    // Copyright (C) 2015  Davis E. King (davis@dlib.net)
    // License: Boost Software License   See LICENSE.txt for the full license.
    #undef DLIB_DNn_TRAINER_ABSTRACT_H_
    #ifdef DLIB_DNn_TRAINER_ABSTRACT_H_

    #include "core_abstract.h"
    #include "solvers_abstract.h"
    #include <vector>
    #include <chrono>

    namespace dlib
    {

    // ----------------------------------------------------------------------------------------

        enum class force_flush_to_disk {
            no = 0,
            yes = 1
        };

    // ----------------------------------------------------------------------------------------

        template <
            typename net_type,
            typename solver_type = sgd
            >
        class dnn_trainer
        {
            /*!
                REQUIREMENTS ON net_type
                    - net_type is an add_loss_layer object.

                REQUIREMENTS ON solver_type
                    - solver_type is an implementation of the EXAMPLE_SOLVER interface defined
                      in solvers_abstract.h

                WHAT THIS OBJECT REPRESENTS
                    This object is a tool training a deep neural network. To use it you supply
                    a neural network type and a solver, then you call train() with your
                    training data and it will output a new network instance that has hopefully
                    learned something useful from your training data.

                    If you are compiling with CUDA then this object will use the GPU that is
                    currently selected (i.e. the one indicated by cudaGetDevice()) when
                    dnn_trainer is constructed.  It will continue to use that device even if
                    you later change it by a call to cudaSetDevice().

                EXCEPTIONS
                    If an exception is thrown by any part of the neural network during training
                    then the exception will be propagated out of the trainer to the user.
                    Moreover, the trainer instance will be unusable and should be destroyed.
            !*/

        public:

            typedef typename net_type::training_label_type training_label_type;
            typedef typename net_type::input_type input_type;
            const static size_t num_computational_layers = net_type::num_computational_layers;

            using threads = std::vector<std::shared_ptr<thread_pool>>;

            dnn_trainer() = delete;
            dnn_trainer(const dnn_trainer&) = delete;
            dnn_trainer& operator=(const dnn_trainer&) = delete;

            dnn_trainer(
                net_type& net,
                const solver_type& solver = solver_type(),
                const std::vector<int>& cuda_extra_devices = {},
                std::shared_ptr<threads> thread_pools = std::shared_ptr<threads>()
            );
            /*!
                requires
                    - for all valid i:
                        - 0 <= cuda_extra_devices[i] < dlib::cuda::get_num_devices()
                ensures
                    - &#get_net() == &net
                      (i.e. The dnn_trainer holds a reference to net, it does not copy it.
                      Therefore, you must ensure net has a lifetime at least as long as the
                      dnn_trainer).
                    - #get_solvers() == a set of solvers that are all initialized with the
                      provided solver instance.
                    - #get_max_num_epochs() == 10000
                    - #get_mini_batch_size() == 128
                    - #get_learning_rate() == 1e-2
                    - #get_min_learning_rate() == 1e-5
                    - #get_iterations_without_progress_threshold() == 2000
                    - #get_test_iterations_without_progress_threshold() == 500
                    - #get_learning_rate_shrink_factor() == 0.1
                    - #get_learning_rate_schedule().size() == 0
                    - #get_train_one_step_calls() == 0
                    - #get_test_one_step_calls() == 0
                    - #get_synchronization_file() == ""
                    - if (cuda_extra_devices.size() > 0) then
                        - This object will use multiple graphics cards to run the learning
                          algorithms.  In particular, it will always use whatever device is
                          currently selected on the calling thread (the device indicated by
                          cudaGetDevice()).  In addition, you can ask to use additional
                          devices, which you do by putting their device numbers into
                          cuda_extra_devices.
                    - if (thread_pools.get() != nullptr) then
                        - Any new threads spun within the trainer will execute within the
                          passed thread pools vector. This means that the same threads can
                          be re-used across different dnn_trainer instances. Otherwise, the
                          CUDA runtime may leak memory. This, however, is relevant only if
                          your program is going to instantiate a large number of trainers,
                          and generally stay up and running for a very long time. If not,
                          then you need not worry about this.
                          NB: Any particular thread pools vector should be passed to max
                              one trainer instance at a time.
                          NB: The mentioned leak isn't happening because dlib is or isn't
                              doing something. Instead, it is a limitation of the CUDA
                              runtime that dlib has no control over.
            !*/

