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) {
	return Parse(bytes);
}


/**
 * LoRaServer decoder function.
 */
function Decode(fPort, bytes) {
    // wrap TTN Decoder:
    return Decoder(bytes, fPort);
}
function Parse(input) {
    var data = bytes(atob(input.data));
    var port = input.fPort;
    var fcnt = input.fCnt;
    var vals = {};
    if( port == 20 ){
        if( (uint16_BE(data, 1)==7) && (uint16_BE(data, 3)==3)){
          if( int16_BE(data, 5)/10 < 0){
             vals["mH2O"] = "Err" // Most propably not under water
          }
          vals["mH2O"] = int16_BE(data, 5)/1000;
        }
        else{
          vals["mH2O"] = "Invalid configuration";
        }
    }
    
    vals["port"] = port;
    vals["data"] = data;
    vals["fcnt"] = fcnt;
    var lastFcnt = Device.getProperty("lastFcnt");
    vals["reset"] = fcnt <= lastFcnt;
    Device.setProperty("lastFcnt", fcnt);
    return vals;
}