Checked LINQ queries and collections for inefficiencies.

- Made Array.IndexOf available via extension method.
- Made ToHashSet extension method.
- Change collections queried often via Contains into sets.
- Avoid Count() extension if Count or Length property exist.
- Made Count() > 0 checks and variations calls to Any() instead.
- Don't call ToList/ToArray if there is no benefit to materializing the sequence.
- If the sequence does benefit from materialization, follow this general pattern:
  - Collection queried often via Contains use ToHashSet to speed up lookups.
  - Short lived variables use ToList. This is because ToArray requires an extra copy to output the final size.
  - Collections persisted into fields or for a long time use ToArray to minimize memory overhead.

OpenRA.Game/Effects/FlashTarget.cs

@@ -25,8 +25,11 @@ namespace OpenRA.Effects
 			this.target = target;
 			player = asPlayer;
 			remainingTicks = ticks;
-			foreach (var e in target.World.Effects.OfType<FlashTarget>().Where(a => a.target == target).ToArray())
-				target.World.Remove(e);
+			target.World.RemoveAll(effect =>
+			{
+				var flashTarget = effect as FlashTarget;
+				return flashTarget != null && flashTarget.target == target;
+			});
 		}
 
 		public void Tick(World world)

OpenRA.Game/Exts.cs

@@ -106,6 +106,11 @@ namespace OpenRA
 			return ret;
 		}
 
+		public static int IndexOf<T>(this T[] array, T value)
+		{
+			return Array.IndexOf(array, value);
+		}
+
 		public static T Random<T>(this IEnumerable<T> ts, MersenneTwister r)
 		{
 			var xs = ts as ICollection<T>;
@@ -299,6 +304,11 @@ namespace OpenRA
 			return ts.Concat(moreTs);
 		}
 
+		public static HashSet<T> ToHashSet<T>(this IEnumerable<T> source)
+		{
+			return new HashSet<T>(source);
+		}
+
 		public static Dictionary<TKey, TSource> ToDictionaryWithConflictLog<TSource, TKey>(this IEnumerable<TSource> source, Func<TSource, TKey> keySelector, string debugName, Func<TKey, string> logKey, Func<TSource, string> logValue)
 		{
 			return ToDictionaryWithConflictLog(source, keySelector, x => x, debugName, logKey, logValue);
@@ -341,7 +351,7 @@ namespace OpenRA
 			// If any duplicates were found, throw a descriptive error
 			if (dupKeys.Count > 0)
 			{
-				var badKeysFormatted = string.Join(", ", dupKeys.Select(p => "{0}: [{1}]".F(logKey(p.Key), string.Join(",", p.Value.ToArray()))).ToArray());
+				var badKeysFormatted = string.Join(", ", dupKeys.Select(p => "{0}: [{1}]".F(logKey(p.Key), string.Join(",", p.Value))));
 				var msg = "{0}, duplicate values found for the following keys: {1}".F(debugName, badKeysFormatted);
 				throw new ArgumentException(msg);
 			}

OpenRA.Game/Graphics/PlayerColorRemap.cs

@@ -32,14 +32,14 @@ namespace OpenRA.Graphics
 			var c1 = new HSLColor(c.H, c.S, (byte)Math.Max(rampRange, c.L)).RGB;
 			var c2 = new HSLColor(c.H, c.S, (byte)Math.Max(0, c.L - rampRange)).RGB;
 			var baseIndex = ramp[0];
-			var remapRamp = ramp.Select(r => r - ramp[0]).ToArray();
+			var remapRamp = ramp.Select(r => r - ramp[0]);
 
 			// reversed remapping
 			if (ramp[0] > ramp[15])
 			{
 				baseIndex = ramp[15];
 				for (var i = 15; i > 0; i--)
-					remapRamp = ramp.Select(r => r - ramp[15]).ToArray();
+					remapRamp = ramp.Select(r => r - ramp[15]);
 			}
 
 			remapColors = remapRamp.Select((x, i) => Pair.New(baseIndex + i, Exts.ColorLerp(x / 16f, c1, c2)))

OpenRA.Game/Graphics/WorldRenderer.cs

@@ -80,12 +80,11 @@ namespace OpenRA.Graphics
 		List<IRenderable> GenerateRenderables()
 		{
 			var actors = World.ScreenMap.ActorsInBox(Viewport.TopLeft, Viewport.BottomRight)
-				.Append(World.WorldActor)
-				.ToList();
+				.Append(World.WorldActor);
 
