// 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.

// This function assumes that a square graphics window of size
// pmax is open. It waits for a mouse click and then converts
// the coordinates of that click into a point in a coordinate
// system where x can range from vx0 to vx1 and y can range
// from vy0 to vy1). These new coordinates are returned in the
// reference parameters wx and wy.
void get_mouse(
    int pmax, double vx0, double vy0, double vx1, double vy1,
    double& x, double& y
    );

// 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 color);

// world(p, v0, v1, p0, p1) is a coordinate transformation,
// similar to the pixel function, but it goes in the other
// direction (converting a pixel coordinate p (ranging
// from p0 to p1) into a world coordinate (ranging
// from v0 to v1).
double world(int p, double v0, double v1, int p0, int p1);
//-------------------------------------------------------------

 
//-------------------------------------------------------------
int main()
{
    double speed;  // Initial speed of the projectile
    double angle;  // Angle (in radians) for launch
    double mx, my; // Position of a mouse click in world coord.
    int color;     // Color of a projectile path
    
    // 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 and angles.
    while (true)
    {
	get_mouse(S, -W, +W, +W, -W, mx, my);
        cout << "That click was at "
	     << "(" << mx << ", " << my << ")"
	     << endl;
	color = YELLOW;
	speed = mx;
	angle = my;
	projectile(S, W, angle, speed, color);
    }

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

//-------------------------------------------------------------
void get_mouse(
    int pmax, double vx0, double vy0, double vx1, double vy1,
    double& x, double& y
    )
{
    int px, py; // Pixel coordinates of a mouse click

    while (!ismouseclick(WM_LBUTTONDOWN))
    {
	delay(100);
    }
    getmouseclick(WM_LBUTTONDOWN, px, py);
    x = world(px, vx0, vx1, 0, pmax);
    y = world(py, vy0, vy1, 0, pmax);
}    
//-------------------------------------------------------------


//-------------------------------------------------------------
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, int color)
{
    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:
    setcolor(color);
    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);
}
//-------------------------------------------------------------

//-------------------------------------------------------------
double world(int p, double v0, double v1, int p0, int p1)
{
    double fraction = double(p - p0)/double(p1 - p0);
    double distance_from_v0 = fraction*(v1 - v0);
    
    return v0 + distance_from_v0;
}
//-------------------------------------------------------------