/**
 * HOA LedgerIQ — Form abuse protection
 *
 * Layered defence for public form endpoints (ROI calculator, lead capture):
 *
 *   1. Cloudflare Turnstile  — real CAPTCHA, active when TURNSTILE_* keys are set.
 *   2. Honeypot field        — a hidden input humans never fill in.
 *   3. Signed form token     — proves the form was actually loaded, and enforces a
 *                              minimum fill time (bots submit instantly).
 *   4. Per-IP rate limiting  — caps bursts and daily volume from one source.
 *
 * Layers 2–4 need no configuration and work on their own; adding Turnstile keys
 * upgrades the protection to a full CAPTCHA challenge.
 */

'use strict';

const crypto = require('crypto');

// ── Config ───────────────────────────────────────────────
const TURNSTILE_SITE_KEY = process.env.TURNSTILE_SITE_KEY   || '';
const TURNSTILE_SECRET   = process.env.TURNSTILE_SECRET_KEY || '';
const TURNSTILE_VERIFY_URL = 'https://challenges.cloudflare.com/turnstile/v0/siteverify';

// If no explicit secret is configured, generate one per process. Tokens then stop
// validating across restarts — harmless, the client just fetches a fresh one.
const FORM_SECRET = process.env.FORM_TOKEN_SECRET || crypto.randomBytes(32).toString('hex');

const MIN_FILL_MS      = 4 * 1000;             // faster than this is not a human
const MAX_TOKEN_AGE_MS = 2 * 60 * 60 * 1000;   // tokens expire after 2 hours

// Sliding-window caps per IP: short burst window + daily ceiling.
const RATE_WINDOWS = [
  { windowMs: 10 * 60 * 1000,      max: 5,  label: '10 minutes' },
  { windowMs: 24 * 60 * 60 * 1000, max: 25, label: '24 hours'   },
];

const turnstileEnabled = Boolean(TURNSTILE_SITE_KEY && TURNSTILE_SECRET);

// ── Signed, single-use form tokens ───────────────────────
const usedTokens = new Map();   // token -> expiry ms
const hits       = new Map();   // ip -> [timestamps]

function sign(payload) {
  return crypto.createHmac('sha256', FORM_SECRET).update(payload).digest('hex').slice(0, 32);
}

function issueFormToken() {
  const payload = `${Date.now()}.${crypto.randomBytes(9).toString('base64url')}`;
  return `${payload}.${sign(payload)}`;
}

function verifyFormToken(token) {
  if (typeof token !== 'string' || token.length > 200) {
    return { ok: false, reason: 'missing_token' };
  }

  const parts = token.split('.');
  if (parts.length !== 3) return { ok: false, reason: 'bad_token' };

  const [tsRaw, nonce, sig] = parts;
  const expected = sign(`${tsRaw}.${nonce}`);
  if (sig.length !== expected.length ||
      !crypto.timingSafeEqual(Buffer.from(sig), Buffer.from(expected))) {
    return { ok: false, reason: 'bad_token' };
  }

  const issuedAt = Number(tsRaw);
  if (!Number.isFinite(issuedAt)) return { ok: false, reason: 'bad_token' };

  const age = Date.now() - issuedAt;
  if (age > MAX_TOKEN_AGE_MS || age < -60_000) return { ok: false, reason: 'expired_token' };
  if (age < MIN_FILL_MS)                        return { ok: false, reason: 'too_fast' };
  if (usedTokens.has(token))                    return { ok: false, reason: 'replayed_token' };

  usedTokens.set(token, Date.now() + MAX_TOKEN_AGE_MS);
  return { ok: true };
}

// ── Rate limiting ────────────────────────────────────────
function clientIp(req) {
  // Requires `app.set('trust proxy', ...)` when running behind nginx.
  return req.ip || req.socket?.remoteAddress || 'unknown';
}

/** Check the caps without consuming a slot. */
function checkRateLimit(ip) {
  const now  = Date.now();
  const list = hits.get(ip) || [];

  for (const { windowMs, max, label } of RATE_WINDOWS) {
    const recent = list.filter(t => now - t < windowMs).length;
    if (recent >= max) return { ok: false, reason: 'rate_limited', label };
  }
  return { ok: true };
}

/** Record a successful submission against the caller's IP. */
function recordSubmission(ip) {
  const now    = Date.now();
  const widest = Math.max(...RATE_WINDOWS.map(w => w.windowMs));
  const list   = (hits.get(ip) || []).filter(t => now - t < widest);
  list.push(now);
  hits.set(ip, list);
}

