#define BITCOUNT(x)     (((BX_(x)+(BX_(x)>>4)) & 0x0F0F0F0F) % 255)
#define  BX_(x)         ((x) - (((x)>>1)&0x77777777)    \
                             - (((x)>>2)&0x33333333)    \
                             - (((x)>>3)&0x11111111))

                -- really weird C code to count the number of bits in a word