/*
	Rocket Simulation Module
*/

import Vec3D;
import Planet;

class Rocket
{
	/*	A Rocket is an object which has a certain position and velocity,
		and is imbedded in a certain environment.  On each Update(),
		it can query its environment for external forces, calculate
		forces (thrust) it exerts itself, and updates its state.
	*/

	public Planet	itsPlanet;		// planet it is bound to
	public double	mass;			// current total mass
	public double	fuel_mass;		// current mass of fuel
	public double	V_exhaust;		// current exhaust velocity
	public double	exhaust_rate;	// current mass of exhaust per sec
	public double	cd;				// coefficient of drag
	public double	area;			// frontal area
	public Vec3D	position;		// position relative to center of planet
	public Vec3D	velocity;		// current velocity

	public Rocket(Planet pPlanet) {
		itsPlanet = pPlanet;
		position = new Vec3D(0, itsPlanet.radius, 0);
		velocity = new Vec3D();
		mass = 1;
		cd = 0.3;
		area = 5;
	}

	public void Update() { Update(1.0); }	// sure wish we had default values...
	
	public void Update( double dt ) {
		// As this is the first attempt, I'll use a very simple
		// forward Euler integration approach.
		
		// First, get the total forces on the rocket.
		
		// Gravity is calculated by the planet, given our position & mass.
		Vec3D Fgrav = itsPlanet.GetGravity( position, mass );
		
		// Thrust is a result of whatever mass we're expelling.
		// For now, we assume we're going straight up (+Y).
		double thrust = V_exhaust * exhaust_rate;
		Vec3D Fthrust = new Vec3D(0, thrust, 0);

		// Drag is opposite in direction to the current velocity,
		// and has a complex magnitude computed by the Planet.
		double speed = velocity.Length();
		Vec3D Fdrag;
		if (speed > 0)
			Fdrag = velocity.Times( - itsPlanet.GetAirDrag(
				position, velocity, cd, area ) / speed );
		else Fdrag = new Vec3D();
		
		// Add the forces together, then do a = F/m, and apply a.
		Vec3D Ftotal = Fgrav.Plus(Fthrust).Plus(Fdrag);
		Vec3D accel = Ftotal.DividedBy(mass);
		
		// actually update our state variables
		velocity.Add( accel.Times(dt) );
		position.Add( velocity.Times(dt) );
		
		mass -= exhaust_rate * dt;
		fuel_mass -= exhaust_rate * dt;
		if (fuel_mass < exhaust_rate * dt)
			exhaust_rate = fuel_mass / dt;

		// check for crash/landing
		if (position.Length() < itsPlanet.radius) {
			velocity.Set(0,0,0);
			// later: do something more intelligent here
		}
	}
}


class test {
	static public void main(String argv[]) {
		System.out.println("simulation started");
		Planet Earth = new Planet();
		Rocket r = new Rocket(Earth);
		r.mass = 120;
		r.fuel_mass = 100;
		r.cd = 0.1;
		// light the engines!
		r.exhaust_rate = 1;		// burn 1 kg/sec
		r.V_exhaust = 2000;		// exhaust speed (in m/s)

		Rocket r2 = new Rocket(Earth);	// for comparison, another rocket 34km up
		r2.position.y += 34000;
		r2.mass = 120;
		r2.fuel_mass = 100;
		r2.cd = 0.1;			// totally frictionless surface!
		// light the engines!
		r2.exhaust_rate = 1;		// burn 1 kg/sec
		r2.V_exhaust = 2000;		// exhaust speed (in m/s)


		System.out.println("t\tvel.1\talt.1\tvel.2\talt.2");
		for (int i=0; i < 500; i++) {

			double altitude = r.position.y - Earth.radius;
			if (altitude < 0) {
				altitude = 0;
				if (r.fuel_mass <= 0) i = 99999;
			}
			
			System.out.println( "" + i + "\t" + 
				r.velocity.y + "\t" +
				altitude + "\t" + 
				r2.velocity.y + "\t" +
				(r2.position.y - Earth.radius)
				 );

			for (int j=0; j<100; j++) {
				r.Update( 0.01 );
				r2.Update( 0.01 );
			}
		}
	}
}