I wrote that code for fun 15-20 years ago (code definitely lost in the mists of time). You can think of it as rolling one random number up to n!, then use x%n

I wrote that code for fun 15-20 years ago (code definitely lost in the mists of time). You can think of it as rolling one random number up to n!, then use x%n to pick the first element and then x/n to pick the next (n-1) elements (so the next step is then to pick the second element with (x/n)%(n-1) and use (x/n)/(n-1) for the remaining (n-2) elements). Though as a sibling comment points out, since you need a random number up to n!, even 64 bits of randomness only get you so far (I think up to 20, but didn't check my math).