Reshaping with unsafe-to-break ⚡

Co-Authored-By: Martin <mhaug@live.de>

1 files changed, 250 insertions, 113 deletions

diff --git a/src/layout/shaping.rs b/src/layout/shaping.rs
index 7ead0dff..b062f602 100644
--- a/src/layout/shaping.rs
+++ b/src/layout/shaping.rs
@@ -1,3 +1,4 @@
+use std::borrow::Cow;
 use std::fmt::{self, Debug, Formatter};
 use std::ops::Range;
 
@@ -5,86 +6,179 @@ use fontdock::FaceId;
 use rustybuzz::UnicodeBuffer;
 use ttf_parser::GlyphId;
 
-use super::{Element, Frame, ShapedText};
+use super::{Element, Frame, Glyph, Text};
 use crate::env::FontLoader;
 use crate::exec::FontProps;
+use crate::font::FaceBuf;
 use crate::geom::{Dir, Length, Point, Size};
+use crate::util::SliceExt;
 
-/// Shape text into a frame containing [`ShapedText`] runs.
-pub fn shape<'a>(
-    text: &'a str,
-    dir: Dir,
-    loader: &mut FontLoader,
-    props: &'a FontProps,
-) -> ShapeResult<'a> {
-    let iter = props.families.iter();
-    let mut results = vec![];
-    shape_segment(&mut results, text, dir, loader, props, iter, None);
+/// 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]>,
+}
 
-    let mut top = Length::ZERO;
-    let mut bottom = Length::ZERO;
-    for result in &results {
-        top = top.max(result.top);
-        bottom = bottom.max(result.bottom);
-    }
+/// 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 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,
+}
 
-    let mut frame = Frame::new(Size::new(Length::ZERO, top + bottom), top);
+impl<'a> ShapedText<'a> {
+    /// Build the shaped text's frame.
+    pub fn build(&self, loader: &mut FontLoader) -> Frame {
+        let mut frame = Frame::new(self.size, self.baseline);
+        let mut x = Length::ZERO;
 
-    for shaped in results {
-        let offset = frame.size.width;
-        frame.size.width += shaped.width;
+        for (face_id, group) in self.glyphs.as_ref().group_by_key(|g| g.face_id) {
+            let face = loader.face(face_id);
 
-        if !shaped.glyphs.is_empty() {
-            frame.push(Point::new(offset, top), Element::Text(shaped));
-        }
-    }
+            let pos = Point::new(x, self.baseline);
+            let mut text = Text {
+                face_id,
+                size: self.props.size,
+                color: self.props.color,
+                glyphs: vec![],
+            };
 
-    ShapeResult { frame, text, dir, props }
-}
+            for glyph in group {
+                let x_advance = face.convert(self.props.size, glyph.x_advance);
+                let x_offset = face.convert(self.props.size, glyph.x_offset);
+                text.glyphs.push(Glyph { id: glyph.id, x_advance, x_offset });
+                x += x_advance;
+            }
 
-#[derive(Clone)]
-pub struct ShapeResult<'a> {
-    frame: Frame,
-    text: &'a str,
-    dir: Dir,
-    props: &'a FontProps,
-}
+            frame.push(pos, Element::Text(text));
+        }
 
-impl<'a> ShapeResult<'a> {
+        frame
+    }
+
+    /// Reshape a range of the shaped text, reusing information from this
+    /// shaping process if possible.
     pub fn reshape(
-        &self,
-        range: Range<usize>,
+        &'a self,
+        text_range: Range<usize>,
         loader: &mut FontLoader,
-    ) -> ShapeResult<'_> {
-        if range.start == 0 && range.end == self.text.len() {
-            self.clone()
+    ) -> ShapedText<'a> {
+        if let Some(glyphs) = self.slice_safe_to_break(text_range.clone()) {
+            let (size, baseline) = measure(glyphs, loader, self.props);
+            Self {
+                text: &self.text[text_range],
+                dir: self.dir,
+                props: self.props,
+                size,
+                baseline,
+                glyphs: Cow::Borrowed(glyphs),
+            }
         } else {
-            shape(&self.text[range], self.dir, loader, self.props)
+            shape(&self.text[text_range], self.dir, loader, self.props)
         }
     }
 
-    pub fn text(&self) -> &'a str {
-        self.text
-    }
+    /// 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 mut start = self.find_safe_to_break(text_range.start)?;
+        let mut end = self.find_safe_to_break(text_range.end)?;
+
+        if !self.dir.is_positive() {
+            std::mem::swap(&mut start, &mut end);
+        }
+
+        // TODO: Expand to left and right if necessary because
+        //       find_safe_to_break may find any glyph with the text_index.
 
