22. Extract Even Bits Only from 32-bit Register

#include <stdio.h>
#include <stdint.h>

uint32_t extract_even_bits(uint32_t reg) {
    // Your code here
    // uint32_t res=0;
    // uint32_t bit=0;
    // uint32_t pos=0;

    // for(int i=0;i<32;i=i+2){
    //     uint32_t bit = (reg >> i)&1;
    //     res = res | (bit << pos);
    //     pos++;
    // }

    //mask + bit compression trick
    uint32_t x = reg & 0x55555555;
    x = (x | (x >> 1)) & 0x33333333;
    x = (x | (x >> 2)) & 0x0F0F0F0F;
    x = (x | (x >> 4)) & 0x00FF00FF;
    x = (x | (x >> 8)) & 0x0000FFFF;
    return x;
}

int main() {
    uint32_t reg;
    scanf("%u", &reg);
    printf("%u", extract_even_bits(reg));
    return 0;
}