oxml_layout/

line.rs

//! Line breaking: converts inline items into laid-out lines.
//!
//! Uses a greedy algorithm with unicode-linebreak for break opportunities.

use crate::error::Result;
use crate::font::FontManager;
use crate::output::{Color, FieldKind, FontId, GroupElement, MediaId, SourceSpan};

/// A tab stop positioned in typographic points.
#[derive(Debug, Clone, Copy, PartialEq)]
pub struct TabStop {
    pub pos_pt: f64,
    pub align: TabAlign,
    pub leader: Option<TabLeader>,
}

/// Paragraph alignment.
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub enum Align {
    Start,
    Center,
    End,
    Justify,
    Distribute,
}

/// Alignment relative to a tab stop.
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub enum TabAlign {
    Left,
    Center,
    Right,
    Decimal,
    Bar,
}

/// Leader style used to fill a tab gap.
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub enum TabLeader {
    None,
    Dot,
    Hyphen,
    Underscore,
    Heavy,
    MiddleDot,
}

/// Underline style applied to a text segment.
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub enum Underline {
    Single,
    Words,
    Double,
    Thick,
    Dotted,
    Dash,
    DotDash,
    DotDotDash,
    Wave,
}

/// Line height rule.
#[derive(Debug, Clone, Copy, PartialEq)]
pub enum LineSpacing {
    Single,
    /// A multiple of the largest text point size on the line.
    Multiple(f64),
    Exact(f64),
    AtLeast(f64),
}

/// An inline item to be placed on a line.
#[derive(Debug, Clone)]
#[non_exhaustive]
pub enum InlineItem {
    /// A shaped text segment.
    Text(TextSegment),
    /// A tab character.
    Tab,
    /// A forced line break.
    LineBreak,
    /// A forced page break.
    PageBreak,
    /// A forced column break.
    ColumnBreak,
    /// An inline image.
    Image {
        width: f64,
        height: f64,
        media_id: MediaId,
    },
    /// A backend-neutral group with child-local coordinates.
    Group {
        width: f64,
        height: f64,
        group: GroupElement,
    },
    /// A numbering marker (rendered before the first line).
    Marker(TextSegment),
}

/// Which stream a note reference belongs to.
///
/// A reference carries only a number in the markup, and the two streams
/// number independently, so a document can hold a footnote and an endnote
/// that share a number. Without the stream the two are indistinguishable and
/// one silently shadows the other.
#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
pub enum NoteStream {
    /// Rendered at the foot of the page carrying the reference.
    Footnote,
    /// Rendered at the end of the document.
    Endnote,
}

/// A reference to one note, unique across both streams.
#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
pub struct NoteRef {
    pub stream: NoteStream,
    pub id: i32,
}

/// A shaped text segment with associated formatting.
#[derive(Debug, Clone)]
pub struct TextSegment {
    pub text: String,
    /// Exact source range for this segment, when directly attributable.
    pub source: Option<SourceSpan>,
    pub font_id: FontId,
    pub font_size: f64,
    pub glyph_ids: Vec<u16>,
    pub advances: Vec<f64>,
    pub width: f64,
    pub ascent: f64,
    pub descent: f64,
    /// Additional font leading included in the natural line advance.
    pub line_gap: f64,
    pub color: Color,
    pub bold: bool,
    pub italic: bool,
    /// Underline style (None = no underline).
    pub underline: Option<Underline>,
    /// Single strikethrough.
    pub strike: bool,
    /// Double strikethrough.
    pub dstrike: bool,
    /// Highlight/background color for the run.
    pub highlight: Option<Color>,
    /// Baseline offset in points (positive = raise, negative = lower).
    pub baseline_offset: f64,
    /// Hyperlink URL if this segment is inside a hyperlink.
    pub hyperlink_url: Option<String>,
    /// If this segment is a field placeholder, the kind of field.
    pub field_kind: Option<FieldKind>,
    /// If this segment is a note reference marker, which note it points at.
    pub note: Option<NoteRef>,
}

/// A single item positioned on a line.
#[derive(Debug, Clone)]
#[non_exhaustive]
pub enum LineItem {
    Text(TextSegment),
    Tab {
        width: f64,
        /// Pre-shaped leader text to fill the tab gap (e.g., dots, hyphens).
        leader: Option<TextSegment>,
    },
    Image {
        width: f64,
        height: f64,
        media_id: MediaId,
    },
    Group {
        width: f64,
        height: f64,
        group: GroupElement,
    },
    Marker(TextSegment),
}

impl LineItem {
    pub fn width(&self) -> f64 {
        match self {
            LineItem::Text(seg) => seg.width,
            LineItem::Tab { width, .. } => *width,
            LineItem::Image { width, .. } => *width,
            LineItem::Group { width, .. } => *width,
            LineItem::Marker(seg) => seg.width,
        }
    }
}

/// A laid-out line within a paragraph.
#[derive(Debug, Clone)]
pub struct LayoutLine {
    pub items: Vec<LineItem>,
    /// Total content width of the line.
    pub width: f64,
    /// Maximum ascent on this line (above baseline).
    pub ascent: f64,
    /// Maximum descent on this line (below baseline).
    pub descent: f64,
    /// Effective leading needed to preserve the tallest run's natural advance.
    pub line_gap: f64,
    /// Total line height.
    pub height: f64,
    /// Left indent for this line.
    pub indent_left: f64,
    /// Available width this line was laid out against.
    pub available_width: f64,
    /// Whether this is the last line of the paragraph.
    pub is_last: bool,
}

impl LayoutLine {
    /// Distance from the line-box top to its text baseline.
    pub fn baseline_offset(&self) -> f64 {
        let leading = self.height - self.ascent - self.descent;
        self.ascent + if leading >= 0.0 { leading / 2.0 } else { 0.0 }
    }
}

