text: ////////////////////////////////////////////////////////////////////////////////
  // Copyright (c) 2014-2019, Lawrence Livermore National Security, LLC.
  // Produced at the Lawrence Livermore National Laboratory.
  // Written by the LBANN Research Team (B. Van Essen, et al.) listed in
  // the CONTRIBUTORS file. <lbann-dev@llnl.gov>
  //
  // LLNL-CODE-697807.
  // All rights reserved.
  //
  // This file is part of LBANN: Livermore Big Artificial Neural Network
  // Toolkit. For details, see http://software.llnl.gov/LBANN or
  // https://github.com/LLNL/LBANN.
  //
  // Licensed under the Apache License, Version 2.0 (the "Licensee"); you
  // may not use this file except in compliance with the License.  You may
  // obtain a copy of the License at:
  //
  // http://www.apache.org/licenses/LICENSE-2.0
  //
  // Unless required by applicable law or agreed to in writing, software
  // distributed under the License is distributed on an "AS IS" BASIS,
  // WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or
  // implied. See the License for the specific language governing
  // permissions and limitations under the license.
  //
  // callback .hpp - Base class for LBANN callbacks
  ////////////////////////////////////////////////////////////////////////////////
  
  #ifndef LBANN_CALLBACKS_CALLBACK_HPP_INCLUDED
  #define LBANN_CALLBACKS_CALLBACK_HPP_INCLUDED
  
  #include "lbann/trainers/trainer.hpp"
  #include "lbann/layers/layer.hpp"
  #include "lbann/models/model.hpp"
  #include "lbann/utils/description.hpp"
  #include "lbann/utils/memory.hpp"
  #include "lbann/utils/summary.hpp"
  #include "lbann/execution_algorithms/sgd_execution_context.hpp"
  
  #include <google/protobuf/message.h>
  
  #include <algorithm>
  #include <string>
  
  /** @brief A utility macro for easily adding default-constructed sub-class
   *  builders.*/
  #define LBANN_ADD_DEFAULT_CALLBACK_BUILDER(Class, FunctionName)  \
    inline std::unique_ptr<callback_base> FunctionName(           \
      const google::protobuf::Message&, std::shared_ptr<lbann_summary> const&) {          \
      return lbann::make_unique<Class>();                          \
    }
  
  namespace cereal
  {
    class access;
  }// namespace cereal
  
  namespace lbann {
  
  /** @class callback_base
   *  @brief Base class for callbacks during training/testing.
   *
   *  The method of each callback is called at a given point during
   *  training or testing by the model. Implement whichever ones you
   *  care about.  Callbacks may be passed a lbann_summary instance,
   *  which they can use to log any relevant information.
   */
  class callback_base {
  public:
  
    /** @name Constructors and destructor */
    ///@{
  
    /** @brief Initialize a callback with an optional batch interval
     */
    callback_base(int batch_interval = 1) :
      m_batch_interval(std::max(batch_interval, 1)) {}
    callback_base(const callback_base&) = default;
    virtual ~callback_base() = default;
  
    ///@}
    /** @name Polymorphic copy */
    ///@{
  
    virtual callback_base* copy() const = 0;
  
    ///@}
    /** @name Modifiers */
    ///@{
  
    /** @brief Called once to set up the callback on the trainer
     */
    virtual void setup(trainer *t) {};
  
    /** @brief Called once to set up the callback on the model
     *         (after all layers are set up).
     */
    virtual void setup(model *m) {};
  
