#undef MODULE_TEST
#ifndef max
#define max(a,b) ((a) > (b) ? (a) : (b))
#endif
#ifndef min
#define min(a,b) ((a) < (b) ? (a) : (b))
#endif

int hexrange(int x1, int y1, int x2, int y2)
{
	/* hexrange() computes the minimum number of hexes that must be
	 * traversed to get from hex (x1, y1) to hex (x2, y2).  The
	 * coordinates relate to an odd-high orthogonally-numbered hex grid,
	 * like that shown below:
	 *
	 *   /~~\    /~~\    /
	 *  /1,1 \__/3,1 \__/
	 *  \    /~~\    /~~\
	 *   \__/2,1 \__/4,1 \
	 *   /~~\    /~~\    /
	 *  /1,2 \__/3,2 \__/
	 *  \    /~~\    /~~\
	 *   \__/2,2 \__/4,2 \
	 *   /~~\    /~~\    /
	 *  /    \__/    \__/
	 */
	int decision;
	int coldiff;
	int rowdiff;
	int cols, rows;
	int dist;
	coldiff = x2 - x1;
	rowdiff = y2 - y1;
	/* There is a correction to be computed, arising from the fact that
	 * *both* (2,0) and (2,1) are adjacent to (1,1).  To determine which
	 * way the correction should point, we check the oddness of the
	 * x-coordinates, then use a switch on the resulting value (oddness
	 * of x1 in bit 1, oddness of x2 in bit 2. */
	decision = ((x1 & 1) << 1) | (x2 & 1);
	switch (decision)
	{
		case 0:
			/* both x coordinates are even - no offsets are
			 * required. */
			break;
		case 1:
			/* x2 is odd, x1 even. Therefore, x101 is below x201.
			 * Therefore, x100 is in an adjacent row to x201.
			 * So, if (y2 > y1) (i.e., hex 2 is "below" hex 1,
			 * we need to adjust the row difference towards
			 * zero. */
			if (rowdiff > 0)
			{
				rowdiff--;
			}
			break;
		case 2:
			/* x1 is odd, x2 even. Therefore, x201 is below x101.
			 * Therefore, x200 is in an adjacent row to x101.
			 * So, if (y2 < y1) (i.e., hex 2 is "above" hex 1,
			 * we need to adjust the row difference towards
			 * zero. */
			if (rowdiff < 0)
			{
				rowdiff++;
			}
			break;
		case 3:
			/* both odd */
			break;
	}
	/* The range is now:
	 *   (hdist) + max(vdist - (hdist / 2), 0)
	 * using the corrected vertical distance we just computed.
	 * cols is (hdist), rows is (vdist).
	 */
	cols = abs(coldiff);
	rows = abs(rowdiff);
	dist = cols + max((rows - (cols/2)), 0);
	return dist;
}

#ifdef MODULE_TEST
int main()
{
	printf("hex range (1,1)->(2,2): %d\n", hexrange(1,1,2,2));
	printf("hex range (1,1)->(7,3): %d\n", hexrange(1,1,7,3));
	return 0;
}

#endif