From 87aec89f25fa752727a2396275a50d63df9e1e15 Mon Sep 17 00:00:00 2001
From: James Jackson-South <james_south@hotmail.com>
Date: Fri, 11 Jun 2021 18:43:15 +1000
Subject: [PATCH] Use GreatestCommonDivisor. Fix #1616

---
 .../Processors/Transforms/Resize/ResizeKernelMap.cs           | 4 ++--
 .../Processing/Processors/Transforms/ResizeKernelMapTests.cs  | 3 +++
 2 files changed, 5 insertions(+), 2 deletions(-)

diff --git a/src/ImageSharp/Processing/Processors/Transforms/Resize/ResizeKernelMap.cs b/src/ImageSharp/Processing/Processors/Transforms/Resize/ResizeKernelMap.cs
index ab6040c17b..2ab1d8b5a7 100644
--- a/src/ImageSharp/Processing/Processors/Transforms/Resize/ResizeKernelMap.cs
+++ b/src/ImageSharp/Processing/Processors/Transforms/Resize/ResizeKernelMap.cs
@@ -130,9 +130,9 @@ namespace SixLabors.ImageSharp.Processing.Processors.Transforms
             int radius = (int)TolerantMath.Ceiling(scale * sampler.Radius);
 
             // 'ratio' is a rational number.
-            // Multiplying it by LCM(sourceSize, destSize)/sourceSize will result in a whole number "again".
+            // Multiplying it by destSize/GCD(sourceSize, destSize) will result in a whole number "again".
             // This value is determining the length of the periods in repeating kernel map rows.
-            int period = Numerics.LeastCommonMultiple(sourceSize, destinationSize) / sourceSize;
+            int period = destinationSize / Numerics.GreatestCommonDivisor(sourceSize, destinationSize);
 
             // the center position at i == 0:
             double center0 = (ratio - 1) * 0.5;
diff --git a/tests/ImageSharp.Tests/Processing/Processors/Transforms/ResizeKernelMapTests.cs b/tests/ImageSharp.Tests/Processing/Processors/Transforms/ResizeKernelMapTests.cs
index f15a6242d5..1d4629ccc1 100644
--- a/tests/ImageSharp.Tests/Processing/Processors/Transforms/ResizeKernelMapTests.cs
+++ b/tests/ImageSharp.Tests/Processing/Processors/Transforms/ResizeKernelMapTests.cs
@@ -80,6 +80,9 @@ namespace SixLabors.ImageSharp.Tests.Processing.Processors.Transforms
             { KnownResamplers.Bicubic, 1680, 1200 },
             { KnownResamplers.Box, 13, 299 },
             { KnownResamplers.Lanczos5, 3032, 600 },
+
+            // Large number. https://github.com/SixLabors/ImageSharp/issues/1616
+            { KnownResamplers.Bicubic, 207773, 51943 }
         };
 
         public static TheoryData<string, int, int> GeneratedImageResizeData =