fusl/src/string/strcspn.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))))

char* __strchrnul(const char*, int);

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

  if (!c[0] || !c[1])
    return __strchrnul(s, *c) - a;

  memset(byteset, 0, sizeof byteset);
  for (; *c && BITOP(byteset, *(unsigned char*)c, |=); c++)
    ;
  for (; *s && !BITOP(byteset, *(unsigned char*)s, &); s++)
    ;
  return s - a;
}