ref: a2619e8323f46432134bed045dd90e3694e93052
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 = (char*)s;
	while(map[*(unsigned char *)s++])
		;
	return s - os - 1;
}