ref: 8d8798e08a6d07094b90b4c06bc3df988e935d6c
dir: /sys/src/ape/lib/ap/gen/strcspn.c/
#include <string.h>
#define	N	256
size_t
strcspn(const char *s, const char *b)
{
	char map[N], *os;
	memset(map, 0, N);
	for(;;) {
		map[*(unsigned char*)b] = 1;
		if(*b++ == 0)
			break;
	}
	os = (char*)s;
	while(map[*(unsigned char*)s++] == 0)
		;
	return s - os - 1;
}