/// Parameters for line breaking.
#[derive(Debug, Clone)]
pub struct LineBreakParams {
    /// Total available width (page width minus margins).
    pub available_width: f64,
    /// Left indentation in points.
    pub ind_left: f64,
    /// Right indentation in points.
    pub ind_right: f64,
    /// First line indent in points (positive = indent, 0 if hanging).
    pub ind_first_line: f64,
    /// Hanging indent in points (positive = text lines indented relative to first).
    pub ind_hanging: f64,
    /// Tab stops.
    pub tab_stops: Vec<TabStop>,
    /// Line spacing rule and value.
    pub line_spacing: LineSpacing,
    /// Paragraph justification.
    pub jc: Option<Align>,
    /// Whether width overflow may create automatic line breaks.
    pub wrap: bool,
    /// Extra width kept clear at the start of individual lines, by line index.
    ///
    /// This is how a floating drawing pushes text aside. An empty vector, the
    /// default, reserves nothing and reproduces unwrapped line breaking
    /// exactly.
    pub line_prefix_widths: Vec<f64>,
    /// Extra width kept clear at the end of individual lines, by line index.
    pub line_suffix_widths: Vec<f64>,
}

impl Default for LineBreakParams {
    fn default() -> Self {
        LineBreakParams {
            available_width: 468.0, // US Letter with 1" margins
            line_prefix_widths: Vec::new(),
            line_suffix_widths: Vec::new(),
            ind_left: 0.0,
            ind_right: 0.0,
            ind_first_line: 0.0,
            ind_hanging: 0.0,
            tab_stops: Vec::new(),
            line_spacing: LineSpacing::Single,
            jc: None,
            wrap: true,
        }
    }
}

/// Break inline items into lines using a greedy algorithm.
pub fn break_into_lines(
    items: &[InlineItem],
    params: &LineBreakParams,
    fm: &FontManager,
) -> Result<Vec<LayoutLine>> {
    if items.is_empty() {
        // Empty paragraph still gets one empty line
        return Ok(vec![LayoutLine {
            items: Vec::new(),
            width: 0.0,
            ascent: 0.0,
            descent: 0.0,
            line_gap: 0.0,
            height: compute_line_height(0.0, 0.0, 0.0, 0.0, params),
            indent_left: line_indent_at(params, 0, true),
            available_width: line_width_at(params, 0, true),
            is_last: true,
        }]);
    }

    let mut lines: Vec<LayoutLine> = Vec::new();
    let mut current_items: Vec<LineItem> = Vec::new();
    let mut current_width: f64 = 0.0;
    let mut current_ascent: f64 = 0.0;
    let mut current_descent: f64 = 0.0;
    let mut current_natural_height: f64 = 0.0;
    let mut current_font_size: f64 = 0.0;
    // The line index drives the per-line reservations a floating drawing
    // creates, so it is tracked rather than a plain first-or-not flag.
    let mut line_index = 0usize;
    let mut is_first_line = true;

    let first_line_width = line_width_at(params, 0, true);

    let mut line_avail = first_line_width;

    // Track the most recent font context for shaping tab leaders
    let mut font_ctx: Option<(FontId, f64)> = None;
    // Initialize from the first text segment if available
    for item in items {
        if let InlineItem::Text(seg) | InlineItem::Marker(seg) = item {
            font_ctx = Some((seg.font_id, seg.font_size));
            break;
        }
    }

    // Build breakable segments from inline items
    let segments = build_breakable_segments(items, fm)?;

    for seg in &segments {
        match seg {
            BreakableSegment::Items(seg_items) => {
                let seg_width: f64 = seg_items.iter().map(inline_item_width).sum();

                if params.wrap
                    && !current_items.is_empty()
                    && current_width + seg_width > line_avail + 0.01
                {
                    // Finish current line
                    let indent = line_indent_at(params, line_index, is_first_line);
                    let line_gap =
                        effective_line_gap(current_ascent, current_descent, current_natural_height);
                    lines.push(LayoutLine {
                        items: std::mem::take(&mut current_items),
                        width: current_width,
                        ascent: current_ascent,
                        descent: current_descent,
                        line_gap,
                        height: compute_line_height(
                            current_ascent,
                            current_descent,
                            line_gap,
                            current_font_size,
                            params,
                        ),
                        indent_left: indent,
                        available_width: line_avail,
                        is_last: false,
                    });
                    current_width = 0.0;
                    current_ascent = 0.0;
                    current_descent = 0.0;
                    current_natural_height = 0.0;
                    current_font_size = 0.0;
                    is_first_line = false;
                    line_index += 1;
                    line_avail = line_width_at(params, line_index, false);
                }

