More efficient variant of joriki/Question4882568.java

MatchScheduler.java

import java.util.ArrayList;
import java.util.BitSet;
import java.util.List;

public class MatchScheduler {
    static boolean [] [] played;
    static int count;
    static int n;
    static List<Integer> factorials = new ArrayList<Integer>();
    
    public static void main (String [] args) {

        factorials.add(1);
        
        for (n = 2;;n += 2) {
            updateFactorials(n);
            count = 0;
            generatePlayed();
            recurse (null,n,0);
            System.out.println(n + " : " + count*factorials.get(n-1));
        }
    }

    private static void updateFactorials(int n){
        int prev = factorials.get(n-2);
        int next = prev*(n-1);
        int next2 = next*n;
        factorials.add(n-1, next);
        factorials.add(n, next2);
    }

    private static void generatePlayed() {
        played = new boolean [n] [n];
    }
    
    static void recurse (BitSet matched,int daysLeft,int unmatched) {
        if (daysLeft == 0){
            count++;
        }
        else if (unmatched == 0){
            int newDaysLeft = daysLeft - 1;
            recurse (generateBitset(newDaysLeft),newDaysLeft,n-2);
        }
        else {
            int toMatch = matched.nextClearBit (0);
            matched.set (toMatch);
            int team = toMatch;
            for (;;) {
                team = matched.nextClearBit (team + 1);
                if (team == n || toMatch == n)
                    break;
                if (!played [toMatch] [team]) {
                    set (toMatch,team,true);
                    matched.set (team);
                    recurse (matched,daysLeft,unmatched - 2);
                    matched.clear (team);
                    set (toMatch,team,false);
                }
            }
            matched.clear (toMatch);
        }
    }

    private static BitSet generateBitset(int daysLeft) {
        if (daysLeft == 0) return null;
        BitSet bitSet = new BitSet (n);
        int opponent = n - daysLeft;
        bitSet.set(0);
        bitSet.set(opponent);
        set(0, opponent, true);
        return bitSet;
    }
    
    static void set (int i,int j,boolean value) {
        played [i] [j] = played [j] [i] = value;
    }
}