This Gist solves the problem specified at: https://www.spotify.com/fi/jobs/tech/reversed-binary/ Unlike most of its counterparts this one runs in constant-time and makes explicit use of bit-hacks and intrinsics (whenever possible).

spotify-reversed-binary-numbers.c

#include <stdlib.h>
#include <stdio.h>

static inline unsigned int reverse_bits(register unsigned int number)
{
        register unsigned int shift = __builtin_clz(number);

        number = (((number & 0xAAAAAAAA) >> 0x01) |
                ((number & 0x55555555) << 0x01));

        number = (((number & 0xCCCCCCCC) >> 0x02) |
                ((number & 0x33333333) << 0x02));

        number = (((number & 0xF0F0F0F0) >> 0x04) |
                ((number & 0x0F0F0F0F) << 0x04));

        return __builtin_bswap32(number) >> shift;
}

int main(int count, char *arguments[])
{
        unsigned int number = 0;

        scanf("%u", &number);
        printf("%u\n", reverse_bits(number));

        return EXIT_SUCCESS;
}