<?php

namespace App\Libraries;

/**
 * Two-Factor Authentication Library
 * Implements TOTP (Time-based One-Time Password) for Google Authenticator
 * Based on RFC 6238
 */
class TwoFactorAuth
{
    protected int $codeLength = 6;
    protected int $period = 30; // 30 seconds validity window

    public function __construct()
    {
        // No CI instance needed
    }

    /**
     * Generate a new secret key for TOTP
     * @return string Base32 encoded secret
     */
    public function generateSecret(int $length = 16): string
    {
        $secret = '';
        $validChars = 'ABCDEFGHIJKLMNOPQRSTUVWXYZ234567'; // Base32 alphabet

        for ($i = 0; $i < $length; $i++) {
            $secret .= $validChars[random_int(0, 31)];
        }

        return $secret;
    }

    /**
     * Generate QR code URL for Google Authenticator
     * @param string $secret The user's TOTP secret
     * @param string $username The username
     * @param string $issuer The application name
     * @return string QR code URL
     */
    public function getQRCodeUrl(string $secret, string $username, string $issuer = 'DBOIS'): string
    {
        $encodedIssuer = rawurlencode($issuer);
        $encodedUsername = rawurlencode($username);

        $otpauthUrl = "otpauth://totp/{$encodedIssuer}:{$encodedUsername}?secret={$secret}&issuer={$encodedIssuer}";

        // Using Google Charts API for QR code generation
        return "https://chart.googleapis.com/chart?chs=200x200&chld=M|0&cht=qr&chl=" . urlencode($otpauthUrl);
    }

    /**
     * Verify a TOTP code
     * @param string $secret The user's secret
     * @param string $code The code to verify
     * @param int $discrepancy Time window (default 1 = +/- 30 seconds)
     * @return bool True if code is valid
     */
    public function verifyCode(string $secret, string $code, int $discrepancy = 1): bool
    {
        $currentTimeSlice = floor(time() / $this->period);

        for ($i = -$discrepancy; $i <= $discrepancy; $i++) {
            $calculatedCode = $this->getCode($secret, (int)($currentTimeSlice + $i));
            if ($this->timingSafeEquals($calculatedCode, $code)) {
                return true;
            }
        }

        return false;
    }

    /**
     * Generate TOTP code for a given time slice
     * @param string $secret The secret key
     * @param int|null $timeSlice The time slice (defaults to current)
     * @return string The generated code
     */
    public function getCode(string $secret, ?int $timeSlice = null): string
    {
        if ($timeSlice === null) {
            $timeSlice = (int)floor(time() / $this->period);
        }

        $secretKey = $this->base32Decode($secret);

        // Pack time into binary string
        $time = chr(0) . chr(0) . chr(0) . chr(0) . pack('N*', $timeSlice);

        // Hash with secret
        $hash = hash_hmac('sha1', $time, $secretKey, true);

        // Use last 4 bits as offset
        $offset = ord(substr($hash, -1)) & 0x0F;

        // Get 4 bytes from offset
        $hashPart = substr($hash, $offset, 4);

        // Unpack binary value
        $value = unpack('N', $hashPart);
        $value = $value[1];

        // Only 32 bits
        $value = $value & 0x7FFFFFFF;

        // Get the required digits
        $modulo = pow(10, $this->codeLength);

        return str_pad($value % $modulo, $this->codeLength, '0', STR_PAD_LEFT);
    }

    /**
     * Decode Base32 string
     * @param string $secret Base32 encoded string
     * @return string Binary string
     */
    protected function base32Decode(string $secret): string
    {
        if (empty($secret)) {
            return '';
        }

        $base32chars = 'ABCDEFGHIJKLMNOPQRSTUVWXYZ234567';
        $base32charsFlipped = array_flip(str_split($base32chars));

        $paddingCharCount = substr_count($secret, '=');
        $allowedValues = [6, 4, 3, 1, 0];

        if (!in_array($paddingCharCount, $allowedValues)) {
            return '';
        }

        for ($i = 0; $i < 4; $i++) {
            if (
                $paddingCharCount == $allowedValues[$i] &&
                substr($secret, -($allowedValues[$i])) != str_repeat('=', $allowedValues[$i])
            ) {
                return '';
            }
        }

        $secret = str_replace('=', '', $secret);
        $secret = str_split($secret);
        $binaryString = '';

        for ($i = 0; $i < count($secret); $i = $i + 8) {
            $x = '';
            if (!isset($base32charsFlipped[$secret[$i]])) {
                return '';
            }

            for ($j = 0; $j < 8; $j++) {
                $char = $secret[$i + $j] ?? '';
                $charValue = $base32charsFlipped[$char] ?? 0;
                $x .= str_pad(base_convert((string)$charValue, 10, 2), 5, '0', STR_PAD_LEFT);
            }

            $eightBits = str_split($x, 8);

            for ($z = 0; $z < count($eightBits); $z++) {
                $y = chr((int)base_convert($eightBits[$z], 2, 10));
                $binaryString .= ($y || ord($y) == 48) ? $y : '';
            }
        }

        return $binaryString;
    }

    /**
     * Timing-safe string comparison
     * @param string $safeString Known string
     * @param string $userString User provided string
     * @return bool True if strings match
     */
    protected function timingSafeEquals(string $safeString, string $userString): bool
    {
        return hash_equals($safeString, $userString);
    }
}