import java.util.HashMap;
import java.util.Map;

// REVIEW
public abstract class Digest {
    private Map<String, byte[]> cache = new HashMap<>();

    public byte[] digest(String input) {
        byte[] result = cache.get(input);
        if (result == null) {
            synchronized (cache) {
                result = cache.get(input);
                if (result == null) {
                    result = doDigest(input);
                    cache.put(input, result);
                }
            }
        }
        return result;
    }

    protected abstract byte[] doDigest(String input);
}

public abstract class Digest {

    private final ReadWriteLock rwl = new ReentrantReadWriteLock();
    private final Lock readLock = rwl.readLock();
    private final Lock writeLock = rwl.writeLock();

    private Map<String, byte[]> cache = new HashMap<>();

    public byte[] digest(String input) {
        byte[] result = null;
        readLock.lock();
        try {
            result = cache.get(input);
        } finally {
            readLock.unlock();
        }
        if (result == null) {
            writeLock.lock();
            try {
                result = cache.get(input);
                if (result == null) {
                    result = doDigest(input);
                    cache.put(input, result);
                }
            } finally {
                writeLock.unlock();
            }
        }

        return result;
    }

    protected abstract byte[] doDigest(String input);
}