运费拆分，数量维度拆分

/**
             * 运费拆分.
             *
             * @param int   $f
             * @param array $arr
             *
             * @return array
             */
            protected function freightSplit($f = 0, $arr = []): array
            {
                $count = count($arr); //总记录数
                $allNum = array_sum(array_column($arr, 'product_number'));
                //排序
                uksort($arr, function ($ak, $bk) use ($arr) {
                    $a = $arr[$ak];
                    $b = $arr[$bk];
                    if ($a['product_number'] === $b['product_number']) {
                        return $ak - $bk;
                    }
                    return $a['product_number'] > $b['product_number'] ? 1 : -1;
                });
                $i = 1;
                $freightCumulative = 0;
                foreach ($arr as &$val) {
                    if ($count == 1) {
                        $val['_freight_amount'] = $f;
                    } elseif ($i == $count) {
                        $val['_freight_amount'] = bcsub($f, $freightCumulative, 2);
                    } else {
                        $val['_freight_amount'] = bcmul(bcdiv($val['product_number'], $allNum, 2), $f, 2);
                        $freightCumulative = bcadd($freightCumulative, $val['_freight_amount'], 2);
                    }
                    ++$i;
                }
                ksort($arr);
                return $arr;
            }