    ///@}
    /** @name Callback hooks */
    ///@{
  
    /** @brief Called at the end of setup. */
    virtual void on_setup_end(model *m) {}
    /** @brief Called at the beginning of training. */
    virtual void on_train_begin(model *m) {}
    /** @brief Called at the end of training. */
    virtual void on_train_end(model *m) {}
    /** @brief Called at the end of every phase (multiple epochs) in a
     *         layer-wise model training
     */
    virtual void on_phase_end(model *m) {}
    /** @brief Called at the beginning of each epoch. */
    virtual void on_epoch_begin(model *m) {}
    /** @brief Called immediate after the end of each epoch. */
    virtual void on_epoch_end(model *m) {}
    /** @brief Called at the beginning of a (mini-)batch. */
    virtual void on_batch_begin(model *m) {}
    /** @brief Called immediately after the end of a (mini-)batch. */
    virtual void on_batch_end(model *m) {}
    /** @brief Called at the beginning of testing. */
    virtual void on_test_begin(model *m) {}
    /** @brief Called immediately after the end of testing. */
    virtual void on_test_end(model *m) {}
    /** @brief Called at the beginning of validation. */
    virtual void on_validation_begin(model *m) {}
    /** @brief Called immediately after the end of validation. */
    virtual void on_validation_end(model *m) {}
    /** @brief Called when a model begins forward propagation. */
    virtual void on_forward_prop_begin(model *m) {}
    /** @brief Called when a layer begins forward propagation. */
    virtual void on_forward_prop_begin(model *m, Layer *l) {}
    /** @brief Called when a model ends forward propagation. */
    virtual void on_forward_prop_end(model *m) {}
    /** @brief Called when a layer ends forward propagation. */
    virtual void on_forward_prop_end(model *m, Layer *l) {}
    /** @brief Called when a model begins backward propagation. */
    virtual void on_backward_prop_begin(model *m) {}
    /** @brief Called when a layer begins backward propagation. */
    virtual void on_backward_prop_begin(model *m, Layer *l) {}
    /** @brief Called when a model ends backward propagation. */
    virtual void on_backward_prop_end(model *m) {}
    /** @brief Called when a layer ends backward propagation. */
    virtual void on_backward_prop_end(model *m, Layer *l) {}
    /** @brief Called when a model begins optimization. */
    virtual void on_optimize_begin(model *m) {}
    /** @brief Called when weights begins optimization. */
    virtual void on_optimize_begin(model *m, weights *w) {}
    /** @brief Called when a model ends optimization. */
    virtual void on_optimize_end(model *m) {}
    /** @brief Called when weights ends optimization. */
    virtual void on_optimize_end(model *m, weights *w) {}
  
    /** @brief Called at the beginning of a (mini-)batch evaluation
     *         (validation / testing).
     */
    virtual void on_batch_evaluate_begin(model *m) {}
    /** @brief Called at the end of a (mini-)batch evaluation
     *         (validation / testing).
     */
    virtual void on_batch_evaluate_end(model *m) {}
    /** @brief Called when a model begins forward propagation for
     *         evaluation (validation / testing).
     */
    virtual void on_evaluate_forward_prop_begin(model *m) {}
    /** @brief Called when a layer begins forward propagation for
     *         evaluation (validation / testing).
     */
    virtual void on_evaluate_forward_prop_begin(model *m, Layer *l) {}
    /** @brief Called when a model ends forward propagation for
     *         evaluation (validation / testing).
     */
    virtual void on_evaluate_forward_prop_end(model *m) {}
    /** @brief Called when a layer ends forward propagation for
     *         evaluation (validation / testing).
     */
    virtual void on_evaluate_forward_prop_end(model *m, Layer *l) {}
  
    ///@}
    /** @name Queries */
    ///@{
  
    /** @brief Return the batch interval. */
    int get_batch_interval() const { return m_batch_interval; }
  
    /** @brief Return this callback's name. */
    virtual std::string name() const = 0;
  
    /** @brief Human-readable description. */
    virtual description get_description() const;
  
    ///@}
    /** @name Serialization */
    ///@{
  
    /** @brief Store state to archive for checkpoint and restart */
    template <class Archive> void serialize(Archive & ar);
  
    ///@}
  
    /** @brief Build a standard directory hierarchy including trainer ID.
     */
    inline std::string get_multi_trainer_path(const model& m,
                                              const std::string& root_dir) {
      std::string dir = root_dir;
      if (dir.empty()) { dir = "./"; }
      if (dir.back() != '/') { dir += "/"; }
  
      return build_string(dir,
                          get_const_trainer().get_name(), '/');
    }
  
    /** @brief Build a standard directory hierachy including trainer,
     * execution context, and model information (in that order).
     */
    inline std::string get_multi_trainer_ec_model_path(const model& m,
                                                       const std::string& root_dir) {
      std::string dir = get_multi_trainer_path(m, root_dir);
      const auto& c = static_cast<const SGDExecutionContext&>(m.get_execution_context());
      return build_string(dir,
                          c.get_state_string(), '/',
                          m.get_name(), '/');
    }
  
    /** @brief Build a standard directory hierachy including trainer,
     * model information in that order.
     */
    inline std::string get_multi_trainer_model_path(const model& m,
                                                    const std::string& root_dir) {
      std::string dir = get_multi_trainer_path(m, root_dir);
      return build_string(dir,
                          m.get_name(), '/');
    }
  
  protected:
  
    /** @brief Copy-assignment operator.
     *
     *  Performs a shallow (pointer) copy of the summarizer.
     */
    callback_base& operator=(const callback_base&) = default;
  
  protected:
    /** @todo Make callback data private */
  
    /** @brief Batch methods should once every this many steps. */
    int m_batch_interval;
  };
  
  }  // namespace lbann
  
  #endif  // LBANN_CALLBACKS_CALLBACK_HPP_INCLUDED
  