Simplify iterators in Kálmán actor

Currently, the Kálmán actor stores two iterators, one forward and
backwards, but this is not necessary. Instead, we can simply keep an
index and advance it forward or backwards, saving space and simplifying
the code.

Author: stephenswat

core/include/traccc/fitting/kalman_filter/kalman_actor.hpp

@@ -54,54 +54,55 @@ struct kalman_actor_state {
     TRACCC_HOST_DEVICE
     typename edm::track_state_collection<algebra_t>::device::proxy_type
     operator()() {
-        if (!backward_mode) {
-            return m_track_states.at(*m_it);
-        } else {
-            return m_track_states.at(*m_it_rev);
-        }
+        return m_track_states.at(m_track.state_indices().at(m_idx));
     }
 
     /// Reset the iterator
     TRACCC_HOST_DEVICE
     void reset() {
-        m_it = m_track.state_indices().begin();
-        m_it_rev = m_track.state_indices().rbegin();
+        if (!backward_mode) {
+            m_idx = 0;
+        } else {
+            m_idx = m_track.state_indices().size() - 1;
+        }
     }
 
     /// Advance the iterator
     TRACCC_HOST_DEVICE
     void next() {
         if (!backward_mode) {
-            m_it++;
+            m_idx++;
         } else {
-            m_it_rev++;
+            m_idx--;
         }
     }
 
     /// @return true if the iterator reaches the end of vector
     TRACCC_HOST_DEVICE
     bool is_complete() {
-        if (!backward_mode && m_it == m_track.state_indices().end()) {
+        if (!backward_mode && m_idx == m_track.state_indices().size()) {
             return true;
-        } else if (backward_mode &&
-                   m_it_rev == m_track.state_indices().rend()) {
+        } else if (backward_mode && m_idx > m_track.state_indices().size()) {
             return true;
         }
         return false;
     }
 
+    TRACCC_HOST_DEVICE
+    bool is_state() {
+        return m_track.state_indices().at(m_idx) !=
+               std::numeric_limits<unsigned int>::max();
+    }
+
     /// Object describing the track fit
     typename edm::track_fit_collection<algebra_t>::device::proxy_type m_track;
     /// All track states in the event
     typename edm::track_state_collection<algebra_t>::device m_track_states;
     /// All measurements in the event
     measurement_collection_types::const_device m_measurements;
 
-    /// Iterator for forward filtering over the track states
-    vecmem::device_vector<unsigned int>::iterator m_it;
-
-    /// Iterator for backward filtering over the track states
-    vecmem::device_vector<unsigned int>::reverse_iterator m_it_rev;
+    /// Index of the current track state
+    unsigned int m_idx;
 
     // The number of holes (The number of sensitive surfaces which do not
     // have a measurement for the track pattern)

core/include/traccc/fitting/kalman_filter/kalman_fitter.hpp

@@ -259,22 +259,23 @@ class kalman_fitter {
             return kalman_fitter_status::ERROR_BARCODE_SEQUENCE_OVERFLOW;
         }
 
-        auto& track = fitter_state.m_fit_actor_state.m_track;
-        auto& track_states = fitter_state.m_fit_actor_state.m_track_states;
+        fitter_state.m_fit_actor_state.backward_mode = true;
+        fitter_state.m_fit_actor_state.reset();
 
         // Since the smoothed track parameter of the last surface can be
         // considered to be the filtered one, we can reversly iterate the
         // algorithm to obtain the smoothed parameter of other surfaces
-        for (auto it = track.state_indices().rbegin();
-             it != track.state_indices().rend(); ++it) {
-            if (!track_states.at(*it).is_hole()) {
-                fitter_state.m_fit_actor_state.m_it_rev = it;
-                break;
-            }
-            // TODO: Return false because there is no valid track state
-            // return false;
+        while (!fitter_state.m_fit_actor_state.is_complete() &&
+               (!fitter_state.m_fit_actor_state.is_state() ||
+                fitter_state.m_fit_actor_state().is_hole())) {
+            fitter_state.m_fit_actor_state.next();
         }
-        auto last = track_states.at(*fitter_state.m_fit_actor_state.m_it_rev);
+
+        if (fitter_state.m_fit_actor_state.is_complete()) {
+            return kalman_fitter_status::SUCCESS;
+        }
+
+        auto last = fitter_state.m_fit_actor_state();
 
         const scalar theta = last.filtered_params().theta();
         if (theta <= 0.f || theta >= constant<traccc::scalar>::pi) {
@@ -322,7 +323,6 @@ class kalman_fitter {
 
         propagation._navigation.set_direction(
             detray::navigation::direction::e_backward);
-        fitter_state.m_fit_actor_state.backward_mode = true;
 
         // Synchronize the current barcode with the input track parameter
         while (propagation._navigation.get_target_barcode() !=