Fastest way to count number of bit transitions in an unsigned int

Question

I\'m looking for the fastest way of counting the number of bit transitions in an unsigned int.

If the int contains: 0b0000000000000000000000000000101

Answer 1

Ok, with transitions you mean if you walk through the string of 0-s and 1-s, you count each occurance that a 0 follows a 1 or a 1 follows a 0.

This is easy by shifting bits out and counting the changes:

transitions(n)
  result = 0
  prev = n mod 2
  n = n div 2
  while n<>0 
    if n mod 2 <> prev then
      result++
      prev = n mod 2
    fi
    n = n div 2
  elihw
  return result

you can replace the mod and div with shifts.