Java/src/main/java/com/thealgorithms/maths/FibonacciJavaStreams.java

package com.thealgorithms.maths;

import java.math.BigDecimal;
import java.util.List;
import java.util.Optional;
import java.util.stream.Stream;

/**
 * @author: caos321
 * @date: 14 October 2021 (Thursday)
 */
public final class FibonacciJavaStreams {
    private FibonacciJavaStreams() {
    }

    public static Optional<BigDecimal> calculate(final BigDecimal index) {
        if (index == null || index.compareTo(BigDecimal.ZERO) < 0) {
            throw new IllegalArgumentException("Input index cannot be null or negative!");
        }

        if (index.compareTo(BigDecimal.ONE) < 0) {
            return Optional.of(BigDecimal.ZERO);
        }

        if (index.compareTo(BigDecimal.TWO) < 0) {
            return Optional.of(BigDecimal.ONE);
        }

        final List<BigDecimal> results = Stream.iterate(index, x -> x.compareTo(BigDecimal.ZERO) > 0, x -> x.subtract(BigDecimal.ONE))
                                             .reduce(List.of(), (list, current) -> list.isEmpty() || list.size() < 2 ? List.of(BigDecimal.ZERO, BigDecimal.ONE) : List.of(list.get(1), list.get(0).add(list.get(1))), (list1, list2) -> list1);

        return results.isEmpty() ? Optional.empty() : Optional.of(results.get(results.size() - 1));
    }
}