随机红包算法（java）

随机红包算法，每个人都有自己的实现思路。

package com.jmmq.load.jim.algorithm;

import JAVA.math.BigDecimal;
import java.util.Arrays;
import java.util.List;
import java.util.Random;

/**
 * 红包算法
 */
public class RedPacketPrc {

    private static final BigDecimal MIN = new BigDecimal("0.01");

    public static void main(String[] args) {

        String[] rs = redPacketRandom(new BigDecimal(1), 100, false);
        BigDecimal sum = BigDecimal.ZERO;
        for(String str: rs){
            System.out.println(str);
            sum = sum.add(new BigDecimal(str));
        }
        System.out.println("sum:" + sum);

        System.out.println("(¯`•._.• •._.•´¯)(¯`•¸•´¯) (¯`•._.• •._.•´¯)(¯`•¸•´¯) (¯`•._.• •._.•´¯)(¯`•¸•´¯)");

        String[] res = redPacketOpen(new BigDecimal(20), 6, false);
        BigDecimal sum2 = BigDecimal.ZERO;
        for(String str: res){
            System.out.println(str);
            sum2 = sum2.add(new BigDecimal(str));
        }
        System.out.println("sum2:" + sum2);
    }

    /**
     *
     * @param faceValue 面值 单位：元
     * @param amount    人数
     * @param avgFlag 是否平均
     * @return
     *   平均分就不写了
     *   思路一：
     *        按百分比进行随机计算，最后一个进行匝差计算
     */
    public static String[] redPacketRandom(BigDecimal faceValue, int amount, boolean avgFlag){

        // 先计算平均数  四舍五入
        BigDecimal perValue = faceValue.divide(new BigDecimal(amount),3, BigDecimal.ROUND_HALF_UP);

        // 平均下来最小的红包必须大于等于 1分钱
        if(perValue.compareTo(MIN) == -1){
            throw new RuntimeException("red packet amount OutOfBounds  min = 0.01");
        }

        String[] rs = new String[amount];

        // 平均
        if(avgFlag){
            // 这里四舍五入重新计算
            perValue = faceValue.divide(new BigDecimal(amount),2, BigDecimal.ROUND_HALF_UP);
            Arrays.fill(rs, perValue.toString());
            // 无法除尽
            if(perValue.multiply(new BigDecimal(amount)).compareTo(faceValue) != 0 ){
                // 随机获取一个下标
                int index = new Random().nextInt(amount);
                rs[index] = String.valueOf(new BigDecimal(rs[index]).add(
                           faceValue.subtract(perValue.multiply(new BigDecimal(amount)))));

            }
            return rs;
        }

        BigDecimal weightSum = BigDecimal.ZERO;
        BigDecimal redSum = BigDecimal.ZERO;
        BigDecimal[] weights = new BigDecimal[amount];
        // 随机
        for (int i=0; i<rs.length; i++){
            BigDecimal seed = new BigDecimal(new Random().nextInt(amount));
            weightSum = weightSum.add(seed);
            weights[i] = seed;

            // 循环最后一个后计算金额
            if(i == rs.length -1){
                for(int j=0; j<rs.length; j++){
                   // 修改为向下取值，这样差额永远都是正数就不存在吃不掉差额的问题了
                   BigDecimal val = faceValue.multiply(weights[j].divide(weightSum,2, BigDecimal.ROUND_HALF_UP));
                   if(val.compareTo(MIN) == -1){
                       val = MIN;
                   }
                    redSum = redSum.add(val);
                   rs[j] = val.toString();
                }
            }
        }

        BigDecimal diff = faceValue.subtract(redSum);
        if(faceValue.compareTo(redSum) != 0){

            // 找到第一个加上或减少diff大于MIN的元素修改金额
            for(int i=0; i<rs.length; i++){
                if(new BigDecimal(rs[i]).add(diff).compareTo(MIN) > -1 ){
                    rs[i] = new BigDecimal(rs[i]).add(diff).toString();
                    break;
                }
            }
        }

        // 若没有任何一个元素能吃掉差额,因此差额需要进行分摊了，
        // 这里简单粗暴的将循环到大于MIN的都改为MIN同时减少差额
        // 这里代码不进行优化，会写另外一个优化的算法
        if(faceValue.compareTo(redSum) != 0){
            for(int i=0; i<rs.length; i++){
                if(new BigDecimal(rs[i]).compareTo(MIN) > 0){
                    if(new BigDecimal(rs[i]).add(diff).compareTo(MIN) > -1){
                        rs[i] = new BigDecimal(rs[i]).add(diff).toString();
                        break;
                    } else {
                        diff.add(new BigDecimal(rs[i])).subtract(MIN);
                        rs[i] = MIN.toString();
                    }
                }
            }
        }
        return rs;
    }


    /**
     *  上面算法进行优化
     * @param faceValue 面值 单位：元
     * @param amount    人数
     * @param avgFlag 是否平均
     * @return
     *   优化思路：
     *       上面算法为了保证红包最小金额处理上花费了大量的循环来处理
     *       优化上就直接先分配最小金额，然后在进行随机分摊剩余的钱
     */
    public static String[] redPacketOpen(BigDecimal faceValue, int amount, boolean avgFlag){

        // 先计算平均数  四舍五入
        BigDecimal perValue = faceValue.divide(new BigDecimal(amount),3, BigDecimal.ROUND_HALF_UP);

        // 平均下来最小的红包必须大于等于 1分钱
        if(perValue.compareTo(MIN) == -1){
            throw new RuntimeException("red packet amount OutOfBounds  min = 0.01");
        }

        String[] rs = new String[amount];

        // 平均
        if(avgFlag){
            // 这里四舍五入重新计算
            perValue = faceValue.divide(new BigDecimal(amount),2, BigDecimal.ROUND_HALF_UP);
            Arrays.fill(rs, perValue.toString());
            // 无法除尽
            if(perValue.multiply(new BigDecimal(amount)).compareTo(faceValue) != 0 ){
                // 随机获取一个下标
                int index = new Random().nextInt(amount);
                rs[index] = String.valueOf(new BigDecimal(rs[index]).add(
                        faceValue.subtract(perValue.multiply(new BigDecimal(amount)))));

            }
            return rs;
        }

        // 随机
        // 先把低保放进去
        Arrays.fill(rs, MIN.toString());

        BigDecimal weightSum = BigDecimal.ZERO;
        BigDecimal redSum = BigDecimal.ZERO;
        BigDecimal[] weights = new BigDecimal[amount];
        // 随机
        for (int i=0; i<rs.length; i++){
            BigDecimal seed = new BigDecimal(new Random().nextInt(amount));
            weightSum = weightSum.add(seed);
            weights[i] = seed;

            // 循环最后一个后计算金额
            if(i == rs.length -1){
                for(int j=0; j<rs.length; j++){
                    BigDecimal val = faceValue.multiply(weights[j].divide(weightSum,2, BigDecimal.ROUND_DOWN));
                    redSum = redSum.add(val);
                    rs[j] = val.add(MIN).toString();
                }
            }
        }

        // 随机获取一个下标
        BigDecimal diff = faceValue.subtract(redSum);
        int index = new Random().nextInt(amount);
        rs[index] = String.valueOf(new BigDecimal(rs[index]).add(diff));

        return rs;
    }

}