Uplink Payload Formatter for TTN Mapper LoRa node (including generic function for converting raw C struct integer values to JavaScript number)

ttn_mapper_uplink_payload_formatter.js

// convert raw data to integer
function getVar(inputBytes, offset, varLen, isSigned = false) {
  let outVal = 0;
  for (let i = 0; i < varLen; i++) {
    outVal |= inputBytes[offset + i] << i * 8;
  }
  
  if (isSigned) { // parse as signed value
    if (inputBytes[offset + varLen - 1] & 0x80) { // sign bit is set
      outVal = ~(outVal) + 1; // calculate two's complement
    }
  }
  
  return outVal;
}

function decodeUplink(input) {
  const ib = input.bytes;
  let decodedPayload = {
    latitude: getVar(ib, 0, 3) / 0xFFFFFF * 180 - 90,
    longitude: getVar(ib, 3, 3) / 0xFFFFFF * 360 - 180,
    altitude: getVar(ib, 6, 2, true) / 10,
    hdop: getVar(ib, 8, 1) / 2.5,
  };
  return {
    data: decodedPayload,
    warnings: [
      (decodedPayload.altitude < -3270) ? "No inital GPS fix!" : null
      ],
    errors: []
  };
}