strspn.c

#include <string.h>

#define BITOP(a,b,op) \
 ((a)[(size_t)(b)/(8*sizeof *(a))] op (size_t)1<<((size_t)(b)%(8*sizeof *(a))))

size_t strspn(const char *s, const char *c)
{
	const char *a = s;
	size_t byteset[32/sizeof(size_t)] = { 0 };

	if (!c[0]) return 0;
	if (!c[1]) {
		for (; *s == *c; s++);
		return s-a;
	}

	for (; *c && BITOP(byteset, *(unsigned char *)c, |=); c++);
	for (; *s && BITOP(byteset, *(unsigned char *)s, &); s++);
	return s-a;
}