Discussion on Algorithm Implementation of WeChat Red Envelopes (Based on PHP)

Tonight, I had a sudden idea and sent a red envelope to the alumni WeChat group. I set the total amount of the red envelope to 10 yuan, allowing 28 people to receive it randomly.

So an interesting result appeared:

A received 0.26 yuan
B received 0.29 yuan
C received 0.02 yuan
D received 0.56 yuan
E received 0.64 yuan
...

What does WeChat use? What kind of algorithm does it do? I simply checked on Baidu and found that there is no official explanation yet. There is only a relatively popular discussion in Zhihu. Click here for the link. However, their discussion is too in-depth and seems to be a trap.

I tried it according to my own logic. This algorithm needs to meet the following requirements:

1. Everyone must be able to receive the red envelope;

2. The sum of the red envelope amount received by each person = the total amount;

3. The amount of red envelopes received by each person varies, but the difference cannot be too outrageous, otherwise it will be uninteresting;

4. The algorithm must be simple, otherwise it will fail Tencent’s signature;

Before formal coding, build one first Progressive model to analyze the pattern

Set the total amount to 10 yuan, and N people randomly receive it:

N=1 
则红包金额=X元； 
N=2 
为保证第二个红包可以正常发出，第一个红包金额=0.01至9.99之间的某个随机数 
第二个红包=10-第一个红包金额； 
N=3 
红包1=0.01至0.98之间的某个随机数 
红包2=0.01至(10-红包1-0.01)的某个随机数 
红包3=10-红包1-红包2 
……

At this point, the pattern appears! Start coding!

header("Content-Type: text/html;charset=utf-8");//输出不乱码，你懂的
$total=10;//红包总额
$num=8;// 分成8个红包，支持8人随机领取
$min=0.01;//每个人最少能收到0.01元
for ($i=1;$i<$num;$i++)
{
    $safe_total=$total-($num-$i)*$min;//随机安全上限
    $money=mt_rand($min*100,$safe_total*100)/100;
    $total=$total-$money;
    echo &#39;第&#39;.$i.&#39;个红包：&#39;.$money.&#39; 元，余额：&#39;.$total.&#39; 元 <br/>&#39;;
}
echo &#39;第&#39;.$num.&#39;个红包：&#39;.$total.&#39; 元，余额：0 元&#39;;

After inputting it, the fluctuation is too big and the data is too boring!

第1个红包：7.48 元，余额：2.52 元 
第2个红包：1.9 元，余额：0.62 元 
第3个红包：0.49 元，余额：0.13 元 
第4个红包：0.04 元，余额：0.09 元 
第5个红包：0.03 元，余额：0.06 元 
第6个红包：0.03 元，余额：0.03 元 
第7个红包：0.01 元，余额：0.02 元 
第8个红包：0.02 元，余额：0 元

Improve it and use the average value as the random safety upper limit to control the fluctuation difference

header("Content-Type: text/html;charset=utf-8");//输出不乱码，你懂的
$total=10;//红包总额
$num=8;// 分成8个红包，支持8人随机领取
$min=0.01;//每个人最少能收到0.01元
for ($i=1;$i<$num;$i++)
{
    $safe_total=($total-($num-$i)*$min)/($num-$i);//随机安全上限
    $money=mt_rand($min*100,$safe_total*100)/100;
    $total=$total-$money;
    echo &#39;第&#39;.$i.&#39;个红包：&#39;.$money.&#39; 元，余额：&#39;.$total.&#39; 元 <br/>&#39;;
}
echo &#39;第&#39;.$num.&#39;个红包：&#39;.$total.&#39; 元，余额：0 元&#39;;

The output results are shown in the figure below

第1个红包：0.06 元，余额：9.94 元 
第2个红包：1.55 元，余额：8.39 元 
第3个红包：0.25 元，余额：8.14 元 
第4个红包：0.98 元，余额：7.16 元 
第5个红包：1.88 元，余额：5.28 元 
第6个红包：1.92 元，余额：3.36 元 
第7个红包：2.98 元，余额：0.38 元 
第8个红包：0.38 元，余额：0 元