Java8のStreamAPIでフィボナッチ関数

Fibonacci.java

import java.math.BigInteger;
import java.util.function.UnaryOperator;
import java.util.stream.Stream;

import static java.math.BigInteger.*;

public class Fibonacci implements UnaryOperator<BigInteger> {

    @Override
    public BigInteger apply(BigInteger n) {

        assert(n != null && n.compareTo(ZERO) >= 0);

        return Stream.iterate(new BigInteger[] { ZERO, ZERO, ZERO }, cont -> {
            BigInteger x = cont[0].add(ONE);
            BigInteger preY = cont[1];
            BigInteger y = (x.equals(ZERO)||x.equals(ONE)) ? x : cont[1].add(cont[2]);
            return new BigInteger[] {x, y, preY};
        })
                .filter(cont -> cont[0].equals(n))
                .findFirst()
                .map(cont -> cont[1])
                .orElseThrow(() -> new RuntimeException("Failed."));


    }
}