Add a hierarchical path finder to improve pathfinding performance.

Replaces the existing bi-directional search between points used by the pathfinder with a guided hierarchical search. The old search was a standard A* search with a heuristic of advancing in straight line towards the target. This heuristic performs well if a mostly direct path to the target exists, it performs poorly it the path has to navigate around blockages in the terrain. The hierarchical path finder maintains a simplified, abstract graph. When a path search is performed it uses this abstract graph to inform the heuristic. Instead of moving blindly towards the target, it will instead steer around major obstacles, almost as if it had been provided a map which ensures it can move in roughly the right direction. This allows it to explore less of the area overall, improving performance.

When a path needs to steer around terrain on the map, the hierarchical path finder is able to greatly improve on the previous performance. When a path is able to proceed in a straight line, no performance benefit will be seen. If the path needs to steer around actors on the map instead of terrain (e.g. trees, buildings, units) then the same poor pathfinding performance as before will be observed.

OpenRA.Mods.Common/Pathfinder/SparsePathGraph.cs

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+#region Copyright & License Information
+/*
+ * Copyright 2007-2022 The OpenRA Developers (see AUTHORS)
+ * This file is part of OpenRA, which is free software. It is made
+ * available to you under the terms of the GNU General Public License
+ * as published by the Free Software Foundation, either version 3 of
+ * the License, or (at your option) any later version. For more
+ * information, see COPYING.
+ */
+#endregion
+
+using System;
+using System.Collections.Generic;
+
+namespace OpenRA.Mods.Common.Pathfinder
+{
+	/// <summary>
+	/// A sparse pathfinding graph that supports a search over provided cells.
+	/// This is a classic graph that supports an arbitrary graph of nodes and edges,
+	/// and does not require a dense grid of cells.
+	/// Costs and any desired connections to a <see cref="Traits.ICustomMovementLayer"/>
+	/// must be provided as input.
+	/// </summary>
+	sealed class SparsePathGraph : IPathGraph
+	{
+		readonly Func<CPos, List<GraphConnection>> edges;
+		readonly Dictionary<CPos, CellInfo> info;
+
+		public SparsePathGraph(Func<CPos, List<GraphConnection>> edges, int estimatedSearchSize = 0)
+		{
+			this.edges = edges;
+			info = new Dictionary<CPos, CellInfo>(estimatedSearchSize);
+		}
+
+		public List<GraphConnection> GetConnections(CPos position)
+		{
+			return edges(position) ?? new List<GraphConnection>();
+		}
+
+		public CellInfo this[CPos pos]
+		{
+			get
+			{
+				if (info.TryGetValue(pos, out var cellInfo))
+					return cellInfo;
+				return default;
+			}
+			set => info[pos] = value;
+		}
+
+		public void Dispose() { }
+	}
+}