shmodot/scripts/orbits/OrbitSystem.cs

using Godot;
using System;
using Vim.Math3d;

[Tool]
public class OrbitSystem : Node, IMassive, ILocation
{
    [Export] private NodePath _a;
    [Export] private NodePath _b;
    [Export] private NodePath _barycenter;

    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();
        }
    }

    #region Point Masses
    private IPointMass Primary
    {
        get
        {
            if (_pointMasses[0] == null)
            {
                InitPointMasses();
            }
            return _pointMasses[0];
        }
    }
    private IPointMass Secondary
    {
        get
        {
            if (_pointMasses[1] == null)
            {
                InitPointMasses();
            }
            return _pointMasses[1];
        }
    }
    private readonly IPointMass[] _pointMasses = new IPointMass[2];
    private void InitPointMasses()
    {
        var a = GetNode<IPointMass>(_a);
        var b = GetNode<IPointMass>(_b);

        if (a.Mass > b.Mass)
        {
            _pointMasses[0] = a;
            _pointMasses[1] = b;
        }
        else
        {
            _pointMasses[0] = b;
            _pointMasses[1] = a;
        }
    }
    #endregion

    public float Mass => Primary.Mass + Secondary.Mass;
    public DVector3 Position
    {
        get => Barycenter;
        set => Barycenter = value;
    }

    public DVector3 Barycenter
    {
        get
        {
            var p0 = Primary.Position;
            var p1 = Secondary.Position;
            return p0.Lerp(p1, .5f);
        }
        // TODO - make setting the berycenter do something sensible?
        set => _ = value;
    }

    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);

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