Cache the timestep field on the SimState.

src/sim.rs

@@ -358,6 +358,7 @@ pub struct SimState<M=GenericMaterial> {
     scratch: Array3<Cell<M>>,
     feature_size: Real,
     step_no: u64,
+    timestep: Real,
 }
 
 impl<M> SimState<M> {
@@ -381,6 +382,12 @@ impl<M> SimState<M> {
         self.feature_size.into()
     }
     pub fn timestep(&self) -> Flt {
+        self.timestep.into()
+    }
+}
+
+impl<M: Default> SimState<M> {
+    pub fn new(size: Index, feature_size: Flt) -> Self {
         // XXX this has to be multiplied by a Courant Number in order to ensure stability.
         // For 3d, we need 3 full timesteps in order to communicate information across the corner
         // of a grid. The distance traveled during that time is \sqrt{3}. Hence each timestep needs
@@ -388,16 +395,13 @@ impl<M> SimState<M> {
         // In 2d, this would be \sqrt{2}/2.
         // It's an upper limit though; it should be safe to go lower.
         let courant = 0.577;
-        (self.feature_size / consts::real::C() * courant).into()
-    }
-}
-
-impl<M: Default> SimState<M> {
-    pub fn new(size: Index, feature_size: Flt) -> Self {
+        let feature_size = Real::from(feature_size);
+        let timestep = feature_size / consts::real::C() * courant;
         Self {
             cells: Array3::default((size.z() as _, size.y() as _, size.x() as _)),
             scratch: Array3::default((size.z() as _, size.y() as _, size.x() as _)),
-            feature_size: feature_size.into(),
+            feature_size,
+            timestep,
             ..Default::default()
         }
     }
@@ -406,7 +410,7 @@ impl<M: Default> SimState<M> {
 impl<M: Material + Clone + Default + Send + Sync + 'static> SimState<M> {
     pub fn step(&mut self) {
         use consts::real::*;
-        let time_step = Real::from_inner(self.timestep());
+        let time_step = self.timestep;
         let half_time_step = HALF() * time_step;
         let feature_size = self.feature_size;
         let inv_feature_size = Real::from_inner(1.0) / feature_size;
@@ -843,7 +847,6 @@ impl<M: Material> Cell<M> {
         use consts::real::{HALF, ONE, TWO, ZERO};
         // let inv_feature_size = Real::from_inner(1.0) / feature_size;
         let twice_delta_t = TWO() * delta_t;
-        let half_delta_t = HALF() * delta_t;
 
         let b_prev = self.b();