Voile/DaggerFramework/Source/SceneGraph/Entities/Particles2d.cs

using System.Drawing;
using System.Numerics;
using DaggerFramework.Rendering;

namespace DaggerFramework.SceneGraph
{
    // TODO: add oneshot parameter.
    public class Particles2d : Drawable2d
    {
        public int MaxParticles => _maxParticles;

        public void BeginEmit(ParticleSettings settings)
        {
            _settings = settings;
            _maxParticles = _settings.MaxParticles;
            _particleIndex = _maxParticles - 1;
            InitializeParticles();
        }
        public void Restart()
        {
            CleanupParticles();
            InitializeParticles();
        }
        public override void OnDraw(Renderer renderer)
        {
            foreach (var particle in _particles)
            {
                if (!particle.Alive) continue;

                var t = particle.LifeTimeRemaining / particle.LifeTime;
                var scale = MathUtils.Lerp(_settings.ScaleEnd, _settings.ScaleBegin, t);
                var color = MathUtils.LerpColor(_settings.ColorEnd, _settings.ColorBegin, t);

                renderer.SetTransform(particle.Position, Vector2.Zero, particle.Rotation);
                renderer.DrawRectangle(Vector2.One * scale, color);
            }
        }
        protected override void OnStart()
        {
            base.OnStart();
            // Emit();
        }
        protected override void OnUpdate(double dt)
        {
            base.OnUpdate(dt);

            var rate = (int)MathUtils.Lerp(1, _maxParticles, _settings.Explosiveness);
            for (int i = 0; i < rate; i++)
            {
                Emit();
            }

            for (int i = 0; i < _maxParticles; i++)
            {
                var particle = _particles[i];
                if (!particle.Alive) continue;

                if (particle.LifeTimeRemaining <= 0.0f)
                {
                    particle.Alive = false;
                    continue;
                }

                particle.LifeTimeRemaining -= (float)dt;
                particle.Velocity += _settings.Gravity * (float)dt;
                particle.Position += particle.Velocity * (float)dt;
                particle.Rotation += particle.AngularVelocity * (float)dt;

                particle.Velocity -= particle.Velocity * _settings.Damping * (float)dt;

                _particles[i] = particle;
            }
        }

        private void InitializeParticles()
        {
            _particles = new Particle[_maxParticles];
        }

        private void Emit()
        {
            Particle particle = _particles[_particleIndex];
            if (!(particle.LifeTimeRemaining <= 0)) return;
            particle.Alive = true;
            particle.Position = GetEmitPosition();
            particle.Velocity = _settings.Direction * _settings.LinearVelocity;

            particle.Velocity += Vector2.One * _settings.LinearVelocityRandom * ((float)_random.NextDouble() - 0.5f);

            particle.AngularVelocity = _settings.AngularVelocity;
            particle.AngularVelocity += 1f * _settings.AngularVelocityRandom * ((float)_random.NextDouble() - 0.5f);

            particle.LifeTime = _settings.LifeTime;
            particle.LifeTimeRemaining = particle.LifeTime;

            _particles[_particleIndex] = particle;
            _particleIndex = --_particleIndex <= 0 ? _maxParticles - 1 : --_particleIndex;
        }

        private void CleanupParticles() => Array.Clear(_particles);
        private Vector2 GetEmitPosition()
        {
            // https://gamedev.stackexchange.com/questions/26713/calculate-random-points-pixel-within-a-circle-image
            var angle = _random.NextDouble() * Math.PI * 2;
            float radius = (float)Math.Sqrt(_random.NextDouble()) * _settings.EmitRadius;

            float x = Position.X + radius * (float)Math.Cos(angle);
            float y = Position.Y + radius * (float)Math.Sin(angle);

            return new Vector2(x, y);
        }

        private ParticleSettings _settings;
        private int _maxParticles;
        private Particle[] _particles;
        private int _particleIndex;

        // TODO: replace a random function for better distribution and performance.
        private LehmerRandom _random = new LehmerRandom();
    }

    public struct ParticleSettings
    {
        public ParticleSettings()
        {
        }
        public float EmitRadius;
        public float LifeTime;
        public float Explosiveness;
        public int MaxParticles;
        public Vector2 Direction;
        public float LinearVelocity;
        public float AngularVelocity = 0.0f;
        public float AngularVelocityRandom;
        public float LinearVelocityRandom;
        public Vector2 Gravity;
        public float ScaleBegin = 16f;
        public float ScaleEnd = 0.0f;
        public Color ColorBegin = Color.White;
        public Color ColorEnd = Color.Black;
        public float Damping = 0.0f;
    }

    public struct Particle
    {
        public Particle()
        {
        }
        public Vector2 Position;
        public Vector2 Velocity;
        public float AngularVelocity;
        public float LifeTime;
        public float LifeTimeRemaining;
        public float Scale;
        public float Rotation;
        public bool Alive = true;
    }
}