Merge pull request #19 from typst/shape-runs 🔀

Text work

5 files changed, 845 insertions, 301 deletions

diff --git a/src/layout/frame.rs b/src/layout/frame.rs
index d3276e99..21fdbf28 100644
--- a/src/layout/frame.rs
+++ b/src/layout/frame.rs
@@ -10,14 +10,16 @@ use crate::geom::{Length, Path, Point, Size};
 pub struct Frame {
     /// The size of the frame.
     pub size: Size,
+    /// The baseline of the frame measured from the top.
+    pub baseline: Length,
     /// The elements composing this layout.
     pub elements: Vec<(Point, Element)>,
 }
 
 impl Frame {
     /// Create a new, empty frame.
-    pub fn new(size: Size) -> Self {
-        Self { size, elements: vec![] }
+    pub fn new(size: Size, baseline: Length) -> Self {
+        Self { size, baseline, elements: vec![] }
     }
 
     /// Add an element at a position.
@@ -38,62 +40,45 @@ impl Frame {
 #[derive(Debug, Clone, PartialEq)]
 pub enum Element {
     /// Shaped text.
-    Text(ShapedText),
+    Text(Text),
     /// A geometric shape.
     Geometry(Geometry),
     /// A raster image.
     Image(Image),
 }
 
-/// A shaped run of text.
+/// A run of shaped text.
 #[derive(Debug, Clone, PartialEq)]
-pub struct ShapedText {
-    /// The font face the text was shaped with.
-    pub face: FaceId,
+pub struct Text {
+    /// The font face the glyphs are contained in.
+    pub face_id: FaceId,
     /// The font size.
     pub size: Length,
-    /// The width.
-    pub width: Length,
-    /// The extent to the top.
-    pub top: Length,
-    /// The extent to the bottom.
-    pub bottom: Length,
     /// The glyph fill color / texture.
     pub color: Fill,
-    /// The shaped glyphs.
-    pub glyphs: Vec<GlyphId>,
-    /// The horizontal offsets of the glyphs. This is indexed parallel to
-    /// `glyphs`. Vertical offsets are not yet supported.
-    pub offsets: Vec<Length>,
+    /// The glyphs.
+    pub glyphs: Vec<Glyph>,
 }
 
-impl ShapedText {
-    /// Create a new shape run with `width` zero and empty `glyphs` and `offsets`.
-    pub fn new(
-        face: FaceId,
-        size: Length,
-        top: Length,
-        bottom: Length,
-        color: Fill,
-    ) -> Self {
-        Self {
-            face,
-            size,
-            width: Length::ZERO,
-            top,
-            bottom,
-            glyphs: vec![],
-            offsets: vec![],
-            color,
-        }
-    }
+/// A glyph in a run of shaped text.
+#[derive(Debug, Copy, Clone, PartialEq)]
+pub struct Glyph {
+    /// The glyph's ID in the face.
+    pub id: GlyphId,
+    /// The advance width of the glyph.
+    pub x_advance: Length,
+    /// The horizontal offset of the glyph.
+    pub x_offset: Length,
+}
 
+impl Text {
     /// Encode the glyph ids into a big-endian byte buffer.
     pub fn encode_glyphs_be(&self) -> Vec<u8> {
         let mut bytes = Vec::with_capacity(2 * self.glyphs.len());
-        for &GlyphId(g) in &self.glyphs {
-            bytes.push((g >> 8) as u8);
-            bytes.push((g & 0xff) as u8);
+        for glyph in &self.glyphs {
+            let id = glyph.id.0;
+            bytes.push((id >> 8) as u8);
+            bytes.push((id & 0xff) as u8);
         }
         bytes
     }
diff --git a/src/layout/pad.rs b/src/layout/pad.rs
index 2c8712af..d24ca654 100644
--- a/src/layout/pad.rs
+++ b/src/layout/pad.rs
@@ -38,6 +38,8 @@ fn pad(frame: &mut Frame, padding: Sides<Linear>) {
     let origin = Point::new(padding.left, padding.top);
 
     frame.size = padded;
+    frame.baseline += origin.y;
+
     for (point, _) in &mut frame.elements {
         *point += origin;
     }
diff --git a/src/layout/par.rs b/src/layout/par.rs
index e0b42821..f7d67981 100644
--- a/src/layout/par.rs
+++ b/src/layout/par.rs
@@ -1,7 +1,14 @@
 use std::fmt::{self, Debug, Formatter};
+use std::mem;
+
+use unicode_bidi::{BidiInfo, Level};
+use xi_unicode::LineBreakIterator;
 
 use super::*;
 use crate::exec::FontProps;
+use crate::util::{RangeExt, SliceExt};
+
+type Range = std::ops::Range<usize>;
 
 /// A node that arranges its children into a paragraph.
 #[derive(Debug, Clone, PartialEq)]
@@ -15,229 +22,534 @@ pub struct ParNode {
 }
 
 /// A child of a paragraph node.
-#[derive(Debug, Clone, PartialEq)]
+#[derive(Clone, PartialEq)]
 pub enum ParChild {
     /// Spacing between other nodes.
     Spacing(Length),
     /// A run of text and how to align it in its line.
-    Text(TextNode, Align),
+    Text(String, FontProps, Align),
     /// Any child node and how to align it in its line.
     Any(AnyNode, Align),
-    /// A forced linebreak.
-    Linebreak,
 }
 
