At Wikipedia we may see a straightforward way to tell whether a year is leap or not. Well here is a more "compact" way (pseudocode)
where MOD is modulo, and XOR is exclusive or.

We are going to calculate points of intersections for 2 circels, given their centers and radiuses on a 2D plane. Say, we will implement the following C function
int intersect(struct circle_t const circles[], struct point_t intersections[]);
where point_t and circle_t are
    The function intersect takes 2 circles as parameter, returns the number of points of intersection, and the details of the points will be put in parameter intersection. Since 2 circles may have up to 2 points of intersection, we may call that function in following way: