shmodot/scripts/orbits/PointMass.cs

using Godot;

public struct PointMass : IMassive, ILocation
{
    public float Mass => _massive == null ? _mass : _massive.Mass;
    public Vector3 Position => _spatial.Translation;

    private IMassive _massive;
    private Spatial _spatial;
    private float _mass;

    public PointMass(Spatial spatial, IMassive massive)
    {
        _spatial = spatial;
        _massive = massive;
        _mass = 0;
    }

    public PointMass(Spatial spatial, float mass)
    {
        _spatial = spatial;
        _massive = null;
        _mass = mass;
    }
}