/
ShepperdTests.cs
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/
ShepperdTests.cs
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/*
* Copyright Lamont Granquist, Sebastien Gaggini and the MechJeb contributors
* SPDX-License-Identifier: MIT-0 OR LGPL-2.1+ OR CC0-1.0
*/
using System;
using System.Collections.Generic;
using AssertExtensions;
using MechJebLib.Core;
using MechJebLib.Core.ODE;
using MechJebLib.Core.TwoBody;
using MechJebLib.Primitives;
using Xunit;
using Xunit.Abstractions;
using static MechJebLib.Statics;
using static System.Math;
namespace MechJebLibTest.MathsTests
{
public class ShepperdTests
{
private readonly ITestOutputHelper _testOutputHelper;
public ShepperdTests(ITestOutputHelper testOutputHelper)
{
_testOutputHelper = testOutputHelper;
}
[Fact]
private void RandomForwardAndBack()
{
const int NTRIALS = 5000;
var random = new Random();
for (int i = 0; i < NTRIALS; i++)
{
var r0 = new V3(4 * random.NextDouble() - 2, 4 * random.NextDouble() - 2, 4 * random.NextDouble() - 2);
var v0 = new V3(4 * random.NextDouble() - 2, 4 * random.NextDouble() - 2, 4 * random.NextDouble() - 2);
double dt = 40 * random.NextDouble() - 20;
// XXX: this probably needs a test to reject random orbits that are near-parabolic. See the
// Farnocchia paper.
(V3 rf, V3 vf) = Shepperd.Solve(1.0, dt, r0, v0);
(V3 rp, V3 vp) = Shepperd.Solve(1.0, -dt, rf, vf);
if (!NearlyEqual(rp, r0, 1e-8) || !NearlyEqual(vp, v0, 1e-8))
{
_testOutputHelper.WriteLine("r0 :" + r0 + " v0:" + v0 + " dt:" + dt + "\nrf:" + rf + " vf:" + vf + "\nrf2:" + rp + " vf2:" +
vp + "\n");
}
if ((rp - r0).magnitude / r0.magnitude > 1e-8 || (vp - v0).magnitude / v0.magnitude > 1e-8)
{
_testOutputHelper.WriteLine(r0 + " " + v0);
}
rp.ShouldEqual(r0, 1e-8);
vp.ShouldEqual(vp, 1e-8);
}
}
[Fact]
private void RandomForwardAndBack2()
{
const int NTRIALS = 5;
var random = new Random();
for (int i = 0; i < NTRIALS; i++)
{
var r0 = new V3(4 * random.NextDouble() - 2, 4 * random.NextDouble() - 2, 4 * random.NextDouble() - 2);
var v0 = new V3(4 * random.NextDouble() - 2, 4 * random.NextDouble() - 2, 4 * random.NextDouble() - 2);
double dt = 40 * random.NextDouble() - 20;
// XXX: this probably needs a test to reject random orbits that are near-parabolic. See the
// Farnocchia paper.
(V3 rf, V3 vf, _, _, _, _) = Shepperd.Solve2(1.0, dt, r0, v0);
(V3 rp, V3 vp, _, _, _, _) = Shepperd.Solve2(1.0, -dt, rf, vf);
if (!NearlyEqual(rp, r0, 1e-8) || !NearlyEqual(vp, v0, 1e-8))
{
_testOutputHelper.WriteLine("r0 :" + r0 + " v0:" + v0 + " dt:" + dt + "\nrf:" + rf + " vf:" + vf + "\nrf2:" + rp + " vf2:" +
vp + "\n");
}
if ((rp - r0).magnitude / r0.magnitude > 1e-8 || (vp - v0).magnitude / v0.magnitude > 1e-8)
{
_testOutputHelper.WriteLine(r0 + " " + v0);
}
rp.ShouldEqual(r0, 1e-8);
vp.ShouldEqual(vp, 1e-8);
}
}
private readonly VacuumKernel _ode = new VacuumKernel();
private class VacuumKernel
{
public int N => 6;
public void dydt(IList<double> yin, double x, IList<double> dyout)
{
var r = new V3(yin[0], yin[1], yin[2]);
var v = new V3(yin[3], yin[4], yin[5]);
double rm2 = r.sqrMagnitude;
double rm = Sqrt(rm2);
double rm3 = rm2 * rm;
V3 dr = v;
V3 dv = -r / rm3;
dyout.Set(0, dr);
dyout.Set(3, dv);
}
}
[Fact]
private void RandomComparedToDormandPrince()
{
var solver = new DP5();
solver.Rtol = 1e-13;
solver.Hmin = EPS;
solver.ThrowOnMaxIter = true;
solver.Maxiter = 2000;
const int NTRIALS = 5000;
var random = new Random();
for (int i = 0; i < NTRIALS; i++)
{
var r0 = new V3(4 * random.NextDouble() - 2, 4 * random.NextDouble() - 2, 4 * random.NextDouble() - 2);
var v0 = new V3(4 * random.NextDouble() - 2, 4 * random.NextDouble() - 2, 4 * random.NextDouble() - 2);
double dt = 10 * random.NextDouble() - 5;
(double _, double ecc, double _, double _, double _, double _, double _) =
Maths.KeplerianFromStateVectors(1.0, r0, v0);
// near-parabolic orbits are difficult for Shepperd, see the Farnocchia paper.
if (ecc < 1.01 && ecc > 0.99)
continue;
// massively hyperbolic orbits are hard for DormandPrince
if (ecc > 8)
continue;
(V3 rf, V3 vf) = Shepperd.Solve(1.0, dt, r0, v0);
V3 rf2, vf2;
using (var y0 = Vn.Rent(6))
using (var yf = Vn.Rent(6))
{
y0.Set(0, r0);
y0.Set(3, v0);
try
{
solver.Solve(_ode.dydt, y0, yf, 0, dt);
}
catch (ArgumentException)
{
// the ODE integrator can throw iterations exceeded for certain initial
// conditions it inherently has issues with.
continue;
}
rf2 = yf.Get(0);
vf2 = yf.Get(3);
}
if (!NearlyEqual(rf, rf2, 1e-5) || !NearlyEqual(vf, vf2, 1e-5))
{
_testOutputHelper.WriteLine("r0 :" + r0 + " v0:" + v0 + " dt:" + dt + " ecc:" + ecc + "\nrf:" + rf + " vf:" + vf + "\nrf2:" +
rf2 + " vf2:" +
vf2 + "\n");
}
rf.ShouldEqual(rf2, 1e-5);
vf.ShouldEqual(vf2, 1e-5);
}
}
}
}