fix: small bug when avoiding iterations while looking for dublets (#2304

)

While looping over the doublets, we find the first SP inside the radius region of interest and update the iterator so we don't need to look at the other SPs again. That mean that we have changed the middle space point (which are sorted in radius) and the radius lower bound has moved accordingly.

However, if that is the last bottom or top spaceport in that grid bin, we continue and we don't move the lower bound. This means that we will continue looking at some spacepoints unnecessarily.
This PR is intended to fix that and update the lower bound also for these cases.

@CarloVarni @noemina let me know if you have a better suggestion on how to fix this

I tested this and improves slightly ~4% performance for ttbar 200 pileUp

Core/include/Acts/Seeding/SeedFinder.ipp

@@ -242,7 +242,27 @@ SeedFinder<external_spacepoint_t, platform_t>::getCompatibleDoublets(
     // we make a copy of the iterator here since we need it to remain
     // the same in the Neighbour object
     auto min_itr = otherSPCol.itr;
-    bool found = false;
+
+    // find the first SP inside the radius region of interest and update
+    // the iterator so we don't need to look at the other SPs again
+    for (; min_itr != otherSPs.end(); ++min_itr) {
+      const auto& otherSP = *min_itr;
+      if constexpr (candidateType == Acts::SpacePointCandidateType::eBottom) {
+        // if r-distance is too big, try next SP in bin
+        if ((rM - otherSP->radius()) <= deltaRMaxSP) {
+          break;
+        }
+      } else {
+        // if r-distance is too small, try next SP in bin
+        if ((otherSP->radius() - rM) >= deltaRMinSP) {
+          break;
+        }
+      }
+    }
+    // We update the iterator in the Neighbour object
+    // that mean that we have changed the middle space point
+    // and the lower bound has moved accordingly
+    otherSPCol.itr = min_itr;
 
     for (; min_itr != otherSPs.end(); ++min_itr) {
       const auto& otherSP = *min_itr;
@@ -254,30 +274,13 @@ SeedFinder<external_spacepoint_t, platform_t>::getCompatibleDoublets(
         if (deltaR < deltaRMinSP) {
           break;
         }
-        // if r-distance is too big, try next SP in bin
-        if (deltaR > deltaRMaxSP) {
-          continue;
-        }
-
       } else {
         deltaR = (otherSP->radius() - rM);
 
         // if r-distance is too big, try next SP in bin
         if (deltaR > deltaRMaxSP) {
           break;
         }
-        // if r-distance is too small, try next SP in bin
-        if (deltaR < deltaRMinSP) {
-          continue;
-        }
-      }
-
-      // We update the iterator in the Neighbour object
-      // that mean that we have changed the middle space point
-      // and the lower bound has moved accordingly
-      if (not found) {
-        found = true;
-        otherSPCol.itr = min_itr;
       }
 
       if constexpr (candidateType == Acts::SpacePointCandidateType::eBottom) {