#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)] = {};

    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;
}