How can I store an integer in a nodejs Buffer?

Question

The nodejs Buffer is pretty swell. However, it seems to be geared towards storing strings. The constructors either take a string, an array of bytes, or a size of bytes to allocate.

I am using version 0.4.12 of Node.js, and I want to store an integer in a buffer. Not integer.toString(), but the actual bytes of the integer. Is there an easy way to do this without looping over the integer and doing some bit-twiddling? I could do that, but I feel like this is a problem someone else must have faced at some time.

Ulster

Answer 1

Since it's not builtin 0.4.12 you could use something like this:

var integer = 1000;
var length = Math.ceil((Math.log(integer)/Math.log(2))/8); // How much byte to store integer in the buffer
var buffer = new Buffer(length);
var arr = []; // Use to create the binary representation of the integer

while (integer > 0) {
    var temp = integer % 2;
    arr.push(temp);
    integer = Math.floor(integer/2);
}

console.log(arr);

var counter = 0;
var total = 0;

for (var i = 0,j = arr.length; i < j; i++) {
   if (counter % 8 == 0 && counter > 0) { // Do we have a byte full ?
       buffer[length - 1] = total;
       total = 0;
       counter = 0;
       length--;      
   }

   if (arr[i] == 1) { // bit is set
      total += Math.pow(2, counter);
   }
   counter++;
}

buffer[0] = total;

console.log(buffer);


/* OUTPUT :

racar $ node test_node2.js 
[ 0, 0, 0, 1, 0, 1, 1, 1, 1, 1 ]
<Buffer 03 e8>

*/

Caudad

Answer 2

var buf = new Buffer(4);
buf.writeUInt8(0x3, 0);

http://nodejs.org/docs/v0.6.0/api/buffers.html#buffer.writeUInt8

Bothnia

Note: these API have only recently been made available in the latest stable release of node (<4 days ago). If you're still stuck 0.4, you'll need to loop/bit-twiddle in order to encode integers. – Ical

0.5.x was never a stable release. I updated the question to indicate I am on 0.4.x, but have an upvote for an answer that will grow more useful in the future. – Ulster

Surely this will only write a single byte. What about a 32bit int? – Frisco

Answer 3

With more recent versions of Node this is much easier. Here's an example for a 2 byte unsigned integer:

let buf = Buffer.allocUnsafe(2);
buf.writeUInt16BE(1234);  // Big endian

Or for a 4 byte signed integer:

let buf = Buffer.allocUnsafe(4);  // Init buffer without writing all data to zeros
buf.writeInt32LE(-123456);  // Little endian this time..

The different writeInt functions were added in node v0.5.5.

Have a look at these docs for a better understanding:
Buffer
writeUInt16BE/LE
writeUIntBE/LE
allocUnsafe

Lance

Answer 4

This is something very efficient but uses some "bit twiddling"

// Filled 8 bits number
const fillByte = 0xff;
function bufferFromInt(num){
    const buffArr = [];
    do {
        buffArr.push(fillByte & num);
    } while(num >>= 8);
    return Buffer.from(buffArr.reverse())
}

Epictetus