#include <string.h>
#include <stdint.h>

#define aliases __attribute__((__may_alias__))
#define byte_repeat(x) ((size_t)~0 / 0xff * (x))
#define word_has_zero(x) (((x) - byte_repeat(0x01)) & ~(x) & byte_repeat(0x80))
#define weak_alias(o, n) extern __typeof__(o) n __attribute__((weak, alias(#o)))

char *__stpncpy(char *restrict d, const char *restrict s, size_t n)
{
	size_t *wd;
	const size_t *ws;

	if (n < sizeof(size_t) * 3 || ((uintptr_t)d | (uintptr_t)s)
	    & sizeof(size_t) - 1) goto bytewise;

	for (; ((uintptr_t)s & sizeof(size_t) - 1) && (*d = *s); d++, s++, n--);
	if (!*s) return memset(d, 0, n);

	wd = (void *)d;
	ws = (const void *)s;
	for (; n >= sizeof(size_t) && !word_has_zero(*ws)
	     ; n -= sizeof(size_t), wd++, ws++) *wd = *ws;
	d = (void *)wd;
	s = (const void *)ws;

bytewise:
	for (; n && (*d = *s); d++, s++, n--);

	return memset(d, 0, n);
}

weak_alias(__stpncpy, stpncpy);