#include<iostream.h>

const double dt=1.0;
const double gravity=0.5;
const double engine_strength=1.0;
const double safe_velocity = -0.5;
const char burn_key='b';

class ship_state {
public:
   ship_state(double h, double v, double f);
   double height();
   double velocity();
   double fuel();
   int landed();
   ship_state& update(double rate);
   void print(ostream& os);
private:
   double height_;
   double velocity_;
   double fuel_;
};

//Class definitions
ship_state::ship_state(double h, double v, double f)
{
   height_ = h;
   velocity_ = v;
   fuel_ = f;
}

double ship_state::height() {return height_;}
double ship_state::velocity(){return velocity_;}
double ship_state::fuel() {return fuel_;}

int ship_state::landed()
{
   if (height_<=0)
      return 1;
   return 0;
}

void ship_state::print(ostream& os)
{
   os<<"Velocity :"<<velocity_<<endl;
   os<<"Height   :"<<height_<<endl;
   os<<"Fuel     :"<<fuel_<<endl;
}

ship_state& ship_state::update(double rate)
{
   double dh;
   double dv;
   if (fuel_<=0) {
      fuel_=0;
      rate=0;
   }
   dh=velocity_*dt;
   dv=engine_strength*rate-gravity;
   fuel_ -= (rate*dt);
   height_ += dh;
   velocity_+=dv;
   return *this;
}

//Regular prototypes
void end_game(ship_state&s);
double get_burn_rate();

void lander_loop(ship_state& s)
{
   s.print(cout);
   if (s.landed())
      end_game(s);
   else
      lander_loop(s.update(get_burn_rate()));
}

void end_game(ship_state& s)
{
   double v=s.velocity();
   cout<<"Final velocity: "<<v;
   if (v>=safe_velocity)
      cout<<"...good landing!\n";
   else
      cout<<"...you crashed!\n";
}

double get_burn_rate()
{
   cout<<"Enter "<<burn_key<<" to burn fuel, any other to float :";
   char ch;
   cin>>ch;
   if (ch==burn_key)
      return 1.0;
   else
      return 0.0;
}

main()
{
   cout << "Welcome to Lunar Lander.  The object is to land the ship with a" 
        << " final" << endl << "velocity greater than " << safe_velocity 
        << endl << endl;

   ship_state s(50.0, 0.0, 20.0);
   lander_loop(s);
}