22. Extract Even Bits Only from 32-bit Register

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

uint32_t extract_even_bits(uint32_t reg) {
    uint32_t even_bits = reg & 0x55555555;

    
    // Merge 2-bit groups
    even_bits = (even_bits | (even_bits >> 1)) & 0x33333333;
    // 4-bit groups
    even_bits = (even_bits | (even_bits >> 2)) & 0x0F0F0F0F;
    // 8-bit groups
    even_bits = (even_bits | (even_bits >> 4)) & 0x00FF00FF;
    // 16-bit groups
    even_bits = (even_bits | (even_bits >> 8)) & 0x0000FFFF;

    return even_bits;
}

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