Fast popcount method.

fast_popcount.cc

typedef unsigned int uint;

uint popcount_short[0xffff];

void popcount_setup()
{
    popcount_short[0] = 0;
    for (uint mask = 1; mask < 0xffff; ++mask) {
        popcount_short[mask] = (mask&1) + popcount_short[mask>>1];
    }
}

inline uint popcount(uint x)
{
    return popcount_short[x&0xffff] + popcount_short[x>>16&0xffff];
}