YY一个拼手气的红包算法

package com.evan;

import java.util.ArrayList;
import java.util.Collections;
import java.util.List;
import java.util.Random;

public class RedPacket {

    private static final int MAX     = 20000; //单个红包不能大于200元
    private static final int BALANCE = 1;    //平衡值，值越大越平衡，最新红包和最大红包的差距越小

    /**
     * 生成随机红包[不包含抢]
     * 
     * @param value 红包总金额[单位分]
     * @param num 红包个数
     * @return
     */
    public static List<Integer> build(int value, int num) {
        if (value <= 0) {
            throw new RuntimeException("红包金额非法!");
        }
        if (num <= 0 || num > value) {
            throw new RuntimeException("红包份数非法!");
        }
        if (num * MAX < value) {
            throw new RuntimeException("每一个红包的金额不能大于[" + (MAX / 100) + "]元!");
        }
        List<Integer> list = new ArrayList<Integer>(num);
        for (int i = 0; i < num; i++) {
            list.add(0);
        }
        while (value > 0) {
            int average = value / num / BALANCE;//BALANCE的值决定瓜分的次数，次数越多越平衡
            int i = 0;
            while (average == 0 && value > 0) {
                //当剩余金额不足以让每一个红包分到钱时，将剩余金额拆成1分钱一次分给红包直到剩余金额为0
                //排在前面的红包会有一分钱的固定优点，须要后期须要对全部红包进行乱序去除这种优点
                int idx = (i++) % num;
                int current = list.get(idx);
                if (current == MAX) {
                    continue;
                }
                list.set(idx, current + 1);
                value--;
            }
            if (value == 0) {
                //当剩余金额为0时中止瓜分
                break;
            }
            for (int idx = 0; idx < num; idx++) {
                int current = list.get(idx);
                if (current == MAX) {
                    continue;
                }
                //从剩余金额中随机瓜分金额到当前红包，Math.min(average, MAX - current)保证当前红包的值不会超过MAX常量
                int randomValue = new Random().nextInt(Math.min(average, MAX - current)) + 1;
                list.set(idx, current + randomValue);
                value = value - randomValue;
                if (value == 0) {
                    break;
                }
            }
        }
        //随机乱序
        Collections.shuffle(list);
        return list;
    }

    public static void main(String[] args) {
        for (int idx = 0; idx < 20; idx++) {

            List<Integer> list = build(90000, 5);
            for (int i : list) {
                System.out.println(i);
            }
            System.out.println("==========");
        }

    }

}