implement a multitrack recording/editing feature

src/BizHawk.Client.Common/inputAdapters/MultitrackAdapter.cs

@@ -0,0 +1,58 @@
+#nullable enable
+
+using System.Collections.Generic;
+using BizHawk.Emulation.Common;
+
+namespace BizHawk.Client.Common
+{
+	/// <summary>
+	/// Used to enable recording a subset of a controller's buttons, while keeping the existing inputs for the other buttons.
+	/// Also used to allow TAStudio to clear (or otherwise edit) a subset of input columns.
+	/// </summary>
+	internal class MultitrackAdapter : IController
+	{
+		/// <summary>
+		/// Input states in this definition will come from <see cref="ActiveSource"/>. All others will come from <see cref="BackingSource"/>.
+		/// </summary>
+		public ControllerDefinition ActiveDefinition { get; set; }
+
+		public IController ActiveSource { get; set; }
+
+		public IController BackingSource { get; set; }
+
+		public ControllerDefinition Definition => BackingSource.Definition;
+
+		public MultitrackAdapter(IController active, IController backing, ControllerDefinition activeDefinition)
+		{
+			ActiveSource = active;
+			BackingSource = backing;
+			ActiveDefinition = activeDefinition;
+		}
+
+		public bool IsPressed(string button)
+		{
+			if (ActiveDefinition.BoolButtons.Contains(button))
+			{
+				return ActiveSource.IsPressed(button);
+			}
+			else
+			{
+				return BackingSource.IsPressed(button);
+			}
+		}
+		public int AxisValue(string name)
+		{
+			if (ActiveDefinition.Axes.ContainsKey(name))
+			{
+				return ActiveSource.AxisValue(name);
+			}
+			else
+			{
+				return BackingSource.AxisValue(name);
+			}
+		}
+
+		public IReadOnlyCollection<(string Name, int Strength)> GetHapticsSnapshot() => throw new NotImplementedException(); // don't use this
+		public void SetHapticChannelStrength(string name, int strength) => throw new NotImplementedException(); // don't use this
+	}
+}

src/BizHawk.Client.Common/movie/bk2/Bk2Movie.cs

@@ -5,7 +5,16 @@ namespace BizHawk.Client.Common
 {
 	public partial class Bk2Movie : BasicMovieInfo, IMovie
 	{
-		private Bk2Controller _adapter;
+		private IController _defaultValueController;
+		protected IController DefaultValueController
+		{
+			get
+			{
+				// LogKey isn't available at construction time, so we have to create this instance when it is accessed.
+				_defaultValueController ??= new Bk2Controller(Session.MovieController.Definition, LogKey);
+				return _defaultValueController;
+			}
+		}
 
 		public Bk2Movie(IMovieSession session, string filename) : base(filename)
 		{
@@ -18,6 +27,17 @@ public virtual void Attach(IEmulator emulator)
 			Emulator = emulator;
 		}
 
+		private ControllerDefinition/*?*/ _activeControllerInputs = null;
+		public virtual ControllerDefinition/*?*/ ActiveControllerInputs
+		{
+			get => _activeControllerInputs;
+			set
+			{
+				value?.AssertImmutable();
+				_activeControllerInputs = value;
+			}
+		}
+
 		protected bool IsAttached() => Emulator != null;
 
 		public IEmulator Emulator { get; private set; }
@@ -48,6 +68,11 @@ public void CopyLog(IEnumerable<string> log)
 
 		public void AppendFrame(IController source)
 		{
+			if (ActiveControllerInputs != null)
+			{
+				source = new MultitrackAdapter(source, new Bk2Controller(Session.MovieController.Definition, LogKey), ActiveControllerInputs);
+			}
+
 			Log.Add(Bk2LogEntryGenerator.GenerateLogEntry(source));
 			Changes = true;
 		}
@@ -62,15 +87,24 @@ public virtual void RecordFrame(int frame, IController source)
 				}
 			}
 
