From 670968f60a51a8626521f32bb2fe852f77100e66 Mon Sep 17 00:00:00 2001
From: "Dylan M. Taylor" <dylan@dylanmtaylor.com>
Date: Tue, 3 Nov 2026 12:59:45 +0000
Subject: [PATCH] perf(text): look only at a paragraph's own runs

---
 crates/text/src/layout.rs |  9 +++++++--
 crates/text/src/shape.rs  |  8 +++++++-
 crates/text/src/tests.rs  | 13 +++++++++++++
 3 files changed, 27 insertions(+), 3 deletions(-)

diff --git a/crates/text/src/layout.rs b/crates/text/src/layout.rs
index 669777687..9d7ed666e 100644
--- a/crates/text/src/layout.rs
+++ b/crates/text/src/layout.rs
@@ -140,7 +140,9 @@ impl Ctx<'_> {
     /// stretch.
     fn char_directions(&self, r: &Range<usize>) -> Vec<(Range<usize>, ParaDirection)> {
         let mut out: Vec<(Range<usize>, ParaDirection)> = Vec::new();
-        for (rr, st) in &self.runs {
+        // Only the paragraph's own runs (they are in order), not every run of the text.
+        let first = self.runs.partition_point(|(rr, _)| rr.end <= r.start);
+        for (rr, st) in self.runs.iter().skip(first).take_while(|(rr, _)| rr.start < r.end) {
             let (a, b) = (rr.start.max(r.start), rr.end.min(r.end));
             let Some(d) = st.char_direction.filter(|_| a < b) else { continue };
             let (a, b) = (a - r.start, b - r.start);
@@ -212,7 +214,10 @@ impl Ctx<'_> {
     fn prepend_marker(&self, pr: &Range<usize>, para: &ParaStyle, text: Option<&str>, rtl: bool, sg: &mut Vec<SGlyph>) -> usize {
         let Some(text) = text.filter(|_| para.list.is_some() && !pr.is_empty()) else { return 0 };
         let st = vectorcraft_doc::text::ParaList::marker_style(self.style_at(pr.start));
-        let Some(run) = self.runs.iter().position(|(r, _)| r.start <= pr.start && pr.start < r.end) else { return 0 };
+        let run = self.runs.partition_point(|(r, _)| r.end <= pr.start);
+        if !self.runs.get(run).is_some_and(|(r, _)| r.start <= pr.start && pr.start < r.end) {
+            return 0;
+        }
         // The paragraph's direction: the marker starts a right-to-left paragraph at its right.
         let level = if rtl { Level::rtl() } else { Level::ltr() };
         let mut marker = Vec::with_capacity(text.len() + 1);
diff --git a/crates/text/src/shape.rs b/crates/text/src/shape.rs
index c5f82071d..a257a3ae8 100644
--- a/crates/text/src/shape.rs
+++ b/crates/text/src/shape.rs
@@ -375,7 +375,13 @@ pub(crate) fn shape_range(
         art: Option<&'a InlineArt>,
     }
     let mut spans: Vec<ShapeSpan<'_>> = Vec::new();
-    for (ri, (rr, st)) in runs.iter().enumerate() {
+    // The runs are in order: start at the first that ends inside the range and stop past it, so
+    // that each paragraph looks only at its own runs.
+    let first = runs.partition_point(|(rr, _)| rr.end <= range.start);
+    for (ri, (rr, st)) in runs.iter().enumerate().skip(first) {
+        if rr.start >= range.end {
+            break;
+        }
         let a = rr.start.max(range.start);
         let b = rr.end.min(range.end);
         if a >= b || text.get(a..b).is_none() {
diff --git a/crates/text/src/tests.rs b/crates/text/src/tests.rs
index 3f86a85bd..002e4b921 100644
--- a/crates/text/src/tests.rs
+++ b/crates/text/src/tests.rs
@@ -555,6 +555,19 @@ fn multiple_runs_and_styles() {
     assert_ne!(l.glyphs[0].font_id, l.glyphs[3].font_id);
 }
 
+#[test]
+fn many_paragraphs_of_their_own_runs_lay_out_in_linear_time() {
+    // 20,000 one-letter paragraphs, each a run of its own: every paragraph looking through every
+    // run would be 400 million steps.
+    let n = 20_000;
+    let mut t = point("", style(10.0));
+    t.runs = (0..n).map(|i| TextRun { text: "a\n".into(), style: style(if i % 2 == 0 { 10.0 } else { 11.0 }), inline: None }).collect();
+    let start = std::time::Instant::now();
+    let l = layout(db(), &t);
+    assert!(start.elapsed() < std::time::Duration::from_secs(60), "{:?}", start.elapsed());
+    assert!(l.glyphs.iter().any(|g| g.run == n - 1));
+}
+
 #[test]
 fn layout_is_fast() {
     let text: String = LOREM.chars().cycle().take(1000).collect();