#include <iostream.h>
#include <math.h>
#include <winbgim.h>
#include <conio.h>

const double DELTA_T = .05; // increment of time in seconds
const double GX = 0;     // x component of gravitational acceleration
const double GY = -32.2; // y component of gravitational acceleration
const int XDIM = 500;
const int YDIM = 500;
void display_state(int t, double px, double py, double vx, double vy);
void run_simulation(double px, double py, double vx, double vy);


void main()
{
   double cannon_angle, vel;
   double px, py, vx, vy;

   // initialize simulation
   cout << "Enter cannon angle: ";
   cin >> cannon_angle;
   cout << "Enter initial velocity of cannonball: ";
   cin >> vel;

   initwindow(XDIM, YDIM);
   px = 0;
   py = 0;
   vx = vel * cos(cannon_angle * 3.14159 / 180);
   vy = vel * sin(cannon_angle * 3.14159 / 180);

   run_simulation(px, py, vx, vy);

   while (!kbhit()) ;
}

////////////////////////// run_simulation //////////////////////////////////////

void run_simulation(double px, double py, double vx, double vy)
{
   int t, crash;

   for (crash=t=0;!crash;++t) 
   {
      display_state(t, px, py, vx, vy);
      if (py < 0 || px >= XDIM) crash = 1;
      vx += DELTA_T * GX;
      vy += DELTA_T * GY;
      px += DELTA_T * vx;
      py += DELTA_T * vy;
   }
}

////////////////////////// display_state //////////////////////////////////////

void display_state(int t, double px, double py, double vx, double vy)
{
   static double prevx = 0, prevy = 0;
   //cout << "t = " << t*DELTA_T
   //cout << " pos = " << px << " " << py;
   //cout << " vel = " << vx << " " << vy;
   //cout << endl;

   circle(int(px),YDIM-int(py),10);
   line(int(px),YDIM-int(py),int(prevx),YDIM-int(prevy));
   prevx = px;
   prevy = py;
}