Bit reverse -- via 簡潔なQ http://qnighy.hatenablog.com/entry/20091006/1254832950

main.c

#include <stdio.h>

unsigned int bitreverse(unsigned int x) {
    x = (x & 0x55555555)<<1 | (x & 0xaaaaaaaa)>>1;
    x = (x & 0x33333333)<<2 | (x & 0xcccccccc)>>2;
    x = (x & 0x0f0f0f0f)<<4 | (x & 0xf0f0f0f0)>>4;
    x = (x & 0x00ff00ff)<<8 | (x & 0xff00ff00)>>8;
    x = (x & 0x0000ffff)<<16 | (x & 0xffff0000)>>16;
    return x;
}
#define bitreverse_range(x,s) (bitreverse((x))>>(32-(s)))

int main()
{
    int i;
    for(i = 0; i < 128; ++i) {
        printf("%d -> %d\n", i, bitreverse_range(i, 7));
    }
    return 0;
}