            net_type& get_net (
                force_flush_to_disk force_flush = force_flush_to_disk::yes
            );
            /*!
                ensures
                    - returns the neural network object used by this trainer.  This is the
                      network that is optimized when you call train() or train_one_step().
                      Recall that the dnn_trainer doesn't contain the net_type object but
                      simply holds a reference to an external network which was provided to the
                      dnn_trainer's constructor.
                    - This function blocks until all threads inside the dnn_trainer have
                      stopped touching the net.
                    - If force_flush is yes, then this function will sync the trainer state to
                      disk if the current state hasn't already been synced to disk since the
                      last network modification.
            !*/

            const std::vector<solver_type>& get_solvers (
            ) const;
            /*!
                ensures
                    - returns the solvers used to optimize each layer of the neural network
                      get_net().  In particular, the first layer's solver is
                      get_solvers()[0], the second layer's solver is
                      get_solvers()[1], and so on.
                    - This function blocks until all threads inside the dnn_trainer have
                      stopped touching the net.
            !*/

            unsigned long get_mini_batch_size (
            ) const;
            /*!
                ensures
                    - During training, we call the network's update() routine over and over
                      with training data.  The number of training samples we give to each call
                      to update is the "mini-batch size", which is defined by
                      get_mini_batch_size().
            !*/

            void set_mini_batch_size (
                unsigned long batch_size
            );
            /*!
                requires
                    - batch_size > 0
                ensures
                    - #get_mini_batch_size() == batch_size
            !*/

            unsigned long get_max_num_epochs (
            ) const;
            /*!
                ensures
                    - train() will execute at most get_max_num_epochs() iterations over the
                      training data before returning.
            !*/

            void set_max_num_epochs (
                unsigned long num
            );
            /*!
                requires
                    - num > 0
                ensures
                    - #get_max_num_epochs() == num
            !*/

            void set_learning_rate (
                double lr
            );
            /*!
                requires
                    - lr > 0
                ensures
                    - #get_learning_rate() == lr
                    - #get_learning_rate_schedule().size() == 0
                    - This function blocks until all threads inside the dnn_trainer have
                      stopped touching the net.
            !*/

            double get_learning_rate(
            ) const;
            /*!
                ensures
                    - During each training step, a solver tells us how to modify the parameters
                      of each layer in the network.  It does this by outputting a step vector
                      that, when added to the parameters, will hopefully result in improved
                      network performance.  The learning rate is one of the inputs to the
                      solver and influences the size of this step vector.  This function
                      returns the current learning rate, that is, the learning rate that will
                      be used during the next training step.
            !*/

            void set_min_learning_rate (
                double lr
            );
            /*!
                requires
                    - lr > 0
                ensures
                    - #get_min_learning_rate() == lr
                    - #get_learning_rate_schedule().size() == 0
                    - This function blocks until all threads inside the dnn_trainer have
                      stopped touching the net.
            !*/

            double get_min_learning_rate (
            ) const;
            /*!
                ensures
                    - During training via this->train(), this object will test if progress is
                      still being made and if it isn't then it will reduce get_learning_rate()
                      by setting it to get_learning_rate()*get_learning_rate_shrink_factor().
                      However, it will not reduce it below get_min_learning_rate().  Once this
                      minimum learning rate is crossed the training will terminate.
                    - get_min_learning_rate() doesn't apply if you are using train_one_step().
                      You can keep calling train_one_step() as many times as you want and the
                      learning rate will drop infinitely close to 0 if you run long enough.
            !*/

            template <typename EXP>
            void set_learning_rate_schedule (
                const matrix_exp<EXP>& schedule
            );
            /*!
                requires
                    - schedule.size() > 0
                    - min(schedule) > 0
                ensures
                    - #get_learning_rate_schedule() == reshape_to_column_vector(schedule)
                    - #get_learning_rate() == schedule(0,0)
                    - #get_min_learning_rate() == min(schedule)
                    - #set_learning_rate_shrink_factor() == 1
            !*/

            const matrix<double,0,1>& get_learning_rate_schedule (
            ) const;
            /*!
                ensures
                    - if (this function returns a non-empty matrix) then
                        - This trainer will use an explicit learning rate schedule defined by
                          the learning rate values in get_learning_rate_schedule().  For
                          example, if get_learning_rate_schedule() returned {0.1, 0.09, 0.08,
                          0.07, 0.06} then the first training mini-batch would use a learning
                          rate of 0.1, then the next training mini-batch uses 0.09, and then
                          0.8, and so on until the end of the schedule is reached.

