package Maths;

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

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

 public static Optional<BigDecimal> calculate(final BigDecimal index) {
 if (index == null || index.compareTo(BigDecimal.ZERO) < 0) {
 return Optional.empty();
 }

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

 if (index.compareTo(new BigDecimal(2)) < 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));
 }

 public static void assertThat(final Object actual, final Object expected) {
 if (!Objects.equals(actual, expected)) {
 throw new AssertionError(String.format("expected=%s but was actual=%s", expected, actual));
 }
 }

 public static void main(final String[] args) {
 {
 final Optional<BigDecimal> result = calculate(new BigDecimal(-1));
 assertThat(result.isEmpty(), true);
 }
 {
 final Optional<BigDecimal> result = calculate(BigDecimal.ZERO);
 assertThat(result.isPresent(), true);
 result.ifPresent(value -> assertThat(value, BigDecimal.ZERO));
 }
 {
 final Optional<BigDecimal> result = calculate(BigDecimal.ONE);
 assertThat(result.isPresent(), true);
 result.ifPresent(value -> assertThat(value, BigDecimal.ONE));
 }
 {
 final Optional<BigDecimal> result = calculate(new BigDecimal(2));
 assertThat(result.isPresent(), true);
 result.ifPresent(value -> assertThat(value, BigDecimal.ONE));
 }
 {
 final Optional<BigDecimal> result = calculate(new BigDecimal(3));
 assertThat(result.isPresent(), true);
 result.ifPresent(value -> assertThat(value, new BigDecimal(2)));
 }
 {
 final Optional<BigDecimal> result = calculate(new BigDecimal(10));
 assertThat(result.isPresent(), true);
 result.ifPresent(value -> assertThat(value, new BigDecimal(55)));
 }
 {
 final Optional<BigDecimal> result = calculate(new BigDecimal(20));
 assertThat(result.isPresent(), true);
 result.ifPresent(value -> assertThat(value, new BigDecimal(6765)));
 }
 {
 final Optional<BigDecimal> result = calculate(new BigDecimal(30));
 assertThat(result.isPresent(), true);
 result.ifPresent(value -> assertThat(value, new BigDecimal(832040)));
 }
 {
 final Optional<BigDecimal> result = calculate(new BigDecimal(40));
 assertThat(result.isPresent(), true);
 result.ifPresent(value -> assertThat(value, new BigDecimal(102334155)));
 }
 {
 final Optional<BigDecimal> result = calculate(new BigDecimal(50));
 assertThat(result.isPresent(), true);
 result.ifPresent(value -> assertThat(value, new BigDecimal(12586269025L)));
 }
 {
 final Optional<BigDecimal> result = calculate(new BigDecimal(100));
 assertThat(result.isPresent(), true);
 result.ifPresent(value -> assertThat(value, new BigDecimal("354224848179261915075")));
 }
 {
 final Optional<BigDecimal> result = calculate(new BigDecimal(200));
 assertThat(result.isPresent(), true);
 result.ifPresent(value -> assertThat(value, new BigDecimal("280571172992510140037611932413038677189525")));
 }
 }
}