rev nibbles and nibbles bits

#include <stdio.h>

void printBits(unsigned x) {
  for (int i = sizeof(x) * 8 - 1; i >= 0; i--) {
    printf("%d", 1 & (x >> i));
    if (i % 4 == 0) {
      putchar(' ');
    }
  }
  puts("");
}

unsigned revNibbles(unsigned a) {
  const int nbNr = sizeof(a) * 2;
  const unsigned mask = (1 << 4) - 1;
  unsigned r = 0;
  for (int nbId = 0; nbId < nbNr; nbId++) {
    unsigned nb = (a >> (nbId * 4)) & mask;
    r |= (nb << ((nbNr - 1) * 4 - nbId * 4));
  }
  return r;
}

unsigned revNibBits(unsigned nb) {
  unsigned r = 0;
  for (int i = 0; i < 4; i++) {
    if ((nb & (1 << i))) {
      r |= 1 << (3 - i);
    } else{
      r &= ~(1 << (3 - i));
    }
  }
  return r;
}

unsigned revAllNibBits(unsigned a) {
  const int nbNr = sizeof(a) * 2;
  const unsigned mask = (1 << 4) - 1;
  unsigned r = 0;
  for (int nbId = 0; nbId < nbNr; nbId++) {
    unsigned nb = (a >> (nbId * 4)) & mask;
    r |= (revNibBits(nb) << (nbId * 4));
  }
  return r;
}

int main(void) {
  unsigned a;
  scanf("%u", &a);

  printBits(a);
  printBits(revNibbles(a));
  printBits(revAllNibBits(a));

  return 0;
}