AdaptiveTimeStepping_impl.hpp

/*
  Copyright 2024 Equinor ASA.

  This file is part of the Open Porous Media project (OPM).

  OPM is free software: you can redistribute it and/or modify
  it under the terms of the GNU General Public License as published by
  the Free Software Foundation, either version 3 of the License, or
  (at your option) any later version.

  OPM is distributed in the hope that it will be useful,
  but WITHOUT ANY WARRANTY; without even the implied warranty of
  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  GNU General Public License for more details.

  You should have received a copy of the GNU General Public License
  along with OPM.  If not, see <http://www.gnu.org/licenses/>.
*/

#ifndef OPM_ADAPTIVE_TIME_STEPPING_IMPL_HPP
#define OPM_ADAPTIVE_TIME_STEPPING_IMPL_HPP

// Improve IDE experience
#ifndef OPM_ADAPTIVE_TIME_STEPPING_HPP
#include <config.h>
#include <opm/simulators/timestepping/AdaptiveTimeStepping.hpp>
#include <opm/simulators/timestepping/AdaptiveSimulatorTimer.hpp>
#endif

#include <dune/common/timer.hh>
#include <dune/istl/istlexception.hh>

#include <opm/common/Exceptions.hpp>
#include <opm/common/ErrorMacros.hpp>
#include <opm/common/OpmLog/OpmLog.hpp>
#include <opm/common/TimingMacros.hpp>

#include <opm/grid/utility/StopWatch.hpp>

#include <opm/input/eclipse/Schedule/Tuning.hpp>

#include <opm/input/eclipse/Units/Units.hpp>
#include <opm/input/eclipse/Units/UnitSystem.hpp>

#include <opm/models/utils/parametersystem.hpp>

#include <opm/simulators/timestepping/EclTimeSteppingParams.hpp>

#include <algorithm>
#include <cassert>
#include <cmath>
#include <sstream>
#include <stdexcept>

#include <boost/date_time/posix_time/posix_time.hpp>
#include <fmt/format.h>
#include <fmt/ranges.h>
namespace Opm {
/*********************************************
 * Public methods of AdaptiveTimeStepping
 * ******************************************/


template<class TypeTag>
AdaptiveTimeStepping<TypeTag>::
AdaptiveTimeStepping(const UnitSystem& unit_system,
                     const SimulatorReport& report,
                     const double max_next_tstep,
                     const bool terminal_output
)
    : time_step_control_{}
    , restart_factor_{Parameters::Get<Parameters::SolverRestartFactor<Scalar>>()} // 0.33
    , growth_factor_{Parameters::Get<Parameters::SolverGrowthFactor<Scalar>>()} // 2.0
    , max_growth_{Parameters::Get<Parameters::SolverMaxGrowth<Scalar>>()} // 3.0
    , max_time_step_{
        Parameters::Get<Parameters::SolverMaxTimeStepInDays<Scalar>>() * 24 * 60 * 60} // 365.25
    , min_time_step_{
        unit_system.to_si(UnitSystem::measure::time,
                          Parameters::Get<Parameters::SolverMinTimeStep<Scalar>>())} // 1e-12;
    , ignore_convergence_failure_{
        Parameters::Get<Parameters::SolverContinueOnConvergenceFailure>()} // false;
    , solver_restart_max_{Parameters::Get<Parameters::SolverMaxRestarts>()} // 10
    , solver_verbose_{Parameters::Get<Parameters::SolverVerbosity>() > 0 && terminal_output} // 2
    , timestep_verbose_{Parameters::Get<Parameters::TimeStepVerbosity>() > 0 && terminal_output} // 2
    , suggested_next_timestep_{
        (max_next_tstep <= 0 ? Parameters::Get<Parameters::InitialTimeStepInDays>()
                             : max_next_tstep) * 24 * 60 * 60} // 1.0
    , full_timestep_initially_{Parameters::Get<Parameters::FullTimeStepInitially>()} // false
    , timestep_after_event_{
        Parameters::Get<Parameters::TimeStepAfterEventInDays<Scalar>>() * 24 * 60 * 60} // 1e30
    , use_newton_iteration_{false}
    , min_time_step_before_shutting_problematic_wells_{
        Parameters::Get<Parameters::MinTimeStepBeforeShuttingProblematicWellsInDays>() * unit::day}
    , report_(report)
{
    init_(unit_system);
}

template<class TypeTag>
AdaptiveTimeStepping<TypeTag>::
AdaptiveTimeStepping(double max_next_tstep,
                     const Tuning& tuning,
                     const UnitSystem& unit_system,
                     const SimulatorReport& report,
                     const bool terminal_output
)
    : time_step_control_{}
    , restart_factor_{tuning.TSFCNV}
    , growth_factor_{tuning.TFDIFF}
    , max_growth_{tuning.TSFMAX}
    , max_time_step_{tuning.TSMAXZ} // 365.25
    , min_time_step_{tuning.TSMINZ} // 0.1;
    , ignore_convergence_failure_{true}
    , solver_restart_max_{Parameters::Get<Parameters::SolverMaxRestarts>()} // 10
    , solver_verbose_{Parameters::Get<Parameters::SolverVerbosity>() > 0 && terminal_output} // 2
    , timestep_verbose_{Parameters::Get<Parameters::TimeStepVerbosity>() > 0 && terminal_output} // 2
    , suggested_next_timestep_{
        max_next_tstep <= 0 ? Parameters::Get<Parameters::InitialTimeStepInDays>() * 24 * 60 * 60
                            : max_next_tstep} // 1.0
    , full_timestep_initially_{Parameters::Get<Parameters::FullTimeStepInitially>()} // false
    , timestep_after_event_{tuning.TMAXWC} // 1e30
    , use_newton_iteration_{false}
    , min_time_step_before_shutting_problematic_wells_{
        Parameters::Get<Parameters::MinTimeStepBeforeShuttingProblematicWellsInDays>() * unit::day}
    , report_(report)
{
    init_(unit_system);
}

template<class TypeTag>
bool
AdaptiveTimeStepping<TypeTag>::
operator==(const AdaptiveTimeStepping<TypeTag>& rhs) const
{
    if (this->time_step_control_type_ != rhs.time_step_control_type_ ||
        (this->time_step_control_ && !rhs.time_step_control_) ||
        (!this->time_step_control_ && rhs.time_step_control_)) {
        return false;
    }

    bool result = false;
    switch (this->time_step_control_type_) {
    case TimeStepControlType::HardCodedTimeStep:
        result = castAndComp<HardcodedTimeStepControl>(rhs);
        break;
    case TimeStepControlType::PIDAndIterationCount:
        result = castAndComp<PIDAndIterationCountTimeStepControl>(rhs);
        break;
    case TimeStepControlType::SimpleIterationCount:
        result = castAndComp<SimpleIterationCountTimeStepControl>(rhs);
        break;
    case TimeStepControlType::PID:
        result = castAndComp<PIDTimeStepControl>(rhs);
        break;
    case TimeStepControlType::General3rdOrder:
        result = castAndComp<General3rdOrderController>(rhs);
        break;
    }

    return result &&
           this->restart_factor_ == rhs.restart_factor_ &&
           this->growth_factor_ == rhs.growth_factor_ &&
           this->max_growth_ == rhs.max_growth_ &&
           this->max_time_step_ == rhs.max_time_step_ &&
           this->min_time_step_ == rhs.min_time_step_ &&
           this->ignore_convergence_failure_ == rhs.ignore_convergence_failure_ &&
           this->solver_restart_max_== rhs.solver_restart_max_ &&
           this->solver_verbose_ == rhs.solver_verbose_ &&
           this->full_timestep_initially_ == rhs.full_timestep_initially_ &&
           this->timestep_after_event_ == rhs.timestep_after_event_ &&
           this->use_newton_iteration_ == rhs.use_newton_iteration_ &&
           this->min_time_step_before_shutting_problematic_wells_ ==
               rhs.min_time_step_before_shutting_problematic_wells_;
}

template<class TypeTag>
void
AdaptiveTimeStepping<TypeTag>::
registerParameters()
{
    registerEclTimeSteppingParameters<Scalar>();
    detail::registerAdaptiveParameters();
}

