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 Camera2D _camera;
    private Tween _cameraTween;

    private Vector2 _cameraZoomTarget;
    private Vector2 _cameraPosTarget;

    private float _maxZoom;
    private Vector2 _spaceSize;
    
    private const float CameraZoomSpeed = 0.5f;

    private ParticleSimulation _particleSimulation;
    public float PhysicsInterpolationFraction;

    public void Initialize(int seed, int nParticles, float zoom)
    {
        _maxZoom = zoom;
        _camera = GetNode<Camera2D>("Camera2D");
        _cameraTween = GetNode<Tween>("CameraTween");
        _particleNodes = GetNode<Node2D>("ParticleNodes");
        _cameraZoomTarget = _camera.Zoom;
        _cameraPosTarget = _camera.Position;
        GD.Seed((ulong)seed);
        GD.Print("Last Seed: " + seed);
        var viewSize = GetViewportRect().Size;
        _camera.Zoom = new Vector2(zoom, zoom);
        var spaceSize = viewSize * zoom;
        _spaceSize = spaceSize;
        _particleSimulation = new ParticleSimulation
        {
            SpaceSize = spaceSize
        };
        _particleSimulation.Initialize(nParticles);
        foreach (var id in _particleSimulation.LastParticlesAdded) CreateParticleNode(id);
    }

    public override void _Process(float delta)
    {
        // Game state inputs
        if (Input.IsActionJustPressed("quit")) 
            GetParent<Main>().ExitToMenu();
        if (Input.IsActionJustPressed("reset")) 
            GetParent<Main>().RestartSimulation();

        var shouldTweenStop = false;
        
        if (Input.IsActionJustReleased("zoom_in"))
        {
            shouldTweenStop = true;

            // Zoom
            _cameraZoomTarget -= new Vector2(0.1f, 0.1f);

            // Movement
            var mousePos = GetGlobalMousePosition();
            var posDelta = _cameraPosTarget - mousePos;
            posDelta = posDelta.Clamped(500f);
            _cameraPosTarget -= posDelta * 0.2f;
        }
        if (Input.IsActionJustReleased("zoom_out"))
        {
            shouldTweenStop = true;

            // Zoom
            _cameraZoomTarget += new Vector2(0.1f, 0.1f);

            // Movement
            var mousePos = GetGlobalMousePosition();
            var posDelta = _cameraPosTarget - mousePos;
            posDelta = posDelta.Clamped(500f);
            _cameraPosTarget += posDelta * 0.2f;
        }
        
        var cameraDir = new Vector2();

        if (Input.IsActionPressed("up"))
            cameraDir += new Vector2(0, -1);
        if (Input.IsActionPressed("down"))
            cameraDir += new Vector2(0, 1);
        if (Input.IsActionPressed("left"))
            cameraDir += new Vector2(-1, 0);
        if (Input.IsActionPressed("right"))
            cameraDir += new Vector2(1, 0);

        if (cameraDir.LengthSquared() != 0)
        {
            shouldTweenStop = true;
            cameraDir = cameraDir.Normalized();
            _cameraPosTarget += cameraDir * 1000f * delta;
        }

        if (Input.IsActionPressed("key_zoom_in"))
        {
            shouldTweenStop = true;
            _cameraZoomTarget -= new Vector2(0.01f, 0.01f) * (delta * 60f);
        }

        if (Input.IsActionPressed("key_zoom_out"))
        {
            shouldTweenStop = true;
            _cameraZoomTarget += new Vector2(0.01f, 0.01f) * (delta * 60f);
        }

        if (shouldTweenStop)
        {
            _cameraTween.StopAll();
            
            _cameraZoomTarget.x = Mathf.Clamp(_cameraZoomTarget.x, 0.01f, _maxZoom);
            _cameraZoomTarget.y = Mathf.Clamp(_cameraZoomTarget.y, 0.01f, _maxZoom);

            _cameraPosTarget.x = Mathf.Clamp(_cameraPosTarget.x,
                0f - (_maxZoom - _cameraZoomTarget.x) * (_spaceSize.x / (Mathf.Sqrt2 * 2)),
                0f + (_maxZoom - _cameraZoomTarget.x) * (_spaceSize.x / (Mathf.Sqrt2 * 2)));
            _cameraPosTarget.y = Mathf.Clamp(_cameraPosTarget.y,
                0f - (_maxZoom - _cameraZoomTarget.y) * (_spaceSize.y / (Mathf.Sqrt2 * 2)),
                0f + (_maxZoom - _cameraZoomTarget.y) * (_spaceSize.y / (Mathf.Sqrt2 * 2)));
            
            _cameraTween.InterpolateProperty(_camera, "zoom", _camera.Zoom, _cameraZoomTarget, CameraZoomSpeed,
                Tween.TransitionType.Quint, Tween.EaseType.Out);
            _cameraTween.InterpolateProperty(_camera, "position", _camera.Position, _cameraPosTarget, CameraZoomSpeed,
                Tween.TransitionType.Quint, Tween.EaseType.Out);
            _cameraTween.Start();
        }
        
        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 - (_spaceSize / 2.0f);
            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);
    }
}