/* RayCast */
// Uses Bresenham's line algorithm to efficiently loop between two points, and find the first solid pixel
// This particular variation always starts from the first point, so collisions don't happen at the wrong end.
// returns an int array
//       ||| x = ([0],[1]) point in empty space before collision point
//       ||| o = ([2],[3]) collision point
//(end)--||ox------- (start)
//       |||
// using http://www.gamedev.net/page/resources/_/reference/programming/sweet-snippets/line-drawing-algorithm-explained-r1275
int[] rayCast(int startX, int startY, int lastX, int lastY) {
  int deltax = (int) abs(lastX - startX);        // The difference between the x's
  int deltay = (int) abs(lastY - startY);        // The difference between the y's
  int x = (int) startX;                       // Start x off at the first pixel
  int y = (int) startY;                       // Start y off at the first pixel
  int xinc1, xinc2, yinc1, yinc2;
  if (lastX >= startX) {                // The x-values are increasing
    xinc1 = 1;
    xinc2 = 1;
  }
  else {                         // The x-values are decreasing
    xinc1 = -1;
    xinc2 = -1;
  }
  
  if (lastY >= startY) {                // The y-values are increasing
    yinc1 = 1;
    yinc2 = 1;
  }
  else {                         // The y-values are decreasing
    yinc1 = -1;
    yinc2 = -1;
  }
  int den, num, numadd, numpixels;
  if (deltax >= deltay) {        // There is at least one x-value for every y-value
    xinc1 = 0;                  // Don't change the x when numerator >= denominator
    yinc2 = 0;                  // Don't change the y for every iteration
    den = deltax;
    num = deltax / 2;
    numadd = deltay;
    numpixels = deltax;         // There are more x-values than y-values
  }
  else {                         // There is at least one y-value for every x-value
    xinc2 = 0;                  // Don't change the x for every iteration
    yinc1 = 0;                  // Don't change the y when numerator >= denominator
    den = deltay;
    num = deltay / 2;
    numadd = deltax;
    numpixels = deltay;         // There are more y-values than x-values
  }
  int prevX = (int)startX;
  int prevY = (int)startY;
  
  for (int curpixel = 0; curpixel <= numpixels; curpixel++) {
    if (terrain.isPixelSolid(x, y))
        return new int[]{prevX, prevY, x, y};
    prevX = x;
    prevY = y;  
    
    num += numadd;              // Increase the numerator by the top of the fraction
    
    if (num >= den) {             // Check if numerator >= denominator
      num -= den;               // Calculate the new numerator value
      x += xinc1;               // Change the x as appropriate
      y += yinc1;               // Change the y as appropriate
    }
    
    x += xinc2;                 // Change the x as appropriate
    y += yinc2;                 // Change the y as appropriate
  }  
  return new int[]{};
}