replace std::max_element with std::ranges::max_element where appropriate

the c++-20 constrained algorithm is safer and code is more readable

opm/simulators/flow/GenericOutputBlackoilModule.cpp

@@ -980,7 +980,7 @@ int GenericOutputBlackoilModule<FluidSystem>::
 regionMax(const std::vector<int>& region,
           const Parallel::Communication& comm)
 {
-    const auto max_value = region.empty() ? 0 : *std::max_element(region.begin(), region.end());
+    const auto max_value = region.empty() ? 0 : *std::ranges::max_element(region);
     return comm.max(max_value);
 }
 

opm/simulators/flow/InterRegFlows.cpp

@@ -34,7 +34,7 @@ InterRegFlowMapSingleFIP(const std::vector<int>& region)
 {
     if (! region.empty()) {
         this->maxLocalRegionID_= this->maxGlobalRegionID_ =
-            *std::max_element(region.begin(), region.end());
+            *std::ranges::max_element(region);
     }
 
     std::ranges::transform(region, this->region_.begin(),

opm/simulators/flow/RegionPhasePVAverage.cpp

@@ -58,7 +58,7 @@ namespace {
         for (auto rset = 0*nset; rset < nset; ++rset) {
             const auto& reg = getRegionArray(regionNames[rset]);
 
-            auto m = std::max_element(reg.begin(), reg.end());
+            const auto m = std::ranges::max_element(reg);
             if (m == reg.end()) { // reg.empty()
                 continue;
             }

opm/simulators/linalg/getQuasiImpesWeights.hpp

@@ -98,8 +98,10 @@ namespace Amg
                 diag_block_transpose.solve(bweights, rhs);
             }
 
-            double abs_max = *std::max_element(
-                bweights.begin(), bweights.end(), [](double a, double b) { return std::fabs(a) < std::fabs(b); });
+            const double abs_max =
+                *std::ranges::max_element(bweights,
+                                          [](double a, double b)
+                                          { return std::fabs(a) < std::fabs(b); });
             bweights /= std::fabs(abs_max);
             weights[row_idx] = bweights;
         }
@@ -208,8 +210,10 @@ namespace Amg
                 }
                 block_transpose.solve(bweights, rhs);
 
-                double abs_max = *std::max_element(
-                    bweights.begin(), bweights.end(), [](double a, double b) { return std::fabs(a) < std::fabs(b); });
+                const double abs_max =
+                    *std::ranges::max_element(bweights,
+                                              [](double a, double b)
+                                              { return std::fabs(a) < std::fabs(b); });
                 // probably a scaling which could give approximately total compressibility would be better
                 bweights /=  std::fabs(abs_max); // given normal densities this scales weights to about 1.
 

opm/simulators/timestepping/AdaptiveSimulatorTimer.cpp

@@ -150,8 +150,8 @@ namespace Opm
     /// \brief return max step length used so far
     double AdaptiveSimulatorTimer::maxStepLength () const
     {
-        if( steps_.empty() ) return 0.0;
-        return *(std::max_element( steps_.begin(), steps_.end() ));
+        if (steps_.empty()) return 0.0;
+        return *std::ranges::max_element(steps_);
     }
 
     /// \brief return min step length used so far

opm/simulators/wells/ParallelPAvgDynamicSourceData.cpp

@@ -157,7 +157,7 @@ void Opm::ParallelPAvgDynamicSourceData<Scalar>::defineCommunication()
     //    permutation of 0..allIndices.size()-1 and the maximum source
     //    location may exceed size()-1.  Resize the storageIndex_ according
     //    to the largest source location ID.
-    if (auto maxIxPos = std::max_element(allIndices.begin(), allIndices.end());
+    if (const auto maxIxPos = std::ranges::max_element(allIndices);
         maxIxPos != allIndices.end())
     {
         // +1 for zero-based indices.

tests/test_tuning_TRGMBE.cpp

@@ -127,7 +127,7 @@ BOOST_AUTO_TEST_CASE(CheckMassBalanceWithinTRGMBE)
     auto mbw = data["MB_Water"];
     auto mbg = data["MB_Gas"];
 
-    const int num_reports = 1 + *std::max_element(rstep.begin(), rstep.end());
+    const int num_reports = 1 + *std::ranges::max_element(rstep);
     std::vector<double> max_mb;
     max_mb.reserve(num_reports);