Permutations

I suggest doing a coding challenge on generating permutations of an array. That can easily be done beautifully and the intuition of the algorithms is usually pretty straightforward. Namely, I'd love to see
- Fisher-Yates shuffle (https://bost.ocks.org/mike/shuffle/)
- Heap's algorithm (http://ruslanledesma.com/2016/06/17/why-does-heap-work.html)
- Next permutation lexicographically (https://www.nayuki.io/page/next-lexicographical-permutation-algorithm)
- And my challenge to you is devise some way of generating permutations of bits of an integer. (That, actually, was the problem which made me research all aforementioned algorithms.)