 			// Include player actor for the rendered player
 			if (World.RenderPlayer != null)
-				actors.Add(World.RenderPlayer.PlayerActor);
+				actors = actors.Append(World.RenderPlayer.PlayerActor);
 
 			var worldRenderables = actors.SelectMany(a => a.Render(this));
 			if (World.OrderGenerator != null)

OpenRA.Game/Group.cs

@@ -15,7 +15,7 @@ namespace OpenRA
 {
 	public class Group
 	{
-		List<Actor> actors;
+		Actor[] actors;
 		int id;
 
 		static int nextGroup;
@@ -24,7 +24,7 @@ namespace OpenRA
 
 		public Group(IEnumerable<Actor> actors)
 		{
-			this.actors = actors.ToList();
+			this.actors = actors.ToArray();
 
 			foreach (var a in actors)
 				a.Group = this;

OpenRA.Game/Map/MapCache.cs

@@ -79,7 +79,7 @@ namespace OpenRA
 			foreach (var p in maps.Values)
 				p.UpdateRemoteSearch(MapStatus.Searching, null);
 
-			var url = Game.Settings.Game.MapRepository + "hash/" + string.Join(",", maps.Keys.ToArray()) + "/yaml";
+			var url = Game.Settings.Game.MapRepository + "hash/" + string.Join(",", maps.Keys) + "/yaml";
 
 			Action<DownloadDataCompletedEventArgs, bool> onInfoComplete = (i, cancelled) =>
 			{

OpenRA.Game/Map/MapPreview.cs

@@ -46,7 +46,7 @@ namespace OpenRA
 
 	public class MapPreview
 	{
-		static readonly List<CPos> NoSpawns = new List<CPos>();
+		static readonly CPos[] NoSpawns = new CPos[] { };
 		MapCache cache;
 
 		public readonly string Uid;
@@ -54,7 +54,7 @@ namespace OpenRA
 		public string Type { get; private set; }
 		public string Author { get; private set; }
 		public int PlayerCount { get; private set; }
-		public List<CPos> SpawnPoints { get; private set; }
+		public CPos[] SpawnPoints { get; private set; }
 		public Rectangle Bounds { get; private set; }
 		public Bitmap CustomPreview { get; private set; }
 		public Map Map { get; private set; }
@@ -112,7 +112,7 @@ namespace OpenRA
 			Author = m.Author;
 			PlayerCount = m.Players.Count(x => x.Value.Playable);
 			Bounds = m.Bounds;
-			SpawnPoints = m.GetSpawnPoints().ToList();
+			SpawnPoints = m.GetSpawnPoints();
 			CustomPreview = m.CustomPreview;
 			Status = MapStatus.Available;
 			Class = classification;
@@ -143,9 +143,9 @@ namespace OpenRA
 						PlayerCount = r.players;
 						Bounds = r.bounds;
 
-						var spawns = new List<CPos>();
+						var spawns = new CPos[r.spawnpoints.Length / 2];
 						for (var j = 0; j < r.spawnpoints.Length; j += 2)
-							spawns.Add(new CPos(r.spawnpoints[j], r.spawnpoints[j + 1]));
+							spawns[j / 2] = new CPos(r.spawnpoints[j], r.spawnpoints[j + 1]);
 						SpawnPoints = spawns;
 
 						CustomPreview = new Bitmap(new MemoryStream(Convert.FromBase64String(r.minimap)));

OpenRA.Game/Orders/UnitOrderGenerator.cs

@@ -37,7 +37,7 @@ namespace OpenRA.Orders
 			var orders = world.Selection.Actors
 				.Select(a => OrderForUnit(a, target, mi))
 				.Where(o => o != null)
-				.ToArray();
+				.ToList();
 
 			var actorsInvolved = orders.Select(o => o.Actor).Distinct();
 			if (actorsInvolved.Any())
@@ -81,8 +81,7 @@ namespace OpenRA.Orders
 
 			var orders = world.Selection.Actors
 				.Select(a => OrderForUnit(a, target, mi))
-				.Where(o => o != null)
-				.ToArray();
+				.Where(o => o != null);
 
 			var cursorName = orders.Select(o => o.Cursor).FirstOrDefault();
 			return cursorName ?? (useSelect ? "select" : "default");

