export enum FrameType {
  /** Used to transmit data. May transmit zero length payloads depending on the flags. */
  Data = 0x0,
  /** Used to updated the senders receive window size. This is used to implement per-session flow control. */
  WindowUpdate = 0x1,
  /** Used to measure RTT. It can also be used to heart-beat and do keep-alive over TCP. */
  Ping = 0x2,
  /** Used to close a session. */
  GoAway = 0x3
}

export enum Flag {
  /** Signals the start of a new stream. May be sent with a data or window update message. Also sent with a ping to indicate outbound. */
  SYN = 0x1,
  /** Acknowledges the start of a new stream. May be sent with a data or window update message. Also sent with a ping to indicate response. */
  ACK = 0x2,
  /** Performs a half-close of a stream. May be sent with a data message or window update. */
  FIN = 0x4,
  /** Reset a stream immediately. May be sent with a data or window update message. */
  RST = 0x8
}

const flagCodes = Object.values(Flag).filter((x) => typeof x !== 'string') as Flag[]

export const YAMUX_VERSION = 0

export enum GoAwayCode {
  NormalTermination = 0x0,
  ProtocolError = 0x1,
  InternalError = 0x2
}

export const HEADER_LENGTH = 12

export interface FrameHeader {
  /**
   * The version field is used for future backward compatibility.
   * At the current time, the field is always set to 0, to indicate the initial version.
   */
  version?: number
  /** The type field is used to switch the frame message type. */
  type: FrameType
  /** The flags field is used to provide additional information related to the message type. */
  flag: number
  /**
   * The StreamID field is used to identify the logical stream the frame is addressing.
   * The client side should use odd ID's, and the server even.
   * This prevents any collisions. Additionally, the 0 ID is reserved to represent the session.
   */
  streamID: number
  /**
   * The meaning of the length field depends on the message type:
   * Data - provides the length of bytes following the header
   * Window update - provides a delta update to the window size
   * Ping - Contains an opaque value, echoed back
   * Go Away - Contains an error code
   */
  length: number
}

export function stringifyHeader (header: FrameHeader): string {
  const flags = flagCodes.filter(f => (header.flag & f) === f).map(f => Flag[f]).join('|')
  return `streamID=${header.streamID} type=${FrameType[header.type]} flag=${flags} length=${header.length}`
}