---
tags: codebook
---

{%hackmd theme-dark %}

# popcount function

int lookup_popcnt(unsigned int n){
# define BIT2(n) n      , n+1      , n+1      , n+2
# define BIT4(n) BIT2(n), BIT2(n+1), BIT2(n+1), BIT2(n+2)
# define BIT6(n) BIT4(n), BIT4(n+1), BIT4(n+1), BIT4(n+2)
# define BIT8(n) BIT6(n), BIT6(n+1), BIT6(n+1), BIT6(n+2)
    static constexpr unsigned char TABLE[256]={BIT8(0)};
    return TABLE[n       & 0xff]+
           TABLE[(n>>8)  & 0xff]+
           TABLE[(n>>16) & 0xff]+
           TABLE[(n>>24) & 0xff];
}