Added lower_bound function that works in device code

Co-authored-by: Andrea Bocci <[email protected]>

Author: ariostas

HeterogeneousCore/AlpakaInterface/interface/alpakastdAlgorithm.h

@@ -7,12 +7,28 @@
 
 #include <alpaka/alpaka.hpp>
 
-// reimplementation of std algorithms able to compile with Alpaka,
-// mostly by declaring them constexpr (until C++20, which will make it
-// constexpr by default. TODO: drop when moving to C++20)
+// reimplementation of std algorithms able to work on device code
 
 namespace alpaka_std {
 
+  template <typename RandomIt, typename T, typename Compare = std::less<T>>
+  ALPAKA_FN_HOST_ACC constexpr RandomIt lower_bound(RandomIt first, RandomIt last, const T &value, Compare comp = {}) {
+    auto count = last - first;
+
+    while (count > 0) {
+      auto it = first;
+      auto step = count / 2;
+      it += step;
+      if (comp(*it, value)) {
+        first = ++it;
+        count -= step + 1;
+      } else {
+        count = step;
+      }
+    }
+    return first;
+  }
+
   template <typename RandomIt, typename T, typename Compare = std::less<T>>
   ALPAKA_FN_HOST_ACC constexpr RandomIt upper_bound(RandomIt first, RandomIt last, const T &value, Compare comp = {}) {
     auto count = last - first;

RecoTracker/LSTCore/src/alpaka/Hit.h

@@ -2,6 +2,7 @@
 #define RecoTracker_LSTCore_src_alpaka_Hit_h
 
 #include "HeterogeneousCore/AlpakaInterface/interface/workdivision.h"
+#include "HeterogeneousCore/AlpakaInterface/interface/alpakastdAlgorithm.h"
 
 #include "RecoTracker/LSTCore/interface/alpaka/Common.h"
 #include "RecoTracker/LSTCore/interface/ModulesSoA.h"
@@ -103,7 +104,7 @@ namespace ALPAKA_ACCELERATOR_NAMESPACE::lst {
             ((ihit_z > 0) - (ihit_z < 0)) *
             alpaka::math::acosh(
                 acc, alpaka::math::sqrt(acc, ihit_x * ihit_x + ihit_y * ihit_y + ihit_z * ihit_z) / hits.rts()[ihit]);
-        auto found_pointer = std::lower_bound(modules.mapdetId(), modules.mapdetId() + nModules, iDetId);
+        auto found_pointer = alpaka_std::lower_bound(modules.mapdetId(), modules.mapdetId() + nModules, iDetId);
         int found_index = std::distance(modules.mapdetId(), found_pointer);
         if (found_pointer == modules.mapdetId() + nModules)
           found_index = -1;
@@ -112,7 +113,7 @@ namespace ALPAKA_ACCELERATOR_NAMESPACE::lst {
         hits.moduleIndices()[ihit] = lastModuleIndex;
 
         if (modules.subdets()[lastModuleIndex] == Endcap && modules.moduleType()[lastModuleIndex] == TwoS) {
-          found_pointer = std::lower_bound(geoMapDetId, geoMapDetId + nEndCapMap, iDetId);
+          found_pointer = alpaka_std::lower_bound(geoMapDetId, geoMapDetId + nEndCapMap, iDetId);
           found_index = std::distance(geoMapDetId, found_pointer);
           if (found_pointer == geoMapDetId + nEndCapMap)
             found_index = -1;