#include <inttypes.h>
#include <limits.h>

#define	NBITS	(sizeof(uint64_t) * CHAR_BIT)
#define MSBMASK	(1ULL << (NBITS - 1))

typedef struct {
	uint64_t hi;
	uint64_t lo;
} uint128_t;

void bitr128(uint128_t *);

void bitr128(uint128_t *n)
{
	uint64_t hiword, loword, lsbit, msbit;

	hiword = n->hi;
	loword = n->lo;
	msbit = hiword & MSBMASK;
	hiword <<= 1;
	for (unsigned i = 0; i < NBITS; i++) {
		lsbit = loword & 1;
		loword >>= 1;
		loword |= msbit;
		msbit = hiword & MSBMASK;
		hiword <<= 1;
		hiword |= lsbit;
	}
	n->hi = hiword;
	n->lo = loword;
}