1#include <string.h>
 2
 3#define BITOP(a,b,op) \
 4 ((a)[(size_t)(b)/(8*sizeof *(a))] op (size_t)1<<((size_t)(b)%(8*sizeof *(a))))
 5
 6size_t strspn(const char *s, const char *c)
 7{
 8	const char *a = s;
 9	size_t byteset[32/sizeof(size_t)] = { 0 };
10
11	if (!c[0]) return 0;
12	if (!c[1]) {
13		for (; *s == *c; s++);
14		return s-a;
15	}
16
17	for (; *c && BITOP(byteset, *(unsigned char *)c, |=); c++);
18	for (; *s && BITOP(byteset, *(unsigned char *)s, &); s++);
19	return s-a;
20}