ref: 72be22cd3ad8236dba47d3c56d1ba8af5b16a1c2
dir: /sys/src/ape/lib/ap/gen/strspn.c/
#include <string.h>
#define N 256
size_t
strspn(const char *s, const char *b)
{
char map[N], *os;
memset(map, 0, N);
while(*b)
map[*(unsigned char *)b++] = 1;
os = s;
while(map[*(unsigned char *)s++])
;
return s - os - 1;
}