// See Doxygen comment on the declaration in AdaptiveTimeStepping.hpp.
template<class TypeTag>
template <class Solver>
SimulatorReport
AdaptiveTimeStepping<TypeTag>::
step(const SimulatorTimer& simulator_timer,
     Solver& solver,
     const bool is_event,
     const TuningUpdateCallback& tuning_updater)
{
    SubStepper<Solver> sub_stepper{
        *this, simulator_timer, solver, is_event, tuning_updater,
    };
    return sub_stepper.run();
}

template<class TypeTag>
template<class Serializer>
void
AdaptiveTimeStepping<TypeTag>::
serializeOp(Serializer& serializer)
{
    serializer(this->time_step_control_type_);
    switch (this->time_step_control_type_) {
    case TimeStepControlType::HardCodedTimeStep:
        allocAndSerialize<HardcodedTimeStepControl>(serializer);
        break;
    case TimeStepControlType::PIDAndIterationCount:
        allocAndSerialize<PIDAndIterationCountTimeStepControl>(serializer);
        break;
    case TimeStepControlType::SimpleIterationCount:
        allocAndSerialize<SimpleIterationCountTimeStepControl>(serializer);
        break;
    case TimeStepControlType::PID:
        allocAndSerialize<PIDTimeStepControl>(serializer);
        break;
    case TimeStepControlType::General3rdOrder:
        allocAndSerialize<General3rdOrderController>(serializer);
        break;
    }
    serializer(this->restart_factor_);
    serializer(this->growth_factor_);
    serializer(this->max_growth_);
    serializer(this->max_time_step_);
    serializer(this->min_time_step_);
    serializer(this->ignore_convergence_failure_);
    serializer(this->solver_restart_max_);
    serializer(this->solver_verbose_);
    serializer(this->timestep_verbose_);
    serializer(this->suggested_next_timestep_);
    serializer(this->full_timestep_initially_);
    serializer(this->timestep_after_event_);
    serializer(this->use_newton_iteration_);
    serializer(this->min_time_step_before_shutting_problematic_wells_);
}

template<class TypeTag>
SimulatorReport&
AdaptiveTimeStepping<TypeTag>::
report()
{
    return report_;
}

template<class TypeTag>
AdaptiveTimeStepping<TypeTag>
AdaptiveTimeStepping<TypeTag>::
serializationTestObjectHardcoded()
{
    return serializationTestObject_<HardcodedTimeStepControl>();
}

template<class TypeTag>
AdaptiveTimeStepping<TypeTag>
AdaptiveTimeStepping<TypeTag>::
serializationTestObjectPID()
{
    return serializationTestObject_<PIDTimeStepControl>();
}

template<class TypeTag>
AdaptiveTimeStepping<TypeTag>
AdaptiveTimeStepping<TypeTag>::
serializationTestObjectPIDIt()
{
    return serializationTestObject_<PIDAndIterationCountTimeStepControl>();
}

template<class TypeTag>
AdaptiveTimeStepping<TypeTag>
AdaptiveTimeStepping<TypeTag>::
serializationTestObjectSimple()
{
    return serializationTestObject_<SimpleIterationCountTimeStepControl>();
}

template<class TypeTag>
AdaptiveTimeStepping<TypeTag>
AdaptiveTimeStepping<TypeTag>::
serializationTestObject3rdOrder()
{
    return serializationTestObject_<General3rdOrderController>();
}


template<class TypeTag>
void
AdaptiveTimeStepping<TypeTag>::
setSuggestedNextStep(const double x)
{
    this->suggested_next_timestep_ = x;
}

template<class TypeTag>
double
AdaptiveTimeStepping<TypeTag>::
suggestedNextStep() const
{
    return this->suggested_next_timestep_;
}

template<class TypeTag>
const TimeStepControlInterface&
AdaptiveTimeStepping<TypeTag>::
timeStepControl() const
{
    return *this->time_step_control_;
}


template<class TypeTag>
void
AdaptiveTimeStepping<TypeTag>::
updateNEXTSTEP(double max_next_tstep)
{
     // \Note Only update next suggested step if TSINIT was explicitly
     //  set in TUNING or NEXTSTEP is active.
    if (max_next_tstep > 0) {
        this->suggested_next_timestep_ = max_next_tstep;
    }
}

template<class TypeTag>
void
AdaptiveTimeStepping<TypeTag>::
updateTUNING(double max_next_tstep, const Tuning& tuning)
{
    this->restart_factor_ = tuning.TSFCNV;
    this->growth_factor_ = tuning.TFDIFF;
    this->max_growth_ = tuning.TSFMAX;
    this->max_time_step_ = tuning.TSMAXZ;
    updateNEXTSTEP(max_next_tstep);
    this->timestep_after_event_ = tuning.TMAXWC;
}

/*********************************************
 * Private methods of AdaptiveTimeStepping
 * ******************************************/

template<class TypeTag>
template<class T, class Serializer>
void
AdaptiveTimeStepping<TypeTag>::
allocAndSerialize(Serializer& serializer)
{
    if (!serializer.isSerializing()) {
        this->time_step_control_ = std::make_unique<T>();
    }
    serializer(*static_cast<T*>(this->time_step_control_.get()));
}

template<class TypeTag>
template<class T>
bool
AdaptiveTimeStepping<TypeTag>::
castAndComp(const AdaptiveTimeStepping<TypeTag>& Rhs) const
{
    const T* lhs = static_cast<const T*>(this->time_step_control_.get());
    const T* rhs = static_cast<const T*>(Rhs.time_step_control_.get());
    return *lhs == *rhs;
}

template<class TypeTag>
void
AdaptiveTimeStepping<TypeTag>::
maybeModifySuggestedTimeStepAtBeginningOfReportStep_(const double original_time_step,
                                                     bool is_event)
{
    // init last time step as a fraction of the given time step
    if (this->suggested_next_timestep_ < 0) {
        this->suggested_next_timestep_ = this->restart_factor_ * original_time_step;
    }

    if (this->full_timestep_initially_) {
        this->suggested_next_timestep_ = original_time_step;
    }

