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<div class="window-title"><h2 style="padding: 0px; margin: 0px;">Learning Rust : Orbitals</h2></div>
<i>How do I store non-owning references to collection members?</i>
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<p>I want to make a data-driven viewer for the solar system, as part of a game inspired by <a href="https://www.aurora2.pentarch.org/">Aurora 4x</a> but Rust is an entirely different beast from C.</p>
<p>So let's start with the types i'll use:</p>
<pre><code class="language-rust">
pub const GRAVITATIONAL_CONSTANT : f64 = 6.67430e-11;
pub type Second = u64;
pub type Meter = f64;
pub type Rad = f64;
pub type Kilogram = f64;
</code></pre>
<p>I have prior experience writing systems like this, so I figured it would be as easy as:</p>
<pre><code class="language-rust">
pub struct Orbital {
//Some optional non-owning reference to the parent orbital
mass : Kilogram,
semi_major_axis : Rad,
eccentricity : Rad,
inclination : Rad,
long_asc_node : Rad,
argument_periapsis : Rad,
mean_anomaly_epoch : Rad
}
</code></pre>
<p>The question then is: How do we store the reference to the parent orbital?</p>
<p>note: by parent orbital, I mean the orbital body that the given orbital body orbits.</p>
<p>In C, we would just store a pointer into the data of the owning container (i.e. a dynamic array).</p>
<p>I understand that the method above could cause problems with object lifetimes and dead references, but in this case we are assuming a static lifetime with a fixed-width container.</p>
<p>So my first instinct is to look in the Rust docs for a non-owning reference type.</p>
<p>I wasted about a day researching Rc's and RefCell's when I had a revelation; what I wanted was over-engineered. Instead of storing a reference to the parent orbital, I could just store an index into the system's orbital container. All I need to do to make this work is pass a borrowed ref to the system object to the orbital's update function.</p>
<p>But not every part of the update function needs the parent orbital's position, so I can split the update into two parts:</p>
<p>1: Update the orbital elements relative to the parent object</p>
<p>2: Use the parent orbital's position to transform the relative coordinates into absolute coordinates</p>
<p>All the tick method needs is an immutable parent object to calculate the standard gravitational parameter of the one-body system being emulated.</p>
<p>So here's what I came up with.</p>
<pre><code class="language-rust">
pub struct Orbital {
origin : Option<usize>,
//Euler angles for a given time (T) relative to origin
rel_pos : Option<(Second, Meter, Rad, Rad, Rad)>
//Cartesian coordinates for a given time (T) relative to (0, 0, 0)
abs_pos : Option<(Second, Meter, Meter, Meter)>
//Orbital parameters
mass : Kilogram,
semi_major_axis : Meter,
eccentricity : Rad,
inclination : Rad,
long_asc_node : Rad,
argument_periapsis : Rad,
mean_anomaly_epoch : Rad
}
//...
impl Orbital {
//...
//Update relative
pub fn tick(&mut self, &system: System, t: Second) {
//...
}
pub fn pos(&self, &system : System, t: Second) -> (Meter, Meter, Meter) {
//...
}
}
</code></pre>
<p>Now comes the daunting task of storing our orbitals in a usable manner. Ideally, I want to iterate through the list of orbitals in a way that also lets me access other members of the orbital list from the tick and pos functions. It's a lot harder than it should be.</p>
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