#ifdef HAVE_CONFIG_H
#include <config.h>
#endif

#include <string.h>

/*
 * True if w contains a zero byte. Subtracting ONES from w propagates a
 * borrow into the high bit of a byte exactly when that byte is zero;
 * ANDing with ~w keeps only bytes that were zero and high-bit-clear in w.
 * ONES and its shifted form are derived from the word width, so this works
 * for both 32- and 64-bit unsigned long.
 */
static unsigned long
haszero(unsigned long w)
{
    const unsigned long ones = (unsigned long)-1 / 0xff;

    return (w - ones) & ~w & (ones << 7);
}

/*
 * Return the length of s, excluding the terminating NUL.
 */
size_t
strlen(const char *s)
{
    const unsigned long word = sizeof(unsigned long);
    const char *p = s;

    /* Check the head byte-wise until p is word-aligned. */
    for (; (unsigned long)p % word != 0; p++)
    {
        if (*p == '\0')
        {
            return (size_t)(p - s);
        }
    }

    /* Scan whole words for a zero byte. */
    for (;;)
    {
        unsigned long w;

        __builtin_memcpy(&w, p, sizeof w);
        if (haszero(w))
        {
            break;
        }
        p += word;
    }

    /* Resolve the terminating word byte-wise. */
    for (;;)
    {
        if (*p == '\0')
        {
            return (size_t)(p - s);
        }
        p++;
    }
}