const SESSION_KEY_STORAGE_KEY = 'vncmail:session-encryption-key';
const ALGORITHM = 'AES-GCM';

let _available: boolean | null = null;

export function isEncryptionAvailable(): boolean {
  if (_available !== null) return _available;
  try {
    if (typeof window === 'undefined') { _available = false; return false; }
    if (!window.crypto || !window.crypto.subtle) { _available = false; return false; }
    _available = true;
    return true;
  } catch {
    _available = false;
    return false;
  }
}

function getOrCreateSessionKey(): Promise<CryptoKey | null> {
  if (!isEncryptionAvailable()) return Promise.resolve(null);
  try {
    let raw = sessionStorage.getItem(SESSION_KEY_STORAGE_KEY);
    if (!raw) {
      const keyBytes = new Uint8Array(32);
      crypto.getRandomValues(keyBytes);
      raw = btoa(String.fromCharCode(...keyBytes));
      sessionStorage.setItem(SESSION_KEY_STORAGE_KEY, raw);
    }
    const keyData = Uint8Array.from(atob(raw), (c) => c.charCodeAt(0));
    return crypto.subtle.importKey('raw', keyData, { name: ALGORITHM }, false, [
      'encrypt',
      'decrypt',
    ]);
  } catch {
    return Promise.resolve(null);
  }
}

let _cachedKey: CryptoKey | null | undefined;

async function getKey(): Promise<CryptoKey | null> {
  if (_cachedKey !== undefined) return _cachedKey;
  _cachedKey = await getOrCreateSessionKey();
  return _cachedKey;
}

function invalidateKey(): void {
  _cachedKey = undefined;
}

export async function encryptValue(plaintext: string): Promise<string> {
  if (!isEncryptionAvailable()) {
    console.warn('[localStorage crypto] Web Crypto unavailable, storing in plaintext');
    return plaintext;
  }

  const key = await getKey();
  if (!key) {
    console.warn('[localStorage crypto] Failed to derive key, storing in plaintext');
    return plaintext;
  }

  try {
    const iv = crypto.getRandomValues(new Uint8Array(12));
    const encoded = new TextEncoder().encode(plaintext);
    const ciphertext = await crypto.subtle.encrypt({ name: ALGORITHM, iv }, key, encoded);
    const combined = new Uint8Array(iv.length + new Uint8Array(ciphertext).length);
    combined.set(iv);
    combined.set(new Uint8Array(ciphertext), iv.length);
    return btoa(String.fromCharCode(...combined));
  } catch (err) {
    console.warn('[localStorage crypto] Encryption failed:', err);
    return plaintext;
  }
}

export async function decryptValue(ciphertext: string): Promise<string | null> {
  if (!isEncryptionAvailable()) {
    return ciphertext;
  }

  const key = await getKey();
  if (!key) {
    return ciphertext;
  }

  try {
    const combined = Uint8Array.from(atob(ciphertext), (c) => c.charCodeAt(0));
    if (combined.length < 13) return null;
    const iv = combined.slice(0, 12);
    const data = combined.slice(12);
    const decrypted = await crypto.subtle.decrypt({ name: ALGORITHM, iv }, key, data);
    return new TextDecoder().decode(decrypted);
  } catch {
    return null;
  }
}

export function resetSessionKey(): void {
  try {
    sessionStorage.removeItem(SESSION_KEY_STORAGE_KEY);
  } catch {
    /* noop */
  }
  invalidateKey();
}