function signed(val, bits) {
    // max positive value possible for signed int with bits:
    var mx = Math.pow(2, bits-1);
    if (val < mx) {
        // is positive value, just return
        return val;
    } else {
        // is negative value, convert to neg:
        return val - (2 * mx);
    }
}

function int16_BE(bytes, idx) {
    bytes = bytes.slice(idx || 0);
    return signed(bytes[0] << 8 | bytes[1] << 0, 2*8);
}

function uint16_BE(bytes, idx) {
    bytes = bytes.slice(idx || 0);
    return bytes[0] << 8 | bytes[1] << 0;
}


/**
 * TTN decoder function.
 */
function Decoder(bytes, port) {
    var vals = {};
    if( port == 20 ){
		// make sure that unit is mH2O and precision is 3 decimals
        if( (uint16_BE(bytes, 1)==7) && (uint16_BE(bytes, 3)==3)){
          if( int16_BE(bytes, 5)/10 < 0){
             vals["mH2O"] = "Err" // Most propably not under water
          }
          else{
            vals["mH2O"] = int16_BE(bytes, 5)/1000;
          }
        }
        else{
          vals["mH2O"] = "Invalid configuration";
        }
    }  
    return vals;
}

/**
 * TTN V3 Wrapper
 */
function decodeUplink(input) {
   return {
    data: {
      values: Decoder(input.bytes, input.fPort)
    },
    warnings: [],
    errors: []
  };
}


/**
 * LoRaServer decoder function.
 */
function Decode(fPort, bytes) {
    // wrap TTN Decoder:
    return Decoder(bytes, fPort);
}

/**
 * Lobaro Platform decoder function.
 */
function Parse(input) {
    var data = bytes(atob(input.data));
    var port = input.fPort;
   
    return Decoder(data, port);
}