<?php declare(strict_types=1);

namespace RobThree\Auth;

use RobThree\Auth\Providers\Qr\IQRCodeProvider;
use RobThree\Auth\Providers\Qr\QRServerProvider;
use RobThree\Auth\Providers\Rng\CSRNGProvider;
use RobThree\Auth\Providers\Rng\HashRNGProvider;
use RobThree\Auth\Providers\Rng\IRNGProvider;
use RobThree\Auth\Providers\Rng\OpenSSLRNGProvider;
use RobThree\Auth\Providers\Time\HttpTimeProvider;
use RobThree\Auth\Providers\Time\ITimeProvider;
use RobThree\Auth\Providers\Time\LocalMachineTimeProvider;
use RobThree\Auth\Providers\Time\NTPTimeProvider;

// Based on / inspired by: https://github.com/PHPGangsta/GoogleAuthenticator
// Algorithms, digits, period etc. explained: https://github.com/google/google-authenticator/wiki/Key-Uri-Format
class TwoFactorAuth
{
    private static string $_base32dict = 'ABCDEFGHIJKLMNOPQRSTUVWXYZ234567=';

    /**
     * @var array<string>
     */
    private static array $_base32;

    /**
     * @var array<string, int>
     */
    private static array $_base32lookup = array();

    public function __construct(
        private ?string $issuer = null,
        private int $digits = 6,
        private int $period = 30,
        private Algorithm $algorithm = Algorithm::Sha1,
        private ?IQRCodeProvider $qrcodeprovider = null,
        private ?IRNGProvider $rngprovider = null,
        private ?ITimeProvider $timeprovider = null
    ) {
        if ($this->digits <= 0) {
            throw new TwoFactorAuthException('Digits must be > 0');
        }

        if ($this->period <= 0) {
            throw new TwoFactorAuthException('Period must be int > 0');
        }

        self::$_base32 = str_split(self::$_base32dict);
        self::$_base32lookup = array_flip(self::$_base32);
    }

    /**
     * Create a new secret
     */
    public function createSecret(int $bits = 80, bool $requirecryptosecure = true): string
    {
        $secret = '';
        $bytes = (int) ceil($bits / 5);  // We use 5 bits of each byte (since we have a 32-character 'alphabet' / BASE32)
        $rngprovider = $this->getRngProvider();
        if ($requirecryptosecure && !$rngprovider->isCryptographicallySecure()) {
            throw new TwoFactorAuthException('RNG provider is not cryptographically secure');
        }
        $rnd = $rngprovider->getRandomBytes($bytes);
        for ($i = 0; $i < $bytes; $i++) {
            $secret .= self::$_base32[ord($rnd[$i]) & 31];  // Mask out left 3 bits for 0-31 values
        }
        return $secret;
    }

    /**
     * Calculate the code with given secret and point in time
     */
    public function getCode(string $secret, ?int $time = null): string
    {
        $secretkey = $this->base32Decode($secret);

        $timestamp = "\0\0\0\0" . pack('N*', $this->getTimeSlice($this->getTime($time)));  // Pack time into binary string
        $hashhmac = hash_hmac($this->algorithm->value, $timestamp, $secretkey, true);  // Hash it with users secret key
        $hashpart = substr($hashhmac, ord(substr($hashhmac, -1)) & 0x0F, 4);  // Use last nibble of result as index/offset and grab 4 bytes of the result
        $value = unpack('N', $hashpart);  // Unpack binary value
        $value = $value[1] & 0x7FFFFFFF;  // Drop MSB, keep only 31 bits

        return str_pad((string) ($value % 10 ** $this->digits), $this->digits, '0', STR_PAD_LEFT);
    }

    /**
     * Check if the code is correct. This will accept codes starting from ($discrepancy * $period) sec ago to ($discrepancy * period) sec from now
     */
    public function verifyCode(string $secret, string $code, int $discrepancy = 1, ?int $time = null, ?int &$timeslice = 0): bool
    {
        $timestamp = $this->getTime($time);

        $timeslice = 0;

        // To keep safe from timing-attacks we iterate *all* possible codes even though we already may have
        // verified a code is correct. We use the timeslice variable to hold either 0 (no match) or the timeslice
        // of the match. Each iteration we either set the timeslice variable to the timeslice of the match
        // or set the value to itself.  This is an effort to maintain constant execution time for the code.
        for ($i = -$discrepancy; $i <= $discrepancy; $i++) {
            $ts = $timestamp + ($i * $this->period);
            $slice = $this->getTimeSlice($ts);
            $timeslice = $this->codeEquals($this->getCode($secret, $ts), $code) ? $slice : $timeslice;
        }

        return $timeslice > 0;
    }

    /**
     * Get data-uri of QRCode
     */
    public function getQRCodeImageAsDataUri(string $label, string $secret, int $size = 200): string
    {
        if ($size <= 0) {
            throw new TwoFactorAuthException('Size must be > 0');
        }

