spirv: remove the ArrayHandleMut artifacts in Step{H,E}Context

this will make it easier to reuse these blocks on the CPU side.

crates/spirv_backend/src/lib.rs

@@ -39,8 +39,10 @@ fn step_h<R: Real, M: Material<R>>(
 ) {
     if id.x() < meta.dim.x() && id.y() < meta.dim.y() && id.z() < meta.dim.z() {
         let sim_state = SerializedStepH::new(meta, stimulus_h, material, e, h, m);
-        let update_state = sim_state.index(id);
+        let (update_state, mut out_h, mut out_m) = sim_state.index(id);
-        update_state.step_h();
+        let (new_h, new_m) = update_state.step_h();
+        out_h.write(new_h);
+        out_m.write(new_m);
     }
 }
 
@@ -55,8 +57,9 @@ fn step_e<R: Real, M: Material<R>>(
     if id.x() < meta.dim.x() && id.y() < meta.dim.y() && id.z() < meta.dim.z() {
         let sim_state = SerializedStepE::new(meta, stimulus_e, material, e, h);
 
-        let update_state = sim_state.index(id);
+        let (update_state, mut out_e) = sim_state.index(id);
-        update_state.step_e();
+        let new_e = update_state.step_e();
+        out_e.write(new_e);
     }
 }
 

crates/spirv_backend/src/sim.rs

@@ -41,7 +41,13 @@ impl<'a, R, M> SerializedStepH<'a, R, M> {
 }
 
 impl<'a, R: Real, M> SerializedStepH<'a, R, M> {
-    pub fn index(self, idx: Vec3u) -> StepHContext<'a, R, M> {
+    /// returns a context which the user can call `step_h` on,
+    /// plus output handles which the the results can be written to.
+    pub fn index(self, idx: Vec3u) -> (
+        StepHContext<'a, R, M>,
+        ArrayHandleMut<'a, Vec3<R>>, // out_h
+        ArrayHandleMut<'a, Vec3<R>>, // out_m
+    ){
         let dim = self.meta.dim;
         let stim_h_matrix = Array3::new(self.stimulus_h, dim);
         let mat_matrix = Array3::new(self.material, dim);
@@ -60,15 +66,19 @@ impl<'a, R: Real, M> SerializedStepH<'a, R, M> {
 
         let mat = mat_matrix.into_handle(idx);
 
-        StepHContext {
+        (
-            inv_feature_size: self.meta.inv_feature_size,
+            StepHContext {
-            time_step: self.meta.time_step,
+                inv_feature_size: self.meta.inv_feature_size,
-            stim_h: stim_h_matrix.get(idx).unwrap(),
+                time_step: self.meta.time_step,
-            mat,
+                stim_h: stim_h_matrix.get(idx).unwrap(),
-            in_e,
+                mat,
+                in_e,
+                in_h: out_h.get(),
+                in_m: out_m.get(),
+            },
             out_h,
             out_m,
-        }
+        )
     }
 }
 
@@ -96,7 +106,10 @@ impl<'a, R, M> SerializedStepE<'a, R, M> {
 }
 
 impl<'a, R: Real, M> SerializedStepE<'a, R, M> {
-    pub fn index(self, idx: Vec3u) -> StepEContext<'a, R, M> {
+    pub fn index(self, idx: Vec3u) -> (
+        StepEContext<'a, R, M>,
+        ArrayHandleMut<'a, Vec3<R>> // out_e
+    ){
         let dim = self.meta.dim;
         let stim_e_matrix = Array3::new(self.stimulus_e, dim);
         let mat_matrix = Array3::new(self.material, dim);
@@ -129,14 +142,17 @@ impl<'a, R: Real, M> SerializedStepE<'a, R, M> {
 
         let mat = mat_matrix.into_handle(idx);
 
-        StepEContext {
+        (
-            inv_feature_size: self.meta.inv_feature_size,
+            StepEContext {
-            time_step: self.meta.time_step,
+                inv_feature_size: self.meta.inv_feature_size,
-            stim_e: stim_e_matrix.get(idx).unwrap(),
+                time_step: self.meta.time_step,
-            mat,
+                stim_e: stim_e_matrix.get(idx).unwrap(),
-            in_h,
+                mat,
+                in_h,
+                in_e: out_e.get(),
+            },
             out_e,
-        }
+        )
     }
 }
 
@@ -279,12 +295,11 @@ pub struct StepEContext<'a, R, M> {
     mat: ArrayHandle<'a, M>,
     /// Input field sampled near this location
     in_h: VolumeSampleNeg<R>,
-    /// Handle to the output field at one specific index.
+    in_e: Vec3<R>,
-    out_e: ArrayHandleMut<'a, Vec3<R>>,
 }
 
 impl<'a, R: Real, M: Material<R>> StepEContext<'a, R, M> {
-    pub fn step_e(mut self) {
+    pub fn step_e(self) -> Vec3<R> {
         let twice_eps0 = R::twice_eps0();
         let deltas = self.in_h.delta_h();
         // \nabla x H
@@ -293,12 +308,12 @@ impl<'a, R: Real, M: Material<R>> StepEContext<'a, R, M> {
         // no-conductivity version:
         //   let delta_e = nabla_h * (self.time_step * EPS0_INV);
         let sigma = self.mat.get_ref().conductivity();
-        let e_prev = self.out_e.get();
+        let e_prev = self.in_e;
         let delta_e = (nabla_h - e_prev.elem_mul(sigma)).elem_div(
             sigma*self.time_step + Vec3::uniform(twice_eps0)
         )*(R::two()*self.time_step);
         // println!("spirv-step_e delta_e: {:?}", delta_e);
-        self.out_e.write(e_prev + delta_e + self.stim_e);
+        e_prev + delta_e + self.stim_e
     }
 }
 
@@ -310,13 +325,12 @@ pub struct StepHContext<'a, R, M> {
     mat: ArrayHandle<'a, M>,
     /// Input field sampled near this location
     in_e: VolumeSamplePos<R>,
-    /// Handle to the output field at one specific index.
+    in_h: Vec3<R>,
-    out_h: ArrayHandleMut<'a, Vec3<R>>,
+    in_m: Vec3<R>,
-    out_m: ArrayHandleMut<'a, Vec3<R>>,
 }
 
 impl<'a, R: Real, M: Material<R>> StepHContext<'a, R, M> {
-    pub fn step_h(mut self) {
+    pub fn step_h(self) -> (Vec3<R>, Vec3<R>) {
         let mu0 = R::mu0();
         let mu0_inv = R::mu0_inv();
         let deltas = self.in_e.delta_e();
@@ -327,8 +341,8 @@ impl<'a, R: Real, M: Material<R>> StepHContext<'a, R, M> {
         let delta_b = nabla_e * (-self.time_step);
 
         // Relation between these is: B = mu0*(H + M)
-        let old_h = self.out_h.get();
+        let old_h = self.in_h;
-        let old_m = self.out_m.get();
+        let old_m = self.in_m;
         let old_b = (old_h + old_m) * mu0;
 
         let new_b = old_b + delta_b + self.stim_h * mu0;
@@ -336,7 +350,6 @@ impl<'a, R: Real, M: Material<R>> StepHContext<'a, R, M> {
         let new_m = mat.move_b_vec(old_m, new_b);
         let new_h = new_b * mu0_inv - new_m;
         // println!("spirv-step_h delta_h: {:?}", delta_h);
-        self.out_h.write(new_h);
+        (new_h, new_m)
-        self.out_m.write(new_m);
     }
 }