                // Add segment items to current line
                for item in seg_items {
                    let (w, a, d, natural_height, font_size) = item_metrics(item);
                    current_width += w;
                    if a > current_ascent {
                        current_ascent = a;
                    }
                    if d > current_descent {
                        current_descent = d;
                    }
                    current_natural_height = current_natural_height.max(natural_height);
                    current_font_size = current_font_size.max(font_size);
                    // Update font context from text segments
                    if let InlineItem::Text(seg) | InlineItem::Marker(seg) = item {
                        font_ctx = Some((seg.font_id, seg.font_size));
                    }
                    current_items.push(inline_to_line_item(
                        item,
                        current_width,
                        &params.tab_stops,
                        fm,
                        font_ctx,
                    ));
                }
            }
            BreakableSegment::ForcedBreak(break_type) => {
                let indent = line_indent_at(params, line_index, is_first_line);
                let line_gap =
                    effective_line_gap(current_ascent, current_descent, current_natural_height);
                lines.push(LayoutLine {
                    items: std::mem::take(&mut current_items),
                    width: current_width,
                    ascent: current_ascent,
                    descent: current_descent,
                    line_gap,
                    height: compute_line_height(
                        current_ascent,
                        current_descent,
                        line_gap,
                        current_font_size,
                        params,
                    ),
                    indent_left: indent,
                    available_width: line_avail,
                    is_last: matches!(break_type, ForcedBreakType::Page | ForcedBreakType::Column),
                });
                current_width = 0.0;
                current_ascent = 0.0;
                current_descent = 0.0;
                current_natural_height = 0.0;
                current_font_size = 0.0;
                is_first_line = false;
                line_index += 1;
                line_avail = line_width_at(params, line_index, false);
            }
        }
    }

    // Flush remaining items as the last line
    let indent = line_indent_at(params, line_index, is_first_line);
    let line_gap = effective_line_gap(current_ascent, current_descent, current_natural_height);
    lines.push(LayoutLine {
        items: current_items,
        width: current_width,
        ascent: current_ascent,
        descent: current_descent,
        line_gap,
        height: compute_line_height(
            current_ascent,
            current_descent,
            line_gap,
            current_font_size,
            params,
        ),
        indent_left: indent,
        available_width: line_avail,
        is_last: true,
    });

    Ok(lines)
}

// ---- Internal helpers ----

#[derive(Debug)]
enum BreakableSegment {
    /// A group of items that should be kept together (word or cluster).
    Items(Vec<InlineItem>),
    /// A forced break.
    ForcedBreak(ForcedBreakType),
}

#[derive(Debug)]
enum ForcedBreakType {
    Line,
    Page,
    Column,
}

/// Build breakable segments by finding break opportunities in text.
///
/// Text items are split at unicode line-break opportunities (word boundaries,
/// hyphens, etc.). Non-text items (tabs, images, markers) are treated as
/// atomic units with break opportunities around them.
fn build_breakable_segments(
    items: &[InlineItem],
    fm: &FontManager,
) -> Result<Vec<BreakableSegment>> {
    let mut segments = Vec::new();
    let mut current_group: Vec<InlineItem> = Vec::new();

    for item in items {
        match item {
            InlineItem::LineBreak => {
                if !current_group.is_empty() {
                    segments.push(BreakableSegment::Items(std::mem::take(&mut current_group)));
                }
                segments.push(BreakableSegment::ForcedBreak(ForcedBreakType::Line));
            }
            InlineItem::PageBreak => {
                if !current_group.is_empty() {
                    segments.push(BreakableSegment::Items(std::mem::take(&mut current_group)));
                }
                segments.push(BreakableSegment::ForcedBreak(ForcedBreakType::Page));
            }
            InlineItem::ColumnBreak => {
                if !current_group.is_empty() {
                    segments.push(BreakableSegment::Items(std::mem::take(&mut current_group)));
                }
                segments.push(BreakableSegment::ForcedBreak(ForcedBreakType::Column));
            }
            InlineItem::Tab => {
                // Tab is a break opportunity
                if !current_group.is_empty() {
                    segments.push(BreakableSegment::Items(std::mem::take(&mut current_group)));
                }
                segments.push(BreakableSegment::Items(vec![item.clone()]));
            }
            InlineItem::Text(seg) => {
                if seg.text.is_empty() {
                    if !current_group.is_empty() {
                        segments.push(BreakableSegment::Items(std::mem::take(&mut current_group)));
                    }
                    segments.push(BreakableSegment::Items(vec![item.clone()]));
                    continue;
                }
                // Use unicode-linebreak to find break opportunities within text
                let breaks = split_text_at_break_opportunities(seg);

                for tb in &breaks {
                    let chunk = &seg.text[tb.start..tb.end];
                    if chunk.is_empty() {
                        continue;
                    }

                    // If this chunk starts with whitespace, treat as a break opportunity
                    if !current_group.is_empty() && chunk.starts_with(|c: char| c.is_whitespace()) {
                        segments.push(BreakableSegment::Items(std::mem::take(&mut current_group)));
                    }

                    // Create a sub-segment for just this chunk (not the entire text)
                    let sub_item = split_text_subsegment(seg, tb.start, tb.end, fm)?;
                    current_group.push(sub_item);

                    if tb.is_break {
                        segments.push(BreakableSegment::Items(std::mem::take(&mut current_group)));
                    }
                }

                // Flush any remaining
                if !current_group.is_empty() {
                    segments.push(BreakableSegment::Items(std::mem::take(&mut current_group)));
                }
            }
            InlineItem::Marker(_) | InlineItem::Image { .. } | InlineItem::Group { .. } => {
                current_group.push(item.clone());
            }
        }
    }

    if !current_group.is_empty() {
        segments.push(BreakableSegment::Items(current_group));
    }

    Ok(segments)
}

/// Create a sub-segment InlineItem from a byte range within a TextSegment.
///
/// Reshapes the selected text and preserves formatting from the parent segment.
fn split_text_subsegment(
    seg: &TextSegment,
    byte_start: usize,
    byte_end: usize,
    fm: &FontManager,
) -> Result<InlineItem> {
    // If this is the full segment, just clone it
    if byte_start == 0 && byte_end == seg.text.len() {
        return Ok(InlineItem::Text(seg.clone()));
    }

    let sub_text = seg.text[byte_start..byte_end].to_string();
    let mut shaped = fm.shape_text(seg.font_id, &sub_text, seg.font_size)?;
    let original = fm.shape_text(seg.font_id, &seg.text, seg.font_size)?;
    let spacing = if original.advances.len() == seg.advances.len() && !original.advances.is_empty()
    {
        (seg.width - original.width) / original.advances.len() as f64
    } else {
        0.0
    };
    for advance in &mut shaped.advances {
        *advance += spacing;
    }
    shaped.width += spacing * shaped.advances.len() as f64;

    let source = seg.source.map(|source| {
        let start = seg.text[..byte_start].chars().count() as u32;
        let end = seg.text[..byte_end].chars().count() as u32;
        SourceSpan {
            node: source.node,
            char_start: source.char_start + start,
            char_end: source.char_start + end,
        }
    });

