Redo some of the terrain editing logic

- Introduce a tileToChange variable
  Holds all of the tiles and which corners to modify
  Is used to calculate tilesPrevious & tilesChanged
- Support editing a single corner
- Edit tool mesh shows edited corners

terrain/Terrain+Editing.cs

@@ -1,4 +1,5 @@
 using System.IO;
+using System.Runtime.InteropServices;
 
 public partial class Terrain
 {
@@ -8,11 +9,15 @@ public partial class Terrain
 	// Set by the terrain editing plugin.
 	public EditorUndoRedoManager EditorUndoRedo { get; set; }
 
+	// Dummy value to satisfy the overly careful compiler.
+	static bool _dummy = false;
+
 	Material _editToolMaterial;
 	public override void _EnterTree()
 	{
 		_editToolMaterial = new StandardMaterial3D {
-			AlbedoColor   = Colors.Blue,
+			VertexColorUseAsAlbedo = true,
+			BlendMode     = BaseMaterial3D.BlendModeEnum.Mix,
 			ShadingMode   = BaseMaterial3D.ShadingModeEnum.Unshaded,
 			DepthDrawMode = BaseMaterial3D.DepthDrawModeEnum.Disabled,
 			NoDepthTest   = true,
@@ -64,46 +69,60 @@ public partial class Terrain
 				center.DistanceSquaredTo(tile.ToCenter()) < distanceSqr);
 		}
 
-		var tiles = (toolShape switch {
-			ToolShape.Corner => [ hover.Position ],
-			ToolShape.Circle => GetTilesInRadius(),
-			ToolShape.Square => GetTilesInSquare(),
-			_ => throw new InvalidOperationException(),
-		}).ToHashSet();
-
 		// TODO: Handle different tool modes, such as painting.
-		// TODO: Finally allow editing single corners.
 		// TODO: Allow click-dragging which doesn't affect already changed tiles / corners.
 		// TODO: Make mesh generation generate vertical walls between disconnected corners.
 		// TODO: Use ArrayMesh instead of ImmediateMesh.
 		// TODO: Dynamically expand terrain instead of having it be a set size.
 
-		// Raise / lower the terrain when left mouse button is pressed.
+		// Holds onto all the tiles and which of their corners corners will be affected by this edit operation.
-		if (ev is InputEventMouseButton { ButtonIndex: MouseButton.Left, Pressed: true }) {
+		var tilesToChange = new Dictionary<TilePos, Corners<bool>>();
-			prevent_default = true;
+		ref Corners<bool> GetTileToChange(TilePos position)
+			// Don't look at this black magic. The Dictionary type should have this by default I swear!
+			=> ref CollectionsMarshal.GetValueRefOrAddDefault(tilesToChange, position, out _dummy);
 
-			var cornersToChange = new HashSet<(TilePos Position, Corner Corner)>();
+		if (toolShape == ToolShape.Corner) {
+			// Modify selected corner itself.
+			GetTileToChange(hover.Position)[hover.Corner] = true;
 
-			// Raise selected tiles themselves.
+			if (isConnected) {
+				var height = GetTile(hover.Position).Height[hover.Corner];
+				foreach (var neighbor in GetNeighbors(hover.Position, hover.Corner)) {
+					var neighborHeight = GetTile(neighbor.Position).Height[neighbor.Corner];
+					if (neighborHeight != height) continue;
+					GetTileToChange(neighbor.Position)[neighbor.Corner] = true;
+				}
+			}
+		} else {
+			var tiles = (toolShape switch {
+				ToolShape.Circle => GetTilesInRadius(),
+				ToolShape.Square => GetTilesInSquare(),
+				_ => throw new InvalidOperationException(),
+			}).ToHashSet();
+
+			// Modify selected tiles themselves.
 			foreach (var pos in tiles)
-				foreach (var corner2 in Enum.GetValues<Corner>())
+				GetTileToChange(pos) = new(true);
-					cornersToChange.Add((pos, corner2));
 
-			if (isConnected) {
+			// If the 'connected_toggle' button is active, move "connected" corners.
-				// If the 'connected_toggle' button is active, move "connected" corners.
+			// Connected corners are the ones that are at the same height as ones already being moved.
-				// Connected corners are the ones that are at the same height as ones already being moved.
+			if (isConnected) foreach (var pos in tiles) {
-				foreach (var pos in tiles) {
+				var tile = GetTile(pos);
-					var tile = GetTile(pos);
+				foreach (var corner in Enum.GetValues<Corner>()) {
-					foreach (var corner in Enum.GetValues<Corner>()) {
+					var height = tile.Height[corner];
-						var height = tile.Height[corner];
+					foreach (var neighbor in GetNeighbors(pos, corner)) {
-						foreach (var (neighborPos, neighborCorner) in GetNeighbors(pos, corner)) {
+						if (tiles.Contains(neighbor.Position)) continue;
-							if (tiles.Contains(neighborPos)) continue;
+						var neighborHeight = GetTile(neighbor.Position).Height[neighbor.Corner];
-							var neighborHeight = GetTile(neighborPos).Height[neighborCorner];
+						if (neighborHeight != height) continue;
-							if (neighborHeight == height) cornersToChange.Add((neighborPos, neighborCorner));
+						GetTileToChange(neighbor.Position)[neighbor.Corner] = true;
-						}
 					}
 				}
 			}
+		}
+
+		// Raise / lower the terrain when left mouse button is pressed.
+		if (ev is InputEventMouseButton { ButtonIndex: MouseButton.Left, Pressed: true }) {
+			prevent_default = true;
 
