Legacy Document

Inside Macintosh: QuickDraw GX Environment and Utilities /

Chapter 8 - QuickDraw GX Mathematics / Using QuickDraw GX Mathematics

Analyzing the Bits in a Number

You can use the FirstBit function to determine the highest bit number that is set in a 32-bit number. The following examples demonstrate the use of this function with the parameter x:

If x is 1, the highest order bit that is set is bit number 0,
so FirstBit(1) = 0, as shown below.

FirstBit(0000000000000000000000000000001) = 0x0000

If x is 2, the highest order bit that is set is bit number 1, so FirstBit(2) = 1, as shown below.

FirstBit(0000000000000000000000000000010) = 0x0001

If x is 3, the highest order bit that is set is bit 1, so FirstBit(3) = 1, as shown below.

FirstBit(0000000000000000000000000000011) = 0x0001

If no bits in the number are set, FirstBit returns a value of -1.

You can also use FirstBit to find the last (= lowest-order) bit that is set in a number. Listing 8-9 is an example of such a function.

Listing 8-9 Determining the lowest bit of a number

short LastBit(unsigned long x)
{
   if (x == 0)
      return 32;
   return FirstBit(x & -x);
}

The FirstBit function is described on page 8-62.