#define same(c1, c2) ((c1.x == c2.x) && (c1.y == c2.y))
#define distance(c1, c2) sqrt(square(c1.x - c2.x) + square(c1.y - c2.y))
#define invalidate(w) w.x = w.y = 0
-#define is_valid(w) (w.x != 0 || w.y != 0)
+#define is_valid(w) (w.x != 0 && w.y != 0)
typedef struct {
coord w;