    // use seperate time step after event
    if (is_event && this->timestep_after_event_ > 0) {
        this->suggested_next_timestep_ = this->timestep_after_event_;
    }
}

template<class TypeTag>
template<class Controller>
AdaptiveTimeStepping<TypeTag>
AdaptiveTimeStepping<TypeTag>::
serializationTestObject_()
{
    AdaptiveTimeStepping<TypeTag> result;

    result.restart_factor_ = 1.0;
    result.growth_factor_ = 2.0;
    result.max_growth_ = 3.0;
    result.max_time_step_ = 4.0;
    result.min_time_step_ = 5.0;
    result.ignore_convergence_failure_ = true;
    result.solver_restart_max_ = 6;
    result.solver_verbose_ = true;
    result.timestep_verbose_ = true;
    result.suggested_next_timestep_ = 7.0;
    result.full_timestep_initially_ = true;
    result.use_newton_iteration_ = true;
    result.min_time_step_before_shutting_problematic_wells_ = 9.0;
    result.time_step_control_type_ = Controller::Type;
    result.time_step_control_ =
                       std::make_unique<Controller>(Controller::serializationTestObject());

    return result;
}

/*********************************************
 * Protected methods of AdaptiveTimeStepping
 * ******************************************/

template<class TypeTag>
void AdaptiveTimeStepping<TypeTag>::
init_(const UnitSystem& unitSystem)
{
    std::tie(time_step_control_type_,
             time_step_control_,
             use_newton_iteration_) = detail::createController(unitSystem);
    // make sure growth factor is something reasonable
    if (this->growth_factor_ < 1.0) {
        OPM_THROW(std::runtime_error,
                  "Growth factor cannot be less than 1.");
    }
}



/************************************************
 * Private class SubStepper public methods
 ************************************************/

template<class TypeTag>
template<class Solver>
AdaptiveTimeStepping<TypeTag>::SubStepper<Solver>::
SubStepper(AdaptiveTimeStepping<TypeTag>& adaptive_time_stepping,
           const SimulatorTimer& simulator_timer,
           Solver& solver,
           const bool is_event,
           const TuningUpdateCallback& tuning_updater)
    : adaptive_time_stepping_{adaptive_time_stepping}
    , simulator_timer_{simulator_timer}
    , solver_{solver}
    , is_event_{is_event}
    , tuning_updater_{tuning_updater}
{
}

template<class TypeTag>
template<class Solver>
AdaptiveTimeStepping<TypeTag>&
AdaptiveTimeStepping<TypeTag>::SubStepper<Solver>::
getAdaptiveTimerStepper()
{
    return adaptive_time_stepping_;
}

template<class TypeTag>
template<class Solver>
SimulatorReport
AdaptiveTimeStepping<TypeTag>::SubStepper<Solver>::
run()
{
#ifdef RESERVOIR_COUPLING_ENABLED
    if (isReservoirCouplingSlave_() && reservoirCouplingSlave_().activated()) {
        return runStepReservoirCouplingSlave_();
    }
    else if (isReservoirCouplingMaster_() && reservoirCouplingMaster_().activated()) {
        return runStepReservoirCouplingMaster_();
    }
    else {
        return runStepOriginal_();
    }
#else
    return runStepOriginal_();
#endif
}

/************************************************
 * Private class SubStepper private methods
 ************************************************/

#ifdef RESERVOIR_COUPLING_ENABLED
template<class TypeTag>
template<class Solver>
bool
AdaptiveTimeStepping<TypeTag>::SubStepper<Solver>::
isReservoirCouplingMaster_() const
{
    return this->solver_.model().simulator().reservoirCouplingMaster() != nullptr;
}

template<class TypeTag>
template<class Solver>
bool
AdaptiveTimeStepping<TypeTag>::SubStepper<Solver>::
isReservoirCouplingSlave_() const
{
    return this->solver_.model().simulator().reservoirCouplingSlave() != nullptr;
}
#endif

template<class TypeTag>
template<class Solver>
void
AdaptiveTimeStepping<TypeTag>::SubStepper<Solver>::
maybeModifySuggestedTimeStepAtBeginningOfReportStep_(const double original_time_step)
{
    this->adaptive_time_stepping_.maybeModifySuggestedTimeStepAtBeginningOfReportStep_(
        original_time_step, this->is_event_
    );

    if (this->adaptive_time_stepping_.time_step_control_type_ != TimeStepControlType::HardCodedTimeStep) {
        return;
    }

    struct ZeroRelativeChange final : RelativeChangeInterface {
        double relativeChange() const override { return 0.0; }
    } zero_relative_change;

    const auto* hardcoded_control = static_cast<const HardcodedTimeStepControl*>(
        this->adaptive_time_stepping_.time_step_control_.get());
    AdaptiveSimulatorTimer report_step_timer{
        this->simulator_timer_.startDateTime(),
        original_time_step,
        this->simulator_timer_.simulationTimeElapsed(),
        original_time_step,
        this->simulator_timer_.reportStepNum(),
        maxTimeStep_()
    };

    const double hardcoded_initial_step = hardcoded_control->computeTimeStepSize(
        original_time_step,
        0,
        zero_relative_change,
        report_step_timer);
    if (std::isfinite(hardcoded_initial_step) && hardcoded_initial_step > 0.0) {
        this->adaptive_time_stepping_.setSuggestedNextStep(hardcoded_initial_step);
    }
}

// The maybeUpdateTuning_() lambda callback is defined in SimulatorFullyImplicit::runStep()
// It has to be called for each substep since TUNING might have been changed for next sub step due
// to ACTIONX (via NEXTSTEP) or WCYCLE keywords.
template<class TypeTag>
template<class Solver>
bool
AdaptiveTimeStepping<TypeTag>::SubStepper<Solver>::
maybeUpdateTuning_(double elapsed, double substep_length, int sub_step_number) const
{
    return this->tuning_updater_(elapsed, substep_length, sub_step_number);
}

template<class TypeTag>
template<class Solver>
double
AdaptiveTimeStepping<TypeTag>::SubStepper<Solver>::
maxTimeStep_() const
{
    return this->adaptive_time_stepping_.max_time_step_;
}

template <class TypeTag>
template <class Solver>
SimulatorReport
AdaptiveTimeStepping<TypeTag>::SubStepper<Solver>::
runStepOriginal_()
{
    const auto elapsed = this->simulator_timer_.simulationTimeElapsed();
    const auto original_time_step = this->simulator_timer_.currentStepLength();
    const auto report_step = this->simulator_timer_.reportStepNum();
    maybeUpdateTuning_(elapsed, suggestedNextTimestep_(), /*substep=*/0);
    maybeModifySuggestedTimeStepAtBeginningOfReportStep_(original_time_step);

    AdaptiveSimulatorTimer substep_timer{
        this->simulator_timer_.startDateTime(),
        original_time_step,
        elapsed,
        suggestedNextTimestep_(),
        report_step,
        maxTimeStep_()
    };
    SubStepIteration<Solver> substepIteration{*this, substep_timer, original_time_step, /*final_step=*/true};
    return substepIteration.run();
}

template <class TypeTag>
template <class Solver>
double
AdaptiveTimeStepping<TypeTag>::SubStepper<Solver>::
suggestedNextTimestep_() const
{
    return this->adaptive_time_stepping_.suggestedNextStep();
}


#ifdef RESERVOIR_COUPLING_ENABLED
// Throw if the slave has already been terminated by the master. A terminated slave has
// disconnected its intercommunicator, so running another coupled substep loop would issue
// an MPI_Recv on a null communicator and abort the job. The run loop in
// SimulatorFullyImplicit::runStep() stops before this can happen, so this guards
// against future regressions of that logic.
template <class TypeTag>
template <class Solver>
void
AdaptiveTimeStepping<TypeTag>::SubStepper<Solver>::
checkIfSlaveIsTerminated_()
{
    if (reservoirCouplingSlave_().terminated()) {
        OPM_THROW(ReservoirCouplingError,
                  "Internal error: attempt to run a coupled substep loop after the slave "
                  "has been terminated by the master process");
    }
}

