Combinations benchmark

[stefanv]

https://gist.github.com/stefanv/954d21cb13f1aad55b0f

For the three methods implemented, I see:

1 loops, best of 3: 1.52 s per loop
10 loops, best of 3: 153 ms per loop
10 loops, best of 3: 148 ms per loop

If you are willing to get your outputs as lists instead of arrays, it's quite a bit quicker still.

[kellieotto]

A more likely use scenario is with the two groups roughly the same size. I changed it to N=20, k=10 and got

1 loops, best of 3: 8.09 s per loop
1 loops, best of 3: 791 ms per loop
1 loops, best of 3: 885 ms per loop