Finding the index of a given permutation

Question

I\'m reading the numbers 0, 1, ..., (N - 1) one by one in some order. My goal is to find the lexicography index of this given permutation, using only O(1)

Answer 1

I just wrote a code using Visual Basic and my program can directly calculate every index or every corresponding permutation to a given index up to 17 elements (this limit is due to the approximation of the scientific notation of numbers over 17! of my compiler).

If you are interested I can I can send the program or publish it somewhere for download. It works fine and It can be useful for testing and paragon the output of your codes.

I used the method of James D. McCaffrey called factoradic and you can read about it here and something also here (in the discussion at the end of the page).