Question

I was reading this answer and it is mentioned that this code;

if (data[c] >= 128)
    sum += data[c];

can be replaced with this one;

int t = (data[c] - 128) >> 31;
sum += ~t & data[c];

I am having hard time grasping this. Can someone explain how bitwise operators achieve what if statement does?

Answer 1

if (data[c] >= 128)
    sum += data[c];

Clearly adds data[c] to sum if and only if data[c] is greater or equal than 128. It’s easy to show that

int t = (data[c] - 128) >> 31;
sum += ~t & data[c];

Is equivalent (when data only holds positive values, which it does):

data[c] - 128 is positive if and only if data[c] is greater or equal than 128. Shifted arithmetically right by 31, it becomes either all ones (if it was smaller than 128) or all zeros (if it was greater or equal to 128).

The second line then adds to sum either 0 & data[c] (so zero) in the case that data[c] < 128 or 0xFFFFFFFF & data[c] (so data[c]) in the case that data[c] >= 128.