'use strict';

const assert = require('assert');

/*  Agnav Tracking binary protocol
trk_pos_type = Record  {40 bytes}
 {Header}
 trk_Header1  : byte      {0xFB}
 trk_Header2  : byte      {0xFB}
 trk_DataID   : byte      {0xF7}
  trk_ACID     : LongInt;  {4 bytes signed Integer}
  trk_Time     : LongInt	// Number of seconds since 1970, 1, 1 to current UTC time
  trk_Lat      : float;    {4 byte floating point}
  trk_Lon      : float;    {4 byte floating point}
  trk_Alt      : integer;  {GPS alt, m, 2 byte signed integer}
  trk_SprayStat: byte;     {0: off, 1: on, unsigned char}
  trk_Speed    : word;     {dm/s, speed * 10 2 bytes signed integer}
  trk_Heading  : integer;  { 0 - 359}
  trk_Temp     : byte;     {C deg, offset by 100, unsigned char}
  trk_Humid    : byte;     {0 - 100%, unsigned char}
  trk_AppRate  : integer   {Application rate L/Ha * 10} 
  trk_Wspd     : integer;  {Windspeed * 10 dm/s}
  trk_Whdg     : integer;  {Wind heading, 0-359}
  trk_Res      : byte;     {reserved = 0, unsigned char}
  trk_Chksum   : word;     {Sum of all bytes start from FB}
  trk_End      : word;     {end of record, 0x0D 0x0A}
  End;          
 
** NOTES:
byte        : 1-byte unsigned integer
word        : 2-byte unsigned int
integer     : 2-byte signed int
LongInt     : 4-byte signed int
float       : 4-byte floating number
 */
class RAPParser {

  /**
  * Parse AgNav Tracking binary response package
  * @param {*} agnPkg The package data in Buffer. Structure: <Cmd><Data>, Cmd = 0xF7
  * @returns a GPS data object
  */
  parse(agnPkg) {
    assert.ok(Buffer.isBuffer(agnPkg), 'agnPkg argument must be an instance of Buffer'); // prettier-ignore
    assert.ok(agnPkg.length == 38, 'agnPkg must have 40 bytes'); // prettier-ignore

    let record, offset;

    record = {};
    offset = 5; // Skip the first Cmd byte (F7) + 4 bytes Aircraft Id

    record.gdt = new Date(agnPkg.readInt32LE(offset) * 1000); offset += 4;
    record.lat = agnPkg.readFloatLE(offset); offset += 4;
    record.lon = agnPkg.readFloatLE(offset); offset += 4;
    record.alt = agnPkg.readInt16LE(offset); offset += 2;
    record.inputs = agnPkg.readUInt8(offset); offset += 1;

    record.speed = agnPkg.readInt16LE(offset); offset += 2;
    if (record.speed > 0)
      record.speed = record.speed * 0.36;   // dm/s to km/h

    record.head = agnPkg.readInt16LE(offset); offset += 2;
    record.temp = agnPkg.readUInt8(offset) - 100; offset += 1;
    record.humid = agnPkg.readUInt8(offset); offset += 1;
    record.appRate = agnPkg.readInt16LE(offset) * 1e-1; offset += 2;
    record.windSpd = agnPkg.readInt16LE(offset); offset += 2;
    record.windHdg = agnPkg.readInt16LE(offset); offset += 2;

    agnPkg = null;
    return record;
  }
}

module.exports = RAPParser;