Jason Gooner - 1 year ago 101
Javascript Question

# Number of bits to represent a number

I'm trying to write a function to return the number of bits a positive integer less that the Javascript limit of (2^53)-1 is. However im being hit by precision problems, and want to avoid big integer libraries.

Method 1:

``````function bitSize(num)
{
return Math.floor( Math.log(num) / Math.log(2) ) + 1;
}

Pass: bitSize( Math.pow(2, 16) -1 ) = 16
Pass: bitSize( Math.pow(2, 16) ) = 17
Fail (Should be 48): bitSize( Math.pow(2, 48) -1 ) = 49
Pass: bitSize( Math.pow(2, 48) ) = 49
``````

Method 2:

``````function bitSize(num)
{
var count = 0;
while(num > 0)
{
num = num >> 1;
count++;
}
return count;
}

Pass: bitSize( Math.pow(2, 16) -1 ) = 16
Pass: bitSize( Math.pow(2, 16) ) = 17
Fail (Should be 48): bitSize( Math.pow(2, 48) -1 ) = 1
Fail (Should be 49): bitSize( Math.pow(2, 48) ) = 1
``````

Both methods fail to precision issues I think.

Can anyone suggest an alternative method that will work for numbers between 0 -> 2^53-1

Thanks.

``````function bitSize(num) {
The `toString()` method of `Number` takes the radix as an optional argument.