unsigned char reverse(unsigned char s){
s = ((s >> 1) & 0x55) | ((s << 1) & 0xaa);
s = ((s >> 2) & 0x33) | ((s << 2) & 0xcc);
s = ((s >> 4) & 0x0f) | ((s << 4) & 0xf0);
return s;
}