-/// A consecutive, styled run of text.
-#[derive(Clone, PartialEq)]
-pub struct TextNode {
-    /// The text.
-    pub text: String,
-    /// Properties used for font selection and layout.
-    pub props: FontProps,
+impl Layout for ParNode {
+    fn layout(&self, ctx: &mut LayoutContext, areas: &Areas) -> Vec<Frame> {
+        // Collect all text into one string used for BiDi analysis.
+        let text = self.collect_text();
+
+        // Find out the BiDi embedding levels.
+        let bidi = BidiInfo::new(&text, Level::from_dir(self.dir));
+
+        // Build a representation of the paragraph on which we can do
+        // linebreaking without layouting each and every line from scratch.
+        let layout = ParLayout::new(ctx, areas, self, bidi);
+
+        // Find suitable linebreaks.
+        layout.build(ctx, areas.clone(), self)
+    }
+}
+
+impl ParNode {
+    /// Concatenate all text in the paragraph into one string, replacing spacing
+    /// with a space character and other non-text nodes with the object
+    /// replacement character. Returns the full text alongside the range each
+    /// child spans in the text.
+    fn collect_text(&self) -> String {
+        let mut text = String::new();
+        for string in self.strings() {
+            text.push_str(string);
+        }
+        text
+    }
+
+    /// The range of each item in the collected text.
+    fn ranges(&self) -> impl Iterator<Item = Range> + '_ {
+        let mut cursor = 0;
+        self.strings().map(move |string| {
+            let start = cursor;
+            cursor += string.len();
+            start .. cursor
+        })
+    }
+
+    /// The string representation of each child.
+    fn strings(&self) -> impl Iterator<Item = &str> {
+        self.children.iter().map(|child| match child {
+            ParChild::Spacing(_) => " ",
+            ParChild::Text(ref piece, _, _) => piece,
+            ParChild::Any(_, _) => "\u{FFFC}",
+        })
+    }
 }
 
-impl Debug for TextNode {
+impl From<ParNode> for AnyNode {
+    fn from(par: ParNode) -> Self {
+        Self::new(par)
+    }
+}
+
+impl Debug for ParChild {
     fn fmt(&self, f: &mut Formatter) -> fmt::Result {
-        write!(f, "Text({})", self.text)
+        match self {
+            Self::Spacing(amount) => write!(f, "Spacing({:?})", amount),
+            Self::Text(text, _, align) => write!(f, "Text({:?}, {:?})", text, align),
+            Self::Any(any, align) => {
+                f.debug_tuple("Any").field(any).field(align).finish()
+            }
+        }
     }
 }
 
-impl Layout for ParNode {
-    fn layout(&self, ctx: &mut LayoutContext, areas: &Areas) -> Vec<Frame> {
-        let mut layouter = ParLayouter::new(self.dir, self.line_spacing, areas.clone());
-        for child in &self.children {
+/// A paragraph representation in which children are already layouted and text
+/// is separated into shapable runs.
+struct ParLayout<'a> {
+    /// The top-level direction.
+    dir: Dir,
+    /// Bidirectional text embedding levels for the paragraph.
+    bidi: BidiInfo<'a>,
+    /// Layouted children and separated text runs.
+    items: Vec<ParItem<'a>>,
+    /// The ranges of the items in `bidi.text`.
+    ranges: Vec<Range>,
+}
+
+impl<'a> ParLayout<'a> {
+    /// Build a paragraph layout for the given node.
+    fn new(
+        ctx: &mut LayoutContext,
+        areas: &Areas,
+        par: &'a ParNode,
+        bidi: BidiInfo<'a>,
+    ) -> Self {
+        // Prepare an iterator over each child an the range it spans.
+        let mut items = vec![];
+        let mut ranges = vec![];
+
+        // Layout the children and collect them into items.
+        for (range, child) in par.ranges().zip(&par.children) {
             match *child {
-                ParChild::Spacing(amount) => layouter.push_spacing(amount),
-                ParChild::Text(ref node, align) => {
-                    let frame = shape(&node.text, &mut ctx.env.fonts, &node.props);
-                    layouter.push_frame(frame, align);
+                ParChild::Spacing(amount) => {
+                    items.push(ParItem::Spacing(amount));
+                    ranges.push(range);
                 }
-                ParChild::Any(ref node, align) => {
-                    for frame in node.layout(ctx, &layouter.areas) {
-                        layouter.push_frame(frame, align);
+                ParChild::Text(_, ref props, align) => {
+                    // TODO: Also split by language and script.
+                    for (subrange, dir) in split_runs(&bidi, range) {
+                        let text = &bidi.text[subrange.clone()];
+                        let shaped = shape(ctx, text, dir, props);
+                        items.push(ParItem::Text(shaped, align));
+                        ranges.push(subrange);
                     }
                 }
-                ParChild::Linebreak => layouter.finish_line(),
+                ParChild::Any(ref node, align) => {
+                    let frames = node.layout(ctx, areas);
+                    assert_eq!(frames.len(), 1);
+
+                    let frame = frames.into_iter().next().unwrap();
+                    items.push(ParItem::Frame(frame, align));
+                    ranges.push(range);
+                }
+            }
+        }
+
+        Self { dir: par.dir, bidi, items, ranges }
+    }
+
+    /// Find first-fit line breaks and build the paragraph.
+    fn build(self, ctx: &mut LayoutContext, areas: Areas, par: &ParNode) -> Vec<Frame> {
+        let mut stack = LineStack::new(par.line_spacing, areas);
+
+        // The current line attempt.
+        // Invariant: Always fits into `stack.areas.current`.
+        let mut last = None;
+
+        // The start of the line in `last`.
+        let mut start = 0;
+
+        // Find suitable line breaks.
+        // TODO: Provide line break opportunities on alignment changes.
+        for (end, mandatory) in LineBreakIterator::new(self.bidi.text) {
+            // Compute the line and its size.
+            let mut line = LineLayout::new(ctx, &self, start .. end);
+
+            // If the line doesn't fit anymore, we push the last fitting attempt
+            // into the stack and rebuild the line from its end. The resulting
+            // line cannot be broken up further.
+            if !stack.areas.current.fits(line.size) {
+                if let Some((last_line, last_end)) = last.take() {
+                    stack.push(last_line);
+                    start = last_end;
+                    line = LineLayout::new(ctx, &self, start .. end);
+                }
+            }
+
+            // If the line does not fit vertically, we start a new area.
+            if !stack.areas.current.height.fits(line.size.height)
+                && !stack.areas.in_full_last()
+            {
+                stack.finish_area(ctx);
+            }
+
+            if mandatory || !stack.areas.current.width.fits(line.size.width) {
+                // If the line does not fit horizontally or we have a mandatory
+                // line break (i.e. due to "\n"), we push the line into the
+                // stack.
+                stack.push(line);
+                start = end;
+                last = None;
+
+                // If there is a trailing line break at the end of the
+                // paragraph, we want to force an empty line.
+                if mandatory && end == self.bidi.text.len() {
+                    stack.push(LineLayout::new(ctx, &self, end .. end));
+                }
+            } else {
+                // Otherwise, the line fits both horizontally and vertically
+                // and we remember it.
+                last = Some((line, end));
             }
         }
-        layouter.finish()
+
+        if let Some((line, _)) = last {
+            stack.push(line);
+        }
+
+        stack.finish(ctx)
+    }
+
+    /// Find the index of the item whose range contains the `text_offset`.
+    fn find(&self, text_offset: usize) -> Option<usize> {
+        self.ranges.binary_search_by(|r| r.locate(text_offset)).ok()
     }
 }
 
