Fun With Gravity

using System;
using System.Drawing;
using System.Xml.Serialization;
using System.Collections.Generic;

namespace ParticleSwarm {
	public class Ghost {
		public Vector Location;
		public Color Color;
		public PointF Velocity1;
		public PointF Velocity2;
		public PointF Acceleration1;
		public PointF Acceleration2;
		public Ghost(Vector location, Color color) {
			this.Location = location;
			this.Color = color;
		}
		public Ghost(Vector location, Color color, PointF vel1, PointF vel2, PointF acc1, PointF acc2) : this(location, color) {
			this.Velocity1 = vel1;
			this.Velocity2 = vel2;
			this.Acceleration1 = acc1;
			this.Acceleration2 = acc2;
		}
	}

	/// <summary></summary>
	public class Particle {
		public static int GhostCount = 40;
		public static int TopAlpha = 70;
		private List<Ghost> ghosts = new List<Ghost>();
		private Vector location;
		private Vector velocity;
		private Vector acceleration;
		private float mass;
		private float size;
		private Color color;

		public Particle(Vector loc, Vector vel) {
			this.location = loc;
			this.velocity = vel;
			this.color = Color.Black;
		}

		public Particle(Vector loc, Vector vel, Color color) : this(loc, vel) {
			this.color = color;
		}

		public Particle(Vector loc, Vector vel, float mass, float size, Color color) : this(loc, vel, color) {
			this.mass = mass;
			this.size = size;
		}

		public void Update() {
			int colorFactor = (TopAlpha - 10) / GhostCount;
			ghosts.Add(new Ghost(location, Color.FromArgb(TopAlpha - colorFactor, color.R, color.G, color.B)));
			if (ghosts.Count > GhostCount && ghosts.Count > 0) {
				ghosts.RemoveAt(0);
			}
			for (int i = 1; i < ghosts.Count; i++) {
				int factor = (ghosts.Count - i + 1) * colorFactor;
				ghosts[i].Color = Color.FromArgb(TopAlpha - (factor), color.R, color.G, color.B);
			}

			this.location += this.velocity;
		}

		public void Update(PointF vel1, PointF vel2, PointF acc1, PointF acc2) {
			int colorFactor = (TopAlpha - 10) / GhostCount;
			ghosts.Add(new Ghost(location, Color.FromArgb(TopAlpha - colorFactor, color.R, color.G, color.B), vel1, vel2, acc1, acc2));
			if (ghosts.Count > GhostCount && ghosts.Count > 0) {
				ghosts.RemoveAt(0);
			}
			for (int i = 1; i < ghosts.Count; i++) {
				int factor = (ghosts.Count - i + 1) * colorFactor;
				ghosts[i].Color = Color.FromArgb(TopAlpha - (factor), color.R, color.G, color.B);
			}

			this.location += this.velocity;
		}

		/// <summary></summary>
		[XmlElement("Location")]
		public Vector Location {
			get { return this.location; }
			set { this.location = value; }
		}

		/// <summary></summary>
		[XmlIgnore]
		public Vector MidLocation {
			get {
				return new Vector(this.location.X + this.size / 2, this.location.Y + this.size / 2, this.location.Z + this.size / 2);
			}
		}

		/// <summary></summary>
		[XmlElement("Velocity")]
		public Vector Velocity {
			get { return this.velocity; }
			set { this.velocity = value; }
		}

		/// <summary></summary>
		[XmlIgnore]
		public Vector Acceleration {
			get { return this.acceleration; }
			set { this.acceleration = value; }
		}

		/// <summary></summary>
		[XmlAttribute("mass")]
		public float Mass {
			get { return this.mass; }
			set { this.mass = value; }
		}

		/// <summary></summary>
		[XmlAttribute("size")]
		public float Size {
			get { return this.size; }
			set { this.size = value; }
		}

		/// <summary></summary>
		[XmlElement("Color")]
		public Color Color {
			get { return this.color; }
			set { this.color = value; }
		}

		[XmlIgnore]
		public List<Ghost> Ghosts {
			get { return ghosts; }
		}
	}
}