                          If you continue to run training after the end of the schedule has
                          been reached then the learning rate will be fixed to 0.99 times the
                          final value.  So in our example, eventually the learning rate would
                          be fixed to 0.99*0.06.  This allows you to test if we have reached the
                          end of the schedule by checking if get_learning_rate() >= 0.06.
            !*/

            unsigned long get_steps_without_progress (
            ) const;
            /*!
                ensures
                    - if (get_learning_rate_shrink_factor() != 1) then
                        - returns an estimate of how many mini-batches have executed without us
                          observing a statistically significant decrease in the training error.
                    - else
                        - returns 0
            !*/

            void set_iterations_without_progress_threshold (
                unsigned long thresh
            );
            /*!
                ensures
                    - #get_iterations_without_progress_threshold() == thresh
                    - #get_learning_rate_schedule().size() == 0
                    - This function blocks until all threads inside the dnn_trainer have
                      stopped touching the net.
            !*/

            unsigned long get_iterations_without_progress_threshold (
            ) const;
            /*!
                ensures
                    - This object monitors the progress of training and estimates if the
                      training error is being reduced.  It does this by looking at the previous
                      get_iterations_without_progress_threshold() mini-batch results and
                      applying the statistical test defined by the running_gradient object to
                      see if the training error is getting smaller.  If it isn't being reduced
                      then get_learning_rate() is made smaller by a factor of get_learning_rate_shrink_factor().

                      Therefore, get_iterations_without_progress_threshold() should always be
                      set to something sensibly large so that this test can be done with
                      reasonably high confidence.  Think of this test as saying "if the loss
                      hasn't decreased for the previous get_iterations_without_progress_threshold()
                      then shrink the learning rate".
            !*/

            void set_learning_rate_shrink_factor (
                double shrink
            );
            /*!
                requires
                    - 0 < shrink && shrink <= 1
                ensures
                    - #get_learning_rate_shrink_factor() == shrink
                    - #get_learning_rate_schedule().size() == 0
                    - This function blocks until all threads inside the dnn_trainer have
                      stopped touching the net.
            !*/

            double get_learning_rate_shrink_factor (
            ) const;
            /*!
                ensures
                    - Whenever the training routine thinks it isn't making progress anymore it
                      will reduce get_learning_rate() by multiplying it by get_learning_rate_shrink_factor().
                    - You can disable the automatic learning rate reduction by setting
                      get_learning_rate_shrink_factor() to 1.
            !*/

            unsigned long long get_train_one_step_calls (
            ) const;
            /*!
                ensures
                    - returns the number of times train_one_step() has been called.
            !*/

            unsigned long long get_test_one_step_calls (
            ) const;
            /*!
                ensures
                    - returns the number of times test_one_step() has been called.
            !*/

            void be_verbose (
            );
            /*!
                ensures
                    - This object will print status messages to standard out so that a
                      user can observe the progress of the algorithm.
            !*/

            void be_quiet (
            );
            /*!
                ensures
                    - This object will not print anything to standard out
            !*/

            void set_synchronization_file (
                const std::string& filename,
                std::chrono::seconds time_between_syncs = std::chrono::minutes(15)
            );
            /*!
                ensures
                    - #get_synchronization_file() == filename
                    - While training is running, either via train() or repeated calls to
                      train_one_step(), this object will save its entire state, including the
                      state of get_net(), to disk in the file named filename every
                      time_between_syncs seconds.
                    - If the filename file already exists then the state of this trainer will
                      be loaded from that file by this call to set_synchronization_file().
                      This allows you to resume a training session which was previously
                      interrupted.
                    - It should be noted that when saving, the trainer will alternate between
                      saving to a file called filename and another file called filename+"_".
                      We do this because it's possible that your computer might crash (not
                      because of dlib, just in general) before the data is safely saved to
                      disk.  This way, you will always have a backup file if the write to disk
                      gets corrupted or is incomplete.  Moreover, when loading, we will always
                      load from the newest of the two possible files.
            !*/

            const std::string& get_synchronization_file (
            );
            /*!
                ensures
                    - Returns the name of the file the dnn_trainer will periodically save it's
                      state to.  If the return value is "" then synchronization is disabled.
            !*/

