Question

I am trying to perform some bitshift operations and dealing with binary numbers in JavaScript.

Here’s what I’m trying to do. A user inputs a value and I do the following with it:

// Square Input and mod with 65536 to keep it below that value
var squaredInput = (inputVal * inputVal) % 65536;
// Figure out how many bits is the squared input number
var bits = Math.floor(Math.log(squaredInput) / Math.log(2)) + 1;
// Convert that number to a 16-bit number using bitshift.
var squaredShifted = squaredInput >>> (16 - bits);

As long as the number is larger than 46, it works. Once it is less than 46, it does not work.
I know the problem is the in bitshift. Now coming from a C background, I know this would be done differently, since all numbers will be stored in 32-bit format (given it is an int). Does JavaScript do the same (since it vars are not typed)?

If so, is it possible to store a 16-bit number? If not, can I treat it as 32-bits and do the required calculations to assume it is 16-bits?

Note: I am trying to extract the middle 4-bits of the 16-bit value in squaredInput.

Another note: When printing out the var, it just prints out the value without the padding so I couldn’t figure it out. Tried using parseInt and toString.

Thanks

Answer 1

Are you looking for this?

function get16bitnumber( inputVal ){
   return ("0000000000000000"+(inputVal * inputVal).toString(2)).substr(-16);
}

This function returns last 16 bits of (inputVal*inputVal) value.By having binary string you could work with any range of bits.