// Pick the master's sync-step length for the next outer-loop iteration of
// `runStepReservoirCouplingMaster_()`.  Includes the chop against slave-
// report dates and emits the user-visible log line.  See the block comment
// above `runStepReservoirCouplingMaster_()` for TSYNC/RSYNC mode definitions.
//
// TSYNC (default):
//   Each iteration is one master time step.  Start from the master's
//   adaptive suggestion, capped at the max time step and at the time
//   remaining until the report-step end, then chop.
//
// RSYNC:
//   The outer loop iterates per chunk between slave-report boundaries.
//   Re-use the previous iteration's chopped value as the candidate for
//   this iteration (the caller seeds the first iteration with
//   `original_time_step`), then chop.
template <class TypeTag>
template <class Solver>
double
AdaptiveTimeStepping<TypeTag>::SubStepper<Solver>::
getRcMasterSyncStepLength_(double prev_step,
                           double current_time,
                           double step_end_time)
{
    const bool sync_at_report_steps = reservoirCouplingMaster_().syncAtReportSteps();
    double current_step_length;
    if (sync_at_report_steps) {
        current_step_length = prev_step;
    } else {
        const double remaining = step_end_time - current_time;
        current_step_length = std::min({suggestedNextTimestep_(), maxTimeStep_(), remaining});
        // NOTE: The substep timer is later constructed (in the caller) with
        //  current_step_length as its span, and after construction,
        //  substep_timer.currentStepLength() should return the same as
        //  current_step_length.  The timer's constructor calls
        //  provideTimeStepEstimate() with suggestedNextTimestep_() as the dt_
        //  estimate and current_step_length as the span.  Since we took
        //    current_step_length = min(suggestedNextTimestep_(), maxTimeStep_(), remaining)
        //  here (and only shrink it further via maybeChopSubStep below), the
        //  estimate is always >= the span, so the timer's snap branch (see
        //  AdaptiveSimulatorTimer::provideTimeStepEstimate) fires and clamps
        //  dt_ to current_step_length.
    }
    current_step_length = reservoirCouplingMaster_().maybeChopSubStep(current_step_length, current_time);
    auto num_active = reservoirCouplingMaster_().numActivatedSlaves();
    OpmLog::info(fmt::format(
        "\nChoosing next sync time{} between master and {} active slave {}: {:.2f} days",
        sync_at_report_steps ? " (RSYNC)" : "",
        num_active, (num_active == 1 ? "process" : "processes"),
        current_step_length / unit::day
    ));
    return current_step_length;
}

template <class TypeTag>
template <class Solver>
ReservoirCouplingMaster<typename AdaptiveTimeStepping<TypeTag>::Scalar>&
AdaptiveTimeStepping<TypeTag>::SubStepper<Solver>::
reservoirCouplingMaster_()
{
    return *(this->solver_.model().simulator().reservoirCouplingMaster());
}

template <class TypeTag>
template <class Solver>
ReservoirCouplingSlave<typename AdaptiveTimeStepping<TypeTag>::Scalar>&
AdaptiveTimeStepping<TypeTag>::SubStepper<Solver>::
reservoirCouplingSlave_()
{
    return *(this->solver_.model().simulator().reservoirCouplingSlave());
}

// Description of the reservoir coupling master and slave substep loop
// -------------------------------------------------------------------
// The master and slave processes attempt to reach the end of the report step using a series of substeps
// (also called timesteps).  Each substep has an upper limit roughly determined by a combination of the
// keywords TUNING (through TSINIT, TSMAXZ), NEXSTEP, WCYCLE, and the start of the next report step
// (the last substep has to coincide with this time).  Note that NEXTSTEP can be updated from an
// ACTIONX keyword.  Although this comment focuses on the maximum substep limit, there is also a lower
// limit on the substep length, and the substep sizes are adjusted automatically (or retried) based on
// the convergence behavior of the solver and other criteria.
//
// The master chooses a "sync step" — the span between successive master<->slave exchanges — via one
// of two modes, selected by the `--rescoup-sync-at-report-steps` CLI flag:
//
//   TSYNC (default, flag=false)
//       Each master time step is a sync step.  Every outer iteration of the master substep
//       loop redistributes group-rate targets to the slaves.  This gives fine sync granularity and
//       matches the reference simulator's coupling semantics.
//
//   RSYNC (flag=true)
//       Each chunk between slave-report-step boundaries is one sync step.  The master's own adaptive
//       timestepping runs inside the chunk, but slaves only hear back from the master once per chunk.
//       Provided as a developer escape hatch.
//
// In both modes the sync step is also limited so as not to overshoot the next slave report date
// (`maybeChopSubStep`).  A second, drift-based limit (GRUPMAST item 4 / RCMASTS) is parsed but not
// yet honored — tracked as a follow-up PR.
//
// Per sync step:
//   - Master receives each activated slave's next report date (`receiveNextReportDateFromSlaves`).
//   - Master picks the sync-step length via `getRcMasterSyncStepLength_()` (mode-dependent) and
//     chops it so as not to overshoot any slave report date.
//   - Master sends the chosen sync-step length to each slave (`sendNextTimeStepToSlaves`).
//
// The slaves use the sync-step end as a fixed point — a "mini report step" — that they reach via one
// or more of their own substeps.  If the slave's current report step extends beyond the received
// sync step, the slave loops waiting for the master to send further sync steps; the loop ends when
// the received sync step coincides with the end of the slave's current report step.

