Bytearray() and structures for floating point values

[smbunn]

I am getting to grips with my FiPy and PySense. Just started sending LoRa packages and used the following code to send the battery voltage.

vt = py.read_battery_voltage()
    print("Battery voltage: " + str(py.read_battery_voltage()))
    data = bytearray(2)
    data = bytearray(struct.pack("f", vt))
    s.send(data)  # send buffer to TTN

The voltage was 4.420753 and the byte array I received was 61 d2 8c 40
I used javascript to decode this at the other end using a standard library function but it says the value is 8.1085 e -315

If I use https://gregstoll.dyndns.org/~gregstoll/floattohex/ as a resource to convert from float to hex is comes out as 40 8d 76 cf so the reverse of my number. If i swap endianness it converts 40 8d 76 cf as 4.42075 so that seems to be the problem.

What is the JavaScript I should use to unpack my payload to read the correct value?

thanks

[livius]

@smbunn
maybe this one
https://stackoverflow.com/questions/29879009/how-to-convert-python-struct-unpack-to-java

[robert-hh]

@smbunn given that you transmitted 4 bytes, you could as well send a 4 byte ascii string, tellung the value in millivolt and still being precise enough for the purpose.

[smbunn]

@robert-hh
True but this is a general question and some of my values will be over vastly different ranges. So it is really a question on float to 4 byte array back to float. I am working on the unpacking now guided by https://stackoverflow.com/questions/29879009/how-to-convert-python-struct-unpack-to-java and will let everyone know how I get on.
From my limited understanding it seems that Python has a great pack and unpack function but JavaScript doesn't so you have to write your own.

[smbunn]

False path. The hyperlinked example is in Java. Almost none of the structures used exist in JavaScript which does not support ByteBuffer as a type. So after an hour I have realized I am wasting my time trying to patch something together.

Anyone else got any ideas? It seems like it should be easy :-)

[livius]

@smbunn
look then maybe here
https://github.com/ronniebasak/JStruct
possibly here if above not work
http://www.jython.org/javadoc/org/python/modules/struct.html

[smbunn]

I found an answer. The function in Python by default packs the four bytes in little endian format. JavaScript expects big endian. By merely changing
data = bytearray(struct.pack("f", vt))

to

data = bytearray(struct.pack(">f", vt))

the unpack at the other end was simple.

Java Script

function decodeFloat(data1) {
    var binary = parseInt(data1, 16).toString(2);
    if (binary.length < 32) 
        binary = ('00000000000000000000000000000000'+binary).substr(binary.length);
    var sign = (binary.charAt(0) == '1')?-1:1;
    var exponent = parseInt(binary.substr(1, 8), 2) - 127;
    var significandBase = binary.substr(9);
    var significandBin = '1'+significandBase;
    var i = 0;
    var val = 1;
    var significand = 0;

    if (exponent == -127) {
        if (significandBase.indexOf('1') == -1)
            return 0;
        else {
            exponent = -126;
            significandBin = '0'+significandBase;
        }
    }

    while (i < significandBin.length) {
        significand += val * parseInt(significandBin.charAt(i));
        val = val / 2;
        i++;
    }

    return sign * significand * Math.pow(2, exponent);
}


// example use
data = '408df220' // is floating point value 4.43580627441406255
value = decodeFloat(data);
log.console('Value=', value);

The code above is from https://stackoverflow.com/questions/4414077/read-write-bytes-of-float-in-js

From a great website application at https://gregstoll.dyndns.org/~gregstoll/floattohex/ I produced the image below