#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;
}