From 7cc73ca9b08ff57640a77018e9ce8268aeb4a68c Mon Sep 17 00:00:00 2001
From: nattthebear <goyuken@gmail.com>
Date: Tue, 20 Jul 2021 16:47:10 -0400
Subject: [PATCH] Fix bug with delta state rewinder (#2871)

---
 .../rewind/ZeldaWinder.cs                     |  4 ++--
 .../rewind/ZwinderBuffer.cs                   | 24 +++++++++++--------
 2 files changed, 16 insertions(+), 12 deletions(-)

diff --git a/src/BizHawk.Client.Common/rewind/ZeldaWinder.cs b/src/BizHawk.Client.Common/rewind/ZeldaWinder.cs
index 3e4dc401b4..56f194fdd7 100644
--- a/src/BizHawk.Client.Common/rewind/ZeldaWinder.cs
+++ b/src/BizHawk.Client.Common/rewind/ZeldaWinder.cs
@@ -106,7 +106,7 @@ namespace BizHawk.Client.Common
 				_count++;
 				return;
 			}
-			if (!_buffer.WillCapture(_masterFrame))
+			if (!_buffer.WouldCapture(frame - _masterFrame))
 				return;
 
 			{
@@ -189,7 +189,7 @@ namespace BizHawk.Client.Common
 						_masterLength = (int)sss.Position;
 						_masterFrame = frame;
 						_count++;
-					});
+					}, force: true);
 				});
 			}
 		}
diff --git a/src/BizHawk.Client.Common/rewind/ZwinderBuffer.cs b/src/BizHawk.Client.Common/rewind/ZwinderBuffer.cs
index 710e2cd2d2..e7780e13e3 100644
--- a/src/BizHawk.Client.Common/rewind/ZwinderBuffer.cs
+++ b/src/BizHawk.Client.Common/rewind/ZwinderBuffer.cs
@@ -148,31 +148,35 @@ namespace BizHawk.Client.Common
 				_backingStoreType == settings.BackingStore;
 		}
 
-		private bool ShouldCapture(int frame)
+		private bool ShouldCaptureForFrameDiff(int frameDiff)
 		{
 			if (Count == 0)
 			{
 				return true;
 			}
-
-			var frameDiff = frame - _states[HeadStateIndex].Frame;
 			if (frameDiff < 1)
+			{
 				// non-linear time is from a combination of other state changing mechanisms and the rewinder
 				// not much we can say here, so just take a state
 				return true;
+			}
+			return frameDiff >= ComputeIdealRewindInterval();		
+		}
 
-			return frameDiff >= ComputeIdealRewindInterval();
+		private bool ShouldCapture(int frame)
+		{
+			var frameDiff = frame - _states[HeadStateIndex].Frame;
+			return ShouldCaptureForFrameDiff(frameDiff);
 		}
 
 		/// <summary>
-		/// Predict whether Capture() will capture a state.  Useful if expensive work needs to happen before
-		/// Capture() is called.
+		/// Predict whether Capture() would capture a state, assuming a particular frame delta.
 		/// </summary>
-		/// <param name="frame">The same frame number to be passed to capture</param>
-		/// <returns>Whether capture will happen, assuming Capture() is passed the same frame and force = false</returns>
-		public bool WillCapture(int frame)
+		/// <param name="frameDelta">The assumed frame delta.  Normally this will be equal to `nextStateFrame - GetState(Count - 1).Frame`.</param>
+		/// <returns>Whether Capture(nextStateFrame) would actually capture, assuming the frameDelta matched.</returns>
+		public bool WouldCapture(int frameDelta)
 		{
-			return ShouldCapture(frame);
+			return ShouldCaptureForFrameDiff(frameDelta);
 		}
 
 		/// <summary>