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

#define SS (sizeof(size_t))
#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* memchr(const void* src, int c, size_t n) {
  const unsigned char* s = src;
  c = (unsigned char)c;
  for (; ((uintptr_t)s & ALIGN) && n && *s != c; s++, n--)
    ;
  if (n && *s != c) {
    const size_t* w = (const void*)s;
#if !__has_feature(address_sanitizer)
    // 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.
    size_t k = ONES * c;
    while (n >= SS && !HASZERO(*w ^ k)) {
      ++w;
      n -= SS;
    }
#endif
    for (s = (const void*)w; n && *s != c; s++, n--)
      ;
  }
  return n ? (void*)s : 0;
}