/**
 * kernel that computes next generation of the game of life
 * @param cells_in input array of (n+2)x(n+2) cells
 * @param cells_out output array of (n+2)x(n+2) cells
 * @param ptr bitmap image updated on CPU
 * @param size dimension of the game (with borders = n+2)
 */
void kernel(CellType *cells_in, CellType *cells_out, ImagePoint *img, const int size) {

	// for each line in [1,n]	
	for (int y=1; y<size-1; ++y) {
		// for each column in [1,n]
		for (int x=1; x<size-1; ++x) {

			// get values of cells surrounding the current cell (c11)
			CellType c00, c01, c02; // line y-1
			CellType c10, c11, c12; // line y
			CellType c20, c21, c22; // line y+1

			c00 = cells_in[(y-1)*size + x-1];
			c01 = cells_in[(y-1)*size + x];
			c02 = cells_in[(y-1)*size + x+1];

			c10 = cells_in[(y)*size + x-1];
			c11 = cells_in[(y)*size + x];
			c12 = cells_in[(y)*size + x+1];

			c20 = cells_in[(y+1)*size + x-1];
			c21 = cells_in[(y+1)*size + x];
			c22 = cells_in[(y+1)*size + x+1];

			// compute number of alive cells around current cell
			int nbr = c00 + c01 + c02 + c10 + c12 + c20 + c21 + c22;

			// compute new state of current cell
			c11 = (((CellType) c11) & (nbr == 2)) | (nbr == 3);

			// set new state of current cell in cells_out
			cells_out[y * size + x] = c11;

			// update image with red (ALIVE) or black(DEAD)
			int offset = y * size + x;
			img[offset].red = c11 * 255;
			img[offset].green = 0;
			img[offset].blue = 0;
			img[offset].alpha = c11 * 255;

		}
	}
}