    Ok(InlineItem::Text(TextSegment {
        text: sub_text,
        source,
        font_id: seg.font_id,
        font_size: seg.font_size,
        glyph_ids: shaped.glyph_ids,
        advances: shaped.advances,
        width: shaped.width,
        ascent: seg.ascent,
        descent: seg.descent,
        line_gap: seg.line_gap,
        color: seg.color,
        bold: seg.bold,
        italic: seg.italic,
        underline: seg.underline,
        strike: seg.strike,
        dstrike: seg.dstrike,
        highlight: seg.highlight,
        baseline_offset: seg.baseline_offset,
        hyperlink_url: seg.hyperlink_url.clone(),
        field_kind: seg.field_kind,
        note: seg.note,
    }))
}

struct TextBreakInfo {
    /// Byte range within the original text.
    start: usize,
    end: usize,
    /// Whether a line break is allowed after this segment.
    is_break: bool,
}

fn split_text_at_break_opportunities(seg: &TextSegment) -> Vec<TextBreakInfo> {
    use unicode_linebreak::{BreakOpportunity, linebreaks};

    let text = &seg.text;
    if text.is_empty() {
        return vec![];
    }

    let mut breaks = Vec::new();
    let mut last_start = 0;

    for (byte_pos, opportunity) in linebreaks(text) {
        if byte_pos == 0 {
            continue;
        }

        let is_break = matches!(
            opportunity,
            BreakOpportunity::Allowed | BreakOpportunity::Mandatory
        );

        breaks.push(TextBreakInfo {
            start: last_start,
            end: byte_pos,
            is_break,
        });
        last_start = byte_pos;
    }

    // If unicode-linebreak didn't produce any breaks, treat as one chunk
    if breaks.is_empty() {
        breaks.push(TextBreakInfo {
            start: 0,
            end: text.len(),
            is_break: true,
        });
    }

    breaks
}

fn inline_item_width(item: &InlineItem) -> f64 {
    match item {
        InlineItem::Text(seg) => seg.width,
        InlineItem::Tab => 36.0, // Default tab width, will be resolved
        InlineItem::Image { width, .. } => *width,
        InlineItem::Group { width, .. } => *width,
        InlineItem::Marker(seg) => seg.width,
        InlineItem::LineBreak | InlineItem::PageBreak | InlineItem::ColumnBreak => 0.0,
    }
}

fn item_metrics(item: &InlineItem) -> (f64, f64, f64, f64, f64) {
    // Returns (width, ascent, descent, natural height, text font size)
    match item {
        InlineItem::Text(seg) => (
            seg.width,
            seg.ascent,
            seg.descent,
            seg.ascent + seg.descent + seg.line_gap,
            seg.font_size,
        ),
        InlineItem::Marker(seg) => (
            seg.width,
            seg.ascent,
            seg.descent,
            seg.ascent + seg.descent + seg.line_gap,
            0.0,
        ),
        InlineItem::Tab => (36.0, 0.0, 0.0, 0.0, 0.0),
        InlineItem::Image { width, height, .. } => (*width, *height, 0.0, *height, 0.0),
        InlineItem::Group { width, height, .. } => (*width, *height, 0.0, *height, 0.0),
        InlineItem::LineBreak | InlineItem::PageBreak | InlineItem::ColumnBreak => {
            (0.0, 0.0, 0.0, 0.0, 0.0)
        }
    }
}

fn inline_to_line_item(
    item: &InlineItem,
    current_x: f64,
    tab_stops: &[TabStop],
    fm: &FontManager,
    font_ctx: Option<(FontId, f64)>,
) -> LineItem {
    match item {
        InlineItem::Text(seg) => LineItem::Text(seg.clone()),
        InlineItem::Marker(seg) => LineItem::Marker(seg.clone()),
        InlineItem::Tab => {
            let (tab_width, leader_char) = resolve_tab_width(current_x, tab_stops);
            let leader = leader_char.and_then(|ch| shape_leader(fm, font_ctx, ch, tab_width));
            LineItem::Tab {
                width: tab_width,
                leader,
            }
        }
        InlineItem::Image {
            width,
            height,
            media_id,
        } => LineItem::Image {
            width: *width,
            height: *height,
            media_id: *media_id,
        },
        InlineItem::Group {
            width,
            height,
            group,
        } => LineItem::Group {
            width: *width,
            height: *height,
            group: group.clone(),
        },
        InlineItem::LineBreak | InlineItem::PageBreak | InlineItem::ColumnBreak => LineItem::Tab {
            width: 0.0,
            leader: None,
        },
    }
}

/// Shape a leader character repeated to fill the given width.
fn shape_leader(
    fm: &FontManager,
    font_ctx: Option<(FontId, f64)>,
    leader_char: char,
    tab_width: f64,
) -> Option<TextSegment> {
    let (font_id, font_size) = font_ctx?;
    if tab_width < 1.0 {
        return None;
    }

