ref: 321d79d6f2f158bf005366a5c5f1756a58dca25d
dir: /lib/c/strspn.c/
#include <string.h>
#undef strspn
size_t
strspn(const char *s1, const char *s2)
{
char buf[__NUMCHARS];
unsigned char ch;
size_t n;
memset(buf, 0, sizeof(buf));
while (ch = *s2++)
buf[ch] = 1;
for (n = 0; (ch = *s1++) && buf[ch]; ++n)
;
return n;
}