Question

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

If the int contains: 0b00000000000000000000000000001010

The number of transitions are: 4

If the int contains: 0b00000000000000000000000000001001

The number of transitions are: 3

Language is C.

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