particles/Main.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Threading.Tasks;
using Godot;

public class Main : Node2D
{
    private const int ParticleScreenPadding = 16;

    private readonly List<ParticleType> _particleTypes = new List<ParticleType>();
    private List<Particle> _particles;

    private const int MaxParticles = 600;

    private const float HealthDelta = 0.002f;
    private const float PositiveHealthMultiplier = 2f;
    private Task[] tasks = new Task[MaxParticles];

    public override void _Ready()
    {
        GD.Randomize();
        InitializeParticleTypes(10);
        InitializeParticles(MaxParticles);

    }

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

    public override void _PhysicsProcess(float delta)
    {
        UpdateParticles();
    }

    private void InitializeParticleTypes(int nTypes)
    {
        for (var i = 0; i < nTypes; i++)
        {
            var type = new ParticleType
            {
                Hue = (float) GD.RandRange(0, 1)
            };
            _particleTypes.Add(type);
        }

        foreach (var type1 in _particleTypes)
            foreach (var type2 in _particleTypes)
                type1.AddRelationship(type2,
                    new ParticleRelationshipProps(20, (float) GD.RandRange(25, 60),
                        (float) GD.RandRange(-0.33 * 2f, 0.35 * 2f)));
    }

    private void InitializeParticles(int nParticles)
    {
        var typeCount = 0;
        var particleScene = GD.Load<PackedScene>("res://Particle.tscn");
        for (var i = 0; i < nParticles; i++)
        {
            var particle = particleScene.Instance<Particle>();
            GetNode<Node2D>("Particles").AddChild(particle);
            particle.Position = GetRandomParticlePosition();
            particle.Type = _particleTypes[typeCount];
            particle.Health = 1f;

            if (typeCount < _particleTypes.Count - 1)
                typeCount++;
            else
                typeCount = 0;
        }
    }

    private void CreateParticle(ParticleType type)
    {
        var particleScene = GD.Load<PackedScene>("res://Particle.tscn");
        var particle = particleScene.Instance<Particle>();
        GetNode<Node2D>("Particles").AddChild(particle);
        particle.Position = GetRandomParticlePosition();
        particle.Type = type;
        particle.Health = 1f;
    }

    private void CreateRandomParticle()
    {
        var randomIndex = (int) (GD.Randi() % _particleTypes.Count);
        var type = _particleTypes[randomIndex];
        CreateParticle(type);
    }

    private Vector2 GetRandomParticlePosition()
    {
        var viewportRect = GetViewportRect();
        var position = new Vector2(
            (float) GD.RandRange(ParticleScreenPadding, viewportRect.Size.x - ParticleScreenPadding),
            (float) GD.RandRange(ParticleScreenPadding, viewportRect.Size.y - ParticleScreenPadding));
        return position;
    }

    private void UpdateParticles()
    {
        _particles = GetNode<Node2D>("Particles").GetChildren().Cast<Particle>().ToList();
        
        var viewportRect = GetViewportRect();
        Array.Resize(ref tasks, _particles.Count);
        for (int i = 0; i < _particles.Count; i++)
            tasks[i] = Task.Factory.StartNew(UpdateParticle, _particles[i]);

        Task.WaitAll(tasks);

        var deletedParticle = false;

        
        foreach (var p in _particles)
        {
            if (deletedParticle == false && p.Health == 0f)
            {
                if (GD.Randf() < 0.2f)
                {
                    p.Free();
                    deletedParticle = true;
                    continue;
                }
            }

            var screenSize = viewportRect.Size;
            var position = p.Position;
            p.Velocity = p.Velocity.Clamped(3f * 4f);
            position += p.Velocity;
            p.Velocity *= 0.875f; // friction
            if (position.x > screenSize.x)
                position.x -= screenSize.x;
            else if (position.x < 0)
                position.x += screenSize.x;
            if (position.y > screenSize.y)
                position.y -= screenSize.y;
            else if (position.y < 0)
                position.y += screenSize.y;
            //position.x = Mathf.Clamp(position.x, ParticleScreenPadding, viewportRect.Size.x - ParticleScreenPadding);
            //position.y = Mathf.Clamp(position.y, ParticleScreenPadding, viewportRect.Size.y - ParticleScreenPadding);
            p.AddAverageSpeedValue(p.Position.DistanceTo(position));

            if (p.AverageSpeed < 0.3f)
                p.Health -= HealthDelta * 1.5f;
            else
                p.Health += HealthDelta;

            if (deletedParticle == false && p.Health == 0f)
            {
                if (GD.Randf() < 0.2f)
                {
                    p.Free();
                    deletedParticle = true;
                    continue;
                }
            }

            p.Position = position;
        }
        

        if (_particles.Count < MaxParticles)
            if (GD.Randf() < 0.3f)
                CreateRandomParticle();
    }

    private void UpdateParticle(object particle)
    {
        var p1 = (Particle) particle;
        var closeCount = 0;
        foreach (var p2 in _particles)
        {
            var viewportRect = GetViewportRect();
            if (p1 == p2)
                continue;
            var position = GetScreenWrapPosition(viewportRect.Size, p1.Position, p2.Position);
            var distanceSquared = p1.Position.DistanceSquaredTo(position);
            if (distanceSquared > (60f * 60f))
                continue;
            var direction = p1.Position.DirectionTo(position);
            var distance = 0f;

            if (distanceSquared < (70f * 70f))
            {
                closeCount++;
            }

            // collision force
            if (distanceSquared < (20f * 20f))
            {
                distance = p1.Position.DistanceTo(position);
                var collisionForce = 1f / (0.6f + Mathf.Pow(Mathf.E, -(distance - 12f))) - 1f / 0.6f;
                p1.Velocity += direction * collisionForce;
            }
            // particle relationship force
            
            var props = p1.Type.GetRelationship(p2.Type);
            if (props.Force == 0f || !(distanceSquared >= props.MinRadius * props.MinRadius) || !(distanceSquared <= props.MaxRadius * props.MaxRadius))
                continue;
            distance = p1.Position.DistanceTo(position);
            var slope = props.Force / ((props.MaxRadius - props.MinRadius) / 2f);
            var particleForce = -slope * Mathf.Abs(distance - (props.MinRadius + props.MaxRadius) / 2f) +
                                props.Force;
            p1.Velocity += direction * particleForce;
        }

        if (closeCount < 3 || closeCount > 35)
            p1.Health -= HealthDelta * 1.5f;
        else
            p1.Health += HealthDelta;

    }

    private Vector2 GetScreenWrapPosition(Vector2 screenSize, Vector2 p1, Vector2 p2)
    {
        var newPosition = p2;
        if (p2.x > (p1.x + (screenSize.x / 2f)))
            newPosition.x = p2.x - screenSize.x;
        else if (p2.x < (p1.x - (screenSize.x / 2f)))
            newPosition.x = p2.x + screenSize.x;
        if (p2.y > (p1.y + (screenSize.y / 2f)))
            newPosition.y = p2.y - screenSize.y;
        else if (p2.y < (p1.y - (screenSize.y / 2f)))
            newPosition.y = p2.y + screenSize.y;
        return newPosition;
    }
}