particles/ParticleSimulation/ParticleSimulationScene.cs

using Godot;
using Particles.ParticleSimulation;

// ReSharper disable once CheckNamespace
// ReSharper disable once ClassNeverInstantiated.Global
public class ParticleSimulationScene : Node2D
{
    private Node2D _particleNodes;

    private ParticleSimulation _particleSimulation;
    public float PhysicsInterpolationFraction;

    public void Initialize(int nParticles)
    {
        _particleNodes = GetNode<Node2D>("ParticleNodes");
        GD.Randomize();
        ulong randomSeed = GD.Randi();
        GD.Seed(randomSeed);
        GD.Print("Last Seed: " + randomSeed);
        var viewSize = GetViewportRect().Size;
        var zoom = GetNode<Camera2D>("Camera2D").Zoom;
        var spaceSize = new Vector2(viewSize.x * zoom.x, viewSize.y * zoom.y);
        _particleSimulation = new ParticleSimulation
        {
            SpaceSize = spaceSize
        };
        _particleSimulation.Initialize(nParticles);
        foreach (var id in _particleSimulation.LastParticlesAdded) CreateParticleNode(id);
    }

    public override void _Process(float delta)
    {
        if (Input.IsActionJustPressed("quit")) GetTree().Quit();
        if (Input.IsActionJustPressed("reset")) GetTree().ReloadCurrentScene();
        PhysicsInterpolationFraction = Engine.GetPhysicsInterpolationFraction();
    }

    public override void _PhysicsProcess(float delta)
    {
        _particleSimulation.Update();
        foreach (var id in _particleSimulation.LastParticlesRemoved)
        {
            var particleNode = _particleNodes.GetNode<ParticleNode>(id.ToString());
            particleNode.Free();
        }

        foreach (var id in _particleSimulation.LastParticlesAdded) CreateParticleNode(id);

        var particleNodes = _particleNodes.GetChildren();
        foreach (ParticleNode particleNode in particleNodes)
        {
            var simulationParticle = _particleSimulation.GetParticle(particleNode.SimulationId);
            particleNode.LastSimulationPosition = particleNode.Position;
            particleNode.CurrentSimulationPosition = simulationParticle.Position;
            particleNode.SetColor(simulationParticle.Type.Hue, simulationParticle.Health,
                Mathf.Clamp(simulationParticle.AverageSpeed / 1.5f, 1f, 1f));
            particleNode.WasTeleportedLast = simulationParticle.WasTeleportedLast;
        }
    }

    private void CreateParticleNode(int id)
    {
        var particleScene = GD.Load<PackedScene>("res://ParticleSimulation/ParticleNode.tscn");
        var particleNode = particleScene.Instance<ParticleNode>();
        particleNode.Name = id.ToString();
        particleNode.SimulationId = id;
        _particleNodes.AddChild(particleNode);
    }
}