OpenRA.Game/Player.cs

@@ -50,7 +50,7 @@ namespace OpenRA
 		{
 			var selectableCountries = world.Map.Rules.Actors["world"].Traits
 				.WithInterface<CountryInfo>().Where(c => c.Selectable)
-				.ToArray();
+				.ToList();
 
 			return selectableCountries.FirstOrDefault(c => c.Race == name)
 				?? selectableCountries.Random(world.SharedRandom);

OpenRA.Game/Scripting/ScriptContext.cs

@@ -80,7 +80,7 @@ namespace OpenRA.Scripting
 			// The 'this.' resolves the actual (subclass) type
 			var type = this.GetType();
 			var names = type.GetCustomAttributes<ScriptGlobalAttribute>(true);
-			if (names.Count() != 1)
+			if (names.Length != 1)
 				throw new InvalidOperationException("[LuaGlobal] attribute not found for global table '{0}'".F(type));
 
 			Name = names.First().Name;
@@ -242,13 +242,13 @@ namespace OpenRA.Scripting
 				runtime.Dispose();
 		}
 
-		static Type[] ExtractRequiredTypes(Type t)
+		static IEnumerable<Type> ExtractRequiredTypes(Type t)
 		{
 			// Returns the inner types of all the Requires<T> interfaces on this type
 			var outer = t.GetInterfaces()
 				.Where(i => i.IsGenericType && i.GetGenericTypeDefinition() == typeof(Requires<>));
 
-			return outer.SelectMany(i => i.GetGenericArguments()).ToArray();
+			return outer.SelectMany(i => i.GetGenericArguments());
 		}
 
 		static readonly object[] NoArguments = new object[0];

OpenRA.Game/Selection.cs

@@ -18,7 +18,7 @@ namespace OpenRA
 {
 	public class Selection
 	{
-		List<Actor> actors = new List<Actor>();
+		readonly HashSet<Actor> actors = new HashSet<Actor>();
 		public void Add(World w, Actor a)
 		{
 			actors.Add(a);
@@ -30,20 +30,32 @@ namespace OpenRA
 
 		public bool Contains(Actor a)
 		{
-			return actors.AsEnumerable().Contains(a);
+			return actors.Contains(a);
 		}
 
 		public void Combine(World world, IEnumerable<Actor> newSelection, bool isCombine, bool isClick)
 		{
-			var oldSelection = actors.AsEnumerable();
-
 			if (isClick)
 			{
 				var adjNewSelection = newSelection.Take(1);	/* TODO: select BEST, not FIRST */
-				actors = (isCombine ? oldSelection.SymmetricDifference(adjNewSelection) : adjNewSelection).ToList();
+				if (isCombine)
+					actors.SymmetricExceptWith(adjNewSelection);
+				else
+				{
+					actors.Clear();
+					actors.UnionWith(adjNewSelection);
+				}
 			}
 			else
-				actors = (isCombine ? oldSelection.Union(newSelection) : newSelection).ToList();
+			{
+				if (isCombine)
+					actors.UnionWith(newSelection);
+				else
+				{
+					actors.Clear();
+					actors.UnionWith(newSelection);
+				}
+			}
 
 			var voicedUnit = actors.FirstOrDefault(a => a.Owner == world.LocalPlayer && a.IsInWorld && a.HasVoices());
 			if (voicedUnit != null)
@@ -57,11 +69,11 @@ namespace OpenRA
 		}
 
 		public IEnumerable<Actor> Actors { get { return actors; } }
-		public void Clear() { actors = new List<Actor>(); }
+		public void Clear() { actors.Clear(); }
 
 		public void Tick(World world)
 		{
-			actors.RemoveAll(a => !a.IsInWorld || (!a.Owner.IsAlliedWith(world.RenderPlayer) && world.FogObscures(a)));
+			actors.RemoveWhere(a => !a.IsInWorld || (!a.Owner.IsAlliedWith(world.RenderPlayer) && world.FogObscures(a)));
 
 			foreach (var cg in controlGroups.Values)
 			{

OpenRA.Game/Server/Server.cs

@@ -406,7 +406,7 @@ namespace OpenRA.Server
 		public void DispatchOrdersToClients(Connection conn, int frame, byte[] data)
 		{
 			var from = conn != null ? conn.PlayerIndex : 0;
-			foreach (var c in Conns.Except(conn).ToArray())
+			foreach (var c in Conns.Except(conn).ToList())
 				DispatchOrdersToClient(c, from, frame, data);
 		}
 
