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

#define ALIGN (sizeof(size_t)-1)
#define ONES ((size_t)-1/UCHAR_MAX)
#define HIGHS (ONES * (UCHAR_MAX/2+1))
#define HASZERO(x) (((x)-ONES) & ~(x) & HIGHS)

char *stpncpy(char *d, const char *s, size_t n)
{
#if defined(__GNUC__) && __GNUC__ >= 4
        typedef size_t __attribute__((__may_alias__)) word;
        word *wd;
        const word *ws;
        if (((uintptr_t)s & ALIGN) == ((uintptr_t)d & ALIGN)) {
                for (; ((uintptr_t)s & ALIGN) && n && (*d=*s); n--, s++, d++);
                if (!n || !*s) goto tail;
                wd=(void *)d; ws=(const void *)s;
                for (; n>=sizeof(size_t) && !HASZERO(*ws);
                       n-=sizeof(size_t), ws++, wd++) *wd = *ws;
                d=(void *)wd; s=(const void *)ws;
        }
#endif
        for (; n && (*d=*s); n--, s++, d++);
tail:
        memset(d, 0, n);
        return d;
}