#include "libc.h"
#include <limits.h>
#include <stdint.h>
#include <string.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* restrict d, const char* restrict s, size_t n) {
    // This reads past the end of the string, which is usually OK since
    // it won't cross a page boundary.  But under ASan, even one byte
    // past the actual end is diagnosed.
    if (!__has_feature(address_sanitizer) &&
        ((uintptr_t)s & ALIGN) == ((uintptr_t)d & ALIGN)) {
        for (; ((uintptr_t)s & ALIGN) && n && (*d = *s); n--, s++, d++)
            ;
        if (!n || !*s)
            goto tail;
        size_t* wd = (void*)d;
        const size_t* 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;
    }
    for (; n && (*d = *s); n--, s++, d++)
        ;
tail:
    memset(d, 0, n);
    return d;
}

weak_alias(__stpncpy, stpncpy);