-			SetFrameAt(frame, Bk2LogEntryGenerator.GenerateLogEntry(source));
-
-			Changes = true;
+			PokeFrame(frame, source);
 		}
 
 		public virtual void Truncate(int frame)
 		{
 			if (frame < Log.Count)
 			{
+				if (ActiveControllerInputs != null)
+				{
+					for (int i = frame; i < Log.Count; i++)
+						PokeFrame(i, DefaultValueController);
+					string defaultEntry = Bk2LogEntryGenerator.EmptyEntry(DefaultValueController);
+					int firstDefault = Log.Count;
+					while (firstDefault > frame && Log[firstDefault - 1] == defaultEntry)
+						firstDefault--;
+					frame = firstDefault;
+				}
+
 				Log.RemoveRange(frame, Log.Count - frame);
 				Changes = true;
 			}
@@ -80,20 +114,28 @@ public IMovieController GetInputState(int frame)
 		{
 			if (frame < FrameCount && frame >= -1)
 			{
-				_adapter ??= new Bk2Controller(Session.MovieController.Definition, LogKey);
-				_adapter.SetFromMnemonic(frame >= 0 ? Log[frame] : Bk2LogEntryGenerator.EmptyEntry(_adapter));
-				return _adapter;
+				Bk2Controller controller = new(Session.MovieController.Definition, LogKey);
+				controller.SetFromMnemonic(frame >= 0 ? Log[frame] : Bk2LogEntryGenerator.EmptyEntry(controller));
+				return controller;
 			}
 
 			return null;
 		}
 
 		public virtual void PokeFrame(int frame, IController source)
 		{
+			if (ActiveControllerInputs != null)
+			{
+				source = new MultitrackAdapter(source, GetInputState(frame) ?? DefaultValueController, ActiveControllerInputs);
+			}
+
 			SetFrameAt(frame, Bk2LogEntryGenerator.GenerateLogEntry(source));
 			Changes = true;
 		}
 
+		/// <summary>
+		/// Does not use <see cref="ActiveControllerInputs"/>.
+		/// </summary>
 		protected void SetFrameAt(int frameNum, string frame)
 		{
 			if (Log.Count > frameNum)

src/BizHawk.Client.Common/movie/interfaces/IMovie.cs

@@ -33,6 +33,13 @@ public enum MovieMode
 	// TODO: consider other event handlers, switching modes?
 	public interface IMovie : IBasicMovieInfo
 	{
+		/// <summary>
+		/// When not null, methods that edit the movie will only affect buttons or axis values present in this definition,
+		/// with the exceoption of adding or removing default-value frames at the end of the movie.
+		/// The instance must be made immutable before assignment.
+		/// </summary>
+		ControllerDefinition/*?*/ ActiveControllerInputs { get; set; }
+
 		/// <summary>
 		/// Gets the current movie mode
 		/// </summary>
@@ -161,6 +168,7 @@ public interface IMovie : IBasicMovieInfo
 
 		/// <summary>
 		/// Instructs the movie to remove all input from its input log starting with the input at frame.
+		/// If <see cref="ActiveControllerInputs"/> is not null, this might not actually change the length of the movie.
 		/// </summary>
 		/// <param name="frame">The frame at which to truncate</param>
 		void Truncate(int frame);

src/BizHawk.Client.Common/movie/interfaces/ITasMovie.cs

@@ -55,11 +55,12 @@ public interface ITasMovie : IMovie, INotifyPropertyChanged, IDisposable
 
 		/// <summary>
 		/// Remove all frames between removeStart and removeUpTo (excluding removeUpTo).
+		/// If <see cref="IMovie.ActiveControllerInputs"/> is not null, this will not actually change the length of the movie.
 		/// </summary>
 		/// <param name="removeStart">The first frame to remove.</param>
 		/// <param name="removeUpTo">The frame after the last frame to remove.</param>
 		void RemoveFrames(int removeStart, int removeUpTo);
-		void SetFrame(int frame, string source);
+		void PokeFrame(int frame, string source);
 
 		void LoadBranch(TasBranch branch);