/* -*- mode: c++; tab-width: 4; indent-tabs-mode: nil; c-basic-offset: 4 -*- */

/*

 Copyright (C) 2003 RiskMap srl

 This file is part of QuantLib, a free-software/open-source library

 for financial quantitative analysts and developers - http://quantlib.org/

 QuantLib is free software: you can redistribute it and/or modify it

 under the terms of the QuantLib license.  You should have received a

 copy of the license along with this program; if not, please email

 <quantlib-dev@lists.sf.net>. The license is also available online at

 <https://www.quantlib.org/license.shtml>.

 This program 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 license for more details.

*/

/*! \file lazyobject.hpp

    \brief framework for calculation on demand and result caching

*/

#ifndef quantlib_lazy_object_h

#define quantlib_lazy_object_h

#include <ql/patterns/observable.hpp>

#include <ql/shared_ptr.hpp>

namespace QuantLib {

    //! Framework for calculation on demand and result caching.

    /*! \ingroup patterns */

    class LazyObject : public virtual Observable,

                       public virtual Observer {

      public:

        LazyObject();

        ~LazyObject() override = default;

        //! \name Observer interface

        //@{

        void update() override;

        //@}

        /*! Returns true if the instrument is calculated */

        bool isCalculated() const;

        /*! Set calculated status */

        void setCalculated(bool c) const;

        /*! \name Calculations

            These methods do not modify the structure of the object

            and are therefore declared as <tt>const</tt>. Data members

            which will be calculated on demand need to be declared as

            mutable.

        */

        //@{

        /*! This method force the recalculation of any results which

            would otherwise be cached. It is not declared as

            <tt>const</tt> since it needs to call the

            non-<tt>const</tt> <i><b>notifyObservers</b></i> method.

            \note Explicit invocation of this method is <b>not</b>

                  necessary if the object registered itself as

                  observer with the structures on which such results

                  depend.  It is strongly advised to follow this

                  policy when possible.

        */

        void recalculate();

        /*! This method constrains the object to return the presently

            cached results on successive invocations, even if

            arguments upon which they depend should change.

        */

        void freeze();

        /*! This method reverts the effect of the <i><b>freeze</b></i>

            method, thus re-enabling recalculations.

        */

        void unfreeze();

      protected:

        /*! This method performs all needed calculations by calling

            the <i><b>performCalculations</b></i> method.

            \warning Objects cache the results of the previous

                     calculation. Such results will be returned upon

                     later invocations of

                     <i><b>calculate</b></i>. When the results depend

                     on arguments which could change between

                     invocations, the lazy object must register itself

                     as observer of such objects for the calculations

                     to be performed again when they change.

            \warning Should this method be redefined in derived

                     classes, LazyObject::calculate() should be called

                     in the overriding method.

        */

        virtual void calculate() const;

        /*! This method must implement any calculations which must be

            (re)done in order to calculate the desired results.

        */

        virtual void performCalculations() const = 0;

        //@}

      public:

        //! \name Notification settings

        //@{

        /*! This method causes the object to forward the first notification received,

            and discard the others until recalculated; the rationale is that observers

            were already notified, and don't need further notifications until they

            recalculate, at which point this object would be recalculated too.

            After recalculation, this object would again forward the first notification

            received.

            Although not always correct, this behavior is a lot faster

            and thus is the current default.  The default can be

            changed at compile time, or at at run time by calling

            `LazyObject::Defaults::instance().alwaysForwardNotifications()`;

            the run-time change won't affect lazy objects already created.

        */

        void forwardFirstNotificationOnly();

        /*! This method causes the object to forward all notifications received.

            Although safer, this behavior is a lot slower and thus

            usually not the default.  The default can be changed at

            compile time, or at run-time by calling

            `LazyObject::Defaults::instance().alwaysForwardNotifications()`;

            the run-time change won't affect lazy objects already

            created.

