<?php
declare(strict_types=1);

namespace App\Model;

class Isbn {
	/**
	 * Returns the ISBN-13 form of a valid ISBN-10 or ISBN-13, or null if the value is neither.
	 */
	public static function toIsbn13(string $value): ?string {
		$digits = strtoupper(str_replace(['-', ' '], '', $value));

		if (preg_match('/^\d{9}[\dX]$/', $digits) === 1) {
			if (!self::isValidIsbn10($digits)) {
				return null;
			}

			$stem = '978' . substr($digits, 0, 9);

			return $stem . self::isbn13CheckDigit($stem);
		}

		if (preg_match('/^97[89]\d{10}$/', $digits) === 1
			&& $digits[12] === self::isbn13CheckDigit(substr($digits, 0, 12))) {
			return $digits;
		}

		return null;
	}

	private static function isValidIsbn10(string $digits): bool {
		$sum = 0;
		foreach (str_split($digits) as $position => $digit) {
			$sum += (10 - $position) * ($digit === 'X' ? 10 : (int)$digit);
		}

		return $sum % 11 === 0;
	}

	private static function isbn13CheckDigit(string $stem): string {
		$sum = 0;
		foreach (str_split($stem) as $position => $digit) {
			$sum += (int)$digit * ($position % 2 === 0 ? 1 : 3);
		}

		return (string)((10 - $sum % 10) % 10);
	}
}