speedup delta stating with disks with simple track tracking, should be within acceptable performance bounds (more or less)

src/BizHawk.Common/DeltaSerializer.cs

@@ -9,7 +9,7 @@ namespace BizHawk.Common
 	/// </summary>
 	public static class DeltaSerializer
 	{
-		public static byte[] GetDelta<T>(ReadOnlySpan<T> original, ReadOnlySpan<T> data)
+		public static ReadOnlySpan<byte> GetDelta<T>(ReadOnlySpan<T> original, ReadOnlySpan<T> data)
 			where T : unmanaged
 		{
 			var orignalAsBytes = MemoryMarshal.AsBytes(original);
@@ -94,7 +94,7 @@ namespace BizHawk.Common
 				}
 			}
 
-			return ret.Slice(0, retSize).ToArray();
+			return ret.Slice(0, retSize);
 		}
 
 		public static void ApplyDelta<T>(ReadOnlySpan<T> original, Span<T> data, ReadOnlySpan<byte> delta)

src/BizHawk.Common/Serializer.cs

@@ -759,7 +759,7 @@ namespace BizHawk.Common
 			}
 			else
 			{
-				var delta = DeltaSerializer.GetDelta<T>(original, data);
+				var delta = DeltaSerializer.GetDelta<T>(original, data).ToArray(); // TODO: don't create array here (need .net update to write span to binary writer)
 				Sync(name, ref delta, useNull: false);
 			}
 		}

src/BizHawk.Emulation.Cores/Computers/Commodore64/Media/Disk.cs

@@ -1,5 +1,6 @@
 using System;
 using System.Collections.Generic;
+using System.Diagnostics;
 using System.Linq;
 using BizHawk.Common;
 
@@ -12,6 +13,7 @@ namespace BizHawk.Emulation.Cores.Computers.Commodore64.Media
 		public const int FluxEntriesPerTrack = FluxBitsPerTrack / FluxBitsPerEntry;
 		private readonly int[][] _tracks;
 		private readonly int[][] _originalMedia;
+		private bool[] _usedTracks;
 		public bool Valid;
 		public bool WriteProtected;
 
@@ -24,6 +26,7 @@ namespace BizHawk.Emulation.Cores.Computers.Commodore64.Media
 			_tracks = new int[trackCapacity][];
 			FillMissingTracks();
 			_originalMedia = _tracks.Select(t => (int[])t.Clone()).ToArray();
+			_usedTracks = new bool[trackCapacity];
 			Valid = true;
 		}
 
@@ -46,6 +49,7 @@ namespace BizHawk.Emulation.Cores.Computers.Commodore64.Media
 			FillMissingTracks();
 			Valid = true;
 			_originalMedia = _tracks.Select(t => (int[])t.Clone()).ToArray();
+			_usedTracks = new bool[trackCapacity];
 		}
 
 		private int[] ConvertToFluxTransitions(int density, byte[] bytes, int fluxBitOffset)
@@ -133,17 +137,27 @@ namespace BizHawk.Emulation.Cores.Computers.Commodore64.Media
 
 		public int[] GetDataForTrack(int halftrack)
 		{
+			_usedTracks[halftrack] = true; // TODO: probably can be smarter about this with the WriteProtected flag
 			return _tracks[halftrack];
 		}
 
 		public void SyncState(Serializer ser)
 		{
 			ser.Sync(nameof(WriteProtected), ref WriteProtected);
+			var oldUsedTracks = _usedTracks; // Sync changes reference if loading state (we don't care in the saving state case)
+			ser.Sync(nameof(_usedTracks), ref _usedTracks, useNull: false);
 
 			for (var i = 0; i < _tracks.Length; i++)
+			{
+				if (_usedTracks[i])
 				{
 					ser.SyncDelta($"MediaState{i}", _originalMedia[i], _tracks[i]);
 				}
+				else if (ser.IsReader && oldUsedTracks[i]) // _tracks[i] might be different, but in the state it wasn't, so just copy _originalMedia[i]
+				{
+					_originalMedia[i].AsSpan().CopyTo(_tracks[i]);
+				}
+			}
 		}
 	}
 }