@@ -517,9 +517,7 @@ namespace OpenRA.Server
 
 		public void DropClient(Connection toDrop, int frame)
 		{
-			if (PreConns.Contains(toDrop))
-				PreConns.Remove(toDrop);
-			else
+			if (!PreConns.Remove(toDrop))
 			{
 				Conns.Remove(toDrop);
 

OpenRA.Game/Traits/Util.cs

@@ -130,10 +130,7 @@ namespace OpenRA.Traits
 
 		public static IEnumerable<CPos> ExpandFootprint(IEnumerable<CPos> cells, bool allowDiagonal)
 		{
-			var result = new Dictionary<CPos, bool>();
-			foreach (var c in cells.SelectMany(c => Neighbours(c, allowDiagonal)))
-				result[c] = true;
-			return result.Keys;
+			return cells.SelectMany(c => Neighbours(c, allowDiagonal)).Distinct();
 		}
 
 		public static IEnumerable<CPos> AdjacentCells(World w, Target target)

OpenRA.Game/Widgets/PerfGraphWidget.cs

@@ -26,7 +26,7 @@ namespace OpenRA.Widgets
 			Game.Renderer.LineRenderer.DrawLine(origin + new float2(100, 0) * basis, origin + new float2(100, 100) * basis, Color.White, Color.White);
 
 			var k = 0;
-			foreach (var item in PerfHistory.Items.Values.ToArray())
+			foreach (var item in PerfHistory.Items.Values)
 			{
 				var n = 0;
 				item.Samples().Aggregate((a, b) =>
@@ -55,7 +55,7 @@ namespace OpenRA.Widgets
 			}
 
 			k = 0;
-			foreach (var item in PerfHistory.Items.Values.ToArray())
+			foreach (var item in PerfHistory.Items.Values)
 			{
 				Game.Renderer.Fonts["Tiny"].DrawText(item.Name, new float2(rect.Left, rect.Top) + new float2(18, 10 * k - 3), Color.White);
 				++k;

OpenRA.Game/Widgets/WidgetUtils.cs

@@ -75,7 +75,7 @@ namespace OpenRA.Widgets
 		public static int[] GetBorderSizes(string collection)
 		{
 			var images = new[] { "border-t", "border-b", "border-l", "border-r" };
-			var ss = images.Select(i => ChromeProvider.GetImage(collection, i)).ToArray();
+			var ss = images.Select(i => ChromeProvider.GetImage(collection, i)).ToList();
 			return new[] { (int)ss[0].Size.Y, (int)ss[1].Size.Y, (int)ss[2].Size.X, (int)ss[3].Size.X };
 		}
 

OpenRA.Game/World.cs

@@ -26,9 +26,16 @@ namespace OpenRA
 
 	public class World
 	{
+		class ActorIDComparer : IComparer<Actor>
+		{
+			public static readonly ActorIDComparer Instance = new ActorIDComparer();
+			private ActorIDComparer() { }
+			public int Compare(Actor x, Actor y) { return x.ActorID.CompareTo(y.ActorID); }
+		}
+
 		static readonly Func<MPos, bool> FalsePredicate = _ => false;
 		internal readonly TraitDictionary TraitDict = new TraitDictionary();
-		readonly HashSet<Actor> actors = new HashSet<Actor>();
+		readonly SortedSet<Actor> actors = new SortedSet<Actor>(ActorIDComparer.Instance);
 		readonly List<IEffect> effects = new List<IEffect>();
 		readonly Queue<Action<World>> frameEndActions = new Queue<Action<World>>();
 
@@ -237,6 +244,7 @@ namespace OpenRA
 
 		public void Add(IEffect b) { effects.Add(b); }
 		public void Remove(IEffect b) { effects.Remove(b); }
+		public void RemoveAll(Predicate<IEffect> predicate) { effects.RemoveAll(predicate); }
 
 		public void AddFrameEndTask(Action<World> a) { frameEndActions.Enqueue(a); }