            void train (
                const std::vector<input_type>& data,
                const std::vector<training_label_type>& labels
            );
            /*!
                requires
                    - data.size() == labels.size()
                    - data.size() > 0
                    - net_type uses a supervised loss.
                      i.e. net_type::training_label_type != no_label_type.
                ensures
                    - Trains a supervised neural network based on the given training data.
                      The goal of training is to find the network parameters that minimize
                      get_net().compute_loss(data.begin(), data.end(), labels.begin()).
                    - The optimizer will run until get_learning_rate() < get_min_learning_rate()
                      or get_max_num_epochs() training epochs have been executed.
                    - Each layer in the network will be optimized by its corresponding solver
                      in get_solvers().
                    - Each call to train DOES NOT reinitialize the state of get_net() or
                      get_solvers().  That is, the existing state of the solvers and network is
                      the starting point for the optimization each time train() is called.  In
                      particular, if you use the set_synchronization_file() method you can
                      resume an interrupted train() call by simply calling train() again and it
                      will pick up from the last synchronization point.
                    - You can obtain the average loss value during the final training epoch by
                      calling get_average_loss().
                    - This function blocks until all threads inside the dnn_trainer have
                      stopped touching the net.
            !*/

            void train (
                const std::vector<input_type>& data
            );
            /*!
                requires
                    - data.size() > 0
                    - net_type uses an unsupervised loss.
                      i.e. net_type::training_label_type == no_label_type.
                ensures
                    - Trains an unsupervised neural network based on the given training data.
                      The goal of training is to find the network parameters that minimize
                      get_net().compute_loss(data.begin(), data.end()).
                    - The optimizer will run until get_learning_rate() < get_min_learning_rate()
                      or get_max_num_epochs() training epochs have been executed.
                    - Each layer in the network will be optimized by its corresponding solver
                      in get_solvers().
                    - Each call to train DOES NOT reinitialize the state of get_net() or
                      get_solvers().  That is, the existing state of the solvers and network is
                      the starting point for the optimization each time train() is called.  In
                      particular, if you use the set_synchronization_file() method you can
                      resume an interrupted train() call by simply calling train() again and it
                      will pick up from the last synchronization point.
                    - You can obtain the average loss value during the final training epoch by
                      calling get_average_loss().
                    - This function blocks until all threads inside the dnn_trainer have
                      stopped touching the net.
            !*/

            void train_one_step (
                const std::vector<input_type>& data,
                const std::vector<training_label_type>& labels
            );
            /*!
                requires
                    - data.size() == labels.size()
                    - data.size() > 0
                    - net_type uses a supervised loss.
                      i.e. net_type::training_label_type != no_label_type.
                ensures
                    - Performs one stochastic gradient update step based on the mini-batch of
                      data and labels supplied to this function.  In particular, calling
                      train_one_step() in a loop is equivalent to calling the train() method
                      defined above.  However, train_one_step() allows you to stream data from
                      disk into the training process while train() requires you to first load
                      all the training data into RAM.  Otherwise, these training methods are
                      equivalent.
                    - You can observe the current average loss value by calling get_average_loss().
                    - The network training will happen in another thread.  Therefore, after
                      calling this function you should call get_net() before you touch the net
                      object from the calling thread to ensure no other threads are still
                      accessing the network.
                    - #get_train_one_step_calls() == get_train_one_step_calls() + 1.
            !*/

            template <
                typename data_iterator,
                typename label_iterator
                >
            void train_one_step (
                data_iterator dbegin,
                data_iterator dend,
                label_iterator lbegin
            );
            /*!
                requires
                    - std::advance(lbegin, std::distance(dbegin, dend) - 1) is dereferencable
                    - std::distance(dbegin, dend) > 0
                    - net_type uses a supervised loss.
                      i.e. net_type::training_label_type != no_label_type.
                ensures
                    - Performs one stochastic gradient update step based on the mini-batch of
                      data and labels supplied to this function.  In particular, calling
                      train_one_step() in a loop is equivalent to calling the train() method
                      defined above.  However, train_one_step() allows you to stream data from
                      disk into the training process while train() requires you to first load
                      all the training data into RAM.  Otherwise, these training methods are
                      equivalent.
                    - You can observe the current average loss value by calling get_average_loss().
                    - The network training will happen in another thread.  Therefore, after
                      calling this function you should call get_net() before you touch the net
                      object from the calling thread to ensure no other threads are still
                      accessing the network.
                    - #get_train_one_step_calls() == get_train_one_step_calls() + 1.
            !*/