// ── Turnstile ────────────────────────────────────────────
async function verifyTurnstile(token, ip) {
  if (!turnstileEnabled) return { ok: true, skipped: true };
  if (typeof token !== 'string' || !token) return { ok: false, reason: 'captcha_missing' };

  try {
    const body = new URLSearchParams({ secret: TURNSTILE_SECRET, response: token });
    if (ip && ip !== 'unknown') body.set('remoteip', ip);

    const resp = await fetch(TURNSTILE_VERIFY_URL, {
      method:  'POST',
      headers: { 'Content-Type': 'application/x-www-form-urlencoded' },
      body,
      signal:  AbortSignal.timeout(8000),
    });
    const data = await resp.json();

    if (!data.success) {
      return { ok: false, reason: 'captcha_failed', codes: data['error-codes'] };
    }
    return { ok: true };
  } catch (err) {
    console.error('Turnstile verify error:', err.message);
    // Fail closed: an unverifiable challenge is not a passed challenge.
    return { ok: false, reason: 'captcha_unavailable' };
  }
}

// ── Combined guard ───────────────────────────────────────
const MESSAGES = {
  honeypot:            'Submission rejected.',
  missing_token:       'Your session expired. Please reload the page and try again.',
  bad_token:           'Your session expired. Please reload the page and try again.',
  expired_token:       'Your session expired. Please reload the page and try again.',
  replayed_token:      'This form was already submitted. Please reload the page to run another estimate.',
  too_fast:            'That was a little too quick — please take a moment and try again.',
  rate_limited:        'Too many submissions from this network. Please try again later.',
  captcha_missing:     'Please complete the verification challenge.',
  captcha_failed:      'Verification failed. Please try the challenge again.',
  captcha_unavailable: 'Verification is temporarily unavailable. Please try again in a moment.',
  email_required:      'Please enter your email address.',
  email_invalid:       'Please enter a valid email address.',
  email_too_long:      'That email address is too long.',
  email_blocked:       'Please use a valid work or personal email address.',
  homesites_too_low:   'Please enter the number of homesites in your community (minimum 10).',
};

const STATUS = { rate_limited: 429, honeypot: 400 };

/** User-facing message for a validation/abuse reason code. */
function messageFor(reason) {
  return MESSAGES[reason] || 'Submission rejected.';
}

// ── Email validation ─────────────────────────────────────
// Deliberately stricter than RFC 5322. The local part is limited to the
// characters real-world addresses actually use, which excludes every shell
// metacharacter (; | & ` $ ( ) < > \ " ' space) and every CSV-injection lead-in
// (= + @ at position 0). RFC-legal oddities like `foo!bar$baz@x.com` are
// rejected — an acceptable trade for a marketing form.
const EMAIL_RX = /^[A-Za-z0-9](?:[A-Za-z0-9._%+-]{0,62}[A-Za-z0-9])?@[A-Za-z0-9](?:[A-Za-z0-9-]{0,61}[A-Za-z0-9])?(?:\.[A-Za-z0-9](?:[A-Za-z0-9-]{0,61}[A-Za-z0-9])?)*\.[A-Za-z]{2,24}$/;

// C0/C1 control characters and DEL — includes the CR/LF used for header injection.
const CONTROL_CHARS_RX = /[\x00-\x1F\x7F-\x9F]/;

// Domains that are never a real lead. These are the IANA reserved documentation
// domains (RFC 2606) plus reserved TLDs, which is what spam bots reach for.
// A submitted domain is blocked when it equals one of these or is a subdomain
// of one (e.g. `mail.example.com`).
const BLOCKED_EMAIL_DOMAINS = new Set([
  'example.com', 'example.org', 'example.net', 'example.edu',
]);
const BLOCKED_EMAIL_TLDS = new Set(['test', 'example', 'invalid', 'localhost']);

function isBlockedDomain(domain) {
  const d = domain.toLowerCase();
  const tld = d.slice(d.lastIndexOf('.') + 1);
  if (BLOCKED_EMAIL_TLDS.has(tld)) return true;

  for (const blocked of BLOCKED_EMAIL_DOMAINS) {
    if (d === blocked || d.endsWith('.' + blocked)) return true;
  }
  return false;
}

/**
 * Validate an email address.
 * Returns { ok: true, email } with the normalised (trimmed, lower-cased) value,
 * or { ok: false, reason }.
 *
 * Pass { required: false } to accept an empty value (the calculator's email
 * field is optional) — an empty result comes back as { ok: true, email: null }.
 */
function validateEmail(raw, { required = true } = {}) {
  if (raw === undefined || raw === null || raw === '') {
    return required ? { ok: false, reason: 'email_required' } : { ok: true, email: null };
  }