-impl From<ParNode> for AnyNode {
-    fn from(par: ParNode) -> Self {
-        Self::new(par)
+/// Split a range of text into runs of consistent direction.
+fn split_runs<'a>(
+    bidi: &'a BidiInfo,
+    range: Range,
+) -> impl Iterator<Item = (Range, Dir)> + 'a {
+    let mut cursor = range.start;
+    bidi.levels[range.clone()]
+        .group_by_key(|&level| level)
+        .map(move |(level, group)| {
+            let start = cursor;
+            cursor += group.len();
+            (start .. cursor, level.dir())
+        })
+}
+
+/// A prepared item in a paragraph layout.
+enum ParItem<'a> {
+    /// Spacing between other items.
+    Spacing(Length),
+    /// A shaped text run with consistent direction.
+    Text(ShapedText<'a>, Align),
+    /// A layouted child node.
+    Frame(Frame, Align),
+}
+
+impl ParItem<'_> {
+    /// The size of the item.
+    pub fn size(&self) -> Size {
+        match self {
+            Self::Spacing(amount) => Size::new(*amount, Length::ZERO),
+            Self::Text(shaped, _) => shaped.size,
+            Self::Frame(frame, _) => frame.size,
+        }
+    }
+
+    /// The baseline of the item.
+    pub fn baseline(&self) -> Length {
+        match self {
+            Self::Spacing(_) => Length::ZERO,
+            Self::Text(shaped, _) => shaped.baseline,
+            Self::Frame(frame, _) => frame.baseline,
+        }
     }
 }
 
-struct ParLayouter {
-    dirs: Gen<Dir>,
-    main: SpecAxis,
-    cross: SpecAxis,
+/// A simple layouter that stacks lines into areas.
+struct LineStack<'a> {
     line_spacing: Length,
     areas: Areas,
     finished: Vec<Frame>,
-    stack: Vec<(Length, Frame, Align)>,
-    stack_size: Gen<Length>,
-    line: Vec<(Length, Frame, Align)>,
-    line_size: Gen<Length>,
-    line_ruler: Align,
+    lines: Vec<LineLayout<'a>>,
+    size: Size,
 }
 
-impl ParLayouter {
-    fn new(dir: Dir, line_spacing: Length, areas: Areas) -> Self {
+impl<'a> LineStack<'a> {
+    fn new(line_spacing: Length, areas: Areas) -> Self {
         Self {
-            dirs: Gen::new(Dir::TTB, dir),
-            main: SpecAxis::Vertical,
-            cross: SpecAxis::Horizontal,
             line_spacing,
             areas,
             finished: vec![],
-            stack: vec![],
-            stack_size: Gen::ZERO,
-            line: vec![],
-            line_size: Gen::ZERO,
-            line_ruler: Align::Start,
-        }
-    }
-
-    fn push_spacing(&mut self, amount: Length) {
-        let cross_max = self.areas.current.get(self.cross);
-        self.line_size.cross = (self.line_size.cross + amount).min(cross_max);
-    }
-
-    fn push_frame(&mut self, frame: Frame, align: Align) {
-        // When the alignment of the last pushed frame (stored in the "ruler")
-        // is further to the end than the new `frame`, we need a line break.
-        //
-        // For example
-        // ```
-        // #align(right)[First] #align(center)[Second]
-        // ```
-        // would be laid out as:
-        // +----------------------------+
-        // |                      First |
-        // |           Second           |
-        // +----------------------------+
-        if self.line_ruler > align {
-            self.finish_line();
-        }
-
-        // Find out whether the area still has enough space for this frame.
-        // Space occupied by previous lines is already removed from
-        // `areas.current`, but the cross-extent of the current line needs to be
-        // subtracted to make sure the frame fits.
-        let fits = {
-            let mut usable = self.areas.current;
-            *usable.get_mut(self.cross) -= self.line_size.cross;
-            usable.fits(frame.size)
-        };
+            lines: vec![],
+            size: Size::ZERO,
+        }
+    }
 
-        if !fits {
-            self.finish_line();
-
-            // Here, we can directly check whether the frame fits into
-            // `areas.current` since we just called `finish_line`.
-            while !self.areas.current.fits(frame.size) {
-                if self.areas.in_full_last() {
-                    // The frame fits nowhere.
-                    // TODO: Should this be placed into the first area or the last?
-                    // TODO: Produce diagnostic once the necessary spans exist.
-                    break;
-                } else {
-                    self.finish_area();
-                }
+    fn push(&mut self, line: LineLayout<'a>) {
+        self.areas.current.height -= line.size.height + self.line_spacing;
+
+        self.size.width = self.size.width.max(line.size.width);
+        self.size.height += line.size.height;
+        if !self.lines.is_empty() {
+            self.size.height += self.line_spacing;
+        }
+
+        self.lines.push(line);
+    }
+
+    fn finish_area(&mut self, ctx: &mut LayoutContext) {
+        let expand = self.areas.expand.horizontal;
+        self.size.width = expand.resolve(self.size.width, self.areas.full.width);
+
+        let mut output = Frame::new(self.size, self.size.height);
+        let mut first = true;
+        let mut offset = Length::ZERO;
+
+        for line in mem::take(&mut self.lines) {
+            let frame = line.build(ctx, self.size.width);
+            let Frame { size, baseline, .. } = frame;
+
+            let pos = Point::new(Length::ZERO, offset);
+            output.push_frame(pos, frame);
+
+            if first {
+                output.baseline = offset + baseline;
+                first = false;
             }
+
+            offset += size.height + self.line_spacing;
         }
 
