<?php

namespace App\Services;

use InvalidArgumentException;

final class EducationGapCalculator
{
    /** @return array{state:string, people:?int} */
    public function observed(?int $demand, ?int $available, bool $comparable, bool $current): array
    {
        if (($demand !== null && $demand < 0) || ($available !== null && $available < 0)) {
            throw new InvalidArgumentException('Quantities cannot be negative.');
        }
        if (! $current) {
            return ['state' => 'expired', 'people' => null];
        }
        if (! $comparable) {
            return ['state' => 'incomparable', 'people' => null];
        }
        if ($demand === null || $available === null) {
            return ['state' => 'insufficient_data', 'people' => null];
        }

        return ['state' => 'observed_unvalidated', 'people' => max(0, $demand - $available)];
    }
}