const PREFIX = 'mhu1_';

export function encode(payload) {
  if (!payload || typeof payload !== 'object') {
    throw new Error('encode: payload must be an object');
  }
  const json = JSON.stringify(payload);
  const b64 = Buffer.from(json, 'utf8').toString('base64url');
  return PREFIX + b64;
}

export function decode(code) {
  if (typeof code !== 'string') {
    throw new Error('Invalid code: expected a string');
  }
  const trimmed = code.trim();
  if (!trimmed.startsWith(PREFIX)) {
    throw new Error('Invalid code: missing "mhu1_" prefix');
  }
  const b64 = trimmed.slice(PREFIX.length);
  if (!b64) {
    throw new Error('Invalid code: empty payload');
  }

  let json;
  try {
    json = Buffer.from(b64, 'base64url').toString('utf8');
  } catch {
    throw new Error('Invalid code: not valid base64url');
  }

  let payload;
  try {
    payload = JSON.parse(json);
  } catch {
    throw new Error('Invalid code: payload is not valid JSON');
  }

  if (!payload || typeof payload !== 'object') {
    throw new Error('Invalid code: payload is not an object');
  }
  if (payload.v !== 1) {
    throw new Error(`Invalid code: unsupported version (expected v=1, got ${payload.v})`);
  }
  if (typeof payload.port !== 'number' || !Number.isFinite(payload.port)) {
    throw new Error('Invalid code: "port" must be a number');
  }
  if (typeof payload.token !== 'string' || payload.token.length === 0) {
    throw new Error('Invalid code: "token" must be a non-empty string');
  }
  if (typeof payload.label !== 'string') {
    throw new Error('Invalid code: "label" must be a string');
  }
  if (payload.fp !== undefined && typeof payload.fp !== 'string') {
    throw new Error('Invalid code: "fp" must be a string when present');
  }

  return payload;
}