<?php

namespace App\Services\Qr;

class QrSerialGenerator
{
    /**
     * PH-{area}-{YYMM}-{batch}-{code}-{checksum}
     * Example: PH-QC042-2604-0012-000847-K9
     */
    public function build(
        string $areaCode,
        \DateTimeInterface $batchDate,
        int $batchSeq,
        int $codeSeq,
    ): string {
        $base = sprintf(
            'PH-%s-%s-%04d-%06d',
            strtoupper($areaCode),
            $batchDate->format('ym'),
            $batchSeq,
            $codeSeq,
        );

        return $base.'-'.$this->checksum($base);
    }

    /**
     * Returns 2-char alphanumeric checksum derived from SHA-256 of the
     * input. Deterministic; designed to catch single-character typos
     * during manual entry, not as a cryptographic guarantee.
     */
    public function checksum(string $base): string
    {
        $hex = hash('sha256', $base);
        // Convert two hex bytes to base-36 alphanumeric for shorter,
        // human-readable output.
        $int = hexdec(substr($hex, 0, 4));
        $code = strtoupper(base_convert((string) $int, 10, 36));

        return str_pad(substr($code, 0, 2), 2, '0', STR_PAD_LEFT);
    }

    public function isValid(string $serial): bool
    {
        $parts = explode('-', $serial);
        if (count($parts) !== 6) {
            return false;
        }

        $checksum = array_pop($parts);
        $base = implode('-', $parts);

        return $this->checksum($base) === strtoupper($checksum);
    }
}