 			const float AdjustHeight = 0.5f;
 			var amount = isFlatten ? GetTile(hover.Position).Height[hover.Corner]
@@ -111,16 +130,15 @@ public partial class Terrain
 
 			var tilesPrevious = new List<(TilePos, Corners<float>)>();
 			var tilesChanged  = new List<(TilePos, Corners<float>)>();
-			foreach (var group in cornersToChange.GroupBy(c => c.Position, c => c.Corner)) {
+			foreach (var (pos, corners) in tilesToChange) {
-				var pos = group.Key;
 				var tile = GetTile(pos);
 				tilesPrevious.Add((pos, tile.Height));
 
 				var newHeight = tile.Height;
-				foreach (var corner in group) {
+				if (corners.TopLeft    ) newHeight.TopLeft     = isFlatten ? amount : newHeight.TopLeft     + amount;
-					if (isFlatten) newHeight[corner]  = amount;
+				if (corners.TopRight   ) newHeight.TopRight    = isFlatten ? amount : newHeight.TopRight    + amount;
-					else           newHeight[corner] += amount;
+				if (corners.BottomRight) newHeight.BottomRight = isFlatten ? amount : newHeight.BottomRight + amount;
-				}
+				if (corners.BottomLeft ) newHeight.BottomLeft  = isFlatten ? amount : newHeight.BottomLeft  + amount;
 				tilesChanged.Add((pos, newHeight));
 			}
 
@@ -140,7 +158,7 @@ public partial class Terrain
 			}
 		}
 
-		UpdateEditToolMesh(tiles);
+		UpdateEditToolMesh(tilesToChange);
 		return prevent_default;
 	}
 
@@ -157,29 +175,28 @@ public partial class Terrain
 	}
 
 
-	void UpdateEditToolMesh(IEnumerable<TilePos> tiles)
+	void UpdateEditToolMesh(Dictionary<TilePos, Corners<bool>> tiles)
 	{
 		var mesh = GetOrCreateMesh("EditToolMesh");
 		mesh.ClearSurfaces();
 		mesh.SurfaceBegin(Mesh.PrimitiveType.Lines);
 
-		void AddLine(Vector3 start, Vector3 end) {
+		void AddLine((Vector3 Position, bool Visible) start,
-			mesh.SurfaceAddVertex(start);
+		             (Vector3 Position, bool Visible) end) {
-			mesh.SurfaceAddVertex(end);
+			mesh.SurfaceSetColor(start.Visible ? Colors.Blue : Colors.Transparent);
+			mesh.SurfaceAddVertex(start.Position);
+			mesh.SurfaceSetColor(end.Visible ? Colors.Blue : Colors.Transparent);
+			mesh.SurfaceAddVertex(end.Position);
 		}
 
-		void AddQuad(Vector3 topLeft    , Vector3 topRight  ,
+		foreach (var (tile, visible) in tiles) {
-		             Vector3 bottomRight, Vector3 bottomLeft) {
+			var positions = GetTileCornerPositions(tile);
-			AddLine(topLeft    , topRight   );
+			foreach (var side in Enum.GetValues<Side>()) {
-			AddLine(topRight   , bottomRight);
+				var (corner1, corner2) = side.GetCorners();
-			AddLine(bottomRight, bottomLeft );
+				if (!visible[corner1] && !visible[corner2]) continue;
-			AddLine(bottomLeft , topLeft    );
+				AddLine((positions[corner1], visible[corner1]),
-		}
+				        (positions[corner2], visible[corner2]));
-
+			}
-		foreach (var tile in tiles) {
-			var (topLeft, topRight, bottomRight, bottomLeft)
-				= GetTileCornerPositions(tile);
-			AddQuad(topLeft, topRight, bottomRight, bottomLeft);
 		}
 
 		mesh.SurfaceEnd();
@@ -211,7 +228,7 @@ public partial class Terrain
 		[Corner.BottomRight] = [(+1, +1, Corner.TopLeft    ), (+1, 0, Corner.BottomLeft ), (0, +1, Corner.TopRight   )],
 		[Corner.BottomLeft ] = [(-1, +1, Corner.TopRight   ), (-1, 0, Corner.BottomRight), (0, +1, Corner.TopLeft    )],
 	};
-	static IEnumerable<(TilePos, Corner)> GetNeighbors(TilePos pos, Corner corner)
+	static IEnumerable<(TilePos Position, Corner Corner)> GetNeighbors(TilePos pos, Corner corner)
 		=> _offsetLookup[corner].Select(e => (new TilePos(pos.X + e.X, pos.Y + e.Y), e.Opposite));
 
 	static byte[] Pack(IEnumerable<(TilePos Position, Corners<float> Corners)> data)