<?php

declare(strict_types=1);

namespace Modules\Helpdesk\Services;

use App\Models\User;
use Illuminate\Database\Eloquent\Builder;
use Modules\Helpdesk\Models\Category;

final class AgentAssignmentService
{
    /**
     * Select the least-loaded available agent for a category.
     *
     * @param int|null $categoryId
     * @return int|null
     */
    public function selectAgentId(?int $categoryId): ?int
    {
        $query = User::query()
            ->where(static function (Builder $query): void {
                $query->where('helpdesk_agent', true)->orWhere('helpdesk_admin', true);
            })
            ->withCount(['helpdeskAssignedTickets as open_helpdesk_tickets_count' => static function (Builder $query): void {
                $query->whereNull('completed_at');
            }]);

        if ($categoryId !== null) {
            $category = Category::query()->find($categoryId);
            $agentIds = $category?->agents()->pluck('users.id')->all() ?? [];

            if ($agentIds !== []) {
                $query->whereIn('id', $agentIds);
            }
        }

        /** @var User|null $agent */
        $agent = $query->orderBy('open_helpdesk_tickets_count')->orderBy('id')->first();

        return $agent?->id;
    }
}