From 9f93ac03faa3f623d485f8310b53b0b0eeb8c5e3 Mon Sep 17 00:00:00 2001
From: "Dylan M. Taylor" <dylan@dylanmtaylor.com>
Date: Thu, 12 Nov 2026 06:29:20 -0500
Subject: [PATCH] Text layout: a wrapped line finds its glyphs and end caret by
 bisection

* Text layout: a wrapped line finds its glyphs and end caret by bisection

* Fix merge of layout tests
---
 crates/text/src/layout.rs       | 37 +++++++++++++++++++++++++++++----
 crates/text/src/layout/tests.rs | 19 +++++++++++++++++
 2 files changed, 52 insertions(+), 4 deletions(-)

diff --git a/crates/text/src/layout.rs b/crates/text/src/layout.rs
index acf292889..9eb19d919 100644
--- a/crates/text/src/layout.rs
+++ b/crates/text/src/layout.rs
@@ -378,6 +378,34 @@ struct Item {
     range: Range<usize>,
     rtl: bool,
     glyphs: Vec<ShapedGlyph>,
+    /// Whether the glyphs' clusters never decrease (`Some(true)`) or never increase
+    /// (`Some(false)`) along `glyphs`: the glyphs of a cluster range are then contiguous.
+    cluster_order: Option<bool>,
+}
+
+impl Item {
+    fn new(range: Range<usize>, rtl: bool, glyphs: Vec<ShapedGlyph>) -> Self {
+        let cluster_order = if glyphs.windows(2).all(|w| w[0].cluster <= w[1].cluster) {
+            Some(true)
+        } else if glyphs.windows(2).all(|w| w[0].cluster >= w[1].cluster) {
+            Some(false)
+        } else {
+            None
+        };
+        Self { range, rtl, glyphs, cluster_order }
+    }
+
+    /// The glyphs whose cluster lies in `run`, in glyph order, found by bisection when the clusters
+    /// are ordered (a line of a long paragraph otherwise scanned the whole item).
+    fn glyphs_in(&self, run: &Range<usize>) -> impl Iterator<Item = &ShapedGlyph> {
+        let g = &self.glyphs;
+        let (lo, hi) = match self.cluster_order {
+            Some(true) => (g.partition_point(|x| x.cluster < run.start), g.partition_point(|x| x.cluster < run.end)),
+            Some(false) => (g.partition_point(|x| x.cluster >= run.end), g.partition_point(|x| x.cluster >= run.start)),
+            None => (0, g.len()),
+        };
+        g[lo..hi].iter().filter(|x| run.contains(&x.cluster))
+    }
 }
 
 fn upper_single(c: char) -> char {
@@ -929,7 +957,7 @@ impl<'a> Para<'a> {
                 idx.reverse();
             }
             for k in idx {
-                for g in self.items[k].glyphs.iter().filter(|g| run.contains(&g.cluster)) {
+                for g in self.items[k].glyphs_in(&run) {
                     let st = &self.runs[g.run].1;
                     let x0 = pen;
                     pen += g.pre;
@@ -990,9 +1018,10 @@ impl<'a> Para<'a> {
             }
         }
         // end of line caret
-        let end_x = match self.chars.iter().rev().find(|c| c.0 < r.end && c.0 >= r.start) {
+        let last_char = self.chars.partition_point(|c| c.0 < r.end).checked_sub(1).and_then(|i| self.chars.get(i)).filter(|c| c.0 >= r.start);
+        let end_x = match last_char {
             Some(c) => {
-                let cs = cluster_starts.iter().rev().find(|s| **s <= c.0).copied();
+                let cs = cluster_starts.partition_point(|s| *s <= c.0).checked_sub(1).and_then(|i| cluster_starts.get(i)).copied();
                 match cs.and_then(|s| extents.get(&s)) {
                     Some(&(x0, x1, rtl)) => {
                         if rtl {
@@ -1095,7 +1124,7 @@ fn build_items(text: &str, chars: &[CharInfo], runs: &[(Range<usize>, TextStyle)
         let mut glyphs = shape_item(&sub, rtl, face, size, style, si, form);
         space_item(&mut glyphs, style, text, true);
         let end = if j < chars.len() { chars[j].0 } else { text.len() };
-        items.push(Item { range: chars[i].0..end, rtl, glyphs });
+        items.push(Item::new(chars[i].0..end, rtl, glyphs));
         i = j;
     }
     items
diff --git a/crates/text/src/layout/tests.rs b/crates/text/src/layout/tests.rs
index c467cb5f9..bf275cad8 100644
--- a/crates/text/src/layout/tests.rs
+++ b/crates/text/src/layout/tests.rs
@@ -506,6 +506,25 @@ fn a_later_vertical_run_keeps_the_first_line_baseline() {
     assert!((mixed.lines[0].baseline - plain.lines[0].baseline).abs() < 1e-3, "{} vs {}", mixed.lines[0].baseline, plain.lines[0].baseline);
 }
 
+/// `Item::glyphs_in` finds a cluster range by bisection when the clusters are ordered, and agrees
+/// with a full scan for ascending, descending and unordered clusters.
+#[test]
+fn item_glyphs_in_matches_a_full_scan() {
+    let glyph = |cluster| ShapedGlyph { face: 0, id: cluster as u32, cluster, adv: 1.0, pre: 0.0, dx: 0.0, dy: 0.0, size: 10.0, run: 0, rise: 0.0 };
+    let asc: Vec<usize> = vec![0, 0, 1, 3, 3, 3, 5, 8];
+    let desc: Vec<usize> = asc.iter().rev().copied().collect();
+    let mixed: Vec<usize> = vec![3, 0, 5, 1, 8, 3, 0, 3];
+    for (clusters, order) in [(&asc, Some(true)), (&desc, Some(false)), (&mixed, None)] {
+        let item = Item::new(0..9, false, clusters.iter().map(|&c| glyph(c)).collect());
+        assert_eq!(item.cluster_order, order);
+        for (a, b) in [(0, 9), (0, 1), (1, 4), (3, 4), (4, 9), (6, 8), (9, 9), (2, 3)] {
+            let got: Vec<usize> = item.glyphs_in(&(a..b)).map(|g| g.cluster).collect();
+            let want: Vec<usize> = clusters.iter().copied().filter(|c| (a..b).contains(c)).collect();
+            assert_eq!(got, want, "{clusters:?} {a}..{b}");
+        }
+    }
+}
+
 #[test]
 fn run_lookup_by_bisection_matches_the_scan() {
     let scan = |runs: &[(Range<usize>, TextStyle)], b: usize| {