context: #region Copyright & License Information
  /*
   * Copyright 2007-2020 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.Collections.Generic;
  using System.Linq;
  using OpenRA.Activities;
  using OpenRA.Mods.Common.Traits;
  using OpenRA.Primitives;
  using OpenRA.Traits;
  
  namespace OpenRA.Mods.Common.Activities
  {
  	public class FlyAttack : Activity, IActivityNotifyStanceChanged
  	{
  		readonly Aircraft aircraft;
  		readonly AttackAircraft attackAircraft;
  		readonly Rearmable rearmable;
  		readonly AttackSource source;
  		readonly bool forceAttack;
  		readonly Color? targetLineColor;
  		readonly WDist strafeDistance;
  
  		Target target;
  		Target lastVisibleTarget;
  		WDist lastVisibleMaximumRange;
  		BitSet<TargetableType> lastVisibleTargetTypes;
  		Player lastVisibleOwner;
  		bool useLastVisibleTarget;
  		bool hasTicked;
  		bool returnToBase;
  
  		public FlyAttack(Actor self, AttackSource source, Target target, bool forceAttack, Color? targetLineColor)
  		{
  			this.source = source;
  			this.target = target;
  			this.forceAttack = forceAttack;
  			this.targetLineColor = targetLineColor;
  
  			aircraft = self.Trait<Aircraft>();
  			attackAircraft = self.Trait<AttackAircraft>();
  			rearmable = self.TraitOrDefault<Rearmable>();
  
  			strafeDistance = attackAircraft.Info.StrafeRunLength;
  
  			// The target may become hidden between the initial order request and the first tick (e.g. if queued)
  			// Moving to any position (even if quite stale) is still better than immediately giving up
  			if ((target.Type == TargetType.Actor && target.Actor.CanBeViewedByPlayer(self.Owner))
  				|| target.Type == TargetType.FrozenActor || target.Type == TargetType.Terrain)
  			{
  				lastVisibleTarget = Target.FromPos(target.CenterPosition);
  				lastVisibleMaximumRange = attackAircraft.GetMaximumRangeVersusTarget(target);
  
  				if (target.Type == TargetType.Actor)
  				{
  					lastVisibleOwner = target.Actor.Owner;
  					lastVisibleTargetTypes = target.Actor.GetEnabledTargetTypes();
  				}
  				else if (target.Type == TargetType.FrozenActor)
  				{
  					lastVisibleOwner = target.FrozenActor.Owner;
  					lastVisibleTargetTypes = target.FrozenActor.TargetTypes;
  				}
  			}
  		}
  
  		public override bool Tick(Actor self)
  		{
  			returnToBase = false;
  
  			// Refuse to take off if it would land immediately again.
  			if (aircraft.ForceLanding)
  				Cancel(self);
  
  			if (IsCanceling)
  				return true;
  
  			// Check that AttackFollow hasn't cancelled the target by modifying attack.Target
  			// Having both this and AttackFollow modify that field is a horrible hack.
  			if (hasTicked && attackAircraft.RequestedTarget.Type == TargetType.Invalid)
  				return true;
  
  			if (attackAircraft.IsTraitPaused)
  				return false;
  
  			target = target.Recalculate(self.Owner, out var targetIsHiddenActor);
  			attackAircraft.SetRequestedTarget(self, target, forceAttack);
  			hasTicked = true;
  
  			if (!targetIsHiddenActor && target.Type == TargetType.Actor)
  			{
  				lastVisibleTarget = Target.FromTargetPositions(target);
  				lastVisibleMaximumRange = attackAircraft.GetMaximumRangeVersusTarget(target);
  				lastVisibleOwner = target.Actor.Owner;
  				lastVisibleTargetTypes = target.Actor.GetEnabledTargetTypes();
  			}
  
  			// The target may become hidden in the same tick the FlyAttack constructor is called,
  			// causing lastVisible* to remain uninitialized.
  			// Fix the fallback values based on the frozen actor properties
  			else if (target.Type == TargetType.FrozenActor && !lastVisibleTarget.IsValidFor(self))
  			{
  				lastVisibleTarget = Target.FromTargetPositions(target);
  				lastVisibleMaximumRange = attackAircraft.GetMaximumRangeVersusTarget(target);
  				lastVisibleOwner = target.FrozenActor.Owner;
  				lastVisibleTargetTypes = target.FrozenActor.TargetTypes;
  			}
  
  			useLastVisibleTarget = targetIsHiddenActor || !target.IsValidFor(self);
  
  			// Target is hidden or dead, and we don't have a fallback position to move towards
  			if (useLastVisibleTarget && !lastVisibleTarget.IsValidFor(self))
  				return true;
  
  			// If all valid weapons have depleted their ammo and Rearmable trait exists, return to RearmActor to reload
  			// and resume the activity after reloading if AbortOnResupply is set to 'false'
  			if (rearmable != null && !useLastVisibleTarget && attackAircraft.Armaments.All(x => x.IsTraitPaused || !x.Weapon.IsValidAgainst(target, self.World, self)))
  			{
  				// Attack moves never resupply
  				if (source == AttackSource.AttackMove)
  					return true;
  
  				// AbortOnResupply cancels the current activity (after resupplying) plus any queued activities
  				if (attackAircraft.Info.AbortOnResupply)
  					NextActivity?.Cancel(self);
  
  				QueueChild(new ReturnToBase(self));
  				returnToBase = true;
  				return attackAircraft.Info.AbortOnResupply;
  			}
  
  			var pos = self.CenterPosition;
  			var checkTarget = useLastVisibleTarget ? lastVisibleTarget : target;
  
