planes return to base after attacking

OpenRa.Game/Traits/Activities/ReturnToBase.cs

@@ -7,14 +7,37 @@ namespace OpenRa.Game.Traits.Activities
 {
 	class ReturnToBase : IActivity
 	{
+		// todo: defer decision-making until we are first scheduled!
 		public IActivity NextActivity { get; set; }
 		bool isCanceled;
 
 		readonly float2 w1, w2, w3;	/* tangent points to turn circles */
 		readonly float2 landPoint;
 
-		public ReturnToBase(Actor self, float2 landPos)
+		public ReturnToBase(Actor self)
+			: this( self, null ) {}
+
+		Actor ChooseAirfield(Actor self)
 		{
+			// todo: handle reservations
+
+			var airfield = Game.world.Actors
+				.Where(a => a.Info == Rules.UnitInfo["AFLD"]
+					&& a.Owner == self.Owner)
+				.FirstOrDefault();
+
+			if (airfield == null)
+				throw new NotImplementedException("nowhere to land; what to do?");
+
+			return airfield;
+		}
+
+		public ReturnToBase(Actor self, Actor dest)
+		{
+			if (dest == null)
+				dest = ChooseAirfield(self);
+
+			var landPos = dest.CenterLocation;
 			var unit = self.traits.Get<Unit>();
 			var speed = .2f * Util.GetEffectiveSpeed(self);
 			var approachStart = landPos - new float2(unit.Altitude * speed, 0);