    // Shape a single leader character to get its advance width
    let single = String::from(leader_char);
    let shaped = fm.shape_text(font_id, &single, font_size).ok()?;
    if shaped.glyph_ids.is_empty() {
        return None;
    }
    let char_advance = shaped.advances[0];
    if char_advance < 0.5 {
        return None;
    }

    // Add a small gap between leader chars (about 50% of char width for dots, less for others)
    let spacing = match leader_char {
        '.' | '\u{00B7}' => char_advance * 0.5,
        _ => char_advance * 0.15,
    };
    let step = char_advance + spacing;
    let count = ((tab_width - spacing) / step).floor() as usize;
    if count == 0 {
        return None;
    }

    // Build the repeated leader text and glyph arrays
    let leader_text: String = std::iter::repeat_n(leader_char, count).collect();
    let mut glyph_ids = Vec::with_capacity(count);
    let mut advances = Vec::with_capacity(count);
    for i in 0..count {
        glyph_ids.push(shaped.glyph_ids[0]);
        if i + 1 < count {
            advances.push(char_advance + spacing);
        } else {
            advances.push(char_advance);
        }
    }

    let metrics = fm.metrics(font_id, font_size).ok()?;

    Some(TextSegment {
        text: leader_text,
        source: None,
        font_id,
        font_size,
        glyph_ids,
        advances,
        width: tab_width, // fill the entire tab gap
        ascent: metrics.ascent,
        descent: metrics.descent,
        line_gap: metrics.line_gap,
        color: Color::BLACK,
        bold: false,
        italic: false,
        underline: None,
        strike: false,
        dstrike: false,
        highlight: None,
        baseline_offset: 0.0,
        hyperlink_url: None,
        field_kind: None,
        note: None,
    })
}

/// Resolve tab stop width and leader character based on current x position and defined stops.
fn resolve_tab_width(current_x: f64, tab_stops: &[TabStop]) -> (f64, Option<char>) {
    // Find the next tab stop after the current position
    for stop in tab_stops {
        let stop_pos = stop.pos_pt;
        if stop_pos > current_x {
            let width = match stop.align {
                TabAlign::Left => stop_pos - current_x,
                TabAlign::Center => (stop_pos - current_x).max(0.0),
                TabAlign::Right => (stop_pos - current_x).max(0.0),
                _ => stop_pos - current_x,
            };
            let leader = stop.leader.and_then(|l| match l {
                TabLeader::Dot => Some('.'),
                TabLeader::Hyphen => Some('-'),
                TabLeader::Underscore => Some('_'),
                TabLeader::MiddleDot => Some('\u{00B7}'),
                TabLeader::Heavy => Some('_'),
                TabLeader::None => None,
            });
            return (width, leader);
        }
    }
    // Default tab stops every 0.5 inches (36pt)
    let default_interval = 36.0;
    let next_stop = ((current_x / default_interval).floor() + 1.0) * default_interval;
    (next_stop - current_x, None)
}

fn compute_first_line_width(params: &LineBreakParams) -> f64 {
    if params.ind_hanging > 0.0 {
        // Hanging indent: first line has MORE width (extends left)
        params.available_width - params.ind_left - params.ind_right + params.ind_hanging
    } else {
        params.available_width - params.ind_left - params.ind_right - params.ind_first_line
    }
}

fn compute_subsequent_line_width(params: &LineBreakParams) -> f64 {
    params.available_width - params.ind_left - params.ind_right
}

/// Width kept clear at the start of a given line.
fn line_prefix_width(params: &LineBreakParams, line_index: usize) -> f64 {
    params
        .line_prefix_widths
        .get(line_index)
        .copied()
        .unwrap_or(0.0)
}

/// Width kept clear at the end of a given line.
fn line_suffix_width(params: &LineBreakParams, line_index: usize) -> f64 {
    params
        .line_suffix_widths
        .get(line_index)
        .copied()
        .unwrap_or(0.0)
}

/// Usable width of a line, once anything floating beside it is taken out.
fn line_width_at(params: &LineBreakParams, line_index: usize, is_first_line: bool) -> f64 {
    let base = if is_first_line {
        compute_first_line_width(params)
    } else {
        compute_subsequent_line_width(params)
    };
    (base - line_prefix_width(params, line_index) - line_suffix_width(params, line_index)).max(0.0)
}

/// Where a line starts, once anything floating to its left is taken out.
fn line_indent_at(params: &LineBreakParams, line_index: usize, is_first_line: bool) -> f64 {
    let base = if is_first_line {
        first_line_indent(params)
    } else {
        subsequent_line_indent(params)
    };
    base + line_prefix_width(params, line_index)
}

fn first_line_indent(params: &LineBreakParams) -> f64 {
    if params.ind_hanging > 0.0 {
        params.ind_left - params.ind_hanging
    } else {
        params.ind_left + params.ind_first_line
    }
}

fn subsequent_line_indent(params: &LineBreakParams) -> f64 {
    params.ind_left
}

/// Compute line height based on spacing rules.
fn effective_line_gap(ascent: f64, descent: f64, natural_height: f64) -> f64 {
    (natural_height - ascent - descent).max(0.0)
}

fn compute_line_height(
    ascent: f64,
    descent: f64,
    line_gap: f64,
    font_size: f64,
    params: &LineBreakParams,
) -> f64 {
    let natural = ascent + descent + line_gap;
    let natural = if natural < 1.0 { 12.0 } else { natural }; // minimum for empty lines
    let font_size = if font_size < 1.0 { 12.0 } else { font_size };

    match params.line_spacing {
        LineSpacing::Single => natural,
        LineSpacing::Multiple(factor) => font_size * factor,
        LineSpacing::Exact(points) => points,
        LineSpacing::AtLeast(points) => natural.max(points),
    }
}

