Fonts: shape long items in bounded pieces

The shaper lets one call grow to 256 glyphs per input character, as an
embedded font's substitutions decide, and every glyph was then copied out.
Items longer than 1 KiB are now shaped in pieces (cut at a space where one
is near, never before a combining mark or after a joiner), each with the
rest of the paragraph as context and the item's script, and a piece that
expands past 32 glyphs per character falls back to one glyph per character
from the cmap.

crates/fonts/src/shape.rs

@@ -37,56 +37,146 @@ pub struct ShapeParams<'a> {
     pub language: Option<&'a str>,
 }
 
+/// Most bytes of text given to the shaper at once. The shaper lets one call grow its buffers to
+/// 256 glyphs per character (a font's substitutions decide how many), so a long item is shaped in
+/// pieces of this size, each with the rest of the paragraph as context.
+const MAX_SHAPE_CHUNK: usize = 1024;
+/// Most glyphs kept per character of a piece: a piece the font expands further is laid out one
+/// glyph per character from the cmap instead.
+const MAX_GLYPHS_PER_CHAR: usize = 32;
+
 /// Shape `para[range]` with `face`, the rest of `para` as context. Glyphs are in visual order
 /// (left to right) and clusters are byte offsets into `para`. `map` substitutes characters (All
 /// Caps) without moving clusters. An empty or out-of-bounds range shapes nothing.
 pub fn shape_range(face: &FontFace, para: &str, range: Range<usize>, params: &ShapeParams, map: impl Fn(char) -> char) -> Vec<ShapedGlyph> {
-    let (Some(pre), Some(text), Some(post)) = (para.get(..range.start), para.get(range.clone()), para.get(range.end..)) else {
+    let (Some(_), Some(text), Some(_)) = (para.get(..range.start), para.get(range.clone()), para.get(range.end..)) else {
         return Vec::new();
     };
-    let mut out = Vec::with_capacity(text.len());
+    let chunks = chunks(text, MAX_SHAPE_CHUNK);
+    let mut pieces: Vec<Vec<ShapedGlyph>> = Vec::with_capacity(chunks.len());
     let shaped = face.hb().map(|hb| {
         let shaper = face.shaper.shaper(&hb).instance(face.instance.as_ref()).build();
-        let mut buf = UnicodeBuffer::new();
-        buf.set_pre_context(pre);
-        buf.set_post_context(post);
-        for (i, c) in text.char_indices() {
-            buf.add(map(c), (range.start + i) as u32);
-        }
-        buf.guess_segment_properties();
-        if let Some(s) = params.script.and_then(|t| Script::from_iso15924_tag(Tag::new(&t))) {
-            buf.set_script(s);
-        }
-        if let Some(l) = params.language.or_else(|| params.script.and_then(crate::script::default_language)).and_then(|l| l.parse::<Language>().ok())
-        {
-            buf.set_language(l);
-        }
-        buf.set_direction(if params.rtl { Direction::RightToLeft } else { Direction::LeftToRight });
-        let gb = shaper.shape(buf, ShapeOptions::new().features(params.features));
-        for (info, pos) in gb.glyph_infos().iter().zip(gb.glyph_positions()) {
-            out.push(ShapedGlyph {
-                gid: info.glyph_id,
-                cluster: info.cluster as usize,
-                x_advance: pos.x_advance,
-                x_offset: pos.x_offset,
-                y_offset: pos.y_offset,
-            });
+        // Pieces of one item share the script the whole item would be guessed to have.
+        let item_script = (chunks.len() > 1 && params.script.is_none()).then(|| {
+            let mut probe = UnicodeBuffer::new();
+            for c in text.chars() {
+                probe.add(map(c), 0);
+            }
+            probe.guess_segment_properties();
+            probe.script()
+        });
+        for c in &chunks {
+            let r = (range.start + c.start)..(range.start + c.end);
+            let (Some(pre), Some(piece), Some(post)) = (para.get(..r.start), para.get(r.clone()), para.get(r.end..)) else { continue };
+            let mut buf = UnicodeBuffer::new();
+            buf.set_pre_context(pre);
+            buf.set_post_context(post);
+            for (i, c) in piece.char_indices() {
+                buf.add(map(c), (r.start + i) as u32);
+            }
+            buf.guess_segment_properties();
+            if let Some(s) = params.script.and_then(|t| Script::from_iso15924_tag(Tag::new(&t))) {
+                buf.set_script(s);
+            } else if let Some(s) = item_script {
+                buf.set_script(s);
+            }
+            if let Some(l) =
+                params.language.or_else(|| params.script.and_then(crate::script::default_language)).and_then(|l| l.parse::<Language>().ok())
+            {
+                buf.set_language(l);
+            }
+            buf.set_direction(if params.rtl { Direction::RightToLeft } else { Direction::LeftToRight });
+            let gb = shaper.shape(buf, ShapeOptions::new().features(params.features));
+            let budget = piece.chars().count().saturating_mul(MAX_GLYPHS_PER_CHAR);
+            if gb.len() > budget {
+                // Font-controlled expansion past anything real text needs: keep it bounded.
+                pieces.push(cmap_glyphs(face, para, r, params.rtl, &map));
+                continue;
+            }
+            pieces.push(
+                gb.glyph_infos()
+                    .iter()
+                    .zip(gb.glyph_positions())
+                    .map(|(info, pos)| ShapedGlyph {
+                        gid: info.glyph_id,
+                        cluster: info.cluster as usize,
+                        x_advance: pos.x_advance,
