conversion of Byte array to 320 bits and then bit extraction 16 bits then 10 bits

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i am hoping some clever people here would be able to help me , i am new to JavaScript and trying to figure out how to code the following as i get stuck when the payload string is not standard 8 bit 1 byte groups, so i receive 320 bit 40 byte array. the first few values are fine to extract as they are 16 bits , then 8 bits then 32 bits of info but the next value is only 10 bits and i get stuck here. so i should probably be converting the 40 bytes into 320 bits and then shifting right for each number of bit per variable? i am not sure how to do this. This is my payload format and in brackets is the number of bits for that variable. any help would be appreciated.

https://www.iotsoundsensor.com/wp-content/uploads/2021/11/310_payload_parser_manual.html

function Decoder(bytes, port) {
  //8 bit single byte values
  var messageinfo = bytes[2]>>[2];
  
  var battery   = bytes[2]>>[0];
  var latitude  = bytes[4]>>[4];
  var longatude = bytes[4]>>[4];
  
  //now have 10 bit values to decode
  
  var LAfast    = bytes[15]>>>[0];
  var LAslow    = bytes[16]>>>[2];
  
  return {
    messageinfo: messageinfo,
    battery: battery/10, 
    LAfast : LAfast/10,
    LAslow : LAslow/10,
    latitude: latitude,
    longatude: longatude
    
  }
}
arrays javascript
1 Answers

I've re-written the Decode() function based on the JavaScript code at iot-source (which is really the bible for decoding these messages).

I've created a getField() function for decoding fields at a given offset, length, and resolution and also a getBits() function to retrieve an array of bits for later decoding (used for the LAxxx fields).

I've used the example from the page above as an input to Decode():

function hexToByteArray(hex) {
    return Array.from({ length: hex.length/2}, (v,n) => parseInt(hex.slice(n * 2, n*2 + 2), 16));
}

function getField(offset, length, bytes, resolution = 1) {
    return bytesToNum(bytes.slice(offset, offset + length)) * resolution;
}

function bytesToNum(bytes) {    
    return bytes.reduce((acc, b, x) => acc | (b << (x * 8)), 0);
}

// Convert a byte segment to bits
function getBits(offset, length, bytes) {
    return bytes.slice(offset, offset + length).map((byt) => { 
        return Array.from({ length: 8 }).map((x, i) => {
            return ((byt * (2 ** i) & 0x80) !== 0 ? 1 : 0);
        });
    }, []).flat();
}

function parseBits(bits) {
    return bits.reduce((acc, b, i) => {
        return acc + 0.1*(bits[bits.length - 1 - i] ? 2**(i): 0); 
    }, 30);
}

function Decoder(bytes, port) {
  let offset = 0;
  let messageinfo = getField(offset, 2, bytes, 1);
  
  let battery   = getField(offset += 2, 1, bytes, 0.1);
  // NB: We should check messageinfo to ensure these are actually present...
  let latitude  = getField(offset += 1, 4, bytes, 1e-7);
  let longitude = getField(offset += 4, 4, bytes, 1e-7);
  
  // Skip parsing timestamp for now...
  offset += 4

  let bitArray = getBits(offset += 4, 20, bytes);
    
  let bitIndex = 0;
  let LAfast = parseBits(bitArray.slice(bitIndex, bitIndex += 10));
  let LAslow = parseBits(bitArray.slice(bitIndex, bitIndex += 10));
  let LCfast = parseBits(bitArray.slice(bitIndex, bitIndex += 10));
  let LCslow = parseBits(bitArray.slice(bitIndex, bitIndex += 10));

  return {
    messageinfo,
    battery, 
    LAfast,
    LAslow,
    LCfast,
    LCslow,
    latitude,
    longitude
  }
}

let bytes = (hexToByteArray('7FFF3F6599341FE38C11036A608C60370DA561584699336CDB55CDB390F44BD4B5B9D5390E34BCE3'))
console.log(Decoder(bytes))
.as-console-wrapper { max-height: 100% !important; top: 0; }

user_72372240
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