/**
 * CSRF Protection Module
 * Implements HMAC-signed double-submit cookie pattern for stateless CSRF protection.
 * The cookie contains a random nonce; the header must carry the HMAC signature
 * of that nonce computed with a server-side secret. An attacker who can inject
 * a cookie still cannot forge the matching header without the secret.
 */

const crypto = require('crypto');
const cryptoUtils = require('./crypto-utils');

const CSRF_TOKEN_LENGTH = 32;
const CSRF_COOKIE_NAME = 'dashcaddy_csrf';
const CSRF_HEADER_NAME = 'x-csrf-token';

/**
 * Generate a cryptographically secure CSRF nonce
 * @returns {string} Base64URL-encoded random nonce
 */
function generateToken() {
  return crypto.randomBytes(CSRF_TOKEN_LENGTH).toString('base64url');
}

/**
 * Compute HMAC signature for a CSRF nonce using the server-side encryption key
 * @param {string} nonce - The random nonce to sign
 * @returns {string} Base64URL-encoded HMAC signature
 */
function signToken(nonce) {
  const key = cryptoUtils.loadOrCreateKey();
  return crypto.createHmac('sha256', key).update(nonce).digest('base64url');
}

/**
 * Parse cookie header string into object
 * @param {string} cookieHeader - Cookie header value
 * @returns {Object} Parsed cookies
 */
function parseCookie(cookieHeader) {
  if (!cookieHeader) return {};

  return cookieHeader.split(';').reduce((cookies, cookie) => {
    const [name, ...rest] = cookie.trim().split('=');
    if (name && rest.length > 0) {
      cookies[name] = rest.join('=');
    }
    return cookies;
  }, {});
}

/**
 * Middleware to set CSRF cookie on all requests.
 * Always generates a fresh nonce server-side (never trusts client-supplied values).
 * The cookie holds the nonce; JavaScript must read it and send the HMAC signature
 * in the x-csrf-token header. The /api/csrf-token endpoint provides the signature.
 */
function csrfCookieMiddleware(req, res, next) {
  // Always generate a fresh server-side nonce
  const csrfNonce = generateToken();

  // Store nonce + signature on request so endpoints can access them
  req.csrfToken = signToken(csrfNonce);
  req.csrfNonce = csrfNonce;

  // Set cookie with the nonce (SameSite=Strict for additional protection)
  res.cookie(CSRF_COOKIE_NAME, csrfNonce, {
    httpOnly: false, // Must be readable by JavaScript for signing
    secure: req.secure || req.protocol === 'https', // Only secure in HTTPS
    sameSite: 'strict',
    path: '/',
    maxAge: 24 * 60 * 60 * 1000 // 24 hours
  });

  next();
}

/**
 * Middleware to validate CSRF token on state-changing requests
 * Validates that the token in the cookie matches the token in the header
 */
function csrfValidationMiddleware(req, res, next) {
  const method = req.method.toUpperCase();

  // Skip validation for safe methods
  if (['GET', 'HEAD', 'OPTIONS'].includes(method)) {
    return next();
  }

  // Skip CSRF validation in test environment
  if (process.env.NODE_ENV === 'test') {
    return next();
  }

  // Excluded paths that don't require CSRF validation
  const excludedPaths = [
    '/api/totp/verify',
    '/api/totp/verify-setup',
    '/health',
    '/api/health'
  ];

  // Check if path starts with excluded prefix
  const isExcluded = excludedPaths.some(path => req.path === path) ||
                     req.path.startsWith('/api/auth/gate/');

  if (isExcluded) {
    return next();
  }

  // Get nonce from cookie
  const cookies = parseCookie(req.headers.cookie);
  const cookieNonce = cookies[CSRF_COOKIE_NAME];

  // Get signed token from header (case-insensitive)
  const headerToken = req.headers[CSRF_HEADER_NAME] ||
                      req.headers[CSRF_HEADER_NAME.toLowerCase()];

  // Skip CSRF for API key-authenticated requests (API keys are not sent automatically by browsers)
  if (req.headers['x-api-key'] || (req.headers.authorization && req.headers.authorization.startsWith('Bearer '))) {
    return next();
  }

  // Validate both values exist
  if (!cookieNonce) {
    console.warn(`[CSRF] Missing CSRF cookie: ${method} ${req.path} from ${req.ip}`);
    return res.status(403).json({
      success: false,
      error: '[DC-100] CSRF token missing',
      message: 'CSRF cookie not found. Please refresh the page (Ctrl+Shift+R) and try again.'
    });
  }

  if (!headerToken) {
    console.warn(`[CSRF] Missing CSRF header: ${method} ${req.path} from ${req.ip}`);
    return res.status(403).json({
      success: false,
      error: '[DC-100] CSRF token missing',
      message: 'CSRF token not provided in request headers. Please refresh the page (Ctrl+Shift+R) and try again.'
    });
  }

  // Validate that the header token is the correct HMAC signature of the cookie nonce
  try {
    const expectedSig = signToken(cookieNonce);
    const expectedBuffer = Buffer.from(expectedSig, 'base64url');
    const headerBuffer = Buffer.from(headerToken, 'base64url');

    if (expectedBuffer.length !== headerBuffer.length) {
      throw new Error('Token length mismatch');
    }

    if (!crypto.timingSafeEqual(expectedBuffer, headerBuffer)) {
      throw new Error('Token mismatch');
    }

    // Signature valid — request is authentic
    next();

  } catch (err) {
    console.warn(`[CSRF] Invalid CSRF token: ${method} ${req.path} from ${req.ip} - ${err.message}`);
    return res.status(403).json({
      success: false,
      error: '[DC-101] CSRF token invalid',
      message: 'CSRF token validation failed. Please refresh the page (Ctrl+Shift+R) and try again.'
    });
  }
}

module.exports = {
  CSRF_TOKEN_LENGTH,
  CSRF_COOKIE_NAME,
  CSRF_HEADER_NAME,
  generateToken,
  signToken,
  parseCookie,
  csrfCookieMiddleware,
  csrfValidationMiddleware
};
