Changes

'''idx''' shows which elements in array will be picked by threads. for example, thread 0 in stage 1 will have (0 / 1) * 1 * 2 + 0 % 1 = 0, and thread 1 will have (1 / 1) * 1 * 2 + 1 % 1 = 2, and thread 2 will have (2 / 1) * 1 * 2 + 2 % 1 = 4 ... 6, 8, 10. because in stage 1, index 0 will be compared with 1, 2 compared with 3, 4 compared with 5 .... another example, if thread 0 in round 1 in stage 2 will have ( 0 / 2)*2*2 + 0%2 = 0, thread 1 will have ( 1/2)*2*2 + 1%2 = 1, thread 2 will have ( 2/2)*2*2 + 2%2 = 4, thread 3 will have (3/2) * 2 * 2 + 3%2 = 5. So threads will have 0, 1, 4, 5, 8, 9 .... because in round 1 in stage 2, index 0 will be compared with 2 and 1 compared with 3, 4 compared with 6 and 5 compared with 7.