source: bootcd/isolinux/syslinux-6.03/com32/lib/strspn.c

/*
 * strspn, strcspn
 */

#include <string.h>
#include <stddef.h>
#include <inttypes.h>
#include <limits.h>

#ifndef LONG_BIT
#define LONG_BIT (CHAR_BIT*sizeof(long))
#endif

static void set_bit(unsigned long *bitmap, unsigned int bit)
{
    bitmap[bit / LONG_BIT] |= 1UL << (bit % LONG_BIT);
}

static int test_bit(unsigned long *bitmap, unsigned int bit)
{
    return (int)(bitmap[bit / LONG_BIT] >> (bit % LONG_BIT)) & 1;
}

static size_t strxspn(const char *s, const char *map, int parity)
{
    unsigned long matchmap[((1 << CHAR_BIT) + LONG_BIT - 1) / LONG_BIT];
    size_t n = 0;

    /* Create bitmap */
    memset(matchmap, 0, sizeof matchmap);
    while (*map)
        set_bit(matchmap, (unsigned char)*map++);

    /* Make sure the null character never matches */
    if (parity)
        set_bit(matchmap, 0);

    /* Calculate span length */
    while (test_bit(matchmap, (unsigned char)*s++) ^ parity)
        n++;

    return n;
}

size_t strspn(const char *s, const char *accept)
{
    return strxspn(s, accept, 0);
}

size_t strcspn(const char *s, const char *reject)
{
    return strxspn(s, reject, 1);
}

char *strpbrk(const char *s, const char *accept)
{
    const char *ss = s + strxspn(s, accept, 1);

    return *ss ? (char *)ss : NULL;
}