// Master-side rescoup outer loop.  See the block comment above for TSYNC
// and RSYNC mode definitions.
//
// TSYNC (default):
//   Each outer iteration is one master time step.  The sync-step
//   length is chosen fresh each iteration by `getRcMasterSyncStepLength_()`
//   from the master's adaptive suggestion (capped at the max time step and
//   at the remaining report-step time), then chopped against slave-report
//   dates.  `SubStepIteration::run()` is still used but normally handles
//   only solver-driven chops within a single master time step.
//
// RSYNC:
//   The outer loop iterates once per chunk between slave-report boundaries.
//   The sync-step length starts at the full report step and only shrinks
//   via `maybeChopSubStep`; the master's adaptive timestepping runs inside
//   `SubStepIteration` as solver-driven substeps within each chunk.
template <class TypeTag>
template <class Solver>
SimulatorReport
AdaptiveTimeStepping<TypeTag>::SubStepper<Solver>::
runStepReservoirCouplingMaster_()
{
    int iteration = 0;
    const double original_time_step = this->simulator_timer_.currentStepLength();
    double current_time{this->simulator_timer_.simulationTimeElapsed()};
    double step_end_time = current_time + original_time_step;
    const double report_step_start_time = current_time;
    int report_step_substep_offset = 0;
    // In RSYNC mode this variable persists across outer iterations and
    // carries the previously-chopped sync span into the next iteration.  In
    // TSYNC mode it is overwritten each iteration by
    // `getRcMasterSyncStepLength_()`; the initial value is only consumed by
    // the first helper call in RSYNC mode (as the candidate span for that
    // iteration).
    auto current_step_length = original_time_step;
    auto report_step_idx = this->simulator_timer_.currentStepNum();
    if (report_step_idx == 0 && iteration == 0) {
        reservoirCouplingMaster_().initTimeStepping();
    }
    SimulatorReport report;
    // The master needs to know which slaves have activated before it can start the substep loop
    reservoirCouplingMaster_().maybeReceiveActivationHandshakeFromSlaves(current_time);
    while (true) {
        reservoirCouplingMaster_().sendDontTerminateSignalToSlaves(); // Tell the slaves to keep running.
        reservoirCouplingMaster_().receiveNextReportDateFromSlaves();
        const bool start_of_report_step = (iteration == 0);
        if (start_of_report_step) {
            reservoirCouplingMaster_().initStartOfReportStep(report_step_idx);
            maybeUpdateTuning_(current_time, suggestedNextTimestep_(), /*substep=*/0);
            maybeModifySuggestedTimeStepAtBeginningOfReportStep_(original_time_step);
        }
        current_step_length = getRcMasterSyncStepLength_(
                                          current_step_length, current_time, step_end_time);
        reservoirCouplingMaster_().sendNextTimeStepToSlaves(current_step_length);
        AdaptiveSimulatorTimer substep_timer{
            this->simulator_timer_.startDateTime(),
            /*stepLength=*/current_step_length,
            /*elapsedTime=*/current_time,
            /*timeStepEstimate=*/suggestedNextTimestep_(),
            /*reportStep=*/this->simulator_timer_.reportStepNum(),
            maxTimeStep_()
        };
        // Make the per-chunk timer log the enclosing report step's span and a
        // cumulative substep counter (see AdaptiveSimulatorTimer "report step
        // view"). Without this, log lines would show one sync chunk only.
        substep_timer.setReportStepStartTime(report_step_start_time);
        substep_timer.setReportStepTotalTime(step_end_time);
        substep_timer.setReportStepSubstepOffset(report_step_substep_offset);
        const bool final_step = ReservoirCoupling::Seconds::compare_gt_or_eq(
            current_time + current_step_length, step_end_time
        );
        // Mark this as the first substep of the "sync" timestep. This flag controls
        // whether master-slave data exchange should occur in beginTimeStep() in the well model.
        // It will be cleared after the first runSubStep_() call.
        reservoirCouplingMaster_().setFirstSubstepOfSyncTimestep(true);
        // After the first master substep completes, timeStepSucceeded() will
        // block until slaves finish the sync step and send production data.
        // This ensures correct summary output for all subsequent substeps.
        reservoirCouplingMaster_().setNeedsSlaveDataReceive(true);
        SubStepIteration<Solver> substepIteration{*this, substep_timer, current_step_length, final_step};
        const auto sub_steps_report = substepIteration.run();
        report += sub_steps_report;
        report_step_substep_offset += substep_timer.currentStepNum();
        current_time += current_step_length;
        if (final_step) {
            break;
        }
        iteration++;
    }
    return report;
}

template <class TypeTag>
template <class Solver>
SimulatorReport
AdaptiveTimeStepping<TypeTag>::SubStepper<Solver>::
runStepReservoirCouplingSlave_()
{
    checkIfSlaveIsTerminated_();
    int iteration = 0;
    const double original_time_step = this->simulator_timer_.currentStepLength();
    double current_time{this->simulator_timer_.simulationTimeElapsed()};
    double step_end_time = current_time + original_time_step;
    const double report_step_start_time = current_time;
    int report_step_substep_offset = 0;
    SimulatorReport report;
    auto report_step_idx = this->simulator_timer_.currentStepNum();
    if (report_step_idx == 0 && iteration == 0) {
        reservoirCouplingSlave_().initTimeStepping();
    }
    while (true) {
        bool start_of_report_step = (iteration == 0);
        if (reservoirCouplingSlave_().maybeReceiveTerminateSignalFromMaster()) {
            // Call MPI_Comm_disconnect() to terminate the MPI communicator, etc..
            break;
        }
        reservoirCouplingSlave_().sendNextReportDateToMasterProcess();
        const auto timestep = reservoirCouplingSlave_().receiveNextTimeStepFromMaster();
        if (start_of_report_step) {
            maybeUpdateTuning_(current_time, suggestedNextTimestep_(), /*substep=*/0);
            maybeModifySuggestedTimeStepAtBeginningOfReportStep_(timestep);
        }
        AdaptiveSimulatorTimer substep_timer{
            this->simulator_timer_.startDateTime(),
            /*step_length=*/timestep,
            /*elapsed_time=*/current_time,
            /*time_step_estimate=*/suggestedNextTimestep_(),
            this->simulator_timer_.reportStepNum(),
            maxTimeStep_()
        };
        // Make the per-chunk timer log the enclosing report step's span and a
        // cumulative substep counter (see AdaptiveSimulatorTimer "report step
        // view"). Without this, log lines would show one sync chunk only.
        substep_timer.setReportStepStartTime(report_step_start_time);
        substep_timer.setReportStepTotalTime(step_end_time);
        substep_timer.setReportStepSubstepOffset(report_step_substep_offset);
        const bool final_step = ReservoirCoupling::Seconds::compare_gt_or_eq(
            current_time + timestep, step_end_time
        );
        // Mark this as the first substep of the "sync" timestep. This flag controls
        // whether master-slave data exchange should occur in beginTimeStep() in the well model.
        // It will be cleared after the first runSubStep_() call.
        reservoirCouplingSlave_().setFirstSubstepOfSyncTimestep(true);
        SubStepIteration<Solver> substepIteration{*this, substep_timer, timestep, final_step};
        const auto sub_steps_report = substepIteration.run();
        report += sub_steps_report;
        report_step_substep_offset += substep_timer.currentStepNum();
        current_time += timestep;
        if (final_step) {
            break;
        }
        iteration++;
    }
    return report;
}
#endif  // RESERVOIR_COUPLING_ENABLED


/************************************************
 * Private class SubStepIteration public methods
 ************************************************/

template<class TypeTag>
template<class Solver>
AdaptiveTimeStepping<TypeTag>::SubStepIteration<Solver>::
SubStepIteration(SubStepper<Solver>& substepper,
                 AdaptiveSimulatorTimer& substep_timer,
                 const double original_time_step,
                 bool final_step)
    : substepper_{substepper}
    , substep_timer_{substep_timer}
    , original_time_step_{original_time_step}
    , final_step_{final_step}
    , adaptive_time_stepping_{substepper.getAdaptiveTimerStepper()}
{
}

template <class TypeTag>
template <class Solver>
SimulatorReport
AdaptiveTimeStepping<TypeTag>::SubStepIteration<Solver>::
run()
{
    auto& simulator = solver_().model().simulator();
    auto& problem = simulator.problem();
    // counter for solver restarts
    int restarts = 0;
    SimulatorReport report;

    // sub step time loop
    while (!this->substep_timer_.done()) {
        // if we just chopped the timestep due to convergence i.e. restarts>0
        // we dont want to update the next timestep based on Tuning
        if (restarts == 0) {
            maybeUpdateTuningAndTimeStep_();
        }
        const double dt = this->substep_timer_.currentStepLength();
        if (timeStepVerbose_()) {
            detail::logTimer(this->substep_timer_);
        }

        maybeUpdateLastSubstepOfSyncTimestep_(dt);  // Needed for reservoir coupling
        auto substep_report = runSubStep_();
        markFirstSubStepAsFinished_();  // Needed for reservoir coupling

        if (substep_report.converged || checkContinueOnUnconvergedSolution_(dt)) {
            Dune::Timer perfTimer;
            perfTimer.start();
            // Pass substep to eclwriter for summary output
            problem.setSubStepReport(substep_report);
            auto& full_report = adaptive_time_stepping_.report();
            full_report += substep_report;
            problem.setSimulationReport(full_report);
            problem.endTimeStep();
            substep_report.pre_post_time += perfTimer.stop();

            report += substep_report;