  			// We don't know where the target actually is, so move to where we last saw it
  			if (useLastVisibleTarget)
  			{
  				// We've reached the assumed position but it is not there - give up
  				if (checkTarget.IsInRange(pos, lastVisibleMaximumRange))
  					return true;
  
  				// Fly towards the last known position
  				QueueChild(new Fly(self, target, WDist.Zero, lastVisibleMaximumRange, checkTarget.CenterPosition, Color.Red));
  				return false;
  			}
  
  			var delta = attackAircraft.GetTargetPosition(pos, target) - pos;
  			var desiredFacing = delta.HorizontalLengthSquared != 0 ? delta.Yaw : aircraft.Facing;
  
  			QueueChild(new TakeOff(self));
  
  			var minimumRange = attackAircraft.Info.AttackType == AirAttackType.Strafe ? WDist.Zero : attackAircraft.GetMinimumRangeVersusTarget(target);
  
  			// Move into range of the target.
  			if (!target.IsInRange(pos, lastVisibleMaximumRange) || target.IsInRange(pos, minimumRange))
  				QueueChild(aircraft.MoveWithinRange(target, minimumRange, lastVisibleMaximumRange, target.CenterPosition, Color.Red));
  
  			// The aircraft must keep moving forward even if it is already in an ideal position.
  			else if (attackAircraft.Info.AttackType == AirAttackType.Strafe)
  				QueueChild(new StrafeAttackRun(self, attackAircraft, target, strafeDistance != WDist.Zero ? strafeDistance : lastVisibleMaximumRange));
  			else if (attackAircraft.Info.AttackType == AirAttackType.Default && !aircraft.Info.CanHover)
  				QueueChild(new FlyAttackRun(self, target, lastVisibleMaximumRange));
  
  			// Turn to face the target if required.
  			else if (!attackAircraft.TargetInFiringArc(self, target, 4 * attackAircraft.Info.FacingTolerance))
  				aircraft.Facing = Util.TickFacing(aircraft.Facing, desiredFacing, aircraft.TurnSpeed);
  
  			return false;
  		}
  
  		protected override void OnLastRun(Actor self)
  		{
  			// Cancel the requested target, but keep firing on it while in range
  			attackAircraft.ClearRequestedTarget();
  		}
  
  		void IActivityNotifyStanceChanged.StanceChanged(Actor self, AutoTarget autoTarget, UnitStance oldStance, UnitStance newStance)
  		{
  			// Cancel non-forced targets when switching to a more restrictive stance if they are no longer valid for auto-targeting
  			if (newStance > oldStance || forceAttack)
  				return;
  
  			// If lastVisibleTarget is invalid we could never view the target in the first place, so we just drop it here too
  			if (!lastVisibleTarget.IsValidFor(self) || !autoTarget.HasValidTargetPriority(self, lastVisibleOwner, lastVisibleTargetTypes))
  				attackAircraft.ClearRequestedTarget();
  		}
  
  		public override IEnumerable<TargetLineNode> TargetLineNodes(Actor self)
  		{
  			if (targetLineColor != null)
  			{
  				if (returnToBase)
  					foreach (var n in ChildActivity.TargetLineNodes(self))
  						yield return n;
  				if (!returnToBase || !attackAircraft.Info.AbortOnResupply)
  					yield return new TargetLineNode(useLastVisibleTarget ? lastVisibleTarget : target, targetLineColor.Value);
  			}
  		}
  	}
  
  	class FlyAttackRun : Activity
  	{
  		Target target;
  		WDist exitRange;
  		bool targetIsVisibleActor;
  
  		public FlyAttackRun(Actor self, Target t, WDist exitRange)
  		{
  			ChildHasPriority = false;
  
  			target = t;
  			this.exitRange = exitRange;
  		}
  
  		protected override void OnFirstRun(Actor self)
  		{
  			// The target may have died while this activity was queued
  			if (target.IsValidFor(self))
  			{
  				QueueChild(new Fly(self, target, target.CenterPosition));
  				QueueChild(new Fly(self, target, exitRange, WDist.MaxValue, target.CenterPosition));
  			}
  			else
  				Cancel(self);
  		}
  
  		public override bool Tick(Actor self)
  		{
  			if (TickChild(self) || IsCanceling)
  				return true;
  
  			// Cancel the run if the target become invalid (e.g. killed) while visible
  			var targetWasVisibleActor = targetIsVisibleActor;
  			target = target.Recalculate(self.Owner, out var targetIsHiddenActor);
  			targetIsVisibleActor = target.Type == TargetType.Actor && !targetIsHiddenActor;
  
  			if (targetWasVisibleActor && !target.IsValidFor(self))
  				Cancel(self);
  
  			return false;
  		}
  	}
  
  	class StrafeAttackRun : Activity
  	{
  		Target target;
  		WDist exitRange;
  		readonly AttackAircraft attackAircraft;
  
  		public StrafeAttackRun(Actor self, AttackAircraft attackAircraft, Target t, WDist exitRange)
  		{
  			ChildHasPriority = false;
  
  			target = t;
  			this.attackAircraft = attackAircraft;
  			this.exitRange = exitRange;
  		}
  
  		protected override void OnFirstRun(Actor self)
  		{
  			// The target may have died while this activity was queued
  			if (target.IsValidFor(self))
  			{
  
gt: 				QueueChild(new Fly(self, target, target.CenterPosition));