export const ADDR = {
  CTRL: 0xdb96,
  OVEN: 0xaa55,
};

export const FUNC = {
  // used by oven
  O_CONNECT: 0x01,
  O_TEMPS: 0x02,
  O_READ: 0x03,
  O_ACK_WRITE: 0x06,

  // used by CTRL
  C_PWM_HEAT: 0x08,
  C_PWM_COOL: 0x0d,
  C_STARTSTOP: 0x10,
  C_REQ_READ: 0x11,
  C_WRITE: 0x20,
  C_ACK_CONNECT: 0xf1,
  C_ACK_READ: 0xf2,
};

export const ARG = {
  // for STARSTOP, REQUEST_READ
  STARTSTOP_START: 0x11,
  STARTSTOP_STOP: 0x22,
  REQUEST_READ: 0x11,

  // for CONNECT / ACK_CONNECT
  CONNECT_CONNECTING: 0x0100,
  CONNECT_CONNECTED: 0x0200,

  // for *_PWM
  PWM_ON: 0x44,
  PWM_OFF: 0x88,
};

const lookup = (tbl, val) => Object.keys(tbl).find(key => tbl[key] === val); 

export class Message {
  constructor(buffer) {
    this.buf = buffer;
    this.view = new DataView(this.buf);
    this.data = new DataView(this.buf, 4, this.buf.byteLength - 6);
  }

  get16(i) { return this.data.getUint16(i, false); }
  set16(i, v) { this.data.setUint16(i, v, false); }

  get address() { return this.view.getUint16(0, false); }
  set address(val) { this.view.setUint16(0, val, false); }

  get func() { return this.view.getUint8(2); }
  set func(val) { this.view.setUint8(2, val)}

  get data_len() { return this.view.getUint8(3); }
  set data_len(val) { this.view.setUint8(3, val); }

  get length() { return this.buf.byteLength; }

  get checksum() { return this.view.getUint16(this.buf.byteLength-2, false); }
  set checksum(val) { this.view.setUint16(this.buf.byteLength-2, val, false); }

  calculateSum() {
    let sum = 0;
    for (let i = 0; i < this.length - 2; i++) {
      sum += this.view.getUint8(i);
    }
    return sum & 0xffff;
  }

  getDataArray() {
    return new Uint8Array(this.buf, 4, this.buf.byteLength - 6);
  }

  toString() {
    const address = lookup(ADDR, this.address) ?? "????";
    const func = lookup(FUNC, this.func) ?? "?_??";
    const data = this.getDataArray();
    return `to ${address}: ${func.substr(2)} [${data}]`;
  }

  static from8(addr, func, data) {
    const len = 2 + 2 + data.length + 2;

    const msg = new Message(new ArrayBuffer(len));
    msg.address = addr;
    msg.func = func;
    msg.data_len = data.length;

    for (let i = 0; i < data.length; i++) {
      msg.data.setUint8(i, data[i]);
    }

    msg.checksum = msg.calculateSum();
    return msg;
  }

  static from16(addr, func, data) {
    const len = 2 + 2 + 2*data.length + 2;

    const msg = new Message(new ArrayBuffer(len));
    msg.address = addr;
    msg.func = func;
    msg.data_len = 2*data.length;

    for (let i = 0; i < data.length; i++) {
      msg.data.setUint16(2*i, data[i], false);
    }

    msg.checksum = msg.calculateSum();
    return msg;
  }

  static empty(addr, func, data_len) {
    const len = 2 + 2 + 2*data_len + 2;

    const msg = new Message(new ArrayBuffer(len));
    msg.address = addr;
    msg.func = func;
    msg.data_len = data_len;

    return msg;
  }

  static tryParse(buffer) {
    if (buffer.length < 4 + 2) return;

    const data_len = buffer[3];
    if (buffer.length < 4 + data_len + 2) return;

    const msg = new Message(buffer.slice(0, 4 + data_len + 2).buffer);
    if (msg.calculateSum() !== msg.checksum) {
      throw new Error(`checksum mismatch: expected ${hex(msg.checksum)} but calculated ${hex(msg.calculateSum())}`);
    }
    return msg;
  };
}