+                        x_offset: pos.x_offset,
+                        y_offset: pos.y_offset,
+                    })
+                    .collect(),
+            );
         }
     });
-    if shaped.is_none()
-        && let Some(f) = face.skrifa()
-    {
+    if shaped.is_none() {
         // No shaper for this face: one glyph per character from the cmap, in visual order.
-        let cmap = f.charmap();
-        let gm = f.glyph_metrics(Size::unscaled(), face.location());
-        for (i, c) in text.char_indices() {
-            let g = cmap.map(map(c)).unwrap_or_default();
-            let adv = gm.advance_width(g).unwrap_or(face.upem as f32 * 0.5);
-            out.push(ShapedGlyph { gid: g.to_u32(), cluster: range.start + i, x_advance: adv.round() as i32, x_offset: 0, y_offset: 0 });
-        }
-        if params.rtl {
-            out.reverse();
+        return cmap_glyphs(face, para, range, params.rtl, &map);
+    }
+    // Each piece is in visual order; right to left, the later pieces come first.
+    if params.rtl {
+        pieces.reverse();
+    }
+    let mut out = Vec::with_capacity(pieces.iter().map(Vec::len).sum());
+    for p in pieces {
+        out.extend(p);
+    }
+    out
+}
+
+/// One glyph per character of `para[range]` from the cmap, in visual order.
+fn cmap_glyphs(face: &FontFace, para: &str, range: Range<usize>, rtl: bool, map: &impl Fn(char) -> char) -> Vec<ShapedGlyph> {
+    let (Some(text), Some(f)) = (para.get(range.clone()), face.skrifa()) else { return Vec::new() };
+    let cmap = f.charmap();
+    let gm = f.glyph_metrics(Size::unscaled(), face.location());
+    let mut out = Vec::with_capacity(text.len());
+    for (i, c) in text.char_indices() {
+        let g = cmap.map(map(c)).unwrap_or_default();
+        let adv = gm.advance_width(g).unwrap_or(face.upem as f32 * 0.5);
+        out.push(ShapedGlyph { gid: g.to_u32(), cluster: range.start + i, x_advance: adv.round() as i32, x_offset: 0, y_offset: 0 });
+    }
+    if rtl {
+        out.reverse();
+    }
+    out
+}
+
+/// `text` cut into byte ranges of at most `max` bytes (or one character, if longer), at the last
+/// space of a piece where there is one, else before a character that starts a cluster (not a
+/// combining mark or joiner, not after a joiner). Text that fits is one piece.
+fn chunks(text: &str, max: usize) -> Vec<Range<usize>> {
+    use unicode_bidi::BidiClass::{BN, NSM};
+    let mut out = Vec::new();
+    let mut start = 0;
+    while text.len() - start > max {
+        let window = text.get(start..).unwrap_or_default();
+        let mut space = None;
+        let mut boundary = None;
+        let mut any = None;
+        let mut prev: Option<char> = None;
+        for (i, c) in window.char_indices() {
+            if i > max {
+                break;
+            }
+            if i > 0 {
+                any = Some(i);
+                let joined = prev.is_some_and(|p| matches!(unicode_bidi::bidi_class(p), BN));
+                if !joined && !matches!(unicode_bidi::bidi_class(c), NSM | BN) {
+                    boundary = Some(i);
+                    if prev.is_some_and(char::is_whitespace) {
+                        space = Some(i);
+                    }
+                }
+            }
+            prev = Some(c);
         }
+        // A piece always moves forward by at least one character.
+        let first = window.chars().next().map_or(window.len(), char::len_utf8);
+        let cut = space.filter(|&i| i >= max / 2).or(boundary).or(any).unwrap_or(first).max(first);
+        out.push(start..start + cut);
+        start += cut;
+    }
+    if start < text.len() || out.is_empty() {
+        out.push(start..text.len());
     }
     out
 }
@@ -121,4 +211,37 @@ mod tests {
         // Not a char boundary.
         assert!(shape_range(&face, "سنة", 1..2, &ShapeParams::default(), |c| c).is_empty());
     }
+
+    #[test]
+    fn long_items_are_shaped_in_bounded_pieces() {
+        for text in ["word ".repeat(2_000), "e\u{301}".repeat(2_000), "x".repeat(5_000), "\u{1f469}\u{200d}\u{1f4bb} ".repeat(500)] {
+            let pieces = chunks(&text, MAX_SHAPE_CHUNK);
+            assert_eq!(pieces.first().map(|r| r.start), Some(0));
+            assert_eq!(pieces.last().map(|r| r.end), Some(text.len()));
+            for w in pieces.windows(2) {
+                assert_eq!(w[0].end, w[1].start);
+            }
+            for r in &pieces {
+                assert!(!r.is_empty() && r.len() <= MAX_SHAPE_CHUNK);
+                // Never before a combining mark, never after a joiner.
+                let rest = &text[r.end..];
+                assert!(!rest.starts_with('\u{301}'));
+                assert!(!text[..r.end].ends_with('\u{200d}') || rest.is_empty());
+            }
+        }
+        assert_eq!(chunks("short", MAX_SHAPE_CHUNK), vec![0..5]);
+        assert_eq!(chunks("", MAX_SHAPE_CHUNK), vec![0..0]);
+    }
+
+    #[test]
+    fn long_items_keep_every_cluster_in_visual_order() {
+        let face = FontDb::global().face(DEFAULT_FAMILY, "Regular");
+        let text = "ab cd ".repeat(1_000);
+        let g = shape_range(&face, &text, 0..text.len(), &ShapeParams::default(), |c| c);
+        assert_eq!(g.len(), text.len());
+        assert!(g.windows(2).all(|w| w[0].cluster < w[1].cluster));
+        let rtl = shape_range(&face, &text, 0..text.len(), &ShapeParams { rtl: true, ..Default::default() }, |c| c);
+        assert_eq!(rtl.len(), text.len());
+        assert!(rtl.windows(2).all(|w| w[0].cluster > w[1].cluster));
+    }
 }