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

int numTransitions(int a)
{
  int b = a >> 1; // sign-extending shift properly counts bits at the ends
  int c = a ^ b;  // xor marks bits that are not the same as their neighbors on the left
  return CountBits(c); // count number of set bits in c
}

For an efficient implementation of CountBits see http://graphics.stanford.edu/~seander/bithacks.html#CountBitsSetParallel