I guess you could predictably enumerate the permutations, so that you could lookup with parameters size/n, position/i, seed/r - where r is the permutation. For

I guess you could predictably enumerate the permutations, so that you could lookup with parameters size/n, position/i, seed/r - where r is the permutation. For size=1, all parameters are 1, and only result is 1. For size 2, r can be 1 or 2, naming the permutations [1,2];[2,1] - and eg: rand_at(n=2,r=2,i=2) would return 2, but i=1, would return 2. I'm not sure how I'd implement this - I suppose it might be possible to generate a predictable "walk" based on n/r/i? But for sizes less than, say, a million i would think that a shuffled array would be easier? r would need to be in the range 1..n!