// Décodeur LoRaWAN officiel : EM500-UDL - Ultrasonic Distance/Level Sensor (Milesight)
// Source : https://github.com/TheThingsNetwork/lorawan-devices/blob/master/vendor/milesight-iot/em500-udl.js
// The Things Network Device Repository, fichier publié par le constructeur, distribué sans modification.
// Format : decodeUplink(input) (TTN v3 / ChirpStack v4). Hébergé par smartbuildings.fr.

/**
 * Ursalink EM500-UDL Payload Decoder
 *
 * definition [channel-id] [channel-type] [channel-data]
 *
 * 01: battery      -> 0x01 0x75 [1 byte]  Unit: %
 * 03: distance     -> 0x03 0x82 [2 bytes] Unit: m
 * ------------------------------------------ EM500
 *
 * Example:
 *   base64: AXVaA4IeAA==
 *   bytes: 01 75 5A 03 82 1E 00
 *
 * After decode:
 * {
 *   "battery": 90,
 *   "distance": 30
 * }
 */
function Decoder(bytes, port) {
  var decoded = {};

  for (var i = 0; i < bytes.length;) {
    var channel_id = bytes[i++];
    var channel_type = bytes[i++];
    // BATTERY
    if (channel_id === 0x01 && channel_type === 0x75) {
      decoded.battery = bytes[i];
      i += 1;
    }
    // DISTANCE
    else if (channel_id === 0x03 && channel_type === 0x82) {
      decoded.distance = readInt16LE(bytes.slice(i, i + 2)) / 1000;
      i += 2;
    } else {
      break;
    }
  }

  return decoded;
}

/*******************************************
 * bytes to number
 ********************************************/
function readUInt16LE(bytes) {
  var value = (bytes[1] << 8) + bytes[0];
  return value & 0xffff;
}

function readInt16LE(bytes) {
  var ref = readUInt16LE(bytes);
  return ref > 0x7fff ? ref - 0x10000 : ref;
}