        */

        void alwaysForwardNotifications();

        //@}

      protected:

        mutable bool calculated_ = false, frozen_ = false, failed_ = false, alwaysForward_;

      private:

        bool updating_ = false;

        class UpdateChecker {  // NOLINT(cppcoreguidelines-special-member-functions)

            LazyObject* subject_;

          public:

            explicit UpdateChecker(LazyObject* subject) : subject_(subject) {

                subject_->updating_ = true;

            }

            ~UpdateChecker() {

                subject_->updating_ = false;

            }

        };

      public:

        class Defaults;

    };

    //! Per-session settings for the LazyObject class

    class LazyObject::Defaults : public Singleton<LazyObject::Defaults> {

        friend class Singleton<LazyObject::Defaults>;

      private:

        Defaults() = default;

      public:

        /*! by default, lazy objects created after calling this method

            will only forward the first notification after successful

            recalculation; see

            LazyObject::forwardFirstNotificationOnly for details.

        */

        void forwardFirstNotificationOnly() {

            forwardsAllNotifications_ = false;

        }

        /*! by default, lazy objects created after calling this method

            will always forward notifications; see

            LazyObject::alwaysForwardNotifications for details.

        */

        void alwaysForwardNotifications() {

            forwardsAllNotifications_ = true;

        }

        //! returns the current default

        bool forwardsAllNotifications() const {

            return forwardsAllNotifications_;

        }

      private:

        #ifdef QL_FASTER_LAZY_OBJECTS

        bool forwardsAllNotifications_ = false;

        #else

        bool forwardsAllNotifications_ = true;

        #endif

    };

    // inline definitions

    inline LazyObject::LazyObject()

    : alwaysForward_(LazyObject::Defaults::instance().forwardsAllNotifications()) {}

    inline void LazyObject::update() {

        if (updating_) {

            #ifdef QL_THROW_IN_CYCLES

            QL_FAIL("recursive notification loop detected; you probably created an object cycle");

            #else

            return;

            #endif

        }

        // This sets updating to true (so the above check breaks the

        // infinite loop if we enter this method recursively) and will

        // set it back to false when we exit this scope, either

        // successfully or because of an exception.

        UpdateChecker checker(this);

        // forwards notifications only the first time

        if (calculated_ || failed_ || alwaysForward_) {

            // set to false early

            // 1) to prevent infinite recursion

            // 2) otherways non-lazy observers would be served obsolete

            //    data because of calculated_ being still true

            calculated_ = false;

            failed_ = false;

            // observers don't expect notifications from frozen objects

            if (!frozen_)

                notifyObservers();

                // exiting notifyObservers() calculated_ could be

                // already true because of non-lazy observers

        }

    }

    inline void LazyObject::recalculate() {

        bool wasFrozen = frozen_;

        calculated_ = frozen_ = failed_ = false;

        try {

            calculate();

        } catch (...) {

            frozen_ = wasFrozen;

            notifyObservers();

            throw;

        }

        frozen_ = wasFrozen;

        notifyObservers();

    }

    inline void LazyObject::freeze() {

        frozen_ = true;

    }

    inline void LazyObject::unfreeze() {

        // send notifications, just in case we lost any,

        // but only once, i.e. if it was frozen

        if (frozen_) {

            frozen_ = false;

            notifyObservers();

        }

    }

    inline void LazyObject::forwardFirstNotificationOnly() {

        alwaysForward_ = false;

    }

    inline void LazyObject::alwaysForwardNotifications() {

        alwaysForward_ = true;

    }

    inline void LazyObject::calculate() const {

        if (!calculated_ && !frozen_) {

            calculated_ = true;   // prevent infinite recursion in

                                  // case of bootstrapping

            try {

                performCalculations();

                failed_ = false;  // needed when calculate() is called

                                  // directly after a prior failure

            } catch (...) {

                calculated_ = false;

                failed_ = true;

                throw;

            }

        }

    }

    inline bool LazyObject::isCalculated() const {

        return calculated_;

    }

    inline void LazyObject::setCalculated(const bool c) const {

        calculated_ = c;

    }

}

#endif