            OPM_TIMEBLOCK(convergenceSucceeded);
            ++this->substep_timer_;   // advance by current dt

            const int iterations = getNumIterations_(substep_report);
            auto dt_estimate = timeStepControlComputeEstimate_(
                                     dt, iterations, this->substep_timer_);

            assert(dt_estimate > 0);
            dt_estimate = maybeRestrictTimeStepGrowth_(dt, dt_estimate, restarts);
            restarts = 0;         // solver converged, reset restarts counter

            maybeReportSubStep_(substep_report);
            if (this->final_step_ && this->substep_timer_.done()) {
                // if the time step is done we do not need to write it as this will be done
                // by the simulator anyway.
            }
            else {
                report.success.output_write_time += writeOutput_();
            }

            // set new time step length
            setTimeStep_(dt_estimate);

            report.success.converged = this->substep_timer_.done();
            this->substep_timer_.setLastStepFailed(false);
        }
        else { // in case of no convergence or time step tolerance test failure
            OPM_TIMEBLOCK(convergenceFailed);
            report += substep_report;
            this->substep_timer_.setLastStepFailed(true);
            checkTimeStepMaxRestartLimit_(restarts);

            double new_time_step = restartFactor_() * dt;
            if (substep_report.time_step_rejected) {
                const double tol = Parameters::Get<Parameters::TimeStepControlTolerance>();
                const double safetyFactor = Parameters::Get<Parameters::TimeStepControlSafetyFactor>();
                const double temp_time_step = std::sqrt(safetyFactor * tol / solver_().model().relativeChange()) * dt;
                if (temp_time_step < dt) { // added in case suggested time step is not a reduction
                    new_time_step = temp_time_step;
                }
            }
            checkTimeStepMinLimit_(new_time_step);
            bool wells_shut = false;
            if (new_time_step > minTimeStepBeforeClosingWells_()) {
                chopTimeStep_(new_time_step);
            } else {
                wells_shut = chopTimeStepOrCloseFailingWells_(new_time_step);
            }
            if (wells_shut) {
                setTimeStep_(dt);  // retry the old timestep
            }
            else {
                restarts++;   // only increase if no wells were shut
            }
        }
        problem.setNextTimeStepSize(this->substep_timer_.currentStepLength());
    }
    updateSuggestedNextStep_();
    return report;
}


/************************************************
 * Private class SubStepIteration private methods
 ************************************************/


template<class TypeTag>
template<class Solver>
bool
AdaptiveTimeStepping<TypeTag>::SubStepIteration<Solver>::
checkContinueOnUnconvergedSolution_(double dt) const
{
    const bool continue_on_uncoverged_solution = ignoreConvergenceFailure_() && dt <= minTimeStep_();
    if (continue_on_uncoverged_solution && solverVerbose_()) {
        // NOTE: This method is only called if the solver failed to converge.
        const auto msg = fmt::format(
            "Solver failed to converge but timestep {} is smaller or equal to {}\n"
            "which is the minimum threshold given by option --solver-min-time-step\n",
            dt, minTimeStep_()
        );
        OpmLog::problem(msg);
    }
    return continue_on_uncoverged_solution;
}

template<class TypeTag>
template<class Solver>
void
AdaptiveTimeStepping<TypeTag>::SubStepIteration<Solver>::
checkTimeStepMaxRestartLimit_(const int restarts) const
{
   // If we have restarted (i.e. cut the timestep) too
    // many times, we have failed and throw an exception.
    if (restarts >= solverRestartMax_()) {
        const auto msg = fmt::format(
            fmt::runtime("Solver failed to converge after cutting timestep {} times."), restarts
        );
        if (solverVerbose_()) {
            OpmLog::error(msg);
        }
        // Use throw directly to prevent file and line
        throw TimeSteppingBreakdown{msg};
    }
}

template<class TypeTag>
template<class Solver>
void
AdaptiveTimeStepping<TypeTag>::SubStepIteration<Solver>::
checkTimeStepMinLimit_(const double new_time_step) const
{
    using Meas = UnitSystem::measure;
    // If we have restarted (i.e. cut the timestep) too
    // much, we have failed and throw an exception.
    if (new_time_step < minTimeStep_()) {
        std::string msg = "Solver failed to converge after cutting timestep to ";
        if (Parameters::Get<Parameters::EnableTuning>()) {
            const UnitSystem& unit_system = solver_().model().simulator().vanguard().eclState().getDeckUnitSystem();
            msg += fmt::format(
                "{:.3E} {}\nwhich is the minimum threshold given by the TUNING keyword\n",
                unit_system.from_si(Meas::time, minTimeStep_()),
                unit_system.name(Meas::time)
            );
        }
        else {
            msg += fmt::format(
                "{:.3E} DAYS\nwhich is the minimum threshold given by option --solver-min-time-step\n",
                minTimeStep_() / 86400.0
            );
        }
        if (solverVerbose_()) {
            OpmLog::error(msg);
        }
        // Use throw directly to prevent file and line
        throw TimeSteppingBreakdown{msg};
    }
}

template<class TypeTag>
template<class Solver>
void
AdaptiveTimeStepping<TypeTag>::SubStepIteration<Solver>::
chopTimeStep_(const double new_time_step)
{
    setTimeStep_(new_time_step);
    if (solverVerbose_()) {
        const auto msg = fmt::format(fmt::runtime("{}\nTimestep chopped to {} days\n"),
                    this->cause_of_failure_,
                    unit::convert::to(this->substep_timer_.currentStepLength(), unit::day));
        OpmLog::problem(msg);
    }
}

template<class TypeTag>
template<class Solver>
bool
AdaptiveTimeStepping<TypeTag>::SubStepIteration<Solver>::
chopTimeStepOrCloseFailingWells_(const double new_time_step)
{
    bool wells_shut = false;
    // We are below the threshold, and will check if there are any
    // wells that fails repeatedly (that means that it fails in the last three steps)
    // we should close rather than chopping again.
    // If we already have chopped the timestep two times that is
    // new_time_step < minTimeStepBeforeClosingWells_()*restartFactor_()*restartFactor_()
    // We also shut wells that fails only on this step.
    const bool requireRepeatedFailures =
        new_time_step > (minTimeStepBeforeClosingWells_() * restartFactor_() * restartFactor_());
    const std::set<std::string> failing_wells =
        detail::consistentlyFailingWells(solver_().model().stepReports(), requireRepeatedFailures);

    if (failing_wells.empty()) {
        // Found no wells to close, chop the timestep
        chopTimeStep_(new_time_step);
    } else {
        // Close all consistently failing wells that are not under group control
        std::vector<std::string> shut_wells;
        for (const auto& well : failing_wells) {
            const bool was_shut =
                solver_().model().wellModel().forceShutWellByName(well,
                                                                  this->substep_timer_.simulationTimeElapsed(),
                                                                  /*dont_shut_grup_wells =*/ true);
            if (was_shut) {
                shut_wells.push_back(well);
            }
        }
        // If no wells are closed we also try to shut wells under group control
        if (shut_wells.empty()) {
            for (const auto& well : failing_wells) {
                const bool was_shut =
                    solver_().model().wellModel().forceShutWellByName(well,
                                                                      this->substep_timer_.simulationTimeElapsed(),
                                                                      /*dont_shut_grup_wells =*/ false);
                if (was_shut) {
                    shut_wells.push_back(well);
                }
            }
        }
        // If still no wells are closed we must fall back to chopping again
        if (shut_wells.empty()) {
            chopTimeStep_(new_time_step);
        } else {
            wells_shut = true;
            if (solverVerbose_()) {
                const std::string msg =
                        fmt::format(fmt::runtime("\nProblematic well(s) were shut: {}"
                                    "(retrying timestep)\n"),
                                    fmt::join(shut_wells, " "));
                OpmLog::problem(msg);
            }
        }
    }
    return wells_shut;
}

