// Masks are only a concern for client writers.
  private val maskKey: ByteArray? = if (isClient) ByteArray(4) else null
  private val maskCursor: Buffer.UnsafeCursor? = if (isClient) Buffer.UnsafeCursor() else null

  /** Send a ping with the supplied [payload]. */
  @Throws(IOException::class)
  fun writePing(payload: ByteString) {
    writeControlFrame(OPCODE_CONTROL_PING, payload)
  }

  // Masks are only a concern for server writers.
  private val maskKey: ByteArray? = if (isClient) null else ByteArray(4)
  private val maskCursor: Buffer.UnsafeCursor? = if (isClient) null else Buffer.UnsafeCursor()

  interface FrameCallback {
    @Throws(IOException::class)
    fun onReadMessage(text: String)

    @Throws(IOException::class)
    fun onReadMessage(bytes: ByteString)

  /** Used when an empty close frame was received (i.e., without a status code). */
  internal const val CLOSE_NO_STATUS_CODE = 1005

  fun toggleMask(
    cursor: Buffer.UnsafeCursor,
    key: ByteArray,
  ) {
    var keyIndex = 0
    val keyLength = key.size
    do {
      val buffer = cursor.data
      var i = cursor.start
      val end = cursor.end