bool
collides( const Circle & c, const LineSegment & l)
{
Vector2 posToCenter = c.GetCenter() - l.GetStart();
Vector2 dirVec = l.GetEnd() - l.GetStart();
float segmentLength = dirVec.Length();
// direction vector must be unit vector (ie. length = 1) in this case!
// otherwise we would need another formula for scalar projection.
dirVec = dirVec / segmentLength;
// scalar projection of posToCenter to direction vector.
float d = dirVec.Dot(posToCenter);
// if d value exceeds original segment length, then we put a cap on it.
// if these two lines are dismissed, then algorithm sees line segment
// as a infinite line.
if ( d > segmentLength ) d = segmentLength;
if ( d < -segmentLength ) d = -segmentLength;
// compute closest point to circle center from line start
// along direction vector.
Vector2 closest_point =l.GetStart() + dirVec * d;
// vectorfrom circle center to closest point on line
Vector2 S = closest_point - c.GetCenter();
return (S.Length() <= c.GetRadius());
}
void
draw_line( SDL_Renderer *renderer, LineSegment & l, int r, int g, int b, int a )
{
// just to make compiler happy
int x1 = static_cast<int>(l.GetStart().GetX());
int y1 = static_cast<int>(l.GetStart().GetY());
int x2 = static_cast<int>(l.GetEnd().GetX());
int y2 = static_cast<int>(l.GetEnd().GetY());
lineRGBA( renderer, x1,y1,x2,y2, r,g,b,a);
}