红包随机算法，给定一定的金额，一定的人数，保证每个人都能随机获得一定的金额。

//题目:请编写一个红包随机算法。需求为：给定一定的金额，一定的人数，保证每个人都能随机获得一定的金额。
//比如100元的红包，10个人抢，每人分得一些金额。
//约束条件为，最佳手气金额不能超过最大金额的90%，每人都有红包可抢。
//请给出java代码实现，返回每个人的分配金额并打印出来。

public class RedEnvelopMain {

    // 最佳手气获得红包金额，最大金额/总金额，的占比
    public static final double BEST_LUCK_PERCENT = 0.9;

    // 单人每次最小抢到的金额，默认为1分钱
    public static final double ONE_PERSON_MIN_DRAW_AMOUNT = 1;

    /**
     * 拆红包方法
     * 红包金额分配算法
     *
     * @param totalAmount 红包总金额
     * @param personNum   抢红包总人数
     */
    public void redEnvelopLuckyDraw(double totalAmount, int personNum) {

        if (totalAmount <= 0 || personNum < 1) {
            System.out.println("输入参数非法，请检查");
            return;
        }

        // 红包总金额 >= 分配人数 * 每人最小中奖金额
        if (totalAmount < (ONE_PERSON_MIN_DRAW_AMOUNT * personNum)) {
            System.out.println("红包总金额不能小于（中奖人数*单人单次中奖金额），请核对红包金额和发放人数");
            return;
        }

        double minDrawAmount = ONE_PERSON_MIN_DRAW_AMOUNT;
        double maxDrawAmount = totalAmount * BEST_LUCK_PERCENT;

        double drawLuckAmount = 0;

        for (Integer i = 0; i < personNum; i++) {

            int remainPersonNum = personNum - i - 1;

            // 假设剩下的人都中了单人最高金额，那么他此次最少能中的金额
            double othersAllDrawMaxAmountBalance = totalAmount - (maxDrawAmount * remainPersonNum);
            minDrawAmount = minDrawAmount > othersAllDrawMaxAmountBalance ? minDrawAmount : othersAllDrawMaxAmountBalance;

            // 每次抽奖前，计算此次抽奖最大可能出现的金额，假设10人分10元，第一人中8元，则剩下9人，要分2元，此时最大中奖金额发生变化
            //double othersAllDrawMinAmountBalance = totalAmount - (minDrawAmount * remainPersonNum);
            double othersAllDrawMinAmountBalance = totalAmount / remainPersonNum * 2;
            maxDrawAmount = maxDrawAmount < othersAllDrawMinAmountBalance ? maxDrawAmount : othersAllDrawMinAmountBalance;

            drawLuckAmount = (int) Math.floor((maxDrawAmount - minDrawAmount) * Math.random() + minDrawAmount);

            // 每个人抢到红包后，红包内的剩余金额
            totalAmount = totalAmount - drawLuckAmount;

            System.out.println("第" + (i + 1) + "个人抢到:" + drawLuckAmount + "元");
        }
    }

    public static void main(String[] args) {

        RedEnvelopMain redEnvelopMain = new RedEnvelopMain();

        redEnvelopMain.redEnvelopLuckyDraw(100, 10);
    }
}

import java.math.BigDecimal;
import java.util.Objects;

public class RedEnvelopStrongerMain {

    // 最佳手气金额不能超过最大金额的90%
    public static final BigDecimal BEST_LUCK_PERCENT = new BigDecimal(0.9);

    // 单人每次最小抢到的金额，默认为1分钱
    public static final BigDecimal ONE_PERSON_MIN_DRAW_AMOUNT = new BigDecimal(1);

    /**
     * 拆红包方法
     * 红包金额分配算法
     *
     * @param totalAmount    红包总金额
     * @param personQuantity 抢红包总人数
     */
    public void redEnvelopLuckyDraw(BigDecimal totalAmount, Integer personQuantity) {

        if (Objects.isNull(totalAmount) || totalAmount.compareTo(BigDecimal.ZERO) <= 0
                || Objects.isNull(personQuantity) || personQuantity < 1) {
            System.out.println("输入参数非法，请检查");
            return;
        }

        BigDecimal personNum = new BigDecimal(personQuantity);
        // 红包总金额 >= 分配人数 * 每人最小中奖金额
        if (totalAmount.compareTo(ONE_PERSON_MIN_DRAW_AMOUNT.multiply(personNum)) < 0) {
            System.out.println("红包总金额不能小于（中奖人数*单人单次中奖金额），请核对红包金额和发放人数");
            return;
        }

        BigDecimal minDrawAmount = ONE_PERSON_MIN_DRAW_AMOUNT;

        BigDecimal drawLuckAmount;

        for (Integer i = 0; i < personQuantity; i++) {

            Integer remainPersonQuantity = personQuantity - i - 1;

            if (remainPersonQuantity == 0) {
                // 最后一个人，直接把剩余金额返回
                drawLuckAmount = totalAmount;
                totalAmount = totalAmount.subtract(drawLuckAmount);
                System.out.println("第" + (i + 1) + "个人抢到:" + drawLuckAmount + "元");
                break;
            }

            BigDecimal remainPersonNum = new BigDecimal(remainPersonQuantity);

            // 最大不超过剩余金额的90%
            BigDecimal maxDrawAmount = totalAmount.multiply(BEST_LUCK_PERCENT).setScale(2, BigDecimal.ROUND_UP);

            // 二倍均值法，使每个人的中奖金额都按均值概率分布
            BigDecimal doubleAverageAmount = totalAmount.divide(remainPersonNum, 2, BigDecimal.ROUND_UP).multiply(new BigDecimal(2)).setScale(2, BigDecimal.ROUND_UP);
            maxDrawAmount = doubleAverageAmount.compareTo(maxDrawAmount) < 0 ? doubleAverageAmount : maxDrawAmount;

            BigDecimal othersAllDrawMaxAmountBalance = totalAmount.subtract(maxDrawAmount.multiply(remainPersonNum));
            minDrawAmount = othersAllDrawMaxAmountBalance.compareTo(minDrawAmount) < 0 ? minDrawAmount : othersAllDrawMaxAmountBalance;

            drawLuckAmount = (maxDrawAmount.subtract(minDrawAmount)).multiply(new BigDecimal(Math.random())).setScale(2, BigDecimal.ROUND_UP);

            drawLuckAmount = drawLuckAmount.compareTo(minDrawAmount) < 0 ? minDrawAmount : drawLuckAmount;

            // 每个人抢到红包后，红包内的剩余金额
            totalAmount = totalAmount.subtract(drawLuckAmount);

            System.out.println("第" + (i + 1) + "个人抢到:" + drawLuckAmount + "元");
        }
    }

    public static void main(String[] args) {

        RedEnvelopStrongerMain redEnvelopMain = new RedEnvelopStrongerMain();

        redEnvelopMain.redEnvelopLuckyDraw(new BigDecimal(100), 3);
    }
}