Delivery-slot capacity booking

<?php

namespace App\Delivery;

use App\Models\DeliverySlot;
use Illuminate\Support\Facades\DB;
use Illuminate\Validation\ValidationException;

final class SlotBooking
{
    /**
     * Claim one place in a slot. The WHERE clause is the lock: the row only
     * updates while booked < capacity, so two buyers can never take the last place.
     */
    public function reserve(int $slotId, int $orderId): void
    {
        DB::transaction(function () use ($slotId, $orderId) {
            $claimed = DeliverySlot::query()
                ->whereKey($slotId)
                ->where('is_open', true)
                ->where('starts_at', '>', now()->addMinutes(30)) // cut-off before the window
                ->whereColumn('booked', '<', 'capacity')
                ->increment('booked');

            if ($claimed === 0) {
                throw ValidationException::withMessages([
                    'slot' => 'That time slot is full or no longer available. Please pick another.',
                ]);
            }

            DB::table('orders')->where('id', $orderId)->update(['delivery_slot_id' => $slotId]);
        });
    }

    /** Give the place back (cancellation or reschedule). Never goes below zero. */
    public function release(int $orderId): void
    {
        DB::transaction(function () use ($orderId) {
            $slotId = DB::table('orders')->where('id', $orderId)->lockForUpdate()->value('delivery_slot_id');
            if (! $slotId) {
                return; // nothing reserved: releasing twice is harmless
            }

            DeliverySlot::whereKey($slotId)->where('booked', '>', 0)->decrement('booked');
            DB::table('orders')->where('id', $orderId)->update(['delivery_slot_id' => null]);
        });
    }

    /** Open slots for a day, with how many places remain. */
    public function available(string $date)
    {
        return DeliverySlot::query()
            ->whereDate('starts_at', $date)
            ->where('is_open', true)
            ->whereColumn('booked', '<', 'capacity')
            ->selectRaw('*, capacity - booked as remaining')
            ->orderBy('starts_at')
            ->get();
    }
}