topcoder - 516-2 - 500 (java)

NetworkXOneTimePad.java

import java.io.*;
import java.util.*;
import java.math.*;

public class NetworkXOneTimePad
{
    Long[] convert(String[] arr) {
        int n = arr.length;
        Long[] res = new Long[n];
        for (int i = 0; i < n; i++)
            res[i] = Long.parseLong(arr[i], 2);
        return res;
    }

    public int crack(String[] plaintexts, String[] ciphertexts) {
        Long[] plains  = convert(plaintexts);
        Long[] ciphers = convert(ciphertexts);

        Vector<Long> keys = new Vector();
        long c0 = ciphers[0];
        for (long p : plains)
            keys.add(c0 ^ p);

        // Long (not long) for hashing
        HashSet<Long> plainset = new HashSet(Arrays.asList(plains));

        for (long c : ciphers) {
            Vector<Long> keys2 = new Vector();
            for (long k : keys)
                if (plainset.contains(c ^ k))
                    keys2.add(k);
            keys = keys2;
        }
        return keys.size();
    }
}