<?php
namespace Hanyuu\OTP;

use InvalidArgumentException;
use Index\Serialisation\Serialiser;

class TOTPGenerator implements IOTPGenerator {
    private const DIGITS = 6;
    private const INTERVAL = 30;
    private const HASH_ALGO = 'sha1';

    public function __construct(
        private string $secretKey,
        private int $digits = self::DIGITS,
        private int $interval = self::INTERVAL,
        private string $hashAlgo = self::HASH_ALGO
    ) {
        if(empty($this->secretKey))
            throw new InvalidArgumentException('$secretKey may not be empty');
        if($this->digits < 1)
            throw new InvalidArgumentException('$digits must be a positive integer');
        if($this->interval < 1)
            throw new InvalidArgumentException('$interval must be a positive integer');
        if(!in_array($this->hashAlgo, hash_hmac_algos(), true))
            throw new InvalidArgumentException('$hashAlgo must be a hashing algorithm suitable for hmac');
    }

    public function getDigits(): int {
        return $this->digits;
    }

    public function getInterval(): int {
        return $this->interval;
    }

    public function getHashAlgo(): string {
        return $this->hashAlgo;
    }

    public function getTimeCode(int $offset = 0, int $timeStamp = -1): int {
        if($timeStamp < 0)
            $timeStamp = time();

        // use -1 and 1 to get the previous and next token for user convenience
        if($offset !== 0)
            $timeStamp += $this->interval * $offset;

        return (int)(($timeStamp * 1000) / ($this->interval * 1000));
    }

    public function generate(int $offset = 0, int $timeStamp = -1): string {
        $timeCode = pack('J', $this->getTimeCode($offset, $timeStamp));
        $secretKey = Serialiser::base32()->deserialise($this->secretKey);
        $hash = hash_hmac($this->hashAlgo, $timeCode, $secretKey, true);

        $offset = ord($hash[strlen($hash) - 1]) & 0x0F;

        $bin  = (ord($hash[$offset])     & 0x7F)  << 24;
        $bin |= (ord($hash[$offset + 1]) & 0xFF) << 16;
        $bin |= (ord($hash[$offset + 2]) & 0xFF) << 8;
        $bin |=  ord($hash[$offset + 3]) & 0xFF;

        $otp = (string)($bin % pow(10, $this->digits));

        return str_pad($otp, $this->digits, '0', STR_PAD_LEFT);
    }

    public static function generateKey(int $bytes = 16): string {
        if($length < 1)
            throw new InvalidArgumentException('$bytes must be a positive integer');

        return Serialiser::base32()->serialise(random_bytes($bytes));
    }
}