From 553be49d9cae302cd4d048d5a5f3803af3de47d4 Mon Sep 17 00:00:00 2001
From: "Dylan M. Taylor" <dylan@dylanmtaylor.com>
Date: Tue, 3 Nov 2026 18:47:22 +0000
Subject: [PATCH] text: caret and line lookups by bisection

---
 crates/text/src/layout.rs       | 23 +++++++++++++++--------
 crates/text/src/layout/tests.rs | 11 +++++++++++
 2 files changed, 26 insertions(+), 8 deletions(-)

diff --git a/crates/text/src/layout.rs b/crates/text/src/layout.rs
index 4c0b5a436..25cdb27f5 100644
--- a/crates/text/src/layout.rs
+++ b/crates/text/src/layout.rs
@@ -278,19 +278,26 @@ pub struct Layout {
 impl Layout {
     /// Line index for a caret at byte `pos`.
     pub fn line_of(&self, pos: usize) -> usize {
-        let mut li = 0;
-        for (i, l) in self.lines.iter().enumerate() {
-            if l.range.start <= pos {
-                li = i;
-            }
-        }
-        li
+        // Lines are in text order: the last one starting at or before `pos`.
+        self.lines.partition_point(|l| l.range.start <= pos).saturating_sub(1)
     }
     /// Caret position `(x, baseline, line)` for byte offset `pos`.
     pub fn caret(&self, pos: usize) -> (f32, f32, usize) {
         let li = self.line_of(pos);
         let Some(l) = self.lines.get(li) else { return (0.0, 0.0, 0) };
-        let x = l.carets.iter().min_by_key(|(b, _)| b.abs_diff(pos)).map_or(l.x, |c| c.1);
+        // Carets are sorted by byte offset and unique: the nearest is next to the insertion point (the lower on a tie).
+        let i = l.carets.partition_point(|c| c.0 < pos);
+        let x = match (i.checked_sub(1).and_then(|k| l.carets.get(k)), l.carets.get(i)) {
+            (Some(p), Some(n)) => {
+                if pos - p.0 <= n.0 - pos {
+                    p.1
+                } else {
+                    n.1
+                }
+            }
+            (Some(c), None) | (None, Some(c)) => c.1,
+            (None, None) => l.x,
+        };
         (x, l.baseline, li)
     }
     /// Nearest caret byte offset to point `(x, y)`.
diff --git a/crates/text/src/layout/tests.rs b/crates/text/src/layout/tests.rs
index 5792c1c16..099680148 100644
--- a/crates/text/src/layout/tests.rs
+++ b/crates/text/src/layout/tests.rs
@@ -171,6 +171,17 @@ fn caps_and_small_caps() {
     assert_eq!(l.underlines.len(), 1);
 }
 
+#[test]
+fn indexed_caret_lookup_matches_a_linear_scan() {
+    let l = layout("fi abc\n\nde fg hi", &st(40.0), &ParagraphStyle::default());
+    for pos in 0..=l.lines.iter().map(|x| x.range.end).max().unwrap_or(0) + 2 {
+        let li = l.lines.iter().enumerate().filter(|(_, x)| x.range.start <= pos).map(|(i, _)| i).next_back().unwrap_or(0);
+        let line = &l.lines[li];
+        let want = line.carets.iter().min_by_key(|(b, _)| b.abs_diff(pos)).map_or(line.x, |c| c.1);
+        assert_eq!(l.caret(pos), (want, line.baseline, li), "pos {pos}");
+    }
+}
+
 #[test]
 fn ligature_caret_interpolates() {
     // Inter has an "fi"-like ligature only via calt in some versions; use "ffi" which most