// File: projectile.cxx 
// Written by: Michael Main (main@colorado.edu) on Sep 27, 2011
// The purpose of this program is to show
// how to draw the path of a free-falling projectile.

//-------------------------------------------------------------
// Directives:
#include <cassert>
#include <iostream>
#include <cmath>
#include <cstdlib>
#include <graphics.h>
using namespace std;
//-------------------------------------------------------------

//-------------------------------------------------------------
const int S = 500;      // Screen size in pixels
const double W = 200.0; // World coordinates go from -W to +W
//-------------------------------------------------------------

//-------------------------------------------------------------
// Function Prototypes.
// pixel(v, v0, v1, p0, p1) converts v from an interval that
// ranges from v0 to v1 into an integer interval that ranges
// from p0 to p1.
int pixel(double v, double v0, double v1, int p0, int p1);

// Draws the path of a projectile in a square graphics window
// of size pmax. The world coordinate system goes from
// -wmax to +wmax, with the origin in the center of the screen.
// The projectile is launched from the origin at an angle of
// theta radians and with a speed of v meters per second. The
// strength of gravity is 9.8 m/sec^2.
void projectile(int pmax, double wmax, double theta, double v);
//-------------------------------------------------------------

 
//-------------------------------------------------------------
int main()
{
    double speed; // Initial speed of the projectile

    // Open the window and draw the cliff
    initwindow(S, S, "Projectile");
    setfillstyle(SOLID_FILL, GREEN);
    bar (0, S/2, S/2, S);
    
    // Try different initial speeds, but all at the same angle
    // of 45 degrees.
    for (speed = 5; speed < 50; speed += 5)
    {
	projectile(S, W, M_PI/4.0, speed);
    }

    // Pause and exit
    delay(20000);
    return EXIT_SUCCESS;
}
//-------------------------------------------------------------

//-------------------------------------------------------------
int pixel(double v, double v0, double v1, int p0, int p1)
{
    return int(p0 + (v - v0)/(v1 - v0) * (p1 - p0));
}
//-------------------------------------------------------------

//-------------------------------------------------------------
void projectile(int pmax, double wmax, double theta, double v)
{
    const double G = -9.8; // Strength of gravity in m/sec^2
    double t;        // Time in seconds
    double wx, wy;   // Position of the projectile in meters
    int px, py;      // Pixel coordinates of the projectile
    double vx = v * cos(theta);  // Initial x velocity in m/sec
    double vy = v * sin(theta);  // Initial y velocity in m/sec

    // Move the drawing pen to the center of the screen:
    moveto(pmax/2, pmax/2);

    // For t values starting at t=0, draw a line to the point
    // of the curve that is determined by t. The curve is drawn
    // until the y coordinate falls off the bottom.
    t = 0;
    do
    {
	wx = vx*t;
	wy = vy*t + 0.5*G*t*t;
	px = pixel(wx, -wmax, +wmax, 0, pmax);
	py = pixel(wy, +wmax, -wmax, 0, pmax);
	lineto(px, py);
	t += 0.01;
    } while (wy > -wmax);
}
//-------------------------------------------------------------