-        // A line can contain frames with different alignments. They exact
-        // positions are calculated later depending on the alignments.
-        let size = frame.size.switch(self.main);
-        self.line.push((self.line_size.cross, frame, align));
-        self.line_size.cross += size.cross;
-        self.line_size.main = self.line_size.main.max(size.main);
-        self.line_ruler = align;
+        self.finished.push(output);
+        self.areas.next();
+        self.size = Size::ZERO;
     }
 
-    fn finish_line(&mut self) {
-        let full_size = {
-            let expand = self.areas.expand.get(self.cross);
-            let full = self.areas.full.get(self.cross);
-            Gen::new(
-                self.line_size.main,
-                expand.resolve(self.line_size.cross, full),
-            )
+    fn finish(mut self, ctx: &mut LayoutContext) -> Vec<Frame> {
+        self.finish_area(ctx);
+        self.finished
+    }
+}
+
+/// A lightweight representation of a line that spans a specific range in a
+/// paragraph's text. This type enables you to cheaply measure the size of a
+/// line in a range before comitting to building the line's frame.
+struct LineLayout<'a> {
+    /// The paragraph the line was created in.
+    par: &'a ParLayout<'a>,
+    /// The range the line spans in the paragraph.
+    line: Range,
+    /// A reshaped text item if the line sliced up a text item at the start.
+    first: Option<ParItem<'a>>,
+    /// Middle items which don't need to be reprocessed.
+    items: &'a [ParItem<'a>],
+    /// A reshaped text item if the line sliced up a text item at the end. If
+    /// there is only one text item, this takes precedence over `first`.
+    last: Option<ParItem<'a>>,
+    /// The ranges, indexed as `[first, ..items, last]`. The ranges for `first`
+    /// and `last` aren't trimmed to the line, but it doesn't matter because
+    /// we're just checking which range an index falls into.
+    ranges: &'a [Range],
+    /// The size of the line.
+    size: Size,
+    /// The baseline of the line.
+    baseline: Length,
+}
+
+impl<'a> LineLayout<'a> {
+    /// Create a line which spans the given range.
+    fn new(ctx: &mut LayoutContext, par: &'a ParLayout<'a>, mut line: Range) -> Self {
+        // Find the items which bound the text range.
+        let last_idx = par.find(line.end.saturating_sub(1)).unwrap();
+        let first_idx = if line.is_empty() {
+            last_idx
+        } else {
+            par.find(line.start).unwrap()
         };
 
-        let mut output = Frame::new(full_size.switch(self.main).to_size());
+        // Slice out the relevant items and ranges.
+        let mut items = &par.items[first_idx ..= last_idx];
+        let ranges = &par.ranges[first_idx ..= last_idx];
 
-        for (before, frame, align) in std::mem::take(&mut self.line) {
-            let child_cross_size = frame.size.get(self.cross);
+        // Reshape the last item if it's split in half.
+        let mut last = None;
+        if let Some((ParItem::Text(shaped, align), rest)) = items.split_last() {
+            // Compute the range we want to shape, trimming whitespace at the
+            // end of the line.
+            let base = par.ranges[last_idx].start;
+            let start = line.start.max(base);
+            let end = start + par.bidi.text[start .. line.end].trim_end().len();
+            let range = start - base .. end - base;
 
-            // Position along the cross axis.
-            let cross = align.resolve(if self.dirs.cross.is_positive() {
-                let after_with_self = self.line_size.cross - before;
-                before .. full_size.cross - after_with_self
-            } else {
-                let before_with_self = before + child_cross_size;
-                let after = self.line_size.cross - (before + child_cross_size);
-                full_size.cross - before_with_self .. after
-            });
+            // Reshape if necessary.
+            if range.len() < shaped.text.len() {
+                // If start == end and the rest is empty, then we have an empty
+                // line. To make that line have the appropriate height, we shape the
+                // empty string.
+                if !range.is_empty() || rest.is_empty() {
+                    // Reshape that part.
+                    let reshaped = shaped.reshape(ctx, range);
+                    last = Some(ParItem::Text(reshaped, *align));
+                }
 
-            let pos = Gen::new(Length::ZERO, cross).switch(self.main).to_point();
-            output.push_frame(pos, frame);
+                items = rest;
+                line.end = end;
+            }
         }
 
-        // Add line spacing, but only between lines.
-        if !self.stack.is_empty() {
-            self.stack_size.main += self.line_spacing;
-            *self.areas.current.get_mut(self.main) -= self.line_spacing;
+        // Reshape the start item if it's split in half.
+        let mut first = None;
+        if let Some((ParItem::Text(shaped, align), rest)) = items.split_first() {
+            // Compute the range we want to shape.
+            let Range { start: base, end: first_end } = par.ranges[first_idx];
+            let start = line.start;
+            let end = line.end.min(first_end);
+            let range = start - base .. end - base;
+
+            // Reshape if necessary.
+            if range.len() < shaped.text.len() {
+                if !range.is_empty() {
+                    let reshaped = shaped.reshape(ctx, range);
+                    first = Some(ParItem::Text(reshaped, *align));
+                }
+
+                items = rest;
+            }
         }
 
