Question

In my project I need to AND two binary array of size 40 bytes(320 bits) and then compute set bit count in C++. I found some algorithms to do this but I want to know what is the fastest way of implementing it in c++. I mean what c++ data type would be proper?(unsinged char*,unsigned int 32,u_int64,…). I know many algorithms are compatible with 32bit integer although my array size is 40 bytes .

what about the algorithms described in this link:
Fast Bit Counting Techniques which one is faster?

Is const type better or there is no difference?

Any help would be much appreciated.

Answer 1

Here’s a version that goes through the array with 4 bytes at once, requiring 10 iterations:

uint32_t *arr1_int = (uint32_t*) arr1;
uint32_t *arr2_int = (uint32_t*) arr2;
int i;
int bits_set = 0;

for (i = 0; i < 10; i++) {
    uint32_t v = arr1_int[i] & arr2_int[i];

    /* http://graphics.stanford.edu/~seander/bithacks.html#CountBitsSetParallel */
    v = v - ((v >> 1) & 0x55555555);                   
    v = (v & 0x33333333) + ((v >> 2) & 0x33333333);    
    bits_set += ((v + (v >> 4) & 0xF0F0F0F) * 0x1010101) >> 24;
}

You can do this a lot faster with a modern CPU, using compiler intrinsics. For example on a 64 bit CPU with Visual C++:

#include <intrin.h>

__int64 *arr1_int = (__int64*) arr1;
__int64 *arr2_int = (__int64*) arr2;
int bits_set = 0;

/* 40 / 8 bytes == 5 iterations */
bits_set += __popcnt64(*arr1_int++ & *arr2_int++);
bits_set += __popcnt64(*arr1_int++ & *arr2_int++);
bits_set += __popcnt64(*arr1_int++ & *arr2_int++);
bits_set += __popcnt64(*arr1_int++ & *arr2_int++);
bits_set += __popcnt64(*arr1_int++ & *arr2_int++);

But this is all with performance in mind, if you simply want some readable code that works definitely go with what Rob suggested.