Planet Simulator

# include <Siv3D.hpp>

struct Planet
{
	Vec2 pos;
	Vec2 v;
	int  id;
};

void Main()
{
	Array<Planet> planets;
	for (int i = 0; i < 200; i++) planets.push_back({ RandomVec2(640.0, 480.0), RandomVec2(1.0),Random(0,5) });
	while (System::Update())
	{
		for (auto& p : planets)
		{
			if (p.pos.x + p.v.x >= 0 && p.pos.x + p.v.x <= 640)
			{
				if (p.pos.y + p.v.y >= 0 && p.pos.y + p.v.y <= 480) p.pos += p.v;
				else p.v.y = -p.v.y;
			}
			else p.v.x = -p.v.x;


			for (auto& pt : planets)
			{
				if ((p.pos - pt.pos).isZero()) continue;
				const double length = (p.pos - pt.pos).length();
				if (length < 8.0)
				{
					if (p.v.length() > pt.v.length()) pt.id = p.id;
					else p.id = pt.id;
				}
				const double f = 100.0 / pow(length, 2.0) * (p.id == pt.id ? -1 : 1);

				p.pos += (pt.pos - p.pos).normalized()*f;
				pt.pos += (p.pos - pt.pos).normalized()*f;
			}
		}

		for (auto& p : planets)
			Circle(p.pos, 4).draw(HSV(p.id * 60));

	}
}