[0, 15] is to be expected:
0xff::16 maps to [0, 0, 0, 0, 0, 0, 0, 0, 1, 1, 1, 1, 1, 1, 1, 1]. Grouping this int groups of 6 bits will give you:
[0, 0, 0, 0, 0, 0], which equals 0[0, 0, 1, 1, 1, 1], which equals 15 and[1, 1, 1, 1], which is too small and discarded.