        $qrcodeprovider = $this->getQrCodeProvider();
        return 'data:'
            . $qrcodeprovider->getMimeType()
            . ';base64,'
            . base64_encode($qrcodeprovider->getQRCodeImage($this->getQRText($label, $secret), $size));
    }

    /**
     * Compare default timeprovider with specified timeproviders and ensure the time is within the specified number of seconds (leniency)
     * @param array<ITimeProvider> $timeproviders
     */
    public function ensureCorrectTime(?array $timeproviders = null, int $leniency = 5): void
    {
        if ($timeproviders === null) {
            $timeproviders = array(
                new NTPTimeProvider(),
                new HttpTimeProvider(),
            );
        }

        // Get default time provider
        $timeprovider = $this->getTimeProvider();

        // Iterate specified time providers
        foreach ($timeproviders as $t) {
            if (!($t instanceof ITimeProvider)) {
                throw new TwoFactorAuthException('Object does not implement ITimeProvider');
            }

            // Get time from default time provider and compare to specific time provider and throw if time difference is more than specified number of seconds leniency
            if (abs($timeprovider->getTime() - $t->getTime()) > $leniency) {
                throw new TwoFactorAuthException(sprintf('Time for timeprovider is off by more than %d seconds when compared to %s', $leniency, get_class($t)));
            }
        }
    }

    /**
     * Builds a string to be encoded in a QR code
     */
    public function getQRText(string $label, string $secret): string
    {
        return 'otpauth://totp/' . rawurlencode($label)
            . '?secret=' . rawurlencode($secret)
            . '&issuer=' . rawurlencode((string) $this->issuer)
            . '&period=' . intval($this->period)
            . '&algorithm=' . rawurlencode(strtoupper($this->algorithm->value))
            . '&digits=' . intval($this->digits);
    }

    /**
     * @throws TwoFactorAuthException
     */
    public function getQrCodeProvider(): IQRCodeProvider
    {
        // Set default QR Code provider if none was specified
        if (null === $this->qrcodeprovider) {
            return $this->qrcodeprovider = new QRServerProvider();
        }
        return $this->qrcodeprovider;
    }

    /**
     * @throws TwoFactorAuthException
     */
    public function getRngProvider(): IRNGProvider
    {
        if (null !== $this->rngprovider) {
            return $this->rngprovider;
        }
        if (function_exists('random_bytes')) {
            return $this->rngprovider = new CSRNGProvider();
        }
        if (function_exists('openssl_random_pseudo_bytes')) {
            return $this->rngprovider = new OpenSSLRNGProvider();
        }
        if (function_exists('hash')) {
            return $this->rngprovider = new HashRNGProvider();
        }
        throw new TwoFactorAuthException('Unable to find a suited RNGProvider');
    }

    /**
     * @throws TwoFactorAuthException
     */
    public function getTimeProvider(): ITimeProvider
    {
        // Set default time provider if none was specified
        if (null === $this->timeprovider) {
            return $this->timeprovider = new LocalMachineTimeProvider();
        }
        return $this->timeprovider;
    }

    /**
     * Timing-attack safe comparison of 2 codes (see http://blog.ircmaxell.com/2014/11/its-all-about-time.html)
     */
    private function codeEquals(string $safe, string $user): bool
    {
        if (function_exists('hash_equals')) {
            return hash_equals($safe, $user);
        }
        // In general, it's not possible to prevent length leaks. So it's OK to leak the length. The important part is that
        // we don't leak information about the difference of the two strings.
        if (strlen($safe) === strlen($user)) {
            $result = 0;
            for ($i = 0; $i < strlen($safe); $i++) {
                $result |= (ord($safe[$i]) ^ ord($user[$i]));
            }
            return $result === 0;
        }
        return false;
    }

    private function getTime(?int $time = null): int
    {
        return ($time === null) ? $this->getTimeProvider()->getTime() : $time;
    }

    private function getTimeSlice(?int $time = null, int $offset = 0): int
    {
        return (int) floor($time / $this->period) + ($offset * $this->period);
    }

    private function base32Decode(string $value): string
    {
        if (strlen($value) == 0) {
            return '';
        }

        if (preg_match('/[^' . preg_quote(self::$_base32dict) . ']/', $value) !== 0) {
            throw new TwoFactorAuthException('Invalid base32 string');
        }

        $buffer = '';
        foreach (str_split($value) as $char) {
            if ($char !== '=') {
                $buffer .= str_pad(decbin(self::$_base32lookup[$char]), 5, '0', STR_PAD_LEFT);
            }
        }
        $length = strlen($buffer);
        $blocks = trim(chunk_split(substr($buffer, 0, $length - ($length % 8)), 8, ' '));

        $output = '';
        foreach (explode(' ', $blocks) as $block) {
            $output .= chr(bindec(str_pad($block, 8, '0', STR_PAD_RIGHT)));
        }
        return $output;
    }
}
