feat(ai): Auto-pilot now stops thrusting upon reaching 99% of max spe…

…ed to save energy (endless-sky#8489)

source/AI.cpp

@@ -2204,7 +2204,14 @@ bool AI::MoveTo(Ship &ship, Command &command, const Point &targetPosition,
 	bool isFacing = (dp.Unit().Dot(angle.Unit()) > .95);
 	if(!isClose || (!isFacing && !shouldReverse))
 		command.SetTurn(TurnToward(ship, dp));
-	if(isFacing)
+	// Drag is not applied when not thrusting, so stop thrusting when close to max speed
+	// to save energy. Work with a slightly lower maximum velocity to avoid border cases.
+	// In order for a ship to use their afterburner, they must also have the forward
+	// command active. Therefore, if this ship should use its afterburner, use the
+	// max velocity with afterburner thrust included.
+	double maxVelocity = ship.MaxVelocity(ShouldUseAfterburner(ship)) * .99;
+	if(isFacing && (velocity.LengthSquared() <= maxVelocity * maxVelocity
+			|| dp.Unit().Dot(velocity.Unit()) < .95))
 		command |= Command::FORWARD;
 	else if(shouldReverse)
 	{

source/Ship.cpp

@@ -2972,13 +2972,14 @@ double Ship::Acceleration() const
 
 
 
-double Ship::MaxVelocity() const
+double Ship::MaxVelocity(bool withAfterburner) const
 {
 	// v * drag / mass == thrust / mass
 	// v * drag == thrust
 	// v = thrust / drag
 	double thrust = attributes.Get("thrust");
-	return (thrust ? thrust : attributes.Get("afterburner thrust")) / Drag();
+	double afterburnerThrust = attributes.Get("afterburner thrust");
+	return (thrust ? thrust + afterburnerThrust * withAfterburner: afterburnerThrust) / Drag();
 }
 
 

source/Ship.h

@@ -384,7 +384,7 @@ class Ship : public Body, public std::enable_shared_from_this<Ship> {
 	double InertialMass() const;
 	double TurnRate() const;
 	double Acceleration() const;
-	double MaxVelocity() const;
+	double MaxVelocity(bool withAfterburner = false) const;
 	double ReverseAcceleration() const;
 	double MaxReverseVelocity() const;