shmodot/scripts/orbits/OrbitSystem.cs

using Godot;
using System;
using System.Collections.Generic;
using Vim.Math3d;

[Tool]
public class OrbitSystem : Node, IPointMass
{
    [Export]
    private NodePath[] _pointMassPaths;

    private double _semiMajorAxis;
    [Export]
    private double SemiMajorAxis
    {
        get => _semiMajorAxis;
        set
        {
            _semiMajorAxis = value;

            if (!Engine.EditorHint) return;

            InvalidateGeometry();
        }
    }


    private double _eccentricity;
    [Export(PropertyHint.Range, "0,1")]
    private double Eccentricity
    {
        get => _eccentricity;
        set
        {
            _eccentricity = value;

            if (!Engine.EditorHint) return;

            InvalidateGeometry();
        }
    }

    private IPointMass[] _pointMasses;
    public IPointMass[] Orbiters
    {
        get
        {
            if (_pointMasses == null)
            {
                _pointMasses = new IPointMass[_pointMassPaths.Length];
                // give each body an orbit around ourselves
                for (int i = 0; i < _pointMassPaths.Length; i++)
                {
                    _pointMasses[i] = GetNode<IPointMass>(_pointMassPaths[i]);
                }
            }
            return _pointMasses;
        }
    }

    public double Mass
    {
        get
        {
            double mass = 0;
            foreach (var pointMass in _pointMasses)
            {
                mass += pointMass.Mass;
            }
            return mass;
        }
    }
    public DVector3 Position { get; set; }

    private Orbit _orbit = null;
    private Orbit Orbit
    {
        get
        {
            if (_orbit == null)
            {
                _orbit = new Orbit();
            }
            return _orbit;
        }
    }

    private ImmediateGeometry _orbitGeometry = null;
    public ImmediateGeometry OrbitGeometry
    {
        get
        {
            if (_orbitGeometry == null)
            {
                _orbitGeometry = new ImmediateGeometry();
                var m = new SpatialMaterial();
                m.VertexColorUseAsAlbedo = true;
                m.FlagsUnshaded = true;
                _orbitGeometry.MaterialOverride = m;
                AddChild(_orbitGeometry);
            }
            return _orbitGeometry;
        }
    }

    private float _time = 0;

    [Export]
    private float _speed = 3f;

    public override void _Process(float delta)
    {
        _time += delta * _speed;
        InvalidateGeometry();
    }

    private void InvalidateGeometry()
    {
        Orbit.Ellipse = new Ellipse(SemiMajorAxis, Eccentricity);
        Orbit.Draw(OrbitGeometry);

        Orbiter.Position = Orbit.GetPosition(_time);
    }
}