#include <limits.h>
#include <stdint.h>
#include <string.h>
#include <zircon/compiler.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)

void* memccpy(void* restrict dest, const void* restrict src, int c, size_t n) {
  unsigned char* d = dest;
  const unsigned char* s = src;

  c = (unsigned char)c;
  // 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) != c; n--, s++, d++)
      ;
    if ((uintptr_t)s & ALIGN)
      goto tail;
    size_t k = ONES * c;
    size_t* wd = (void*)d;
    const size_t* ws = (const void*)s;
    for (; n >= sizeof(size_t) && !HASZERO(*ws ^ k); n -= sizeof(size_t), ws++, wd++)
      *wd = *ws;
    d = (void*)wd;
    s = (const void*)ws;
  }
  for (; n && (*d = *s) != c; n--, s++, d++)
    ;
tail:
  if (*s == c)
    return d + 1;
  return 0;
}