strspn.c

#define SYSCLIB_DISABLE_BUILTINS
#include <sysclib.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;
}