-        // Update metrics of paragraph and reset for line.
-        self.stack.push((self.stack_size.main, output, self.line_ruler));
-        self.stack_size.main += full_size.main;
-        self.stack_size.cross = self.stack_size.cross.max(full_size.cross);
-        *self.areas.current.get_mut(self.main) -= full_size.main;
-        self.line_size = Gen::ZERO;
-        self.line_ruler = Align::Start;
+        let mut width = Length::ZERO;
+        let mut top = Length::ZERO;
+        let mut bottom = Length::ZERO;
+
+        // Measure the size of the line.
+        for item in first.iter().chain(items).chain(&last) {
+            let size = item.size();
+            let baseline = item.baseline();
+            width += size.width;
+            top = top.max(baseline);
+            bottom = bottom.max(size.height - baseline);
+        }
+
+        Self {
+            par,
+            line,
+            first,
+            items,
+            last,
+            ranges,
+            size: Size::new(width, top + bottom),
+            baseline: top,
+        }
     }
 
-    fn finish_area(&mut self) {
-        let full_size = self.stack_size;
-        let mut output = Frame::new(full_size.switch(self.main).to_size());
+    /// Build the line's frame.
+    fn build(&self, ctx: &mut LayoutContext, width: Length) -> Frame {
+        let full_width = self.size.width.max(width);
+        let full_size = Size::new(full_width, self.size.height);
+        let free_width = full_width - self.size.width;
 
-        for (before, line, cross_align) in std::mem::take(&mut self.stack) {
-            let child_size = line.size.switch(self.main);
+        let mut output = Frame::new(full_size, self.baseline);
+        let mut ruler = Align::Start;
+        let mut offset = Length::ZERO;
 
-            // Position along the main axis.
-            let main = if self.dirs.main.is_positive() {
-                before
-            } else {
-                full_size.main - (before + child_size.main)
+        self.reordered(|item| {
+            let frame = match *item {
+                ParItem::Spacing(amount) => {
+                    offset += amount;
+                    return;
+                }
+                ParItem::Text(ref shaped, align) => {
+                    ruler = ruler.max(align);
+                    shaped.build(ctx)
+                }
+                ParItem::Frame(ref frame, align) => {
+                    ruler = ruler.max(align);
+                    frame.clone()
+                }
             };
 
-            // Align along the cross axis.
-            let cross = cross_align.resolve(if self.dirs.cross.is_positive() {
-                Length::ZERO .. full_size.cross - child_size.cross
-            } else {
-                full_size.cross - child_size.cross .. Length::ZERO
-            });
+            let Frame { size, baseline, .. } = frame;
+            let pos = Point::new(
+                ruler.resolve(self.par.dir, offset .. free_width + offset),
+                self.baseline - baseline,
+            );
 
-            let pos = Gen::new(main, cross).switch(self.main).to_point();
-            output.push_frame(pos, line);
+            output.push_frame(pos, frame);
+            offset += size.width;
+        });
+
+        output
+    }
+
+    /// Iterate through the line's items in visual order.
+    fn reordered(&self, mut f: impl FnMut(&ParItem<'a>)) {
+        // The bidi crate doesn't like empty lines.
+        if self.line.is_empty() {
+            return;
         }
 
-        self.finished.push(output);
-        self.areas.next();
+        // Find the paragraph that contains the line.
+        let para = self
+            .par
+            .bidi
+            .paragraphs
+            .iter()
+            .find(|para| para.range.contains(&self.line.start))
+            .unwrap();
+
+        // Compute the reordered ranges in visual order (left to right).
+        let (levels, runs) = self.par.bidi.visual_runs(para, self.line.clone());
+
+        // Find the items for each run.
+        for run in runs {
+            let first_idx = self.find(run.start).unwrap();
+            let last_idx = self.find(run.end - 1).unwrap();
+            let range = first_idx ..= last_idx;
 
-        // Reset metrics for the whole paragraph.
-        self.stack_size = Gen::ZERO;
+            // Provide the items forwards or backwards depending on the run's
+            // direction.
+            if levels[run.start].is_ltr() {
+                for item in range {
+                    f(self.get(item).unwrap());
+                }
+            } else {
+                for item in range.rev() {
+                    f(self.get(item).unwrap());
+                }
+            }
+        }
     }
 
-    fn finish(mut self) -> Vec<Frame> {
-        self.finish_line();
-        self.finish_area();
-        self.finished
+    /// Find the index of the item whose range contains the `text_offset`.
+    fn find(&self, text_offset: usize) -> Option<usize> {
+        self.ranges.binary_search_by(|r| r.locate(text_offset)).ok()
+    }
+
+    /// Get the item at the index.
+    fn get(&self, index: usize) -> Option<&ParItem<'a>> {
+        self.first.iter().chain(self.items).chain(&self.last).nth(index)
+    }
+}
+
+/// Helper methods for BiDi levels.
+trait LevelExt: Sized {
+    fn from_dir(dir: Dir) -> Option<Self>;
+    fn dir(self) -> Dir;
+}
+
+impl LevelExt for Level {
+    fn from_dir(dir: Dir) -> Option<Self> {
+        match dir {
+            Dir::LTR => Some(Level::ltr()),
+            Dir::RTL => Some(Level::rtl()),
+            _ => None,
+        }
+    }
+
+    fn dir(self) -> Dir {
+        if self.is_ltr() { Dir::LTR } else { Dir::RTL }
     }
 }
diff --git a/src/layout/shaping.rs b/src/layout/shaping.rs
index 8d035516..faa178d3 100644
--- a/src/layout/shaping.rs
+++ b/src/layout/shaping.rs
@@ -1,30 +1,219 @@
+use std::borrow::Cow;
+use std::fmt::{self, Debug, Formatter};
+use std::ops::Range;
+
 use fontdock::FaceId;
 use rustybuzz::UnicodeBuffer;
 use ttf_parser::GlyphId;
 
