diff --git a/PKHeX.Core/Legality/RNG/Algorithms/XorShift128.cs b/PKHeX.Core/Legality/RNG/Algorithms/XorShift128.cs
index c9091e3c6..6bb0d96b3 100644
--- a/PKHeX.Core/Legality/RNG/Algorithms/XorShift128.cs
+++ b/PKHeX.Core/Legality/RNG/Algorithms/XorShift128.cs
@@ -63,6 +63,26 @@ public uint Next()
         return w = w ^ (w >> 19) ^ t ^ (t >> 8);
     }
 
+    /// <summary>
+    /// Gets the previous random <see cref="ulong"/>.
+    /// </summary>
+    public uint Prev()
+    {
+        var t = w ^ z ^ (z >> 19);
+        t ^= t >> 8;
+        t ^= t >> 16;
+
+        w = z;
+        z = y;
+        y = x;
+
+        t ^= t << 11;
+        t ^= t << 22;
+
+        x = t;
+        return w;
+    }
+
     [MethodImpl(MethodImplOptions.AggressiveInlining)]
     public uint NextUInt() => (uint)NextInt();
 
diff --git a/Tests/PKHeX.Core.Tests/General/XorShiftTests.cs b/Tests/PKHeX.Core.Tests/General/XorShiftTests.cs
new file mode 100644
index 000000000..272e05ce1
--- /dev/null
+++ b/Tests/PKHeX.Core.Tests/General/XorShiftTests.cs
@@ -0,0 +1,48 @@
+using FluentAssertions;
+using PKHeX.Core;
+using Xunit;
+
+namespace PKHeX.Tests;
+
+public static class XorShiftTests
+{
+    [Theory]
+    [InlineData(0x7F996375F5A791FC, 0xF3A995138EC18148, 0x50188547081C7BEC, 0x0DA245AA536BEF36, 99, 1000)]
+    public static void Forward(ulong s0, ulong s1, ulong n0, ulong n1, int frames, int loop)
+    {
+        var adv = GetFramesForward(s0, s1, n0, n1, loop);
+        adv.Should().Be(frames);
+    }
+
+    [Theory]
+    [InlineData(0x7F996375F5A791FC, 0xF3A995138EC18148, 0x50188547081C7BEC, 0x0DA245AA536BEF36, 99, 1000)]
+    public static void Reverse(ulong s0, ulong s1, ulong n0, ulong n1, int frames, int loop)
+    {
+        var adv = GetFramesReverse(s0, s1, n0, n1, loop);
+        adv.Should().Be(frames);
+    }
+
+    private static int GetFramesReverse(ulong s0, ulong s1, ulong n0, ulong n1, int loop)
+    {
+        var rand = new XorShift128(n0, n1);
+        for (int i = 0; i < loop; i++)
+        {
+            _ = rand.Prev();
+            if (rand.GetState64() == (s0, s1))
+                return i;
+        }
+        return -1;
+    }
+
+    private static int GetFramesForward(ulong s0, ulong s1, ulong n0, ulong n1, int loop)
+    {
+        var rand = new XorShift128(s0, s1);
+        for (int i = 0; i < loop; i++)
+        {
+            _ = rand.Next();
+            if (rand.GetState64() == (n0, n1))
+                return i;
+        }
+        return -1;
+    }
+}