sudoku-verifier-rxjava/src/main/java/rxjava/SudokuVerifier.java

package rxjava;

import io.reactivex.rxjava3.annotations.NonNull;
import io.reactivex.rxjava3.core.Observable;
import io.reactivex.rxjava3.core.Single;

import java.util.concurrent.atomic.AtomicInteger;
import java.util.stream.Stream;

public class SudokuVerifier {

	@NonNull Observable<String> digitObservable;

	public SudokuVerifier(Stream<String> digitStream) {
		// Observable on the digit stream
		digitObservable = Observable.fromStream(digitStream);
	}

	public boolean isSolved() {
		AtomicInteger row = new AtomicInteger(1);
		AtomicInteger column = new AtomicInteger(0);
		@NonNull Observable<DigitField> digitFieldObservable = digitObservable
			.map(digit -> digitField(row, column, digit));
		@NonNull Observable<DigitField> filter = digitFieldObservable
			.filter(digitField -> digitField.row == 1);
		return isSolved(filter).blockingGet();
	}

	@NonNull Single<Boolean> isSolved(@NonNull Observable<DigitField> filter) {
		return filter
			.map(digitField -> digitField.digit)
			.distinct().count()
			.map(count -> count == 9);
	}

	DigitField digitField(AtomicInteger row, AtomicInteger column, String digit) {
		if (column.getAndIncrement() > 9) {
			row.getAndIncrement();
			column.set(1);
		}
		return new DigitField(row.get(), column.get(), digit);
	}

	static class DigitField {
		int row;
		int column;
		String digit;

		public DigitField(int row, int column, String digit) {
			this.row = row;
			this.column = column;
			this.digit = digit;
		}
	}
}