/* ellipseMidpoint, Chapter 3, pp. 109-110 */

/* EXAMPLE STARTS HERE */
#include "device.h"

#define ROUND(a) ((int)(a+0.5))

void ellipseMidpoint (int xCenter, int yCenter, int Rx, int Ry)
{
  int Rx2 = Rx*Rx;
  int Ry2 = Ry*Ry;
  int twoRx2 = 2*Rx2;
  int twoRy2 = 2*Ry2;
  int p;
  int x = 0;
  int y = Ry;
  int px = 0;
  int py = twoRx2 * y;
  void ellipsePlotPoints (int, int, int, int);

  /* Plot the first set of points */
  ellipsePlotPoints (xCenter, yCenter, x, y);

  /* Region 1 */
  p = ROUND (Ry2 - (Rx2 * Ry) + (0.25 * Rx2));
  while (px < py) {
    x++;
    px += twoRy2;
    if (p < 0)
      p += Ry2 + px;
    else {
      y--;
      py -= twoRx2;
      p += Ry2 + px - py;
    }
    ellipsePlotPoints (xCenter, yCenter, x, y);
  }

  /* Region 2 */
  p = ROUND (Ry2*(x+0.5)*(x+0.5) + Rx2*(y-1)*(y-1) - Rx2*Ry2);
  while (y > 0) {
    y--;
    py -= twoRx2;
    if (p > 0) 
      p += Rx2 - py;
    else {
      x++;
      px += twoRy2;
      p += Rx2 - py + px;
    }
    ellipsePlotPoints (xCenter, yCenter, x, y);
  }
}

void ellipsePlotPoints (int xCenter, int yCenter, int x, int y)
{
  setPixel (xCenter + x, yCenter + y); 
  setPixel (xCenter - x, yCenter + y); 
  setPixel (xCenter + x, yCenter - y); 
  setPixel (xCenter - x, yCenter - y); 
}
/* EXAMPLE ENDS HERE */

void main (int argc, char ** argv)
{
  long windowID = openGraphics (*argv, WINDOW_WIDTH, WINDOW_HEIGHT);
  setBackground (WHITE);
  setColor (BLACK);
  ellipseMidpoint
    (WINDOW_WIDTH/2, WINDOW_HEIGHT/2, WINDOW_WIDTH/4, WINDOW_WIDTH/5);
  sleep (10);
  closeGraphics (windowID);
}