  // Anything that isn't a plain string is a structured-injection attempt
  // (arrays and objects can survive into places a string wouldn't).
  if (typeof raw !== 'string') return { ok: false, reason: 'email_invalid' };

  const email = raw.trim();
  if (email === '') {
    return required ? { ok: false, reason: 'email_required' } : { ok: true, email: null };
  }

  // Length caps first — bounds every check that follows.
  if (email.length > 254) return { ok: false, reason: 'email_too_long' };

  // Control characters, including the newlines used for header injection.
  if (CONTROL_CHARS_RX.test(email)) return { ok: false, reason: 'email_invalid' };

  const at = email.indexOf('@');
  if (at < 1 || at !== email.lastIndexOf('@')) return { ok: false, reason: 'email_invalid' };

  const local  = email.slice(0, at);
  const domain = email.slice(at + 1);

  if (local.length > 64 || domain.length > 253) return { ok: false, reason: 'email_too_long' };
  if (email.includes('..'))                     return { ok: false, reason: 'email_invalid' };
  if (!EMAIL_RX.test(email))                    return { ok: false, reason: 'email_invalid' };
  if (isBlockedDomain(domain))                  return { ok: false, reason: 'email_blocked' };

  return { ok: true, email: email.toLowerCase() };
}

// ── Homesites validation ─────────────────────────────────
// Real associations have at least this many units; smaller values are the
// placeholder junk (0, 1, 2…) the spam submissions use.
const MIN_HOMESITES = 10;

/**
 * Validate a homesites count.
 * Returns { ok: true, homesites } (a finite number) or { ok: false, reason }.
 */
function validateHomesites(raw) {
  const n = typeof raw === 'number' ? raw : parseFloat(raw);
  if (!Number.isFinite(n) || n < MIN_HOMESITES) {
    return { ok: false, reason: 'homesites_too_low' };
  }
  return { ok: true, homesites: n };
}

/**
 * Run every protection layer for a public form POST.
 * Returns { ok: true, ip } or { ok: false, status, error, reason }.
 */
async function guardSubmission(req, { honeypotField = 'hp_company_url' } = {}) {
  const ip   = clientIp(req);
  const body = req.body ?? {};

  const fail = ({ reason, label }) => ({
    ok:     false,
    reason,
    status: STATUS[reason] ?? 403,
    error:  label ? `${MESSAGES[reason]} (limit: ${label})` : MESSAGES[reason],
  });

  // 1. Honeypot — any value at all means a bot filled every field it found.
  if (typeof body[honeypotField] === 'string' && body[honeypotField].trim() !== '') {
    console.warn(`[abuse] honeypot tripped from ${ip}`);
    return fail({ reason: 'honeypot' });
  }

  // 2. Rate limit (checked before the outbound Turnstile call).
  const rate = checkRateLimit(ip);
  if (!rate.ok) {
    console.warn(`[abuse] rate limit hit by ${ip}`);
    return fail(rate);
  }

  // 3. Signed single-use token + minimum fill time.
  const tok = verifyFormToken(body.formToken);
  if (!tok.ok) {
    console.warn(`[abuse] form token rejected (${tok.reason}) from ${ip}`);
    return fail(tok);
  }

  // 4. CAPTCHA.
  const captcha = await verifyTurnstile(body.captchaToken, ip);
  if (!captcha.ok) {
    console.warn(`[abuse] captcha rejected (${captcha.reason}) from ${ip}`);
    return fail(captcha);
  }

  recordSubmission(ip);
  return { ok: true, ip };
}

function publicConfig() {
  return { turnstileSiteKey: turnstileEnabled ? TURNSTILE_SITE_KEY : null };
}

// ── Housekeeping ─────────────────────────────────────────
const sweep = setInterval(() => {
  const now = Date.now();
  for (const [token, exp] of usedTokens) if (exp < now) usedTokens.delete(token);

  const widest = Math.max(...RATE_WINDOWS.map(w => w.windowMs));
  for (const [ip, list] of hits) {
    const kept = list.filter(t => now - t < widest);
    if (kept.length) hits.set(ip, kept); else hits.delete(ip);
  }
}, 10 * 60 * 1000);
sweep.unref?.();

module.exports = {
  turnstileEnabled,
  issueFormToken,
  guardSubmission,
  validateEmail,
  validateHomesites,
  MIN_HOMESITES,
  messageFor,
  publicConfig,
  clientIp,
};