// Compute and log the measured periods of the moons.
void LogPeriods(Ephemeris<KSP> const& ephemeris) {
auto const position = [this, &ephemeris](
not_null<MassiveBody const*> body, Instant const& t) {
return ephemeris.trajectory(body)->EvaluatePosition(t, nullptr);
};
auto const barycentre = [this, &position](Instant const& t) {
BarycentreCalculator<Position<KSP>, Mass> result;
for (auto const body : jool_system_) {
result.Add(position(body, t), body->mass());
}
return result.Get();
};
auto const barycentric_position =
[this, &barycentre, &ephemeris, &position](
not_null<MassiveBody const*> body,
Instant const& t) {
return position(body, t) - barycentre(t);
};
for (auto const moon : {laythe_, vall_, tylo_}) {
auto const moon_y =
[this, &barycentric_position, moon](Instant const& t) {
return barycentric_position(moon, t).coordinates().y;
};
LOG(INFO) << (moon == laythe_ ? "Laythe" : moon == vall_ ? "Vall"
: "Tylo");
Sign const s0(moon_y(game_epoch_));
Instant t0 = game_epoch_;
Time const Δt = 45 * Minute;
while (Sign(moon_y(t0)) == s0) {
t0 += Δt;
}
// The moon crosses the xz plane between t0 and t0 - Δt.
Instant t1 = t0;
int const orbits = moon == laythe_ ? 8 : moon == vall_ ? 4 : 2;
for (int i = 0; i < orbits; ++i) {
while (Sign(moon_y(t1)) != s0) {
t1 += Δt;
}
// The crossing of the xz plane halfway through the orbit occurs between
// t1 and t1 - Δt.
while (Sign(moon_y(t1)) == s0) {
t1 += Δt;
}
// The |i|th orbit ends between t1 and t1 - Δt.
}
Time const actual_period =
(Bisect(moon_y, t1 - Δt, t1) - Bisect(moon_y, t0 - Δt, t0)) / orbits;
Time const expected_period = (2 * π * Radian) /
*elements_[moon].mean_motion;
LOG(INFO) << "actual period : " << actual_period;
LOG(INFO) << "expected period : " << expected_period;
LOG(INFO) << "error : "
<< RelativeError(expected_period, actual_period);
}
}
T Barycentre(std::vector<T> const& ts, std::vector<Scalar> const& weights) {
CHECK_EQ(ts.size(), weights.size()) << "Ts and weights of unequal sizes";
CHECK(!ts.empty()) << "Empty input";
BarycentreCalculator<T, Scalar> calculator;
for (std::size_t i = 0; i < ts.size(); ++i) {
calculator.Add(ts[i], weights[i]);
}
return calculator.Get();
}