block.rs

use crate::font::registry::FontRegistry;
use crate::font::resolve::{ResolvedFont, resolve_font};
use crate::layout::line::LayoutLine;
use crate::layout::paragraph::{Alignment, break_into_lines};
use crate::shaping::run::ShapedRun;
use crate::shaping::shaper::{FontMetricsPx, font_metrics_px, shape_text};

/// Computed layout for a single block (paragraph).
pub struct BlockLayout {
    pub block_id: usize,
    /// Document character position of the block start.
    pub position: usize,
    /// Laid out lines within the block.
    pub lines: Vec<LayoutLine>,
    /// Top edge relative to document start (set by flow layout).
    pub y: f32,
    /// Total height: top_margin + sum(line heights) + bottom_margin.
    pub height: f32,
    pub top_margin: f32,
    pub bottom_margin: f32,
    pub left_margin: f32,
    pub right_margin: f32,
    /// Shaped list marker (positioned to the left of the content area).
    /// None if the block is not a list item.
    pub list_marker: Option<ShapedListMarker>,
    /// Block background color (RGBA). None means transparent.
    pub background_color: Option<[f32; 4]>,
}

/// A shaped list marker ready for rendering.
pub struct ShapedListMarker {
    pub run: ShapedRun,
    /// X position of the marker (relative to block left edge, before content indent).
    pub x: f32,
}

/// Parameters extracted from text-document's BlockFormat / TextFormat.
/// This is a plain struct so block layout doesn't depend on text-document types.
pub struct BlockLayoutParams {
    pub block_id: usize,
    pub position: usize,
    pub text: String,
    pub fragments: Vec<FragmentParams>,
    pub alignment: Alignment,
    pub top_margin: f32,
    pub bottom_margin: f32,
    pub left_margin: f32,
    pub right_margin: f32,
    pub text_indent: f32,
    /// List marker text (e.g., "1.", "•", "a)"). Empty if not a list item.
    pub list_marker: String,
    /// Additional left indent for list items (in pixels).
    pub list_indent: f32,
    /// Tab stop positions in pixels from the left margin.
    pub tab_positions: Vec<f32>,
    /// Line height multiplier. 1.0 = normal (from font metrics), 1.5 = 150%, 2.0 = double.
    /// None means use font metrics (ascent + descent + leading).
    pub line_height_multiplier: Option<f32>,
    /// If true, prevent line wrapping. The entire block is one long line.
    pub non_breakable_lines: bool,
    /// Checkbox marker: None = no checkbox, Some(false) = unchecked, Some(true) = checked.
    pub checkbox: Option<bool>,
    /// Block background color (RGBA). None means transparent.
    pub background_color: Option<[f32; 4]>,
}

/// A text fragment with its formatting parameters.
pub struct FragmentParams {
    pub text: String,
    pub offset: usize,
    pub length: usize,
    pub font_family: Option<String>,
    pub font_weight: Option<u32>,
    pub font_bold: Option<bool>,
    pub font_italic: Option<bool>,
    pub font_point_size: Option<u32>,
    pub underline: bool,
    pub overline: bool,
    pub strikeout: bool,
    pub is_link: bool,
    /// Extra space added after each glyph (in pixels). From TextFormat::letter_spacing.
    pub letter_spacing: f32,
    /// Extra space added after space glyphs (in pixels). From TextFormat::word_spacing.
    pub word_spacing: f32,
}

/// Lay out a single block: resolve fonts, shape fragments, break into lines.
pub fn layout_block(
    registry: &FontRegistry,
    params: &BlockLayoutParams,
    available_width: f32,
) -> BlockLayout {
    let effective_left_margin = params.left_margin + params.list_indent;
    let content_width = (available_width - effective_left_margin - params.right_margin).max(0.0);

    // Resolve fonts and shape each fragment
    let mut shaped_runs = Vec::new();
    let mut default_metrics: Option<FontMetricsPx> = None;

    for frag in &params.fragments {
        let resolved = resolve_font(
            registry,
            frag.font_family.as_deref(),
            frag.font_weight,
            frag.font_bold,
            frag.font_italic,
            frag.font_point_size,
        );

        if let Some(resolved) = resolved {
            // Capture default metrics from the first resolved font
            if default_metrics.is_none() {
                default_metrics = font_metrics_px(registry, &resolved);
            }

            if let Some(mut run) = shape_text(registry, &resolved, &frag.text, frag.offset) {
                run.underline = frag.underline;
                run.overline = frag.overline;
                run.strikeout = frag.strikeout;
                run.is_link = frag.is_link;

                // Apply letter_spacing and word_spacing post-shaping
                if frag.letter_spacing != 0.0 || frag.word_spacing != 0.0 {
                    apply_spacing(&mut run, &frag.text, frag.letter_spacing, frag.word_spacing);
                }

                // Apply tab stops
                if !params.tab_positions.is_empty() {
                    apply_tab_stops(&mut run, &frag.text, &params.tab_positions);
                }

                shaped_runs.push(run);
            }
        }
    }

    // Fallback metrics if no fragments resolved
    let metrics = default_metrics.unwrap_or_else(|| get_default_metrics(registry));

    // Non-breakable lines: use infinite width to prevent wrapping
    let wrap_width = if params.non_breakable_lines {
        f32::INFINITY
    } else {
        content_width
    };

    // Break shaped runs into lines
    let mut lines = break_into_lines(
        shaped_runs,
        &params.text,
        wrap_width,
        params.alignment,
        params.text_indent,
        &metrics,
    );

    // Apply line height multiplier
    let line_height_mul = params.line_height_multiplier.unwrap_or(1.0).max(0.1);

