def _interleave_bits_16b(x,y)
      x = (x | (x << 8)) & Magic[3]
      x = (x | (x << 4)) & Magic[2]
      x = (x | (x << 2)) & Magic[1]
      x = (x | (x << 1)) & Magic[0]
      y = (y | (y << 8)) & Magic[3]
      y = (y | (y << 4)) & Magic[2]
      y = (y | (y << 2)) & Magic[1]
      y = (y | (y << 1)) & Magic[0]
      z = x | (y << 1)
    end
def _interleave_bits_16b(x,y)
      x = (x | (x << 8)) & Magic[3]
      x = (x | (x << 4)) & Magic[2]
      x = (x | (x << 2)) & Magic[1]
      x = (x | (x << 1)) & Magic[0]
      y = (y | (y << 8)) & Magic[3]
      y = (y | (y << 4)) & Magic[2]
      y = (y | (y << 2)) & Magic[1]
      y = (y | (y << 1)) & Magic[0]
      z = x | (y << 1)
    end
Interleave lower 16 bits of x and y, so the bits of x
 are in the even positions and bits from y in the odd;
 z gets the resulting 32-bit Morton Number.
 x and y must initially be less than 65536.
 Rubyfied from http://graphics.stanford.edu/~seander/bithacks.html