package sd.lab.concurrency.exercise;

import java.math.BigInteger;
import java.util.concurrent.CompletableFuture;
import java.util.concurrent.ExecutorService;

public class AsyncCalculatorImpl implements AsyncCalculator {

	private final ExecutorService executor;
	private volatile BigInteger value = BigInteger.ONE;

	public AsyncCalculatorImpl(ExecutorService executor) {
		this.executor = executor;
	}

	@Override
	public CompletableFuture<BigInteger> factorial(BigInteger valueOf) {
		if (valueOf.signum() == -1) throw new IllegalArgumentException("Cannot compute factorial for negative numbers");
		final CompletableFuture<BigInteger> resultPromise = new CompletableFuture<>();
		executor.execute(() -> factorialImpl(valueOf, resultPromise));
		return resultPromise;
	}

	private void factorialImpl(BigInteger valueOf, CompletableFuture<BigInteger> resultPromise) {
        if (valueOf.equals(BigInteger.ZERO)) {
        	resultPromise.complete(value);
        } else {
        	value.multiply(valueOf);
        	System.out.println(value.toString());
            executor.execute(() -> factorialImpl(valueOf.subtract(BigInteger.ONE), resultPromise));
        }
	}

}