            void train_one_step (
                const std::vector<input_type>& data
            );
            /*!
                requires
                    - data.size() > 0
                    - net_type uses an unsupervised loss.
                      i.e. net_type::training_label_type == no_label_type.
                ensures
                    - Performs one stochastic gradient update step based on the mini-batch of
                      data supplied to this function.  In particular, calling train_one_step()
                      in a loop is equivalent to calling the train() method defined above.
                      However, train_one_step() allows you to stream data from disk into the
                      training process while train() requires you to first load all the
                      training data into RAM.  Otherwise, these training methods are
                      equivalent.
                    - You can observe the current average loss value by calling get_average_loss().
                    - The network training will happen in another thread.  Therefore, after
                      calling this function you should call get_net() before you touch the net
                      object from the calling thread to ensure no other threads are still
                      accessing the network.
                    - #get_train_one_step_calls() == get_train_one_step_calls() + 1.
            !*/

            template <
                typename data_iterator
                >
            void train_one_step (
                data_iterator dbegin,
                data_iterator dend
            );
            /*!
                requires
                    - std::distance(dbegin, dend) > 0
                    - net_type uses an unsupervised loss.
                      i.e. net_type::training_label_type == no_label_type.
                ensures
                    - Performs one stochastic gradient update step based on the mini-batch of
                      data supplied to this function.  In particular, calling train_one_step()
                      in a loop is equivalent to calling the train() method defined above.
                      However, train_one_step() allows you to stream data from disk into the
                      training process while train() requires you to first load all the
                      training data into RAM.  Otherwise, these training methods are
                      equivalent.
                    - You can observe the current average loss value by calling get_average_loss().
                    - The network training will happen in another thread.  Therefore, after
                      calling this function you should call get_net() before you touch the net
                      object from the calling thread to ensure no other threads are still
                      accessing the network.
                    - #get_train_one_step_calls() == get_train_one_step_calls() + 1.
            !*/

            double get_average_loss (
            ) const;
            /*!
                ensures
                    - returns the average loss value observed during previous calls to
                      train_one_step() or train().  That is, the average output of
                      net_type::update() during the previous mini-batch updates.
                    - Note that, if be_verbose() has been called, then this object will
                      automatically call clear_average_loss() periodically when it logs the
                      loss to the console.
                    - This function blocks until all threads inside the dnn_trainer have
                      stopped touching the net.
            !*/

            void clear_average_loss (
            );
            /*!
                ensures
                    - #get_average_loss() == 0
                    - get_average_loss() uses a dlib::running_stats object to keep a running
                      average of the loss values seen during the previous mini-batch updates
                      applied during training.  Calling clear_average_loss() resets the
                      running_stats object so it forgets about all previous loss values
                      observed.
                    - This function blocks until all threads inside the dnn_trainer have
                      stopped touching the net.
            !*/

        // ----------------------

            double get_average_test_loss (
            ) const;
            /*!
                ensures
                    - returns the average loss value observed during previous calls to
                      test_one_step().
                    - This function blocks until all threads inside the dnn_trainer have
                      stopped touching the net.
            !*/

            void test_one_step (
                const std::vector<input_type>& data,
                const std::vector<training_label_type>& labels
            );
            /*!
                requires
                    - data.size() == labels.size()
                    - data.size() > 0
                    - net_type uses a supervised loss.
                      i.e. net_type::training_label_type != no_label_type.
                ensures
                    - Runs the given data through the network and computes and records the loss.
                    - This call does not modify network parameters.  The point of
                      test_one_step() is two fold, to allow you to observe the accuracy of the
                      network on hold out data during training, and to allow the trainer to
                      automatically adjust the learning rate when the test loss stops
                      improving.  It should be noted that you are not required to use
                      test_one_step() at all, but if you want to do this kind of thing it is
                      available.
                    - You can observe the current average loss value by calling get_average_test_loss().
                    - The computation will happen in another thread.  Therefore, after calling
                      this function you should call get_net() before you touch the net object
                      from the calling thread to ensure no other threads are still accessing
                      the network.
                    - #get_test_one_step_calls() == get_test_one_step_calls() + 1.
            !*/

            template <
                typename data_iterator,
                typename label_iterator
                >
            void test_one_step (
                data_iterator dbegin,
                data_iterator dend,
                label_iterator lbegin
            );
            /*!
                requires
                    - std::advance(lbegin, std::distance(dbegin, dend) - 1) is dereferencable
                    - std::distance(dbegin, dend) > 0
                    - net_type uses a supervised loss.
                      i.e. net_type::training_label_type != no_label_type.
                ensures
                    - Runs the given data through the network and computes and records the loss.
                    - This call does not modify network parameters.  The point of
                      test_one_step() is two fold, to allow you to observe the accuracy of the
                      network on hold out data during training, and to allow the trainer to
                      automatically adjust the learning rate when the test loss stops
                      improving.  It should be noted that you are not required to use
                      test_one_step() at all, but if you want to do this kind of thing it is
                      available.
                    - You can observe the current average loss value by calling get_average_test_loss().
                    - The computation will happen in another thread.  Therefore, after calling
                      this function you should call get_net() before you touch the net object
                      from the calling thread to ensure no other threads are still accessing
                      the network.
                    - #get_test_one_step_calls() == get_test_one_step_calls() + 1.
            !*/

