// This program bounces multiple balls at once

#include <math.h>
#include <time.h>
#include <stdio.h>
#include <stdlib.h>
#include <winbgim.h>

#define XDIM 600 // x dimension of table
#define YDIM 400 // y dimension of table
#define BALL_RADIUS 20 // ball radius
#define TABLE_COLOR COLOR(0,50,0) // color of table
#define N_BALLS 100

// function prototypes

void initialize_screen();
void initialize_ball(double &ball_loc_x, double &ball_loc_y,
                     double &ball_direction, double &ball_velocity);
void update_ball_loc(double &ball_loc_x, double &ball_loc_y,
                     double &ball_direction, double ball_velocity);
void update_screen(int old_ball_screen_x, int old_ball_screen_y,
                   int new_ball_screen_x, int new_ball_screen_y,
                   int ball_color);
int random_in_range(int lower, int upper);



int main ()
{
    int i;

    double ball_loc_x[N_BALLS];      // ball x coordinate in screen units
    double ball_loc_y[N_BALLS];           // ball y coordinate in screen units
    double ball_direction[N_BALLS];       // ball angle in radians
    double ball_velocity[N_BALLS];        // ball velocity in screen units
    double old_ball_loc_x;
    double old_ball_loc_y;

    initialize_screen();

    for (i=0; i<N_BALLS; ++i)
        initialize_ball(ball_loc_x[i], ball_loc_y[i],
                                ball_direction[i], ball_velocity[i]);

    do
    {
        for (i=0; i<N_BALLS; ++i) {
            old_ball_loc_x = ball_loc_x[i];
            old_ball_loc_y = ball_loc_y[i];

            update_ball_loc(ball_loc_x[i], ball_loc_y[i],
                                    ball_direction[i], ball_velocity[i]);

            update_screen(int(old_ball_loc_x), int(old_ball_loc_y),
                        int(ball_loc_x[i]), int(ball_loc_y[i]), (i%15)+1);
        }

        //delay(1); // sleep for 1 millisecond

    } while (!kbhit()); // loop until key is hit
}


/************************************ initialize_screen **********************/

void initialize_screen()
{
   // open window

   initwindow(XDIM, YDIM);

   // draw table :  filled rectangle the size of the screen

   setfillstyle(SOLID_FILL, TABLE_COLOR);
   bar(0,0,XDIM-1,YDIM-1); 
}


/************************************ initialize_ball ************************/

void initialize_ball(double &ball_loc_x, double &ball_loc_y, 
                double &ball_direction, double &ball_velocity)
{
   // initialize ball location, direction, and velocity

   ball_loc_x = (double) random_in_range(BALL_RADIUS, XDIM - BALL_RADIUS);
   ball_loc_y = (double) random_in_range(BALL_RADIUS, YDIM - BALL_RADIUS);
   ball_direction = (double) random_in_range(0,359) * 3.14159 / 180.;
   ball_velocity = (double) random_in_range(5, 15);
}


/************************************ update_ball_loc ************************/

void update_ball_loc(double &ball_loc_x, 
                     double &ball_loc_y,
                     double &ball_direction,
                     double ball_velocity)
{
   double delta_x;
   double delta_y;

   delta_x = cos(ball_direction) * ball_velocity;
   delta_y = sin(ball_direction) * ball_velocity;

   // check whether ball has hit left or right wall

   if ((ball_loc_x + delta_x >= XDIM - BALL_RADIUS) || 
                                 (ball_loc_x + delta_x < BALL_RADIUS))  
      ball_direction = 3.14159 - ball_direction;
   else 
      ball_loc_x += delta_x;

   // check whether ball has hit upper or lower wall

   if ((ball_loc_y + delta_y >= YDIM - BALL_RADIUS) || 
                                 (ball_loc_y + delta_y < BALL_RADIUS)) 
      ball_direction = -ball_direction;
   else 
      ball_loc_y += delta_y;
}

/************************************ update_screen **************************/

void update_screen(int old_ball_screen_x,
                   int old_ball_screen_y,
                   int new_ball_screen_x, 
                   int new_ball_screen_y,
                   int ball_color)
{
    // check to see if ball has moved on screen

    if (old_ball_screen_x != new_ball_screen_x || 
                         old_ball_screen_y != new_ball_screen_y) 
    {

        // erase ball in old location

        setfillstyle(SOLID_FILL, TABLE_COLOR);
        setcolor(TABLE_COLOR);
        fillellipse(old_ball_screen_x, old_ball_screen_y, 
                                                BALL_RADIUS, BALL_RADIUS);

        // draw ball in new location

        setfillstyle(SOLID_FILL, ball_color);
        setcolor(ball_color);
        fillellipse(new_ball_screen_x, new_ball_screen_y, 
                        BALL_RADIUS, BALL_RADIUS);
    }
}

/************************************ random_in_range ************************/

// Generate a random number in the integer range [lower, upper]

int random_in_range(int lower, int upper)
{
   static int first_time = 1;

   if (first_time) {
      srand(time(0));
      first_time = 0;
   }

   return (lower + rand()%(upper-lower+1));
}