-use super::{Element, Frame, ShapedText};
+use super::{Element, Frame, Glyph, LayoutContext, Text};
 use crate::env::FontLoader;
 use crate::exec::FontProps;
-use crate::geom::{Point, Size};
+use crate::font::FaceBuf;
+use crate::geom::{Dir, Length, Point, Size};
+use crate::util::SliceExt;
+
+/// The result of shaping text.
+///
+/// This type contains owned or borrowed shaped text runs, which can be
+/// measured, used to reshape substrings more quickly and converted into a
+/// frame.
+pub struct ShapedText<'a> {
+    /// The text that was shaped.
+    pub text: &'a str,
+    /// The text direction.
+    pub dir: Dir,
+    /// The properties used for font selection.
+    pub props: &'a FontProps,
+    /// The font size.
+    pub size: Size,
+    /// The baseline from the top of the frame.
+    pub baseline: Length,
+    /// The shaped glyphs.
+    pub glyphs: Cow<'a, [ShapedGlyph]>,
+}
+
+/// A single glyph resulting from shaping.
+#[derive(Debug, Copy, Clone)]
+pub struct ShapedGlyph {
+    /// The font face the glyph is contained in.
+    pub face_id: FaceId,
+    /// The glyph's ID in the face.
+    pub glyph_id: GlyphId,
+    /// The advance width of the glyph.
+    pub x_advance: i32,
+    /// The horizontal offset of the glyph.
+    pub x_offset: i32,
+    /// The start index of the glyph in the source text.
+    pub text_index: usize,
+    /// Whether splitting the shaping result before this glyph would yield the
+    /// same results as shaping the parts to both sides of `text_index`
+    /// separately.
+    pub safe_to_break: bool,
+}
+
+/// A visual side.
+enum Side {
+    Left,
+    Right,
+}
+
+impl<'a> ShapedText<'a> {
+    /// Build the shaped text's frame.
+    pub fn build(&self, ctx: &mut LayoutContext) -> Frame {
+        let mut frame = Frame::new(self.size, self.baseline);
+        let mut offset = Length::ZERO;
+
+        for (face_id, group) in self.glyphs.as_ref().group_by_key(|g| g.face_id) {
+            let pos = Point::new(offset, self.baseline);
+            let mut text = Text {
+                face_id,
+                size: self.props.size,
+                color: self.props.color,
+                glyphs: vec![],
+            };
+
+            let face = ctx.env.fonts.face(face_id);
+            for glyph in group {
+                let x_advance = face.convert(glyph.x_advance).scale(self.props.size);
+                let x_offset = face.convert(glyph.x_offset).scale(self.props.size);
+                text.glyphs.push(Glyph { id: glyph.glyph_id, x_advance, x_offset });
+                offset += x_advance;
+            }
+
+            frame.push(pos, Element::Text(text));
+        }
+
+        frame
+    }
+
+    /// Reshape a range of the shaped text, reusing information from this
+    /// shaping process if possible.
+    pub fn reshape(
+        &'a self,
+        ctx: &mut LayoutContext,
+        text_range: Range<usize>,
+    ) -> ShapedText<'a> {
+        if let Some(glyphs) = self.slice_safe_to_break(text_range.clone()) {
+            let (size, baseline) = measure(&mut ctx.env.fonts, glyphs, self.props);
+            Self {
+                text: &self.text[text_range],
+                dir: self.dir,
+                props: self.props,
+                size,
+                baseline,
+                glyphs: Cow::Borrowed(glyphs),
+            }
+        } else {
+            shape(ctx, &self.text[text_range], self.dir, self.props)
+        }
+    }
+
+    /// Find the subslice of glyphs that represent the given text range if both
+    /// sides are safe to break.
+    fn slice_safe_to_break(&self, text_range: Range<usize>) -> Option<&[ShapedGlyph]> {
+        let Range { mut start, mut end } = text_range;
+        if !self.dir.is_positive() {
+            std::mem::swap(&mut start, &mut end);
+        }
+
+        let left = self.find_safe_to_break(start, Side::Left)?;
+        let right = self.find_safe_to_break(end, Side::Right)?;
+        Some(&self.glyphs[left .. right])
+    }
+
+    /// Find the glyph offset matching the text index that is most towards the
+    /// given side and safe-to-break.
+    fn find_safe_to_break(&self, text_index: usize, towards: Side) -> Option<usize> {
+        let ltr = self.dir.is_positive();
+
+        // Handle edge cases.
+        let len = self.glyphs.len();
+        if text_index == 0 {
+            return Some(if ltr { 0 } else { len });
+        } else if text_index == self.text.len() {
+            return Some(if ltr { len } else { 0 });
+        }
+
+        // Find any glyph with the text index.
+        let mut idx = self
+            .glyphs
+            .binary_search_by(|g| {
+                let ordering = g.text_index.cmp(&text_index);
+                if ltr { ordering } else { ordering.reverse() }
+            })
+            .ok()?;
+
+        let next = match towards {
+            Side::Left => usize::checked_sub,
+            Side::Right => usize::checked_add,
+        };
+
+        // Search for the outermost glyph with the text index.
+        while let Some(next) = next(idx, 1) {
+            if self.glyphs.get(next).map_or(true, |g| g.text_index != text_index) {
+                break;
+            }
+            idx = next;
+        }
+
+        // RTL needs offset one because the left side of the range should be
+        // exclusive and the right side inclusive, contrary to the normal
+        // behaviour of ranges.
+        if !ltr {
+            idx += 1;
+        }
+
+        self.glyphs[idx].safe_to_break.then(|| idx)
+    }
+}
 