            void test_one_step (
                const std::vector<input_type>& data
            );
            /*!
                requires
                    - data.size() > 0
                    - net_type uses an unsupervised loss.
                      i.e. net_type::training_label_type == no_label_type.
                ensures
                    - Runs the given data through the network and computes and records the loss.
                    - This call does not modify network parameters.  The point of
                      test_one_step() is two fold, to allow you to observe the accuracy of the
                      network on hold out data during training, and to allow the trainer to
                      automatically adjust the learning rate when the test loss stops
                      improving.  It should be noted that you are not required to use
                      test_one_step() at all, but if you want to do this kind of thing it is
                      available.
                    - You can observe the current average loss value by calling get_average_test_loss().
                    - The computation will happen in another thread.  Therefore, after calling
                      this function you should call get_net() before you touch the net object
                      from the calling thread to ensure no other threads are still accessing
                      the network.
                    - #get_test_one_step_calls() == get_test_one_step_calls() + 1.
            !*/

            template <
                typename data_iterator
                >
            void test_one_step (
                data_iterator dbegin,
                data_iterator dend
            );
            /*!
                requires
                    - std::distance(dbegin, dend) > 0
                    - net_type uses an unsupervised loss.
                      i.e. net_type::training_label_type == no_label_type.
                ensures
                    - Runs the given data through the network and computes and records the loss.
                    - This call does not modify network parameters.  The point of
                      test_one_step() is two fold, to allow you to observe the accuracy of the
                      network on hold out data during training, and to allow the trainer to
                      automatically adjust the learning rate when the test loss stops
                      improving.  It should be noted that you are not required to use
                      test_one_step() at all, but if you want to do this kind of thing it is
                      available.
                    - You can observe the current average loss value by calling get_average_test_loss().
                    - The computation will happen in another thread.  Therefore, after calling
                      this function you should call get_net() before you touch the net object
                      from the calling thread to ensure no other threads are still accessing
                      the network.
                    - #get_test_one_step_calls() == get_test_one_step_calls() + 1.
            !*/

            void set_test_iterations_without_progress_threshold (
                unsigned long thresh
            );
            /*!
                ensures
                    - #get_test_iterations_without_progress_threshold() == thresh
                    - #get_learning_rate_schedule().size() == 0
                    - This function blocks until all threads inside the dnn_trainer have
                      stopped touching the net.
            !*/

            unsigned long get_test_iterations_without_progress_threshold (
            ) const;
            /*!
                ensures
                    - This object monitors the progress of training and estimates if the
                      testing error is being reduced.  It does this by looking at the previous
                      get_test_iterations_without_progress_threshold() mini-batch results from
                      test_one_step() and applying the statistical test defined by the
                      running_gradient object to see if the testing error is getting smaller.
                      If it isn't being reduced then get_learning_rate() is made smaller by a
                      factor of get_learning_rate_shrink_factor().

                      Therefore, get_test_iterations_without_progress_threshold() should always be
                      set to something sensibly large so that this test can be done with
                      reasonably high confidence.  Think of this test as saying "if the testing loss
                      hasn't decreased for the previous get_test_iterations_without_progress_threshold()
                      calls to test_one_step() then shrink the learning rate".
            !*/

            unsigned long get_test_steps_without_progress (
            ) const;
            /*!
                ensures
                    - if (get_learning_rate_shrink_factor() != 1) then
                        - returns an estimate of how many mini-batches have executed without us
                          observing a statistically significant decrease in the testing error
                          (i.e. the error on the data given to the trainer via test_one_step()
                          calls).
                    - else
                        - returns 0
            !*/

        };

    // ----------------------------------------------------------------------------------------

        template <
            typename net_type,
            typename solver_type
            >
        std::ostream& operator<< (
            std::ostream& out,
            dnn_trainer<net_type,solver_type>& trainer
        );
        /*!
            ensures
                - Prints a log of the current parameters of trainer to out.
        !*/

    // ----------------------------------------------------------------------------------------

    }

    #endif // DLIB_DNn_TRAINER_ABSTRACT_H_