[timeloop] Suggest the timestep size one before reaching the check point

(cherry picked from commit c6875957)

dumux/common/timeloop.hh

@@ -282,6 +282,12 @@ public:
     int timeStepIndex() const
     { return timeStepIdx_; }
 
+    /*!
+     * \brief The previous time step size
+     */
+    Scalar previousTimeStepSize() const
+    { return previousTimeStepSize_; }
+
     /*!
      * \brief Specify whether the simulation is finished
      *
@@ -438,8 +444,16 @@ public:
             isCheckPoint_ = false;
         }
 
+        const auto previousTimeStepSize = this->previousTimeStepSize();
+
         // advance the time step like in the parent class
         TimeLoop<Scalar>::advanceTimeStep();
+
+        // if this is a check point we might have reduced the time step to reach this check point
+        // reset the time step size to the time step size before this time step
+        using std::max;
+        if (isCheckPoint_)
+            this->setTimeStepSize(max(dt, previousTimeStepSize));
     }
 
     /*!

test/common/timeloop/test_timeloop.cc

@@ -142,6 +142,24 @@ int main(int argc, char* argv[]) try
 
     }
 
+    // check if time loops remembers time step before check point
+    {
+        if (mpiHelper.rank() == 0) std::cout << std::endl << "------- Test check point time loop ----------" << std::endl;
+        Dumux::CheckPointTimeLoop<double> timeLoop(tStart, dt, tEnd);
+        timeLoop.setCheckPoint(0.101);
+        timeLoop.start();
+        timeLoop.advanceTimeStep();
+        timeLoop.reportTimeStep();
+        timeLoop.advanceTimeStep();
+        timeLoop.reportTimeStep();
+        if (!(std::abs(timeLoop.timeStepSize()-0.1) < 1e-14))
+            DUNE_THROW(Dune::InvalidStateException, "Time Loop reduced time step size to " << timeLoop.timeStepSize()
+                         << " after check point unnecessarily!");
+
+        timeLoop.advanceTimeStep();
+        timeLoop.reportTimeStep();
+    }
+
     return 0;
 }
 // //////////////////////////////////