stim: use a Visitor instead of a FoldOp for eval

boosts perf from 420 -> 530 fps

crates/coremem/src/stim.rs

@@ -1,7 +1,7 @@
 use crate::real::Real as _;
 use crate::cross::vec::Vec3;
 use crate::geom::{Coord as _, HasCrossSection, Index, Meters, Region};
-use coremem_cross::compound::list::{Fold, FoldOp};
+use coremem_cross::compound::list::{Visit, Visitor};
 use coremem_cross::dim::OffsetDimSlice;
 use rand;
 
@@ -48,23 +48,25 @@ pub trait AbstractStimulus: Sync {
 
 /// used as a MapVisitor in order to evaluate each Stimulus in a List at a specific time/place.
 struct StimulusEvaluator {
+    fields: Fields,
     t_sec: f32,
     pos: Meters,
 }
 
-impl<S: AbstractStimulus> FoldOp<Fields, &S> for StimulusEvaluator {
-    type Output = Fields;
-    fn feed(&mut self, prev: Fields, next: &S) -> Self::Output {
-        prev + next.at(self.t_sec, self.pos)
+impl<S: AbstractStimulus> Visitor<&S> for &mut StimulusEvaluator {
+    fn visit(&mut self, next: &S) {
+        self.fields += next.at(self.t_sec, self.pos);
     }
 }
 
 impl<L: Sync> AbstractStimulus for L
 where
-    for<'a> &'a L: Fold<StimulusEvaluator, Fields, Output=Fields>,
+    for<'a, 'b> &'a L: Visit<&'b mut StimulusEvaluator>,
 {
     fn at(&self, t_sec: f32, pos: Meters) -> Fields {
-        self.fold(StimulusEvaluator { t_sec, pos}, Fields::default())
+        let mut ev = StimulusEvaluator { t_sec, pos, fields: Fields::default()};
+        self.visit(&mut ev);
+        ev.fields
     }
 }