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

    fn make_text_segment(text: &str, width: f64) -> TextSegment {
        TextSegment {
            text: text.to_string(),
            source: None,
            font_id: FontId(0),
            font_size: 12.0,
            glyph_ids: vec![],
            advances: vec![],
            width,
            ascent: 10.0,
            descent: 3.0,
            line_gap: 0.0,
            color: Color::BLACK,
            bold: false,
            italic: false,
            underline: None,
            strike: false,
            dstrike: false,
            highlight: None,
            baseline_offset: 0.0,
            hyperlink_url: None,
            field_kind: None,
            note: None,
        }
    }

    fn deterministic_font_manager() -> FontManager {
        FontManager::new_deterministic().expect("bundled fonts should load")
    }

    fn shaped_text_segment(fm: &mut FontManager, text: &str, spacing: f64) -> TextSegment {
        let font_id = fm
            .resolve_font(Some("Carlito"), false, false)
            .expect("bundled Carlito should resolve");
        let metrics = fm.metrics(font_id, 30.0).expect("Carlito metrics");
        let mut shaped = fm.shape_text(font_id, text, 30.0).expect("shape text");
        for advance in &mut shaped.advances {
            *advance += spacing;
        }
        shaped.width += spacing * shaped.advances.len() as f64;
        TextSegment {
            text: text.to_owned(),
            source: None,
            font_id,
            font_size: 30.0,
            glyph_ids: shaped.glyph_ids,
            advances: shaped.advances,
            width: shaped.width,
            ascent: metrics.ascent,
            descent: metrics.descent,
            line_gap: metrics.line_gap,
            color: Color::BLACK,
            bold: false,
            italic: false,
            underline: None,
            strike: false,
            dstrike: false,
            highlight: None,
            baseline_offset: 0.0,
            hyperlink_url: None,
            field_kind: None,
            note: None,
        }
    }

    #[test]
    fn empty_paragraph_gets_one_line() {
        let fm = deterministic_font_manager();
        let lines = break_into_lines(&[], &LineBreakParams::default(), &fm).unwrap();
        assert_eq!(lines.len(), 1);
        assert!(lines[0].is_last);
        assert!(lines[0].items.is_empty());
    }

    #[test]
    fn single_word_fits_one_line() {
        let fm = deterministic_font_manager();
        let items = vec![InlineItem::Text(make_text_segment("Hello", 50.0))];
        let lines = break_into_lines(&items, &LineBreakParams::default(), &fm).unwrap();
        assert_eq!(lines.len(), 1);
        assert!(lines[0].is_last);
    }

    #[test]
    fn words_wrap_to_multiple_lines() {
        let fm = deterministic_font_manager();
        // Each word is 200pt wide, line is 468pt → should wrap
        let mut items = vec![
            InlineItem::Text(make_text_segment("Word1", 200.0)),
            InlineItem::Text(make_text_segment("Word2", 200.0)),
        ];
        items.push(InlineItem::Text(make_text_segment("Word3", 200.0)));

        let lines = break_into_lines(&items, &LineBreakParams::default(), &fm).unwrap();
        assert!(lines.len() >= 2);
    }

    #[test]
    fn ligature_runs_reshape_each_break_chunk_without_duplicate_glyphs() {
        let mut fm = deterministic_font_manager();
        let text = "by providing opportunities to crawl in cluttered spaces and handle 3-dimensional objects";
        let spacing = 0.4;
        let segment = shaped_text_segment(&mut fm, text, spacing);
        assert_ne!(segment.glyph_ids.len(), text.chars().count());

        let lines = break_into_lines(
            &[InlineItem::Text(segment)],
            &LineBreakParams {
                available_width: 260.0,
                ..LineBreakParams::default()
            },
            &fm,
        )
        .expect("wrap ligature-bearing text");
        assert!(lines.len() > 1);

        let mut rendered_text = String::new();
        for text_segment in lines.iter().flat_map(|line| {
            line.items.iter().filter_map(|item| match item {
                LineItem::Text(segment) => Some(segment),
                _ => None,
            })
        }) {
            rendered_text.push_str(&text_segment.text);
            let exact = fm
                .shape_text(
                    text_segment.font_id,
                    &text_segment.text,
                    text_segment.font_size,
                )
                .expect("reshape emitted chunk");
            assert_eq!(text_segment.glyph_ids, exact.glyph_ids);
            assert_eq!(text_segment.advances.len(), exact.advances.len());
            for (actual, unspaced) in text_segment.advances.iter().zip(exact.advances) {
                assert!((actual - (unspaced + spacing)).abs() < 1.0e-10);
            }
        }
        assert_eq!(rendered_text, text);
    }

    #[test]
    fn line_splitting_preserves_contiguous_unicode_source_ranges() {
        let mut fm = deterministic_font_manager();
        let node = crate::SourceNodeId::new(7).expect("a non-zero source id");
        let mut segment = shaped_text_segment(&mut fm, "ab 🚀界 cd", 0.0);
        segment.source = Some(crate::SourceSpan {
            node,
            char_start: 11,
            char_end: 19,
        });

        let lines = break_into_lines(
            &[InlineItem::Text(segment)],
            &LineBreakParams {
                available_width: 55.0,
                ..LineBreakParams::default()
            },
            &fm,
        )
        .expect("split mixed Unicode text");
        let sourced = lines
            .iter()
            .flat_map(|line| &line.items)
            .filter_map(|item| match item {
                LineItem::Text(segment) => segment.source,
                _ => None,
            })
            .collect::<Vec<_>>();

        assert!(sourced.len() > 1, "the fixture must cross a line boundary");
        assert_eq!(sourced.first().expect("first range").char_start, 11);
        assert_eq!(sourced.last().expect("last range").char_end, 19);
        for pair in sourced.windows(2) {
            assert_eq!(pair[0].node, node);
            assert_eq!(pair[0].char_end, pair[1].char_start);
        }
    }

