Finding the ranking of a word (permutations) with duplicate letters

Question

I\'m posting this although much has already been posted about this question. I didn\'t want to post as an answer since it\'s not working. The answer to this post (Finding th

Answer 1

Java version of unrank for a String:

public static String unrankperm(String letters, int rank) {
    Map charCounts = new java.util.HashMap<>();
    int permcount = 1;
    for(int i = 0; i < letters.length(); i++) {
        char x = letters.charAt(i);
        int xCount = charCounts.containsKey(x) ? charCounts.get(x) + 1 : 1;
        charCounts.put(x, xCount);

        permcount = (permcount * (i + 1)) / xCount;
    }
    // charCounts is the histogram of letters
    // permcount is the number of distinct perms of letters
    StringBuilder perm = new StringBuilder();

    for(int i = 0; i < letters.length(); i++) {
        List sorted = new ArrayList<>(charCounts.keySet());
        Collections.sort(sorted);

        for(Character x : sorted) {
            // suffixcount is the number of distinct perms that begin with x
            Integer frequency = charCounts.get(x);
            int suffixcount = permcount * frequency / (letters.length() - i); 

            if (rank <= suffixcount) {
                perm.append(x);

                permcount = suffixcount;

                if(frequency == 1) {
                    charCounts.remove(x);
                } else {
                    charCounts.put(x, frequency - 1);
                }
                break;
            }
            rank -= suffixcount;
        }
    }
    return perm.toString();
}

See also n-th-permutation-algorithm-for-use-in-brute-force-bin-packaging-parallelization.