Updated the way that steps are handled

src/controller.rs

@@ -1,5 +1,31 @@
+#[derive(Debug, Clone, Copy, PartialEq)]
+pub enum TryStep {
+    Accepted(f64, f64),
+    NotYetAccepted(f64),
+}
+
+impl TryStep {
+    pub fn extract(&self) -> f64 {
+        match self {
+            TryStep::Accepted(h, _) => *h,
+            TryStep::NotYetAccepted(h) => *h,
+        }
+    }
+
+    pub fn is_accepted(&self) -> bool {
+        matches!(self, TryStep::Accepted(_, _))
+    }
+
+    pub fn reset(&mut self) -> Result<TryStep, &str> {
+        match self {
+            TryStep::Accepted(_, h) => Ok(TryStep::NotYetAccepted(*h)),
+            TryStep::NotYetAccepted(_) => Err("Cannot reset a NotYetAccepted TryStep"),
+        }
+    }
+}
+
 pub trait Controller<const D: usize> {
-    fn determine_step(&mut self, h: f64, err: f64) -> (bool, f64);
+    fn determine_step(&mut self, h: f64, err: f64) -> TryStep;
 }
 
 #[derive(Debug, Clone, Copy)]
@@ -11,32 +37,30 @@ pub struct PIController {
     pub factor_old: f64,
     pub h_max: f64,
     pub safety_factor: f64,
-    pub old_h: f64,
+    pub next_step_guess: TryStep,
 }
 
 impl<const D: usize> Controller<D> for PIController {
     /// Determines if the previously run step size and error were valid or not. Either way, it also
     /// returns what the next step size should be
-    fn determine_step(&mut self, h: f64, err: f64) -> (bool, f64) {
+    fn determine_step(&mut self, prev_step: f64, err: f64) -> TryStep {
         let factor_11 = err.powf(self.alpha);
         let factor = self.factor_c2.max(
             self.factor_c1
                 .min(factor_11 * self.factor_old.powf(-self.beta) / self.safety_factor),
         );
-        let mut h_new = h / factor;
         if err <= 1.0 {
-            // Accept the stepsize
+            let mut h = prev_step / factor;
+            // Accept the stepsize and provide what the next step size should be
             self.factor_old = err.max(1.0e-4);
-            if h_new.abs() > self.h_max {
-                // If the step is too big
-                h_new = self.h_max.copysign(h_new);
+            if h.abs() > self.h_max {
+                // If the step goes past the maximum allowed, though, we shrink it
+                h = self.h_max.copysign(h);
             }
-            (true, h_new)
-            // (true, h_new)
+            TryStep::Accepted(prev_step, h)
         } else {
-            // Reject the stepsize and propose a smaller one
-            h_new = h / (self.factor_c1.min(factor_11 / self.safety_factor));
-            (false, h_new)
+            // Reject the stepsize and propose a smaller one for the current step
+            TryStep::NotYetAccepted(prev_step / (self.factor_c1.min(factor_11 / self.safety_factor)))
         }
     }
 }
@@ -59,7 +83,7 @@ impl PIController {
             factor_old: 1.0e-4,
             h_max: h_max.abs(),
             safety_factor,
-            old_h: initial_h,
+            next_step_guess: TryStep::NotYetAccepted(initial_h),
         }
     }
 }
@@ -85,6 +109,6 @@ mod tests {
         assert!(controller.factor_old == 1.0e-4);
         assert!(controller.h_max == 10.0);
         assert!(controller.safety_factor == 0.9);
-        assert!(controller.old_h == 1e-4);
+        assert!(controller.next_step_guess == TryStep::NotYetAccepted(1e-4));
     }
 }