________ | 8 | 4 | 2 | 1 | 0 | <<--
____ -->> | 6 | 3 | 12 | 5 |
________ | 19 | 9 | 20 | 10 | <<--
____ -->> | 16 | 7 | 13 | 14 |
________ | 18 | 17 | 21 | 15 | <<--
____-->> | 11 |