    #[test]
    fn forced_line_break() {
        let fm = deterministic_font_manager();
        let items = vec![
            InlineItem::Text(make_text_segment("Before", 50.0)),
            InlineItem::LineBreak,
            InlineItem::Text(make_text_segment("After", 50.0)),
        ];
        let lines = break_into_lines(&items, &LineBreakParams::default(), &fm).unwrap();
        assert!(lines.len() >= 2);
    }

    #[test]
    fn line_height_exact() {
        let params = LineBreakParams {
            line_spacing: LineSpacing::Exact(24.0),
            ..Default::default()
        };
        let h = compute_line_height(10.0, 3.0, 5.0, 12.0, &params);
        assert!((h - 24.0).abs() < 0.01);
    }

    #[test]
    fn line_height_auto() {
        let params = LineBreakParams {
            line_spacing: LineSpacing::Multiple(2.0),
            ..Default::default()
        };
        let h = compute_line_height(10.0, 3.0, 2.0, 15.0, &params);
        assert!((h - 30.0).abs() < 0.01); // 15 * 2.0
    }

    #[test]
    fn first_line_indent() {
        let params = LineBreakParams {
            ind_first_line: 36.0,
            ..Default::default()
        };
        let first_w = compute_first_line_width(&params);
        let subseq_w = compute_subsequent_line_width(&params);
        assert!(first_w < subseq_w);
    }

    #[test]
    fn hanging_indent() {
        let params = LineBreakParams {
            ind_left: 36.0,
            ind_hanging: 36.0,
            ..Default::default()
        };
        let first_indent = super::first_line_indent(&params);
        let subseq_indent = super::subsequent_line_indent(&params);
        assert!(first_indent < subseq_indent);
    }

    #[test]
    fn tab_stop_resolution() {
        let stops = vec![TabStop {
            pos_pt: 72.0,
            align: TabAlign::Left,
            leader: None,
        }];
        let (w, leader) = resolve_tab_width(36.0, &stops);
        assert!((w - 36.0).abs() < 0.01);
        assert!(leader.is_none());
    }

    #[test]
    fn default_tab_stops() {
        let (w, _) = resolve_tab_width(10.0, &[]);
        assert!((w - 26.0).abs() < 0.01); // next stop at 36pt
    }

    #[test]
    fn tab_stop_with_dot_leader() {
        let stops = vec![TabStop {
            pos_pt: 400.0,
            align: TabAlign::Right,
            leader: Some(TabLeader::Dot),
        }];
        let (w, leader) = resolve_tab_width(100.0, &stops);
        assert!((w - 300.0).abs() < 0.01);
        assert_eq!(leader, Some('.'));
    }

    #[test]
    fn the_eleven_line_tests_pass_with_owned_types() {
        assert!(LineBreakParams::default().wrap);
        empty_paragraph_gets_one_line();
        single_word_fits_one_line();
        words_wrap_to_multiple_lines();
        forced_line_break();
        line_height_exact();
        line_height_auto();
        first_line_indent();
        hanging_indent();
        tab_stop_resolution();
        default_tab_stops();
        tab_stop_with_dot_leader();
    }

    #[test]
    fn line_spacing_variants_preserve_existing_height_rules() {
        let height = |line_spacing| {
            compute_line_height(
                10.0,
                3.0,
                2.0,
                11.0,
                &LineBreakParams {
                    line_spacing,
                    ..Default::default()
                },
            )
        };

        assert!((height(LineSpacing::Single) - 15.0).abs() < 0.01);
        assert!((height(LineSpacing::Multiple(1.5)) - 16.5).abs() < 0.01);
        assert!((height(LineSpacing::Exact(8.25)) - 8.25).abs() < 0.01);
        assert!((height(LineSpacing::AtLeast(8.25)) - 15.0).abs() < 0.01);
        assert!((height(LineSpacing::AtLeast(18.5)) - 18.5).abs() < 0.01);
    }

    #[test]
    fn mixed_font_line_uses_tallest_full_natural_advance() {
        let fm = deterministic_font_manager();
        let mut first = make_text_segment("first", 20.0);
        first.ascent = 10.0;
        first.descent = 2.0;
        first.line_gap = 4.0;
        let mut second = make_text_segment("second", 20.0);
        second.ascent = 8.0;
        second.descent = 5.0;
        second.line_gap = 1.0;

        let lines = break_into_lines(
            &[InlineItem::Text(first), InlineItem::Text(second)],
            &LineBreakParams::default(),
            &fm,
        )
        .expect("lay out mixed-font line");

        assert_eq!(lines.len(), 1);
        assert!((lines[0].ascent - 10.0).abs() < 0.01);
        assert!((lines[0].descent - 5.0).abs() < 0.01);
        assert!((lines[0].line_gap - 1.0).abs() < 0.01);
        assert!((lines[0].height - 16.0).abs() < 0.01);
        assert!((lines[0].baseline_offset() - 10.5).abs() < 0.01);
    }

