15bit popcount from Hacker's Delight, p. 72

gistfile1.cpp

//Special for 15-bit values on 64bit processors
//with fast multiplication
//From Hacker's Delight, p. 72
inline uint32_t popcount15(uint32_t x)
{
    uint64_t y;
    y = x * 0x0002000400080010;
    y = y & 0x1111111111111111;
    y = y * 0x1111111111111111;
    y = y >> 60;
    return y;
}