// Authentication for the admin panel — no external crypto deps, all from node:crypto.
//
//  • Passwords are stored as scrypt hashes ("scrypt$salt$hash", both base64url) in
//    the ADMIN_PASS_HASH env var. Never a plaintext password anywhere.
//  • Sessions are stateless, signed cookies: base64url(payload).base64url(hmac).
//    Tamper-proof (HMAC-SHA256 with SESSION_SECRET) and self-expiring. No server
//    session store to leak or lose on restart.
//  • Every compare is constant-time (timingSafeEqual) to avoid timing oracles.

import crypto from "node:crypto";

const SCRYPT = { N: 16384, r: 8, p: 1, keylen: 32 };

/** Hash a plaintext password → "scrypt$<salt>$<hash>" (for storing in .env). */
export function hashPassword(password) {
  const salt = crypto.randomBytes(16);
  const hash = crypto.scryptSync(password, salt, SCRYPT.keylen, SCRYPT);
  return `scrypt$${salt.toString("base64url")}$${hash.toString("base64url")}`;
}

/** Verify a plaintext password against a stored "scrypt$salt$hash" string. */
export function verifyPassword(password, stored) {
  if (typeof stored !== "string") return false;
  const parts = stored.split("$");
  if (parts.length !== 3 || parts[0] !== "scrypt") return false;
  let salt, expected;
  try {
    salt = Buffer.from(parts[1], "base64url");
    expected = Buffer.from(parts[2], "base64url");
  } catch {
    return false;
  }
  if (expected.length !== SCRYPT.keylen) return false;
  const got = crypto.scryptSync(password, salt, SCRYPT.keylen, SCRYPT);
  return crypto.timingSafeEqual(got, expected);
}

/** Constant-time string compare (for the username). */
export function safeEqual(a, b) {
  const ba = Buffer.from(String(a));
  const bb = Buffer.from(String(b));
  if (ba.length !== bb.length) return false;
  return crypto.timingSafeEqual(ba, bb);
}

// ---- signed session cookies -------------------------------------------------

const COOKIE_NAME = "exd_admin";

function sign(data, secret) {
  return crypto.createHmac("sha256", secret).update(data).digest("base64url");
}

/** Build a signed session token that expires after `ttlMs`. */
export function issueToken(username, secret, ttlMs) {
  const payload = JSON.stringify({ u: username, exp: Date.now() + ttlMs });
  const body = Buffer.from(payload).toString("base64url");
  return `${body}.${sign(body, secret)}`;
}

/** Verify a session token → { u } if valid & unexpired, else null. */
export function verifyToken(token, secret) {
  if (typeof token !== "string" || !token.includes(".")) return null;
  const [body, mac] = token.split(".");
  if (!body || !mac) return null;
  const expected = sign(body, secret);
  const a = Buffer.from(mac);
  const b = Buffer.from(expected);
  if (a.length !== b.length || !crypto.timingSafeEqual(a, b)) return null;
  let payload;
  try {
    payload = JSON.parse(Buffer.from(body, "base64url").toString("utf8"));
  } catch {
    return null;
  }
  if (!payload || typeof payload.exp !== "number" || Date.now() > payload.exp) return null;
  return { u: payload.u };
}

/** Parse the admin session cookie out of a raw Cookie header. */
export function readSessionCookie(req) {
  const raw = req.headers.cookie || "";
  for (const part of raw.split(";")) {
    const i = part.indexOf("=");
    if (i === -1) continue;
    if (part.slice(0, i).trim() === COOKIE_NAME) {
      return decodeURIComponent(part.slice(i + 1).trim());
    }
  }
  return null;
}

export function setSessionCookie(res, token, { secure, ttlMs }) {
  const attrs = [
    `${COOKIE_NAME}=${encodeURIComponent(token)}`,
    "HttpOnly",
    "SameSite=Strict",
    "Path=/",
    `Max-Age=${Math.floor(ttlMs / 1000)}`,
  ];
  if (secure) attrs.push("Secure");
  res.append("Set-Cookie", attrs.join("; "));
}

export function clearSessionCookie(res, { secure }) {
  const attrs = [`${COOKIE_NAME}=`, "HttpOnly", "SameSite=Strict", "Path=/", "Max-Age=0"];
  if (secure) attrs.push("Secure");
  res.append("Set-Cookie", attrs.join("; "));
}

/** Express middleware: allow through only requests with a valid session. */
export function requireAuth(secret) {
  return (req, res, next) => {
    const tok = readSessionCookie(req);
    const sess = tok && verifyToken(tok, secret);
    if (!sess) {
      // req.path is stripped of the mount prefix, so match on originalUrl
      const url = req.originalUrl || req.url || "";
      if (url.startsWith("/admin/api") || url.endsWith(".csv")) {
        return res.status(401).json({ error: "unauthorized" });
      }
      return res.redirect("/admin/login");
    }
    req.admin = sess;
    next();
  };
}

export { COOKIE_NAME };