fix: Merge cells to unique set before clustering in Examples (acts-project#4759)

As discovered in acts-project#4745 and acts-project#4736, cells are not merged correctly right now. This PR fixes the merging by using a `std::map` intermediately and adding up the activations.

Author: andiwand

Examples/Algorithms/Digitization/include/ActsExamples/Digitization/ModuleClusters.hpp

@@ -9,7 +9,6 @@
 #pragma once
 
 #include "Acts/Clusterization/Clusterization.hpp"
-#include "Acts/Definitions/Algebra.hpp"
 #include "Acts/Definitions/TrackParametrization.hpp"
 #include "Acts/Utilities/BinUtility.hpp"
 #include "ActsExamples/Digitization/MeasurementCreation.hpp"

Examples/Algorithms/Digitization/src/ModuleClusters.cpp

@@ -11,16 +11,13 @@
 #include "Acts/Clusterization/Clusterization.hpp"
 #include "Acts/Utilities/Helpers.hpp"
 #include "ActsExamples/Digitization/MeasurementCreation.hpp"
-#include "ActsFatras/Digitization/Channelizer.hpp"
 
 #include <array>
 #include <cmath>
-#include <cstdint>
 #include <cstdlib>
 #include <limits>
-#include <memory>
+#include <map>
 #include <stdexcept>
-#include <type_traits>
 
 namespace ActsExamples {
 
@@ -88,11 +85,26 @@ void clusterAddCell(std::vector<ModuleValue>& cl, const ModuleValue& ce) {
 }
 
 std::vector<ModuleValue> ModuleClusters::createCellCollection() {
-  std::vector<ModuleValue> cells;
-  for (ModuleValue& mval : m_moduleValues) {
-    if (std::holds_alternative<Cluster::Cell>(mval.value)) {
-      cells.push_back(mval);
+  std::map<ActsFatras::Segmentizer::Bin2D, ModuleValue> uniqueCells;
+  for (const ModuleValue& mval : m_moduleValues) {
+    if (!std::holds_alternative<Cluster::Cell>(mval.value)) {
+      continue;
     }
+    const auto& cell = std::get<ActsExamples::Cluster::Cell>(mval.value).bin;
+
+    if (const auto it = uniqueCells.find(cell); it != uniqueCells.end()) {
+      // Cell already exists, so merge the hit sources
+      std::get<Cluster::Cell>(it->second.value).activation +=
+          std::get<Cluster::Cell>(mval.value).activation;
+    } else {
+      // New cell
+      uniqueCells[cell] = mval;
+    }
+  }
+  std::vector<ModuleValue> cells;
+  cells.reserve(uniqueCells.size());
+  for (const auto& [_, mval] : uniqueCells) {
+    cells.push_back(mval);
   }
   return cells;
 }

Fatras/include/ActsFatras/Digitization/Segmentizer.hpp

@@ -68,9 +68,9 @@ struct Segmentizer {
     /// The bin of this segment
     Bin2D bin = {0, 0};
     /// The segment start, end points
-    Segment2D path2D;
+    Segment2D path2D = {Acts::Vector2::Zero(), Acts::Vector2::Zero()};
     /// The (clipped) value (uncorrected: path length)
-    double activation = 0.;
+    double activation = 0;
 
     /// Constructor with arguments
     ///