template<class TypeTag>
template<class Solver>
boost::posix_time::ptime
AdaptiveTimeStepping<TypeTag>::SubStepIteration<Solver>::
currentDateTime_() const
{
    return simulatorTimer_().currentDateTime();
}

template<class TypeTag>
template<class Solver>
int
AdaptiveTimeStepping<TypeTag>::SubStepIteration<Solver>::
getNumIterations_(const SimulatorReportSingle &substep_report) const
{
    if (useNewtonIteration_()) {
        return substep_report.total_newton_iterations;
    }
    else {
        return substep_report.total_linear_iterations;
    }
}

template<class TypeTag>
template<class Solver>
double
AdaptiveTimeStepping<TypeTag>::SubStepIteration<Solver>::
growthFactor_() const
{
    return this->adaptive_time_stepping_.growth_factor_;
}

template<class TypeTag>
template<class Solver>
bool
AdaptiveTimeStepping<TypeTag>::SubStepIteration<Solver>::
ignoreConvergenceFailure_() const
{
    return adaptive_time_stepping_.ignore_convergence_failure_;
}

template<class TypeTag>
template<class Solver>
bool
AdaptiveTimeStepping<TypeTag>::SubStepIteration<Solver>::
isReservoirCouplingMaster_() const
{
    return this->substepper_.isReservoirCouplingMaster_();
}

template<class TypeTag>
template<class Solver>
bool
AdaptiveTimeStepping<TypeTag>::SubStepIteration<Solver>::
isReservoirCouplingSlave_() const
{
    return this->substepper_.isReservoirCouplingSlave_();
}

template<class TypeTag>
template<class Solver>
void
AdaptiveTimeStepping<TypeTag>::SubStepIteration<Solver>::
markFirstSubStepAsFinished_() const
{
#ifdef RESERVOIR_COUPLING_ENABLED
    // Clear the first-substep flag after the first runSubStep_() call.
    // This ensures that master-slave synchronization only happens once per sync timestep,
    // not on retry attempts after convergence-driven timestep chops.
    if (isReservoirCouplingMaster_()) {
        reservoirCouplingMaster_().setFirstSubstepOfSyncTimestep(false);
    }
    else if (isReservoirCouplingSlave_()) {
        reservoirCouplingSlave_().setFirstSubstepOfSyncTimestep(false);
    }
#endif
    return;
}

template<class TypeTag>
template<class Solver>
double
AdaptiveTimeStepping<TypeTag>::SubStepIteration<Solver>::
maxGrowth_() const
{
    return this->adaptive_time_stepping_.max_growth_;
}

template<class TypeTag>
template<class Solver>
void
AdaptiveTimeStepping<TypeTag>::SubStepIteration<Solver>::
maybeReportSubStep_(SimulatorReportSingle substep_report) const
{
    if (timeStepVerbose_()) {
        std::ostringstream ss;
        substep_report.reportStep(ss);
        OpmLog::info(ss.str());
    }
}

template<class TypeTag>
template<class Solver>
double
AdaptiveTimeStepping<TypeTag>::SubStepIteration<Solver>::
maybeRestrictTimeStepGrowth_(const double dt, double dt_estimate, const int restarts) const
{
    // limit the growth of the timestep size by the growth factor
    dt_estimate = std::min(dt_estimate, double(maxGrowth_() * dt));
    assert(dt_estimate > 0);
    // further restrict time step size growth after convergence problems
    if (restarts > 0) {
        dt_estimate = std::min(growthFactor_() * dt, dt_estimate);
    }

    return dt_estimate;
}


template<class TypeTag>
template<class Solver>
void
AdaptiveTimeStepping<TypeTag>::SubStepIteration<Solver>::
maybeUpdateLastSubstepOfSyncTimestep_([[maybe_unused]] const double dt)
{
#ifdef RESERVOIR_COUPLING_ENABLED
    // For reservoir coupling slaves: predict if this substep will complete
    // the sync timestep.  If so, timeStepSucceeded() will send production
    // data to the master (which is blocking on receive after its first substep).
    // This is used for summary data synchronization between slaves and master.
    if (isReservoirCouplingSlave_()) {
        const bool is_last = ReservoirCoupling::Seconds::compare_gt_or_eq(
            this->substep_timer_.simulationTimeElapsed() + dt,
            this->substep_timer_.totalTime()
        );
        reservoirCouplingSlave_().setLastSubstepOfSyncTimestep(is_last);
    }
#endif
}

// The maybeUpdateTuning_() lambda callback is defined in SimulatorFullyImplicit::runStep()
// It has to be called for each substep since TUNING might have been changed for next sub step due
// to ACTIONX (via NEXTSTEP) or WCYCLE keywords.
template<class TypeTag>
template<class Solver>
void
AdaptiveTimeStepping<TypeTag>::SubStepIteration<Solver>::
maybeUpdateTuningAndTimeStep_()
{
    // TODO: This function is currently only called if NEXTSTEP is activated from ACTIONX or
    // if the WCYCLE keyword needs to modify the current timestep. So this method should rather
    // be named maybeUpdateTimeStep_() or similar, since it should not update the tuning. However,
    // the current definition of the maybeUpdateTuning_() callback is actually calling
    // adaptiveTimeStepping_->updateTUNING(max_next_tstep, tuning) which is updating the tuning
    // see SimulatorFullyImplicit::runStep() for more details.
    const auto old_value = suggestedNextTimestep_();
    if (this->substepper_.maybeUpdateTuning_(this->substep_timer_.simulationTimeElapsed(),
                                             this->substep_timer_.currentStepLength(),
                                             this->substep_timer_.currentStepNum()))
    {
        // Either NEXTSTEP and WCYCLE wants to change the current time step, but they cannot
        // change the current time step directly. Instead, they change the suggested next time step
        // by calling updateNEXTSTEP() via the maybeUpdateTuning() callback. We now need to update
        // the current time step to the new suggested time step and reset the suggested time step
        // to the old value.
        setTimeStep_(suggestedNextTimestep_());
        setSuggestedNextStep_(old_value);
    }
}

template<class TypeTag>
template<class Solver>
double
AdaptiveTimeStepping<TypeTag>::SubStepIteration<Solver>::
minTimeStepBeforeClosingWells_() const
{
    return this->adaptive_time_stepping_.min_time_step_before_shutting_problematic_wells_;
}

template<class TypeTag>
template<class Solver>
double
AdaptiveTimeStepping<TypeTag>::SubStepIteration<Solver>::
minTimeStep_() const
{
    return this->adaptive_time_stepping_.min_time_step_;
}

