Question

I have a function that receives a power of two value.

I need to convert it to an enum range (0, 1, 2, 3, and so on), and then shift it back to the power of two range.

 0         1
 1         2
 2         4
 3         8
 4        16
 5        32
 6        64
 7       128
 8       256
 9       512
10      1024
... and so on.

If my function receives a value of 1024, I need to convert it to 10. What is the best way to do this in C#? Should I just keep dividing by 2 in a loop and count the iterations?

I know I can put it back with (1 << 10).

Answer 1

Just use the logarithm of base 2:

Math.Log(/* your number */, 2)

For example, Math.Log(1024, 2) returns 10.

Update:

Here’s a rather robust version that checks if the number passed in is a power of two:

public static int Log2(uint number)
{
  var isPowerOfTwo = number > 0 && (number & (number - 1)) == 0;
  if (!isPowerOfTwo)
  {
    throw new ArgumentException("Not a power of two", "number");
  }

  return (int)Math.Log(number, 2);
}

The check for number being a power of two is taken from http://graphics.stanford.edu/~seander/bithacks.html#DetermineIfPowerOf2

There are more tricks to find log2 of an integer on that page, starting here:
http://graphics.stanford.edu/~seander/bithacks.html#IntegerLogObvious