    #[test]
    fn multiple_spacing_uses_largest_text_point_size_on_each_line() {
        let fm = deterministic_font_manager();
        let mut first = make_text_segment("first", 20.0);
        first.font_size = 12.0;
        first.line_gap = 4.0;
        let mut second = make_text_segment("second", 20.0);
        second.font_size = 20.0;
        second.line_gap = 1.0;

        let lines = break_into_lines(
            &[InlineItem::Text(first), InlineItem::Text(second)],
            &LineBreakParams {
                line_spacing: LineSpacing::Multiple(1.25),
                ..LineBreakParams::default()
            },
            &fm,
        )
        .expect("lay out percentage-spaced mixed-size line");

        assert_eq!(lines.len(), 1);
        assert!((lines[0].height - 25.0).abs() < 0.01);
    }

    #[test]
    fn positive_leading_is_split_and_below_natural_exact_spacing_is_not_clamped() {
        let positive = LayoutLine {
            items: Vec::new(),
            width: 0.0,
            ascent: 10.0,
            descent: 3.0,
            line_gap: 5.0,
            height: 18.0,
            indent_left: 0.0,
            available_width: 100.0,
            is_last: true,
        };
        let below_natural = LayoutLine {
            height: 8.0,
            ..positive.clone()
        };

        assert!((positive.baseline_offset() - 12.5).abs() < 0.01);
        assert!((below_natural.height - 8.0).abs() < 0.01);
        assert!((below_natural.baseline_offset() - 10.0).abs() < 0.01);
    }

    #[test]
    fn zero_gap_and_empty_segment_preserve_natural_height_rules() {
        let fm = deterministic_font_manager();
        let zero_gap = make_text_segment("zero", 20.0);
        let mut empty = make_text_segment("", 0.0);
        empty.line_gap = 4.0;

        let zero_gap_line = break_into_lines(
            &[InlineItem::Text(zero_gap)],
            &LineBreakParams::default(),
            &fm,
        )
        .expect("lay out zero-gap line");
        let empty_line =
            break_into_lines(&[InlineItem::Text(empty)], &LineBreakParams::default(), &fm)
                .expect("lay out styled empty line");

        assert!((zero_gap_line[0].height - 13.0).abs() < 0.01);
        assert!((empty_line[0].height - 17.0).abs() < 0.01);
    }

    #[test]
    fn wrap_false_only_breaks_on_an_explicit_break() {
        let fm = deterministic_font_manager();
        let params = LineBreakParams {
            available_width: 100.0,
            wrap: false,
            ..Default::default()
        };

        for forced_break in [
            InlineItem::LineBreak,
            InlineItem::PageBreak,
            InlineItem::ColumnBreak,
        ] {
            let items = vec![
                InlineItem::Text(make_text_segment("one", 80.0)),
                InlineItem::Text(make_text_segment("two", 80.0)),
                forced_break,
                InlineItem::Text(make_text_segment("three", 80.0)),
                InlineItem::Text(make_text_segment("four", 80.0)),
            ];
            let lines = break_into_lines(&items, &params, &fm).unwrap();

            assert_eq!(lines.len(), 2);
            assert!((lines[0].width - 160.0).abs() < 0.01);
            assert!((lines[1].width - 160.0).abs() < 0.01);
        }
    }

    #[test]
    fn tab_stops_use_point_positions_and_owned_leaders() {
        let mut fm = deterministic_font_manager();
        let font_id = fm
            .resolve_font(Some("Carlito"), false, false)
            .expect("bundled Carlito should resolve");
        let stop = TabStop {
            pos_pt: 72.25,
            align: TabAlign::Decimal,
            leader: Some(TabLeader::Dot),
        };

        let item = inline_to_line_item(&InlineItem::Tab, 12.0, &[stop], &fm, Some((font_id, 12.0)));

        let LineItem::Tab {
            width,
            leader: Some(leader),
        } = item
        else {
            panic!("owned dot leader should shape into a tab line item");
        };
        assert!((width - 60.25).abs() < 0.01);
        assert!((leader.width - 60.25).abs() < 0.01);
        assert!(!leader.glyph_ids.is_empty());
        assert!(leader.text.chars().all(|ch| ch == '.'));
    }

    #[test]
    fn staged_image_types_use_media_id_instead_of_embed_id() {
        let media_id = crate::MediaId::from_bytes(b"image");
        let item = inline_to_line_item(
            &InlineItem::Image {
                width: 10.0,
                height: 20.0,
                media_id,
            },
            0.0,
            &[],
            &deterministic_font_manager(),
            None,
        );
        let LineItem::Image {
            media_id: actual, ..
        } = item
        else {
            panic!("image should remain an image");
        };
        assert_eq!(actual, media_id);
    }

    #[test]
    fn group_inline_item_breaks_and_positions_like_an_image() {
        use crate::{GroupElement, PositionedElement, Transform};

        let group = GroupElement {
            transform: Transform::IDENTITY,
            clip: None,
            opacity: 1.0,
            effects: Vec::new(),
            children: vec![PositionedElement::FilledRect {
                rect: crate::Rect {
                    x: 2.0,
                    y: 3.0,
                    width: 4.0,
                    height: 5.0,
                },
                color: crate::Color::BLACK,
            }],
        };
        let items = vec![InlineItem::Group {
            width: 80.0,
            height: 40.0,
            group: group.clone(),
        }];
        let lines = break_into_lines(
            &items,
            &LineBreakParams {
                available_width: 80.0,
                ..Default::default()
            },
            &deterministic_font_manager(),
        )
        .expect("group line breaking");

        assert_eq!(lines.len(), 1);
        assert_eq!(lines[0].width, 80.0);
        assert_eq!(lines[0].height, 40.0);
        let LineItem::Group {
            width,
            height,
            group: actual,
        } = &lines[0].items[0]
        else {
            panic!("inline group should remain a group line item");
        };
        assert_eq!((*width, *height), (80.0, 40.0));
        assert_eq!(actual, &group);
    }
}