spirv: re-order the SimBackend parameters to be more consistent

crates/coremem/src/sim/spirv/cpu.rs

@@ -13,14 +13,14 @@ impl<R: Real, M: Material<R>> SimBackend<R, M> for CpuBackend {
     }
     fn step_n(
         &self,
-        num_steps: u32,
         meta: SimMeta<R>,
+        mat: &[M],
         stim_e: &[Vec3<R>],
         stim_h: &[Vec3<R>],
-        mat: &[M],
         e: &mut [Vec3<R>],
         h: &mut [Vec3<R>],
         m: &mut [Vec3<R>],
+        num_steps: u32,
     ) {
         for _ in 0..num_steps {
             // step E field

crates/coremem/src/sim/spirv/gpu.rs

@@ -63,14 +63,14 @@ impl<R: Copy, M: Send + Sync + HasEntryPoints<R>> SimBackend<R, M> for WgpuBacke
 
     fn step_n(
         &self,
-        num_steps: u32,
         meta: SimMeta<R>,
+        mat: &[M],
         stim_cpu_e: &[Vec3<R>],
         stim_cpu_h: &[Vec3<R>],
-        mat: &[M],
         e: &mut [Vec3<R>],
         h: &mut [Vec3<R>],
         m: &mut [Vec3<R>],
+        num_steps: u32,
     ) {
         let field_bytes = meta.dim.product_sum() as usize * std::mem::size_of::<Vec3<f32>>();
 

crates/coremem/src/sim/spirv/mod.rs

@@ -16,18 +16,24 @@ mod gpu;
 pub use cpu::CpuBackend;
 pub use gpu::WgpuBackend;
 
+/// while the SpirvSim type owns all the stateful data,
+/// it dispatches to a pluggable backend for advancing that state.
+///
+/// this backend type abstracts over the specific compute used to advance the state.
+/// it could advance the state locally, on the CPU,
+/// or it could dispatch to a SPIR-V device, like a GPU.
 pub trait SimBackend<R, M> {
     fn new(volume: u64) -> Self;
     fn step_n(
         &self,
-        num_steps: u32,
         meta: SimMeta<R>,
+        mat: &[M],
         stim_e: &[Vec3<R>],
         stim_h: &[Vec3<R>],
-        mat: &[M],
         e: &mut [Vec3<R>],
         h: &mut [Vec3<R>],
         m: &mut [Vec3<R>],
+        num_steps: u32,
     );
 }
 
@@ -137,14 +143,14 @@ where
         let (stim_e, stim_h) = self.eval_stimulus(stim);
         let backend = &**self.backend.as_ref().unwrap();
         backend.step_n(
-            num_steps,
             self.meta,
+            self.mat.as_slice(),
             stim_e.as_slice().unwrap(),
             stim_h.as_slice().unwrap(),
-            self.mat.as_slice(),
             self.e.as_mut_slice(),
             self.h.as_mut_slice(),
             self.m.as_mut_slice(),
+            num_steps,
         );
         self.step_no += num_steps as u64;
     }