-/// Shape text into a frame containing [`ShapedText`] runs.
-pub fn shape(text: &str, loader: &mut FontLoader, props: &FontProps) -> Frame {
-    let mut frame = Frame::new(Size::ZERO);
-    shape_segment(&mut frame, text, loader, props, props.families.iter(), None);
-    frame
+impl Debug for ShapedText<'_> {
+    fn fmt(&self, f: &mut Formatter) -> fmt::Result {
+        write!(f, "Shaped({:?})", self.text)
+    }
+}
+
+/// Shape text into [`ShapedText`].
+pub fn shape<'a>(
+    ctx: &mut LayoutContext,
+    text: &'a str,
+    dir: Dir,
+    props: &'a FontProps,
+) -> ShapedText<'a> {
+    let loader = &mut ctx.env.fonts;
+
+    let mut glyphs = vec![];
+    let families = props.families.iter();
+    if !text.is_empty() {
+        shape_segment(loader, &mut glyphs, 0, text, dir, props, families, None);
+    }
+
+    let (size, baseline) = measure(loader, &glyphs, props);
+
+    ShapedText {
+        text,
+        dir,
+        props,
+        size,
+        baseline,
+        glyphs: Cow::Owned(glyphs),
+    }
 }
 
-/// Shape text into a frame with font fallback using the `families` iterator.
+/// Shape text with font fallback using the `families` iterator.
 fn shape_segment<'a>(
-    frame: &mut Frame,
-    text: &str,
     loader: &mut FontLoader,
+    glyphs: &mut Vec<ShapedGlyph>,
+    base: usize,
+    text: &str,
+    dir: Dir,
     props: &FontProps,
     mut families: impl Iterator<Item = &'a str> + Clone,
-    mut first: Option<FaceId>,
+    mut first_face: Option<FaceId>,
 ) {
     // Select the font family.
-    let (id, fallback) = loop {
+    let (face_id, fallback) = loop {
         // Try to load the next available font family.
         match families.next() {
             Some(family) => match loader.query(family, props.variant) {
@@ -33,97 +222,140 @@ fn shape_segment<'a>(
             },
             // We're out of families, so we don't do any more fallback and just
             // shape the tofus with the first face we originally used.
-            None => match first {
+            None => match first_face {
                 Some(id) => break (id, false),
                 None => return,
             },
         }
     };
 
-    // Register that this is the first available font.
-    if first.is_none() {
-        first = Some(id);
-    }
-
-    // Find out some metrics and prepare the shaped text container.
-    let face = loader.face(id);
-    let ttf = face.ttf();
-    let units_per_em = f64::from(ttf.units_per_em().unwrap_or(1000));
-    let convert = |units| f64::from(units) / units_per_em * props.size;
-    let top = convert(i32::from(props.top_edge.lookup(ttf)));
-    let bottom = convert(i32::from(props.bottom_edge.lookup(ttf)));
-    let mut shaped = ShapedText::new(id, props.size, top, bottom, props.color);
+    // Remember the id if this the first available face since we use that one to
+    // shape tofus.
+    first_face.get_or_insert(face_id);
 
     // Fill the buffer with our text.
     let mut buffer = UnicodeBuffer::new();
     buffer.push_str(text);
-    buffer.guess_segment_properties();
-
-    // Find out the text direction.
-    // TODO: Replace this once we do BiDi.
-    let rtl = matches!(buffer.direction(), rustybuzz::Direction::RightToLeft);
+    buffer.set_direction(match dir {
+        Dir::LTR => rustybuzz::Direction::LeftToRight,
+        Dir::RTL => rustybuzz::Direction::RightToLeft,
+        _ => unimplemented!(),
+    });
 
     // Shape!
-    let glyphs = rustybuzz::shape(face.buzz(), &[], buffer);
-    let info = glyphs.glyph_infos();
-    let pos = glyphs.glyph_positions();
-    let mut iter = info.iter().zip(pos).peekable();
-
-    while let Some((info, pos)) = iter.next() {
-        // Do font fallback if the glyph is a tofu.
-        if info.codepoint == 0 && fallback {
-            // Flush what we have so far.
-            if !shaped.glyphs.is_empty() {
-                place(frame, shaped);
-                shaped = ShapedText::new(id, props.size, top, bottom, props.color);
-            }
+    let buffer = rustybuzz::shape(loader.face(face_id).ttf(), &[], buffer);
+    let infos = buffer.glyph_infos();
+    let pos = buffer.glyph_positions();
+
+    // Collect the shaped glyphs, doing fallback and shaping parts again with
+    // the next font if necessary.
+    let mut i = 0;
+    while i < infos.len() {
+        let info = &infos[i];
+        let cluster = info.cluster as usize;
 
-            // Determine the start and end cluster index of the tofu sequence.
-            let mut start = info.cluster as usize;
-            let mut end = info.cluster as usize;
-            while let Some((info, _)) = iter.peek() {
-                if info.codepoint != 0 {
-                    break;
+        if info.codepoint != 0 || !fallback {
+            // Add the glyph to the shaped output.
+            // TODO: Don't ignore y_advance and y_offset.
+            glyphs.push(ShapedGlyph {
+                face_id,
+                glyph_id: GlyphId(info.codepoint as u16),
+                x_advance: pos[i].x_advance,
+                x_offset: pos[i].x_offset,
+                text_index: base + cluster,
+                safe_to_break: !info.unsafe_to_break(),
+            });
+        } else {
+            // Determine the source text range for the tofu sequence.
+            let range = {
+                // First, search for the end of the tofu sequence.
+                let k = i;
+                while infos.get(i + 1).map_or(false, |info| info.codepoint == 0) {
+                    i += 1;
                 }
-                end = info.cluster as usize;
-                iter.next();
-            }
 
