Ideal gas

# include <Siv3D.hpp> // OpenSiv3D v0.1.5

struct Ball
{
	Ball(const Vec2& _p, const Vec2& _v) : p(_p), v(_v) {}
	Vec2 p;
	Vec2 v;
};

void Main()
{
	Graphics::SetBackground(Palette::Darkviolet);
	Window::SetTitle(L"Ideal gas");
	Window::Resize(1280, 720);

	const double r = 5;

	double wx = Window::Size().x;
	Array<Ball> balls;

	for (double x = r * 2; x < wx - r * 2; x += r * 12)
		for (double y = r * 2; y < Window::Size().y - r * 2; y += r * 12)
			balls.emplace_back(Vec2(x, y), RandomVec2(1));


	while (System::Update())
	{
		for (auto& b : balls)
		{
			b.p += b.v;

			for (auto& t : balls)
			{
				const double length = (b.p - t.p).length();
				if (length != 0 && length < r * 2)
				{
					const Vec2 p = (t.p - b.p).normalized();
					const Vec2 f = p*(b.v - t.v).dot(p);
					t.v += f;
					b.v -= f;
					b.p += p*(length - r * 2) / 2.0;
					t.p -= p*(length - r * 2) / 2.0;
				}
			}
			if (b.p.x < r) { b.v.x = -b.v.x; b.p.x = r; }
			if (b.p.y < r) { b.v.y = -b.v.y; b.p.y = r; }
			if (b.p.x > wx - r) { b.v.x = -b.v.x; b.p.x = wx - r; }
			if (b.p.y > Window::Size().y - r) { b.v.y = -b.v.y; b.p.y = Window::Size().y - r; }
		}

		if (KeyRight.pressed()) wx++;
		if (KeyLeft.pressed()) wx--;
		if (KeySpace.down()) for (auto& b : balls) b.v *= 2;
		if (KeyEnter.down()) for (auto& b : balls) b.v /= 2;

		for (const auto& b : balls) Circle(b.p, r).draw(Palette::White);

		Rect(int(wx), 0, 16, Window::Size().y).draw(Palette::Black);
	}
}