C programs to rank and unrank k-permutations in linear time.

Usage: rank n <k>
Description:
  n is the length of the set.
  k is the length of the permutation. If omitted, k is set to n. k must be <= n.
  rank reads k numbers from stdin, which must be 0 to k - 1, separated by \n.
  output is the rank of the input k-permutation of n.

Usage: unrank r n <k>
  r is the desired rank.
  n is the length of the set.
  k is the length of the permutation. If omitted, k is set to n. k must be <= n.
  output is the k-permutation of n of rank r.