/*
    Name:    ex20-10.c
    Purpose: Reverse the bits in a given unsigned integer.
    Author:  Bill Slough

    Notes:   The main() function assumes 32-bit quantities are in use.
 */

#include <stdio.h>
#include <stdbool.h>

#define NBITS (8 * sizeof(unsigned int))

unsigned int reverse_bits(unsigned int n);

int main(void) {
    unsigned int x;

    while (true) {
        printf("Enter an unsigned integer (in hex): ");
        scanf("%x", &x);
        printf("x = %8.8x reversed = %8.8x twice reversed = %8.8x\n",
               x, reverse_bits(x), reverse_bits(reverse_bits(x)));
    }

    return 0;
}

/* return the bits of n, in reverse order */
unsigned int reverse_bits(unsigned int n) {
    int i;
    unsigned int r = 0;

    for (i = 0; i < NBITS; i++) {
        r = (r << 1) | (n & 1);   /* append the least significant bit of n */
        n = n >> 1;
    }
    return r;
}