PHP Implementation of WeChat Red Packet Splitting Algorithm

The article introduces the problem of splitting a WeChat red packet (hongbao) into multiple smaller packets using PHP, describing the two main constraints: each packet amount is limited to two decimal places and a minimum amount can be set for each packet.

It then presents a complete PHP class named Coupon that encapsulates the algorithm. The class stores the total amount, the number of packets, the minimum per‑packet amount, and the resulting items array. It provides a constructor for initialization, a handle() method that validates inputs and performs the allocation, and several protected helper methods ( apportion() , calcCouponAmount() , calcCouponAmountMax() , apportionRandRatio() , and decimal_number() ) that implement the random distribution logic while respecting the constraints.

<?php
class Coupon {
    protected $amount;
    protected $num;
    protected $coupon_min;
    protected $items = [];

    /**
     * Initialize the object
     * @param float $amount Total amount (max 2 decimal places)
     * @param int   $num    Number of packets
     * @param float $coupon_min Minimum amount per packet
     */
    public function __construct($amount, $num = 1, $coupon_min = 0.01) {
        $this->amount = $amount;
        $this->num = $num;
        $this->coupon_min = $coupon_min;
    }

    /**
     * Process and return the allocation result
     * @return array
     */
    public function handle() {
        // A. Validation
        if ($this->amount < $validAmount = $this->coupon_min * $this->num) {
            throw new Exception('红包总金额必须≥' . $validAmount . '元');
        }
        // B. Allocate red packets
        $this->apportion();
        return ['items' => $this->items];
    }

    /**
     * Allocate red packets
     */
    protected function apportion() {
        $num = $this->num;
        $amount = $this->amount;
        while ($num >= 1) {
            if ($num == 1) {
                $coupon_amount = $this->decimal_number($amount);
            } else {
                $avg_amount = $this->decimal_number($amount / $num);
                $coupon_amount = $this->calcCouponAmount($avg_amount, $amount, $num);
            }
            $this->items[] = $coupon_amount;
            $amount -= $coupon_amount;
            $num--;
        }
        shuffle($this->items);
    }

    /**
     * Calculate the amount for a single packet
     * @param float $avg_amount Average amount for remaining packets
     * @param float $amount     Remaining total amount
     * @param int   $num        Remaining packet count
     * @return float
     */
    protected function calcCouponAmount($avg_amount, $amount, $num) {
        if ($avg_amount <= $this->coupon_min) {
            return $this->coupon_min;
        }
        $coupon_amount = $this->decimal_number($avg_amount * (1 + $this->apportionRandRatio()));
        if ($coupon_amount < $this->coupon_min || $coupon_amount > $this->calcCouponAmountMax($amount, $num)) {
            return $this->calcCouponAmount($avg_amount, $amount, $num);
        }
        return $coupon_amount;
    }

    /**
     * Maximum possible amount for a packet
     * @param float $amount
     * @param int   $num
     */
    protected function calcCouponAmountMax($amount, $num) {
        return $this->coupon_min + $amount - $num * $this->coupon_min;
    }

    /**
     * Random fluctuation ratio (60% chance for ±70%, otherwise ±30%)
     */
    protected function apportionRandRatio() {
        if (rand(1, 100) <= 60) {
            return rand(-70, 70) / 100;
        }
        return rand(-30, 30) / 100;
    }

    /**
     * Format a number to two decimal places
     * @param float $amount
     * @return float
     */
    protected function decimal_number($amount) {
        return sprintf('%01.2f', round($amount, 2));
    }
}
?>

An example usage creates a Coupon instance with a total of 200 CNY, 10 packets, and a minimum of 10 CNY per packet, calls handle() , and prints the resulting array of allocated amounts.

<?php
$coupon = new Coupon(200, 10, 10);
$res = $coupon->handle();
print_r($res);
?>

The printed result shows an array of ten amounts that sum to 200 CNY, each respecting the minimum value constraint, demonstrating that the algorithm works as intended.