    // Compute y positions for each line (relative to block content top)
    let mut y = 0.0f32;
    for line in &mut lines {
        if line_height_mul != 1.0 {
            line.line_height *= line_height_mul;
        }
        line.y = y + line.ascent; // y is the baseline position
        y += line.line_height;
    }

    let content_height = y;
    let total_height = params.top_margin + content_height + params.bottom_margin;

    // Shape list marker or checkbox marker
    let list_marker = if params.checkbox.is_some() {
        shape_checkbox_marker(registry, &metrics, params)
    } else if !params.list_marker.is_empty() {
        shape_list_marker(registry, &metrics, params)
    } else {
        None
    };

    BlockLayout {
        block_id: params.block_id,
        position: params.position,
        lines,
        y: 0.0, // set by flow layout
        height: total_height,
        top_margin: params.top_margin,
        bottom_margin: params.bottom_margin,
        left_margin: effective_left_margin,
        right_margin: params.right_margin,
        list_marker,
        background_color: params.background_color,
    }
}

/// Add letter_spacing (to all glyphs) and word_spacing (to space glyphs).
fn apply_spacing(run: &mut ShapedRun, text: &str, letter_spacing: f32, word_spacing: f32) {
    let mut extra_advance = 0.0f32;
    for glyph in &mut run.glyphs {
        glyph.x_advance += letter_spacing;
        extra_advance += letter_spacing;

        // Add word_spacing to space characters.
        // Detect spaces by mapping cluster back to the text.
        if word_spacing != 0.0 {
            let byte_offset = glyph.cluster as usize;
            if let Some(ch) = text.get(byte_offset..).and_then(|s| s.chars().next())
                && ch == ' '
            {
                glyph.x_advance += word_spacing;
                extra_advance += word_spacing;
            }
        }
    }
    run.advance_width += extra_advance;
}

/// Shape the list marker text and position it in the indent area.
fn shape_list_marker(
    registry: &FontRegistry,
    _metrics: &FontMetricsPx,
    params: &BlockLayoutParams,
) -> Option<ShapedListMarker> {
    // Use the default font for the marker
    let resolved = resolve_font(registry, None, None, None, None, None)?;
    let run = shape_text(registry, &resolved, &params.list_marker, 0)?;

    // Position the marker: right-aligned within the indent area, with a small gap
    let gap = 4.0; // pixels between marker and content
    let marker_x = params.left_margin + params.list_indent - run.advance_width - gap;
    let marker_x = marker_x.max(params.left_margin);

    Some(ShapedListMarker { run, x: marker_x })
}

/// Expand tab character advances to reach the next tab stop position.
fn apply_tab_stops(run: &mut ShapedRun, text: &str, tab_positions: &[f32]) {
    let default_tab = 48.0; // default tab width if no stops defined
    let mut pen_x = 0.0f32;

    for glyph in &mut run.glyphs {
        let byte_offset = glyph.cluster as usize;
        if let Some(ch) = text.get(byte_offset..).and_then(|s| s.chars().next())
            && ch == '\t'
        {
            // Find the next tab stop after the current pen position
            let next_stop = tab_positions
                .iter()
                .find(|&&stop| stop > pen_x + 1.0)
                .copied()
                .unwrap_or_else(|| {
                    // Past all defined stops: use default tab increments
                    let last = tab_positions.last().copied().unwrap_or(0.0);
                    let increment = if tab_positions.len() >= 2 {
                        tab_positions[1] - tab_positions[0]
                    } else {
                        default_tab
                    };
                    let mut stop = last + increment;
                    while stop <= pen_x + 1.0 {
                        stop += increment;
                    }
                    stop
                });

            let tab_advance = next_stop - pen_x;
            let delta = tab_advance - glyph.x_advance;
            glyph.x_advance = tab_advance;
            run.advance_width += delta;
        }
        pen_x += glyph.x_advance;
    }
}

/// Shape a checkbox marker (unchecked or checked) for rendering in the margin.
fn shape_checkbox_marker(
    registry: &FontRegistry,
    _metrics: &FontMetricsPx,
    params: &BlockLayoutParams,
) -> Option<ShapedListMarker> {
    let checked = params.checkbox?;
    let marker_text = if checked { "\u{2611}" } else { "\u{2610}" }; // ballot box with/without check

    let resolved = resolve_font(registry, None, None, None, None, None)?;
    let run = shape_text(registry, &resolved, marker_text, 0)?;

    // If the font doesn't have the ballot box characters, use ASCII fallback
    let run = if run.glyphs.iter().any(|g| g.glyph_id == 0) {
        let fallback_text = if checked { "[x]" } else { "[ ]" };
        shape_text(registry, &resolved, fallback_text, 0)?
    } else {
        run
    };

    let gap = 4.0;
    let marker_x = params.left_margin + params.list_indent - run.advance_width - gap;
    let marker_x = marker_x.max(params.left_margin);

    Some(ShapedListMarker { run, x: marker_x })
}

fn get_default_metrics(registry: &FontRegistry) -> FontMetricsPx {
    if let Some(default_id) = registry.default_font() {
        let resolved = ResolvedFont {
            font_face_id: default_id,
            size_px: registry.default_size_px(),
            face_index: registry.get(default_id).map(|e| e.face_index).unwrap_or(0),
            swash_cache_key: registry
                .get(default_id)
                .map(|e| e.swash_cache_key)
                .unwrap_or_default(),
        };
        if let Some(m) = font_metrics_px(registry, &resolved) {
            return m;
        }
    }
    // Absolute fallback: synthetic metrics for 16px
    FontMetricsPx {
        ascent: 14.0,
        descent: 4.0,
        leading: 0.0,
        underline_offset: -2.0,
        strikeout_offset: 5.0,
        stroke_size: 1.0,
    }
}