Minor fixes to the track building kernel (#859)

This commit makes some minor fixes and improvements to the track
building kernel. In particular, it:

1. Fixes an error where we were comparing measurements indices to see if
   they were _larger_ than the total number of measurements, but they
   should be check also for _equality_.
2. Fail more gracefully if we cannot find any true measurements, i.e.
   register the failed track as a candidate for pruning.
3. Simplify the control flow of the kernel by removing an unnecessary
   break statement.

Co-authored-by: beomki-yeo <63090140+beomki-yeo@users.noreply.github.com>

Author: stephenswat

core/include/traccc/finding/details/find_tracks.hpp

@@ -362,7 +362,7 @@ track_candidate_container_types::host find_tracks(
         unsigned int n_skipped{0u};
         while (true) {
 
-            if (L.meas_idx > n_meas) {
+            if (L.meas_idx >= n_meas) {
                 n_skipped++;
             }
 
@@ -393,20 +393,19 @@ track_candidate_container_types::host find_tracks(
         for (auto it = cands_per_track.rbegin(); it != cands_per_track.rend();
              it++) {
 
-            while (L.meas_idx > n_meas) {
+            while (L.meas_idx >= n_meas) {
                 const auto link_pos =
                     param_to_link[L.previous.first][L.previous.second];
 
                 L = links[L.previous.first][link_pos];
             }
 
             // Break if the measurement is still invalid
-            if (L.meas_idx > measurements.size()) {
+            if (L.meas_idx >= measurements.size()) {
                 break;
             }
 
-            auto& cand = *it;
-            cand = measurements.at(L.meas_idx);
+            *it = measurements.at(L.meas_idx);
 
             // Break the loop if the iterator is at the first candidate and
             // fill the seed
@@ -416,13 +415,12 @@ track_candidate_container_types::host find_tracks(
 
                 // Add seed and track candidates to the output container
                 output_candidates.push_back(cand_seed, cands_per_track);
-                break;
-            }
-
-            const auto l_pos =
-                param_to_link[L.previous.first][L.previous.second];
+            } else {
+                const auto l_pos =
+                    param_to_link[L.previous.first][L.previous.second];
 
-            L = links[L.previous.first][l_pos];
+                L = links[L.previous.first][l_pos];
+            }
         }
     }
 

device/common/include/traccc/finding/device/impl/build_tracks.ipp

@@ -65,11 +65,13 @@ TRACCC_DEVICE inline void build_tracks(const global_index_t globalIndex,
     // Resize the candidates with the exact size
     cands_per_track.resize(n_cands);
 
+    bool success = true;
+
     // Reversely iterate to fill the track candidates
     for (auto it = cands_per_track.rbegin(); it != cands_per_track.rend();
          it++) {
 
-        while (L.meas_idx > n_meas &&
+        while (L.meas_idx >= n_meas &&
                L.previous.first !=
                    std::numeric_limits<
                        candidate_link::link_index_type::first_type>::max()) {
@@ -78,26 +80,27 @@ TRACCC_DEVICE inline void build_tracks(const global_index_t globalIndex,
         }
 
         // Break if the measurement is still invalid
-        if (L.meas_idx > measurements.size()) {
+        if (L.meas_idx >= measurements.size()) {
+            success = false;
             break;
         }
 
-        auto& cand = *it;
-        cand = {measurements.at(L.meas_idx)};
+        *it = {measurements.at(L.meas_idx)};
 
         // Break the loop if the iterator is at the first candidate and fill the
         // seed
         if (it == cands_per_track.rend() - 1) {
             seed = seeds.at(L.previous.second);
-            break;
+        } else {
+            L = links[L.previous.first][L.previous.second];
         }
-
-        L = links[L.previous.first][L.previous.second];
     }
 
+    // NOTE: We may at some point want to assert that `success` is true
+
     // Criteria for valid tracks
     if (n_cands >= cfg.min_track_candidates_per_track &&
-        n_cands <= cfg.max_track_candidates_per_track) {
+        n_cands <= cfg.max_track_candidates_per_track && success) {
 
         vecmem::device_atomic_ref<unsigned int> num_valid_tracks(
             *payload.n_valid_tracks);