Convert 4 bit byte array to int using JavaScript

Question

Using web sockets I send a 4 bit byte array to my JS library. This byte array represents a number and I want to read that number back but all in JavaScript. I have found plenty of examples doing the opposite and example of reading a byte array into a string.

In my C# code:

    public static void SendImage(byte[] blobArray,
        byte[] imageArray , string ts)
    {
        var blobSize = BitConverter.GetBytes(blobArray.Length);
        var tsHdr = Encoding.ASCII.GetBytes(ts);
        byte[] newPacket = new byte[imageArray.Length + tsHdr.Length + blobArray.Length+4];
        Buffer.BlockCopy(tsHdr, 0, newPacket, 0, tsHdr.Length);
        Buffer.BlockCopy(blobSize, 0, newPacket, 17, 4);
        Buffer.BlockCopy(blobArray, 0, newPacket, tsHdr.Length+4, blobArray.Length);
        Buffer.BlockCopy(imageArray, 0, newPacket, tsHdr.Length+4+ blobArray.Length, imageArray.Length);

     }

This pass a timestamp (taking 17 bits). Then 4 bits for the len of the 1st image. Then the image itself. Then the second image.

So, reading it all back in from JS:

ws.onmessage = function (e) {
        try {
                var ts = String.fromCharCode.apply(String, new Uint8Array(e.data, 0, 17));
                var year = ts.substr(0, 4);
                var month = ts.substr(4, 2);
                var day = ts.substr(6, 2);
                var hour = ts.substr(8, 2);
                var min = ts.substr(10, 2);
                var second = ts.substr(12, 2);
                var mil = ts.substr(14, 3);

                liveTimeStamp = hour + "-" + min + "-" + second + "-" + mil + "  " + day + "/" + month + "/" + year;
                var blobLen= e.data.slice(17, 4);
                vat img1 = 'data:image/jpeg;base64,' + arrayBufferToBase64(e.data.slice(21, blobLen ));
               var img2 = "data:image/jpeg;base64," + arrayBufferToBase64(e.data.slice(21 + blobLen, len - blobLen + 21)); 
    };

can you please provide the example of the input you are receiving? — Oleksandr Fedotov
– Oleksandr Fedotov, Commented Nov 11, 2018 at 20:32
The answer depends on the endianness of the byte encoding. And I assume by "4 bit" you mean "4-element." — cdhowie
– cdhowie, Commented Nov 11, 2018 at 20:32
Could you show some code? I'm not clear on what you mean by a 4 bit byte array. 4 bit bytes are... unusual... — Blunt Jackson
– Blunt Jackson, Commented Nov 11, 2018 at 20:33

Alnitak · Accepted Answer · 2018-11-11 20:50:23Z

3

Assuming that your C# code is generating little-endian data, you should be able to extract a Uint32 from your Uint8Array using the DataView type like this:

var data = new Uint8Array(e.data);
var dataView = new DataView(data.buffer);
var blobLen = dataView.getUint32(17, true); // false for big-endian

answered Nov 11, 2018 at 20:50

Alnitak

341k72 gold badges420 silver badges503 bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

Andrew Simpson Over a year ago

Thanks. This looks like it will do the job but will have to test tomorrow morning as beat. Thanks will update later :)

Collectives™ on Stack Overflow

Convert 4 bit byte array to int using JavaScript

1 Answer 1

1 Comment

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

1 Comment

Your Answer

Sign up or log in

Post as a guest

Related