From d036e02bb007a6aeaff1beb12451d954a17d9b44 Mon Sep 17 00:00:00 2001
From: +merlan #flirora <flirora@flirora.xyz>
Date: Thu, 10 Apr 2025 16:24:27 -0400
Subject: [PATCH] Fix panic when sampling across two coincident gradient stops

Fixes #6162.
---
 .../typst-library/src/visualize/gradient.rs   |  12 ++++++++
 ...2-coincident-gradient-stops-export-png.png | Bin 0 -> 522 bytes
 tests/suite/visualize/gradient.typ            |  26 ++++++++++++++++++
 3 files changed, 38 insertions(+)
 create mode 100644 tests/ref/issue-6162-coincident-gradient-stops-export-png.png

diff --git a/crates/typst-library/src/visualize/gradient.rs b/crates/typst-library/src/visualize/gradient.rs
index 45f388ccd..438ab66e9 100644
--- a/crates/typst-library/src/visualize/gradient.rs
+++ b/crates/typst-library/src/visualize/gradient.rs
@@ -1290,9 +1290,21 @@ fn sample_stops(stops: &[(Color, Ratio)], mixing_space: ColorSpace, t: f64) -> C
     if low == 0 {
         low = 1;
     }
+    if t == 1.0 {
+        while stops[low - 1].1 >= stops[low].1 {
+            low -= 1;
+        }
+    } else {
+        while stops[low - 1].1 >= stops[low].1 {
+            low += 1;
+        }
+    }
 
     let (col_0, pos_0) = stops[low - 1];
     let (col_1, pos_1) = stops[low];
+    debug_assert!(pos_0 < pos_1);
+    debug_assert!(pos_0.get() <= t);
+    debug_assert!(t <= pos_1.get());
     let t = (t - pos_0.get()) / (pos_1.get() - pos_0.get());
 
     Color::mix_iter(
diff --git a/tests/ref/issue-6162-coincident-gradient-stops-export-png.png b/tests/ref/issue-6162-coincident-gradient-stops-export-png.png
new file mode 100644
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diff --git a/tests/suite/visualize/gradient.typ b/tests/suite/visualize/gradient.typ
index 811b8b605..8446ca030 100644
--- a/tests/suite/visualize/gradient.typ
+++ b/tests/suite/visualize/gradient.typ
@@ -666,3 +666,29 @@ $ A = mat(
 #let _ = gradient.linear(..my-gradient.stops())
 #let my-gradient2 = gradient.linear(red, blue).repeat(5, mirror: true)
 #let _ = gradient.linear(..my-gradient2.stops())
+
+--- issue-6162-coincident-gradient-stops-export-png ---
+// Ensure that multiple gradient stops with the same position
+// don't cause a panic.
+#rect(
+  fill: gradient.linear(
+    (red, 0%),
+    (green, 0%),
+    (blue, 100%),
+  )
+)
+#rect(
+  fill: gradient.linear(
+    (red, 0%),
+    (green, 100%),
+    (blue, 100%),
+  )
+)
+#rect(
+  fill: gradient.linear(
+    (white, 0%),
+    (red, 50%),
+    (green, 50%),
+    (blue, 100%),
+  )
+)