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package com.penglecode.codeforce.common.util;

import org.springframework.util.Assert;

import java.util.*;
import java.util.stream.Collectors;

/**
 * @author pengpeng
 * @version 1.0
 */
@SuppressWarnings("ConstantConditions")
public class CollectionUtils extends org.springframework.util.CollectionUtils {

    /**
     * 如果collection为null/empty则返回defaultValue否则原值返回
     * @param collection
     * @param defaultValue
     * @return
     */
    public static  Collection defaultIfEmpty(Collection collection, Collection defaultValue) {
        return isEmpty(collection) ? defaultValue : collection;
    }

    /**
     * 如果collection为null/empty则返回defaultValue否则原值返回
     * @param collection
     * @param defaultValue
     * @return
     */
    public static  List defaultIfEmpty(List collection, List defaultValue) {
        return isEmpty(collection) ? defaultValue : collection;
    }

    /**
     * 如果collection为null/empty则返回defaultValue否则原值返回
     * @param collection
     * @param defaultValue
     * @return
     */
    public static  Set defaultIfEmpty(Set collection, Set defaultValue) {
        return isEmpty(collection) ? defaultValue : collection;
    }

    /**
     * 如果map为null/empty则返回defaultValue否则原值返回
     * @param map
     * @param defaultValue
     * @return
     */
    public static  Map defaultIfEmpty(Map map, Map defaultValue) {
        return isEmpty(map) ? defaultValue : map;
    }

    /**
     * Adapts an {@code Iterator} to the {@code Enumeration} interface.
     *
     * @param iterator
     * @param 
     * @return
     */
    public static  Enumeration asEnumeration(final Iterator iterator) {
        Objects.requireNonNull(iterator);
        return new Enumeration() {
            @Override
            public boolean hasMoreElements() {
                return iterator.hasNext();
            }

            @Override
            public T nextElement() {
                return iterator.next();
            }
        };
    }

    /**
     * 求笛卡尔积
     *
     * @param groupedLists
     * @param 
     * @return
     */
    public static  List> cartesianProduct(List> groupedLists) {
        if(isEmpty(groupedLists)) {
            return Collections.emptyList();
        }
        Assert.noNullElements(groupedLists, "The element of 'groupedLists' must be not empty!");
        //利用LinkedList的poll()特性
        LinkedList> groupedElements = new LinkedList<>(groupedLists);
        //为递归中的flatMap创造条件,假设groupedElements.poll() => ["1","2","3"], 则startElements => [["1"],["2"],["3"]]
        List> startElements = groupedElements.poll().stream()
                .map(Collections::singletonList)
                .collect(Collectors.toList());
        List> resultElements = new ArrayList<>();
        recursiveCartesianProduct(startElements, groupedElements, resultElements);
        return resultElements;
    }

    /**
     * 递归求取笛卡尔积
     * @param startElements
     * @param remainingElements
     * @param resultElements
     * @param 
     */
    private static  void recursiveCartesianProduct(List> startElements, LinkedList> remainingElements, List> resultElements) {
        if(!isEmpty(remainingElements)) {
            List remainingFirst = remainingElements.poll();
            List> newStartElements = startElements.stream()
                    //利用flatMap将诸如：[["1"],"红"] ==flatMap()==> ["1","红"]
                    //利用flatMap将诸如：[["1","红"],"A"] ==flatMap()==> ["1","红","A"]
                    .flatMap(startElement -> remainingFirst.stream().map(element -> {
                List subCartesian = new ArrayList<>(startElement);
                subCartesian.add(element);
                return subCartesian;
            })).collect(Collectors.toList());
            recursiveCartesianProduct(newStartElements, remainingElements, resultElements);
        } else {
            resultElements.addAll(startElements);
        }
    }

}