16-bit binary arithmetic in Javascript

Question

Javascript has only one number type: a 64-bit floating point.

Using Javascript, I need to implement a hashing algorithm which is designed to be written in C with 16 bit unsigned integers.

The main operation is something like this (pseudocode):

uint16 n = 0;
string s = "abcd1234";

for (int i = 0; i < s.length; i += 1) {
    n ^= (n << 2) + (n >> 3) + s[i];
}

return n;

Of course, this produces one result when using uint16 values and a different result if n is a 64-bit floating point.

My best solution to the problem so far is to convert the results of each bitwise operation to <= 16 bits, using a function like this (javascript):

function uint16 (n) {
    return parseInt(n.toString(2).slice(-16), 2);
}

and performing the operation something like this (javascript):

for (var i = 0; i < s.length; i +=1 ) {
    n ^= uint16(uint16(n << 2) + uint16(n >>> 3) + s.charCodeAt(i));
}

but I'm not 100% confident this will always produce the correct results.

Is there any standard way to emulate 16-bit unsigned bitwise operations on a number value in Javascript?

Answer 1

You can use bitwise AND.

It operates on 32 bit integers, but you can just "and" with 0xffff.

function uint16 (n) {
  return n & 0xFFFF;
}

Additionally, bit shift operations (<<, >>>) also operate on 32 bit integers, so you only really need to call the uint16 function before assignment:

for (var i = 0; i < s.length; i +=1 ) {
  n ^= uint16((n << 2) + (n >>> 3) + s.charCodeAt(i));
}

Answer 2

I suspect @Amit's version will execute faster but this is a workaround I have had success with:

//create a typed container, we'll work only in the first index.
const idx = 0;
const n = new Uint16Array(1);

for (int i = 0; i < s.length; i += 1) {
    n[idx] ^= (n[idx] << 2) + (n[idx] >> 3) + s[i];
}