From 757ac4e27ef5654453eda392a2d31792462c3ee6 Mon Sep 17 00:00:00 2001
From: nattthebear <goyuken@gmail.com>
Date: Wed, 23 Sep 2020 22:03:37 -0400
Subject: [PATCH] Fix crash saving zwinder state manager when things align a
 certain way

---
 .../rewind/ZwinderBuffer.cs                   |  2 +-
 .../Movie/ZwinderStateManagerTests.cs         | 24 +++++++++++++++++++
 2 files changed, 25 insertions(+), 1 deletion(-)

diff --git a/src/BizHawk.Client.Common/rewind/ZwinderBuffer.cs b/src/BizHawk.Client.Common/rewind/ZwinderBuffer.cs
index 0c8629a5a8..3f0a421677 100644
--- a/src/BizHawk.Client.Common/rewind/ZwinderBuffer.cs
+++ b/src/BizHawk.Client.Common/rewind/ZwinderBuffer.cs
@@ -256,7 +256,7 @@ namespace BizHawk.Client.Common
 			if (Count != 0)
 			{
 				var startByte = _states[_firstStateIndex].Start;
-				var endByte = _states[HeadStateIndex].Start + _states[HeadStateIndex].Size;
+				var endByte = (_states[HeadStateIndex].Start + _states[HeadStateIndex].Size) & _sizeMask;
 				// TODO: Use spans to avoid these extra copies in .net core
 				if (startByte > endByte)
 				{
diff --git a/src/BizHawk.Tests/Client.Common/Movie/ZwinderStateManagerTests.cs b/src/BizHawk.Tests/Client.Common/Movie/ZwinderStateManagerTests.cs
index 8683d2c028..6d6e88282d 100644
--- a/src/BizHawk.Tests/Client.Common/Movie/ZwinderStateManagerTests.cs
+++ b/src/BizHawk.Tests/Client.Common/Movie/ZwinderStateManagerTests.cs
@@ -418,6 +418,30 @@ namespace BizHawk.Tests.Client.Common.Movie
 			Assert.AreEqual(0, zw.AllStates().Single().Frame);
 		}
 
+		[TestMethod]
+		public void WhatIfTheHeadStateWrapsAround()
+		{
+			var ss = new StateSource
+			{
+				PaddingData = new byte[400 * 1000]
+			};
+			using var zw = new ZwinderBuffer(new RewindConfig
+			{
+				BufferSize = 1,
+				TargetFrameLength = 1
+			});
+
+			// Need to get data in the zwinderbuffer so that the last state, and the last state in particular, wraps around
+			ss.Frame = 1;
+			zw.Capture(1, s => ss.SaveStateBinary(new BinaryWriter(s)), null, true);
+			ss.Frame = 2;
+			zw.Capture(2, s => ss.SaveStateBinary(new BinaryWriter(s)), null, true);
+			ss.Frame = 3;
+			zw.Capture(3, s => ss.SaveStateBinary(new BinaryWriter(s)), null, true);
+
+			zw.SaveStateBinary(new BinaryWriter(new MemoryStream()));
+		}
+
 		private class StateSource : IStatable
 		{
 			public int Frame { get; set; }