#include <string.h>

char *strtok(char *s1, const char *s2) {
	static char *tmp = NULL;
	const char *p = s2;

	if (s1 != NULL) {
		/* new string */
		tmp = s1;
	} else {
		/* old string continued */
		if (tmp == NULL) {
			/* No old string, no new string, nothing to do */
			return NULL;
		}
		s1 = tmp;
	}

	/* skipping leading s2 characters */
	while (*p && *s1) {
		if ( *s1 == *p ) {
			/* found seperator; skip and start over */
			++s1;
			p = s2;
			continue;
		}
		++p;
	}

	if (!*s1) {
		/* no more to parse */
		return ( tmp = NULL );
	}

	/* skipping non-s2 characters */
	tmp = s1;
	while (*tmp) {
		p = s2;
		while (*p) {
			if (*tmp == *p++) {
				/* found seperator; overwrite with '\0', position tmp, return */
				*tmp++ = '\0';
				return s1;
			}
		}
		++tmp;
	}

	/* parsed to end of string */
	tmp = NULL;
	return s1;
}