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#3692: math text runs find character boundaries in one walk

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Finding 23: Ctx::glyphs found each character's first glyph by scanning the shaped glyphs from the start (quadratic in a long run). The first glyph at or past a boundary never moves back as the boundary grows, so one walk gives the same cuts.

Test: math::tests::a_long_text_run_lays_out_in_linear_time.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
This commit is contained in:
Martin Ball
2026-10-26 00:19:16 +00:00
co-authored by Claude Opus 5.5
parent 7d31c01f6f
commit 4d027de402
1 changed files with 23 additions and 9 deletions
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crates/layout/src/math.rs
+23 -9
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@@ -631,19 +631,18 @@ impl Ctx {
last_gid = Some(g.gid);
}
b.w = x;
// Character boundaries: where the first glyph of each character starts.
// Character boundaries: where the first glyph of each character starts. The glyph for a
// boundary is never before the previous boundary's, so one walk of the glyphs finds all.
let mut cuts = Vec::with_capacity(text.len() + 1);
let (mut j, mut gx) = (0usize, 0.0f32);
for (cb, _) in text.char_indices() {
let mut gx = 0.0f32;
let mut found = None;
for g in &shaped {
if g.cluster >= cb {
found = Some(gx);
break;
}
while let Some(g) = shaped.get(j)
&& g.cluster < cb
{
gx += g.x_advance as f32 * k;
j += 1;
}
cuts.push(found.unwrap_or(x));
cuts.push(if j < shaped.len() { gx } else { x });
}
cuts.push(x);
b.cuts = cuts;
@@ -2374,4 +2373,19 @@ mod tests {
}
}
}
/// Finding 23: each character's boundary in a run of text was found by walking the run's
/// glyphs from the first.
#[test]
fn a_long_text_run_lays_out_in_linear_time() {
use crate::tests::{SCALE, cost_ratio};
use wordcraft_doc::math::*;
let eq = |n: usize| Math { nodes: vec![MNode::Run(MRun { nor: true, ..MRun::new("a".repeat(n)) })], ..Default::default() };
let (small, big) = (eq(400), eq(SCALE * 400));
// Each timed 16 times over, so the timings are long against the clock's tick.
let lay_16 = |m: &Math| (0..16).for_each(|_| drop(std::hint::black_box(lay_in(m, "", false, 1e6, &MathProps::default()))));
let (t1, tk, ratio) = cost_ratio(SCALE, || lay_16(&small), || lay_16(&big));
eprintln!("t(n) {t1:.4} s, t(8n) {tk:.4} s, ratio {ratio:.1}");
assert!(ratio < 24.0, "8x the text took {ratio:.1}x the time ({t1:.4} s → {tk:.4} s): not linear");
}
}