cross: step: finish backfilling step_h tests

crates/cross/src/step/mod.rs

@@ -189,6 +189,7 @@ impl<'a, R: Real, M: Material<R>> StepHContext<'a, R, M> {
         let mu0_inv = R::mu0_inv();
         let deltas = self.in_e.delta_e();
         // \nabla x E
+        // TODO: inv_feature_size and time_step could be folded
         let nabla_e = deltas.nabla() * self.inv_feature_size;
         let delta_b = nabla_e * (-self.time_step);
 
@@ -216,6 +217,7 @@ mod test {
     use crate::compound::Optional;
     use crate::mat::Vacuum;
     use float_eq::assert_float_eq;
+    use std::cell::Cell;
 
     fn assert_vec_eq(a: Vec3<f32>, b: Vec3<f32>) {
         assert_float_eq!(a.x(), b.x(), r2nd <= 1e-6);
@@ -286,12 +288,112 @@ mod test {
         //    $  2.0 - 4.0 ]$
         // this gets negated, and then scaled by $\mu_0^{-1}$
         //```
+        let delta_b = -Vec3::new(-2.0, 4.0, -2.0);
 
         let (new_h, new_m) = ctx.step_h();
-        let b = -Vec3::new(-2.0, 4.0, -2.0);
-        assert_vec_eq(new_h, b * f32::mu0_inv());
+        assert_vec_eq(new_h, delta_b * f32::mu0_inv());
         assert_vec_eq(new_m, Vec3::zero());
     }
-    // TODO: backfill step_h tests
+    #[test]
+    fn step_h_understands_time_scale_feature_size() {
+        let mid = Vec3::new(4.0, 8.0, 12.0);
+        let ctx = StepHContext {
+            inv_feature_size: 5.0, // $\Delta x = 0.2$
+            time_step: 1e-3,
+            stim_h: Vec3::zero(),
+            mat: &Vacuum,
+            in_e: VolumeSamplePos {
+                mid,
+                xp1: Optional::some(mid + Vec3::new(1.0, 2.0, 3.0)),
+                yp1: Optional::some(mid + Vec3::new(4.0, 5.0, 6.0)),
+                zp1: Optional::some(mid + Vec3::new(7.0, 8.0, 9.0)),
+            },
+            in_h: Vec3::zero(),
+            in_m: Vec3::zero(),
+        };
+
+        // see step_h_understands_nabla_e
+        // the nabla is dE/dx, so divide by \Delta x.
+        // nabla relates to dB/dt, so multiply by \Delta t to get \Delta B
+        let delta_b = -Vec3::new(-2.0, 4.0, -2.0) * 1e-3 / 0.2;
+
+        let (new_h, new_m) = ctx.step_h();
+        assert_vec_eq(new_h, delta_b * f32::mu0_inv());
+        assert_vec_eq(new_m, Vec3::zero());
+    }
+
+    struct MockMaterial {
+        response: Vec3<f32>,
+        called_with: Cell<(Vec3<f32>, Vec3<f32>)>,
+    }
+    impl Material<f32> for MockMaterial {
+        fn move_b_vec(&self, m: Vec3<f32>, target_b: Vec3<f32>) -> Vec3<f32> {
+            self.called_with.set((m, target_b));
+            self.response
+        }
+    }
+    #[test]
+    fn step_h_respects_material() {
+        let mock_mat = MockMaterial {
+            response: Vec3::new(1e-6, 2e-6, 3e-6),
+            called_with: Default::default(),
+        };
+        let ctx = StepHContext {
+            inv_feature_size: 1.0,
+            time_step: 1.0,
+            stim_h: Vec3::zero(),
+            mat: &mock_mat,
+            in_e: VolumeSamplePos {
+                mid: Vec3::zero(),
+                xp1: Optional::some(Vec3::new(1.0, 2.0, 3.0)),
+                yp1: Optional::some(Vec3::new(4.0, 5.0, 6.0)),
+                zp1: Optional::some(Vec3::new(7.0, 8.0, 9.0)),
+            },
+            in_h: Vec3::zero(),
+            in_m: Vec3::zero(),
+        };
+        // see step_h_understands_nabla_e
+        let delta_b = -Vec3::new(-2.0, 4.0, -2.0);
+
+        let (new_h, new_m) = ctx.step_h();
+        assert_vec_eq(mock_mat.called_with.get().0, Vec3::zero()); // not magnetized
+        assert_vec_eq(mock_mat.called_with.get().1, delta_b);
+        assert_vec_eq(new_m, mock_mat.response);
+        assert_vec_eq(new_h, delta_b * f32::mu0_inv() - new_m);
+    }
+    #[test]
+    fn step_h_understands_previous_m_h() {
+        let mock_mat = MockMaterial {
+            response: Vec3::new(1e-6, 2e-6, 3e-6),
+            called_with: Default::default(),
+        };
+        let in_h = Vec3::new(4e-6, 5e-6, 6e-6);
+        let in_m = Vec3::new(7e-6, 8e-6, 9e-6);
+        let ctx = StepHContext {
+            inv_feature_size: 1.0,
+            time_step: 1.0,
+            stim_h: Vec3::zero(),
+            mat: &mock_mat,
+            in_e: VolumeSamplePos {
+                mid: Vec3::zero(),
+                xp1: Optional::some(Vec3::new(1.0, 2.0, 3.0)),
+                yp1: Optional::some(Vec3::new(4.0, 5.0, 6.0)),
+                zp1: Optional::some(Vec3::new(7.0, 8.0, 9.0)),
+            },
+            in_h,
+            in_m,
+        };
+        // see step_h_understands_nabla_e
+        let delta_b = -Vec3::new(-2.0, 4.0, -2.0);
+        let prev_b = (in_h + in_m) * f32::mu0();
+        let new_b = prev_b + delta_b;
+
+        let (new_h, new_m) = ctx.step_h();
+        assert_vec_eq(mock_mat.called_with.get().0, in_m);
+        assert_vec_eq(mock_mat.called_with.get().1, new_b);
+        assert_vec_eq(new_m, mock_mat.response);
+        assert_vec_eq(new_h, new_b * f32::mu0_inv() - new_m);
+    }
+
     // TODO: backfill step_e tests
 }