-            // Because Harfbuzz outputs glyphs in visual order, the start
-            // cluster actually corresponds to the last codepoint in
-            // right-to-left text.
-            if rtl {
-                assert!(end <= start);
-                std::mem::swap(&mut start, &mut end);
-            }
+                // Then, determine the start and end text index.
+                //
+                // Examples:
+                // Everything is shown in visual order. Tofus are written as "_".
+                // We want to find out that the tofus span the text `2..6`.
+                // Note that the clusters are longer than 1 char.
+                //
+                // Left-to-right:
+                // Text:     h a l i h a l l o
+                // Glyphs:   A   _   _   C   E
+                // Clusters: 0   2   4   6   8
+                //              k=1 i=2
+                //
+                // Right-to-left:
+                // Text:     O L L A H I L A H
+                // Glyphs:   E   C   _   _   A
+                // Clusters: 8   6   4   2   0
+                //                  k=2 i=3
+
+                let ltr = dir.is_positive();
+                let first = if ltr { k } else { i };
+                let start = infos[first].cluster as usize;
+
+                let last = if ltr { i.checked_add(1) } else { k.checked_sub(1) };
+                let end = last
+                    .and_then(|last| infos.get(last))
+                    .map_or(text.len(), |info| info.cluster as usize);
 
-            // The end cluster index points right before the last character that
-            // mapped to the tofu sequence. So we have to offset the end by one
-            // char.
-            let offset = text[end ..].chars().next().unwrap().len_utf8();
-            let range = start .. end + offset;
+                start .. end
+            };
 
             // Recursively shape the tofu sequence with the next family.
-            shape_segment(frame, &text[range], loader, props, families.clone(), first);
-        } else {
-            // Add the glyph to the shaped output.
-            // TODO: Don't ignore y_advance and y_offset.
-            let glyph = GlyphId(info.codepoint as u16);
-            shaped.glyphs.push(glyph);
-            shaped.offsets.push(shaped.width + convert(pos.x_offset));
-            shaped.width += convert(pos.x_advance);
+            shape_segment(
+                loader,
+                glyphs,
+                base + range.start,
+                &text[range],
+                dir,
+                props,
+                families.clone(),
+                first_face,
+            );
         }
-    }
 
-    if !shaped.glyphs.is_empty() {
-        place(frame, shaped)
+        i += 1;
     }
 }
 
-/// Place shaped text into a frame.
-fn place(frame: &mut Frame, shaped: ShapedText) {
-    let offset = frame.size.width;
-    frame.size.width += shaped.width;
-    frame.size.height = frame.size.height.max(shaped.top - shaped.bottom);
-    frame.push(Point::new(offset, shaped.top), Element::Text(shaped));
+/// Measure the size and baseline of a run of shaped glyphs with the given
+/// properties.
+fn measure(
+    loader: &mut FontLoader,
+    glyphs: &[ShapedGlyph],
+    props: &FontProps,
+) -> (Size, Length) {
+    let mut width = Length::ZERO;
+    let mut top = Length::ZERO;
+    let mut bottom = Length::ZERO;
+    let mut expand_vertical = |face: &FaceBuf| {
+        top = top.max(face.vertical_metric(props.top_edge).scale(props.size));
+        bottom = bottom.max(-face.vertical_metric(props.bottom_edge).scale(props.size));
+    };
+
+    if glyphs.is_empty() {
+        // When there are no glyphs, we just use the vertical metrics of the
+        // first available font.
+        for family in props.families.iter() {
+            if let Some(face_id) = loader.query(family, props.variant) {
+                expand_vertical(loader.face(face_id));
+                break;
+            }
+        }
+    } else {
+        for (face_id, group) in glyphs.group_by_key(|g| g.face_id) {
+            let face = loader.face(face_id);
+            expand_vertical(face);
+
+            for glyph in group {
+                width += face.convert(glyph.x_advance).scale(props.size);
+            }
+        }
+    }
+
+    (Size::new(width, top + bottom), top)
 }
diff --git a/src/layout/stack.rs b/src/layout/stack.rs
index 79fde72d..b69936ba 100644
--- a/src/layout/stack.rs
+++ b/src/layout/stack.rs
@@ -28,7 +28,13 @@ impl Layout for StackNode {
             match *child {
                 StackChild::Spacing(amount) => layouter.push_spacing(amount),
                 StackChild::Any(ref node, aligns) => {
-                    for frame in node.layout(ctx, &layouter.areas) {
+                    let mut frames = node.layout(ctx, &layouter.areas).into_iter();
+                    if let Some(frame) = frames.next() {
+                        layouter.push_frame(frame, aligns);
+                    }
+
+                    for frame in frames {
+                        layouter.finish_area();
                         layouter.push_frame(frame, aligns);
                     }
                 }
@@ -116,32 +122,39 @@ impl StackLayouter {
                 size = Size::new(width, width / aspect);
             }
 
-            size.switch(self.main)
+            size
         };
 
-        let mut output = Frame::new(full_size.switch(self.main).to_size());
+        let mut output = Frame::new(full_size, full_size.height);
+        let mut first = true;
 
+        let full_size = full_size.switch(self.main);
         for (before, frame, aligns) in std::mem::take(&mut self.frames) {
             let child_size = frame.size.switch(self.main);
 
             // Align along the main axis.
-            let main = aligns.main.resolve(if self.dirs.main.is_positive() {
-                let after_with_self = self.size.main - before;
-                before .. full_size.main - after_with_self
-            } else {
-                let before_with_self = before + child_size.main;
-                let after = self.size.main - (before + child_size.main);
-                full_size.main - before_with_self .. after
-            });
+            let main = aligns.main.resolve(
+                self.dirs.main,
+                if self.dirs.main.is_positive() {
+                    before .. before + full_size.main - self.size.main
+                } else {
+                    self.size.main - (before + child_size.main)
+                        .. full_size.main - (before + child_size.main)
+                },
+            );
 
             // Align along the cross axis.
-            let cross = aligns.cross.resolve(if self.dirs.cross.is_positive() {
-                Length::ZERO .. full_size.cross - child_size.cross
-            } else {
-                full_size.cross - child_size.cross .. Length::ZERO
-            });
+            let cross = aligns.cross.resolve(
+                self.dirs.cross,
+                Length::ZERO .. full_size.cross - child_size.cross,
+            );
 
             let pos = Gen::new(main, cross).switch(self.main).to_point();
+            if first {
+                output.baseline = pos.y + frame.baseline;
+                first = false;
+            }
+
             output.push_frame(pos, frame);
         }