// 2D yes-or-no intersection test for a pair of line segments
public static bool LineSegmentIntersection2D(Vec2 a_begin, Vec2 a_end, Vec2 b_begin, Vec2 b_end)
{
Vec2 a_dir = a_end - a_begin;
Vec2 b_dir = b_end - b_begin;
Vec2 a_normal = new Vec2 {
x = a_dir.y, y = -a_dir.x
};
a_normal /= a_normal.ComputeMagnitude();
Vec2 b_normal = new Vec2 {
x = b_dir.y, y = -b_dir.x
};
b_normal /= b_normal.ComputeMagnitude();
double a_offset = Vec2.Dot(a_normal, a_begin);
double b_offset = Vec2.Dot(b_normal, b_begin);
double a_dir_dot = Vec2.Dot(a_dir, b_normal);
double a_origin_dot = Vec2.Dot(a_begin, b_normal);
double a_hit = (b_offset - a_origin_dot) / a_dir_dot;
double b_dir_dot = Vec2.Dot(b_dir, a_normal);
double b_origin_dot = Vec2.Dot(b_begin, a_normal);
double b_hit = (a_offset - b_origin_dot) / b_dir_dot;
Vec2 a_intersect = a_begin + a_dir * a_hit;
Vec2 b_intersect = b_begin + b_dir * b_hit;
return(a_hit >= 0 && a_hit <= 1 && b_hit >= 0 && b_hit <= 1);
}
