ref: 72f9c4e49acb9d8d7397b6ea7de53ec9899a054c
dir: /sys/src/libc/port/strspn.c/
#include <u.h>
#include <libc.h>
#define	N	256
long
strspn(char *s, char *b)
{
	char map[N], *os;
	memset(map, 0, N);
	while(*b)
		map[*(uchar *)b++] = 1;
	os = s;
	while(map[*(uchar *)s++])
		;
	return s - os - 1;
}