// This version of pong bounces a ball on the screen.
// We don't have a paddle yet.
// There are no global variables in this version.

#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 BALL_COLOR RED            // color of ball

// 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 new_ball_screen_x, int new_ball_screen_y);
int random_in_range(int lower, int upper);


int main ()
{
   double ball_loc_x;             // ball x coordinate in screen units
   double ball_loc_y;           // ball y coordinate in screen units
   double ball_direction;       // ball angle in radians
   double ball_velocity;        // ball velocity in screen units

   initialize_screen();
   initialize_ball(ball_loc_x, ball_loc_y, ball_direction, ball_velocity);

   do  // loop forever 
   {
     update_ball_loc(ball_loc_x, ball_loc_y, ball_direction, ball_velocity);

     update_screen(int(ball_loc_x), int(ball_loc_y));

     delay(1); // sleep for 1 millisecond

   } while (!kbhit()); // loop until a 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 new_ball_screen_x, int new_ball_screen_y)
{
   static int old_ball_screen_x = -1;  // previous displayed x position of ball 
   static int old_ball_screen_y = -1;  // previous displayed y position of ball

   // 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);

      // update stored ball position

      old_ball_screen_x = new_ball_screen_x;
      old_ball_screen_y = new_ball_screen_y;
   }
}

/************************************ 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));
}