<?php

declare(strict_types=1);

namespace App\Services;

final class TotpService
{
    private const ALPHABET = 'ABCDEFGHIJKLMNOPQRSTUVWXYZ234567';

    public function generateSecret(): string
    {
        $secret = '';
        for ($i = 0; $i < 16; $i++) {
            $secret .= self::ALPHABET[random_int(0, 31)];
        }
        return $secret;
    }

    public function verify(string $secret, string $code): bool
    {
        $currentTime = floor(time() / 30);
        
        // Check current, previous and next window (allow 30s clock drift)
        for ($i = -1; $i <= 1; $i++) {
            if ($this->calculateCode($secret, (int)($currentTime + $i)) === $code) {
                return true;
            }
        }

        return false;
    }

    private function calculateCode(string $secret, int $time): string
    {
        $key = $this->base32Decode($secret);
        $timeHex = str_pad(dechex($time), 16, '0', STR_PAD_LEFT);
        $timeBin = pack('H*', $timeHex);
        
        $hash = hash_hmac('sha1', $timeBin, $key, true);
        $offset = ord($hash[19]) & 0xf;
        
        $otp = (
            ((ord($hash[$offset]) & 0x7f) << 24) |
            ((ord($hash[$offset + 1]) & 0xff) << 16) |
            ((ord($hash[$offset + 2]) & 0xff) << 8) |
            (ord($hash[$offset + 3]) & 0xff)
        ) % 1000000;
        
        return str_pad((string)$otp, 6, '0', STR_PAD_LEFT);
    }

    private function base32Decode(string $base32): string
    {
        $base32 = strtoupper($base32);
        $buffer = 0;
        $bufferSize = 0;
        $decoded = '';

        for ($i = 0; $i < strlen($base32); $i++) {
            $char = $base32[$i];
            $pos = strpos(self::ALPHABET, $char);
            if ($pos === false) continue;

            $buffer = ($buffer << 5) | $pos;
            $bufferSize += 5;

            if ($bufferSize >= 8) {
                $bufferSize -= 8;
                $decoded .= chr(($buffer >> $bufferSize) & 0xff);
            }
        }

        return $decoded;
    }

    public function getQrCodeUrl(string $userEmail, string $secret, string $issuer = 'Musadaq'): string
    {
        $label = urlencode($issuer . ':' . $userEmail);
        $otpauth = "otpauth://totp/{$label}?secret={$secret}&issuer=" . urlencode($issuer);
        return "https://api.qrserver.com/v1/create-qr-code/?size=200x200&data=" . urlencode($otpauth);
    }
}