-    pub fn measure(&self) -> (Size, Length) {
-        (self.frame.size, self.frame.baseline)
+        Some(&self.glyphs[start .. end])
     }
 
-    pub fn build(&self) -> Frame {
-        self.frame.clone()
+    /// Find the glyph slice offset at the text index if it's safe to break.
+    fn find_safe_to_break(&self, text_index: usize) -> 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 });
+        }
+
+        // TODO: Do binary search. Take care that RTL needs reversed ordering.
+        let idx = self
+            .glyphs
+            .iter()
+            .position(|g| g.text_index == text_index)
+            .filter(|&i| self.glyphs[i].safe_to_break)?;
+
+        // RTL needs offset one because the the start of the range should
+        // be exclusive and the end inclusive.
+        Some(if ltr { idx } else { idx + 1 })
     }
 }
 
-impl Debug for ShapeResult<'_> {
+impl Debug for ShapedText<'_> {
     fn fmt(&self, f: &mut Formatter) -> fmt::Result {
         write!(f, "Shaped({:?})", self.text)
     }
 }
 
-/// Shape text into a frame with font fallback using the `families` iterator.
+/// Shape text into [`ShapedText`].
+pub fn shape<'a>(
+    text: &'a str,
+    dir: Dir,
+    loader: &mut FontLoader,
+    props: &'a FontProps,
+) -> ShapedText<'a> {
+    let mut glyphs = vec![];
+    let families = props.families.iter();
+    if !text.is_empty() {
+        shape_segment(&mut glyphs, 0, text, dir, loader, props, families, None);
+    }
+
+    let (size, baseline) = measure(&glyphs, loader, props);
+    ShapedText {
+        text,
+        dir,
+        props,
+        size,
+        baseline,
+        glyphs: Cow::Owned(glyphs),
+    }
+}
+
+/// Shape text with font fallback using the `families` iterator.
 fn shape_segment<'a>(
-    results: &mut Vec<ShapedText>,
+    glyphs: &mut Vec<ShapedGlyph>,
+    base: usize,
     text: &str,
     dir: Dir,
     loader: &mut FontLoader,
@@ -93,7 +187,7 @@ fn shape_segment<'a>(
     mut first: 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) {
@@ -111,22 +205,7 @@ fn shape_segment<'a>(
 
     // 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);
-
-    // For empty text, we want a zero-width box with the correct height.
-    if text.is_empty() {
-        results.push(shaped);
-        return;
+        first = Some(face_id);
     }
 
     // Fill the buffer with our text.
@@ -139,59 +218,117 @@ fn shape_segment<'a>(
     });
 
     // 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() {
-                results.push(shaped);
-                shaped = ShapedText::new(id, props.size, top, bottom, props.color);
-            }
+    let buffer = rustybuzz::shape(loader.face(face_id).buzz(), &[], buffer);
+    let infos = buffer.glyph_infos();
+    let pos = buffer.glyph_positions();
 
-            // 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;
-                }
-                end = info.cluster as usize;
-                iter.next();
-            }
+    // Collect the shaped glyphs, reshaping with the next font if necessary.
+    let mut i = 0;
+    while i < infos.len() {
+        let info = &infos[i];
+        let cluster = info.cluster as usize;
 
-            // Because Harfbuzz outputs glyphs in visual order, the start
-            // cluster actually corresponds to the last codepoint in
-            // right-to-left text.
-            if !dir.is_positive() {
-                assert!(end <= start);
-                std::mem::swap(&mut start, &mut end);
+        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,
+                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 {
+            // Do font fallback if the glyph is a tofu.
+            //
+            // 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;
             }
 
-            // 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;
-            let part = &text[range];
+            // Determine the source text range for the tofu sequence.
+            let range = {
+                // Examples
+                //
+                // Here, _ is a tofu.
+                // Note that the glyph cluster length is greater than 1 char!
+                //
+                // Left-to-right clusters:
+                // h a l i h a l l o
+                // A   _   _   C   E
+                // 0   2   4   6   8
+                //
+                // Right-to-left clusters:
+                // O L L A H I L A H
+                // E   C   _   _   A
+                // 8   6   4   2   0
+
+                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(|info| info.cluster as usize)
+                    .unwrap_or(text.len());
+
+                start .. end
+            };
 
             // Recursively shape the tofu sequence with the next family.
-            shape_segment(results, part, dir, 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(
+                glyphs,
+                base + range.start,
+                &text[range],
+                dir,
+                loader,
+                props,
+                families.clone(),
+                first,
+            );
         }
+
+        i += 1;
     }
+}
+
+/// Measure the size and baseline of a run of shaped glyphs with the given
+/// properties.
+fn measure(
+    glyphs: &[ShapedGlyph],
+    loader: &mut FontLoader,
+    props: &FontProps,
+) -> (Size, Length) {
+    let mut top = Length::ZERO;
+    let mut bottom = Length::ZERO;
+    let mut width = Length::ZERO;
+    let mut vertical = |face: &FaceBuf| {
+        top = top.max(face.vertical_metric(props.size, props.top_edge));
+        bottom = bottom.max(-face.vertical_metric(props.size, props.bottom_edge));
+    };
+
+    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) {
+                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);
+            vertical(face);
 
-    if !shaped.glyphs.is_empty() {
-        results.push(shaped);
+            for glyph in group {
+                width += face.convert(props.size, glyph.x_advance);
+            }
+        }
     }
+
+    (Size::new(width, top + bottom), top)
 }