Split an order per vendor

<?php

namespace App\Orders;

use App\Models\Order;
use App\Models\SubOrder;
use Illuminate\Support\Arr;
use Illuminate\Support\Collection;
use Illuminate\Support\Facades\DB;

final class OrderSplitter
{
    /**
     * @param  Collection<int, array{vendor_id:int, product_id:int, qty:int, unit_price:int}>  $lines
     * @param  int  $discount  order-level discount in cents
     */
    public function split(Order $order, Collection $lines, int $discount = 0): Collection
    {
        $groups = $lines->groupBy('vendor_id');
        $subtotals = $groups->map(fn ($g) => $g->sum(fn ($l) => $l['qty'] * $l['unit_price']));
        $shares = $this->allocate($discount, $subtotals->all());

        return DB::transaction(fn () => $groups->map(function ($group, $vendorId) use ($order, $subtotals, $shares) {
            $sub = SubOrder::create([
                'order_id' => $order->id,
                'vendor_id' => $vendorId,
                'subtotal' => $subtotals[$vendorId],
                'discount' => $shares[$vendorId],
                'total' => $subtotals[$vendorId] - $shares[$vendorId],
                'status' => 'pending',
            ]);
            $sub->items()->createMany($group->map(fn (array $line) => Arr::only($line, ['product_id', 'qty', 'unit_price']))->all());

            return $sub;
        }));
    }

    /**
     * Largest-remainder method: split $amount in proportion to $weights so
     * the parts always add up to exactly $amount (no lost or extra cent).
     *
     * @param  array<int,int>  $weights
     * @return array<int,int>
     */
    private function allocate(int $amount, array $weights): array
    {
        $total = array_sum($weights);
        $out = $rema = [];

        foreach ($weights as $key => $w) {
            $exact = $total > 0 ? $amount * $w / $total : 0;
            $out[$key] = (int) floor($exact);
            $rema[$key] = $exact - $out[$key];
        }

        arsort($rema);
        $left = $amount - array_sum($out);
        foreach (array_keys($rema) as $key) {
            if ($left-- <= 0) {
                break;
            }
            $out[$key]++;
        }

        return $out;
    }
}