From ae19ce25a7dad3d9365f21d468da1b6c55bf19ad Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Marek=20Lema=C5=84czyk?= <marek@lemanczyk.eu>
Date: Sun, 21 Sep 2025 11:23:52 +0200
Subject: [PATCH] Implement lookup-based Mod10 for GPU compatibility

---
 PerfectNumbers.Core/ULongExtensions.cs | 25 +++++++++++++++++++++++--
 1 file changed, 23 insertions(+), 2 deletions(-)

diff --git a/PerfectNumbers.Core/ULongExtensions.cs b/PerfectNumbers.Core/ULongExtensions.cs
index 77f4241..118617d 100644
--- a/PerfectNumbers.Core/ULongExtensions.cs
+++ b/PerfectNumbers.Core/ULongExtensions.cs
@@ -137,8 +137,29 @@ public static class ULongExtensions
         [MethodImpl(MethodImplOptions.AggressiveInlining)]
         public static ulong Mod10(this ulong value)
         {
-                ulong quotient = (ulong)(((UInt128)value * 0xCCCCCCCCCCCCCCCDUL) >> 67);
-                return value - quotient * 10UL;
+                ulong parity = value & 1UL;
+                ulong mod5 = value.Mod5();
+
+                if (parity == 0UL)
+                {
+                        return mod5 switch
+                        {
+                                0UL => 0UL,
+                                1UL => 6UL,
+                                2UL => 2UL,
+                                3UL => 8UL,
+                                _ => 4UL,
+                        };
+                }
+
+                return mod5 switch
+                {
+                        0UL => 5UL,
+                        1UL => 1UL,
+                        2UL => 7UL,
+                        3UL => 3UL,
+                        _ => 9UL,
+                };
         }
 
         [MethodImpl(MethodImplOptions.AggressiveInlining)]