sudoku-verifier-rxjava/src/main/java/reactivestreams/DigitBlockChecker.java

package reactivestreams;

import java.util.Collection;
import java.util.HashSet;
import java.util.concurrent.Flow;
import java.util.concurrent.SubmissionPublisher;

/**
 * Processor consuming a flow of digit Strings and emitting a single Boolean which is true if applying 9 digits are unique.
 */
public abstract class DigitBlockChecker extends SubmissionPublisher<Boolean> implements Flow.Processor<String, Boolean> {

	int currentRow;
	int currentColumn;
	Flow.Subscription subscription;
	Collection<String> digitsProcessed;

	@Override
	public void onSubscribe(Flow.Subscription subscription) {
		currentRow = 1;
		currentColumn = 0;
		this.subscription = subscription;
		digitsProcessed = new HashSet<>();
		subscription.request(9*9);
	}

	@Override
	public void onNext(String digit) {
		currentColumn++;
		if (currentColumn > 9) {
			currentRow++;
			currentColumn = 1;
		}
		if (currentDigitApplies(currentRow, currentColumn)) {
			if (digitsProcessed.contains(digit)) {
				submit(false);
				subscription.cancel();
			} else {
				digitsProcessed.add(digit);
				if (digitsProcessed.size() == 9) {
					submit(true);
					subscription.cancel();
				}
			}
		}
	}

	protected abstract boolean currentDigitApplies(int currentRow, int currentColumn);

	@Override
	public void onError(Throwable throwable) {
		System.err.println(throwable.fillInStackTrace());
		closeExceptionally(throwable);
	}

	@Override
	public void onComplete() {
		// not called
	}

}