#ifdef RESERVOIR_COUPLING_ENABLED
template<class TypeTag>
template<class Solver>
ReservoirCouplingMaster<typename AdaptiveTimeStepping<TypeTag>::Scalar>&
AdaptiveTimeStepping<TypeTag>::SubStepIteration<Solver>::
reservoirCouplingMaster_() const
{
    return this->substepper_.reservoirCouplingMaster_();
}

template<class TypeTag>
template<class Solver>
ReservoirCouplingSlave<typename AdaptiveTimeStepping<TypeTag>::Scalar>&
AdaptiveTimeStepping<TypeTag>::SubStepIteration<Solver>::
reservoirCouplingSlave_() const
{
    return this->substepper_.reservoirCouplingSlave_();
}
#endif

template<class TypeTag>
template<class Solver>
double
AdaptiveTimeStepping<TypeTag>::SubStepIteration<Solver>::
restartFactor_() const
{
    return this->adaptive_time_stepping_.restart_factor_;
}

template<class TypeTag>
template<class Solver>
SimulatorReportSingle
AdaptiveTimeStepping<TypeTag>::SubStepIteration<Solver>::
runSubStep_()
{
    OPM_TIMEFUNCTION();
    SimulatorReportSingle substep_report;

    auto handleFailure = [this, &substep_report]
            (const std::string& failure_reason, const std::exception& e, bool log_exception = true)
    {
        substep_report = solver_().failureReport();
        this->cause_of_failure_ = failure_reason;
        if (log_exception && solverVerbose_()) {
            OpmLog::debug(std::string("Caught Exception: ") + e.what());
        }
    };

    try {
        substep_report = solver_().step(this->substep_timer_, &this->adaptive_time_stepping_.timeStepControl());
        if (solverVerbose_()) {
            // report number of linear iterations
            OpmLog::debug("Overall linear iterations used: "
                          + std::to_string(substep_report.total_linear_iterations));
        }
    }
    catch (const TooManyIterations& e) {
        handleFailure("Solver convergence failure - Iteration limit reached", e);
    }
    catch (const TimeSteppingBreakdown& e) {
        handleFailure(e.what(), e);
    }
    catch (const ConvergenceMonitorFailure& e) {
        handleFailure("Convergence monitor failure", e, /*log_exception=*/false);
    }
    catch (const LinearSolverProblem& e) {
        handleFailure("Linear solver convergence failure", e);
    }
    catch (const NumericalProblem& e) {
        handleFailure("Solver convergence failure - Numerical problem encountered", e);
    }
    catch (const std::runtime_error& e) {
        handleFailure("Runtime error encountered", e);
    }
    catch (const Dune::ISTLError& e) {
        handleFailure("ISTL error - Time step too large", e);
    }
    catch (const Dune::MatrixBlockError& e) {
        handleFailure("Matrix block error", e);
    }

    return substep_report;
}

template<class TypeTag>
template<class Solver>
void
AdaptiveTimeStepping<TypeTag>::SubStepIteration<Solver>::
setTimeStep_(double dt_estimate)
{
    this->substep_timer_.provideTimeStepEstimate(dt_estimate);
}

template<class TypeTag>
template<class Solver>
Solver&
AdaptiveTimeStepping<TypeTag>::SubStepIteration<Solver>::
solver_() const
{
    return this->substepper_.solver_;
}


template<class TypeTag>
template<class Solver>
int
AdaptiveTimeStepping<TypeTag>::SubStepIteration<Solver>::
solverRestartMax_() const
{
    return this->adaptive_time_stepping_.solver_restart_max_;
}

template<class TypeTag>
template<class Solver>
void
AdaptiveTimeStepping<TypeTag>::SubStepIteration<Solver>::
setSuggestedNextStep_(double step)
{
    this->adaptive_time_stepping_.setSuggestedNextStep(step);
}

template <class TypeTag>
template <class Solver>
const SimulatorTimer&
AdaptiveTimeStepping<TypeTag>::SubStepIteration<Solver>::
simulatorTimer_() const
{
    return this->substepper_.simulator_timer_;
}

template <class TypeTag>
template <class Solver>
bool
AdaptiveTimeStepping<TypeTag>::SubStepIteration<Solver>::
solverVerbose_() const
{
    return this->adaptive_time_stepping_.solver_verbose_;
}

template<class TypeTag>
template<class Solver>
boost::posix_time::ptime
AdaptiveTimeStepping<TypeTag>::SubStepIteration<Solver>::
startDateTime_() const
{
    return simulatorTimer_().startDateTime();
}

template <class TypeTag>
template <class Solver>
double
AdaptiveTimeStepping<TypeTag>::SubStepIteration<Solver>::
suggestedNextTimestep_() const
{
    return this->adaptive_time_stepping_.suggestedNextStep();
}

template <class TypeTag>
template <class Solver>
double
AdaptiveTimeStepping<TypeTag>::SubStepIteration<Solver>::
timeStepControlComputeEstimate_(const double dt, const int iterations,
                                const AdaptiveSimulatorTimer& substepTimer) const
{
    // create object to compute the time error, simply forwards the call to the model
    const SolutionTimeErrorSolverWrapper<Solver> relative_change{solver_()};
    return this->adaptive_time_stepping_.time_step_control_->computeTimeStepSize(
        dt, iterations, relative_change, substepTimer);
}

template <class TypeTag>
template <class Solver>
bool
AdaptiveTimeStepping<TypeTag>::SubStepIteration<Solver>::
timeStepVerbose_() const
{
    return this->adaptive_time_stepping_.timestep_verbose_;
}

// The suggested time step is the stepsize that will be used as a first try for
// the next sub step. It is updated at the end of each substep. It can also be
// updated by the TUNING or NEXTSTEP keywords at the beginning of each report step or
// at the beginning of each substep by the ACTIONX keyword (via NEXTSTEP), this is
// done by the maybeUpdateTuning_() method which is called at the beginning of each substep
// (and the begginning of each report step). Note that the WCYCLE keyword can also update the
// suggested time step via the maybeUpdateTuning_() method.
template <class TypeTag>
template <class Solver>
void
AdaptiveTimeStepping<TypeTag>::SubStepIteration<Solver>::
updateSuggestedNextStep_()
{
    auto suggested_next_step = this->substep_timer_.currentStepLength();
    if (! std::isfinite(suggested_next_step)) { // check for NaN
        suggested_next_step = this->original_time_step_;
    }
    if (timeStepVerbose_()) {
        std::ostringstream ss;
        this->substep_timer_.report(ss);
        ss << "Suggested next step size = "
           << unit::convert::to(suggested_next_step, unit::day) << " (days)" << std::endl;
        OpmLog::debug(ss.str());
    }
    setSuggestedNextStep_(suggested_next_step);
}

template <class TypeTag>
template <class Solver>
bool
AdaptiveTimeStepping<TypeTag>::SubStepIteration<Solver>::
useNewtonIteration_() const
{
    return this->adaptive_time_stepping_.use_newton_iteration_;
}

template <class TypeTag>
template <class Solver>
double
AdaptiveTimeStepping<TypeTag>::SubStepIteration<Solver>::
writeOutput_() const
{
    time::StopWatch perf_timer;
    perf_timer.start();
    auto& problem = solver_().model().simulator().problem();
    problem.writeOutput(true);
    return perf_timer.secsSinceStart();
}

/************************************************
 * Private class SolutionTimeErrorSolverWrapper
 * **********************************************/

template<class TypeTag>
template<class Solver>
AdaptiveTimeStepping<TypeTag>::
SolutionTimeErrorSolverWrapper<Solver>::
SolutionTimeErrorSolverWrapper(const Solver& solver)
    : solver_{solver}
{}

template<class TypeTag>
template<class Solver>
double AdaptiveTimeStepping<TypeTag>::SolutionTimeErrorSolverWrapper<Solver>::relativeChange() const
{
    // returns:   || u^n+1 - u^n || / || u^n+1 ||
    return solver_.model().relativeChange();
}

} // namespace Opm

#endif // OPM_ADAPTIVE_TIME_STEPPING_IMPL_HPP