    // Copyright (C) 2007  Davis E. King (davis@dlib.net)
    // License: Boost Software License   See LICENSE.txt for the full license.
    #undef DLIB_MLp_ABSTRACT_
    #ifdef DLIB_MLp_ABSTRACT_

    #include "../algs.h"
    #include "../serialize.h"
    #include "../matrix/matrix_abstract.h"

    namespace dlib
    {

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

        class mlp : noncopyable
        {
            /*!
                INITIAL VALUE
                    The network is initially initialized with random weights

                WHAT THIS OBJECT REPRESENTS
                    This object represents a multilayer layer perceptron network that is
                    trained using the back propagation algorithm.  The training algorithm also
                    incorporates the momentum method.  That is, each round of back propagation
                    training also adds a fraction of the previous update.  This fraction
                    is controlled by the momentum term set in the constructor.

                    The activation function used at each node is the sigmoid function.  I.e.
                    sigmoid(x) = 1/(1 + pow(e,-x)).  Thus the output of the network is
                    always in the range [0,1]
            !*/

        public:

            mlp (
                long nodes_in_input_layer,
                long nodes_in_first_hidden_layer,
                long nodes_in_second_hidden_layer = 0,
                long nodes_in_output_layer = 1,
                double alpha = 0.1,
                double momentum = 0.8
            );
            /*!
                requires
                    - nodes_in_input_layer > 0
                    - nodes_in_first_hidden_layer > 0
                    - nodes_in_second_hidden_layer >= 0
                    - nodes_in_output_layer > 0
                ensures
                    - #*this is properly initialized
                    - #input_layer_nodes() == nodes_in_input_layer
                    - #first_hidden_layer_nodes() == nodes_in_first_hidden_layer
                    - #second_hidden_layer_nodes() == nodes_in_second_hidden_layer
                    - #output_layer_nodes() == nodes_in_output_layer
                    - #get_alpha() == alpha
                    - #get_momentum() == momentum
                throws
                    - std::bad_alloc
                        if this is thrown the mlp will be unusable but
                        will not leak memory
            !*/

            virtual ~mlp (
            );
            /*!
                ensures
                    - all resources associated with #*this have been released
            !*/

            void reset (
            ) const;
            /*!
                ensures
                    - reinitialize the network with random weights
            !*/

            long input_layer_nodes (
            ) const;
            /*!
                ensures
                    - returns the number of nodes in the input layer
            !*/

            long first_hidden_layer_nodes (
            ) const;
            /*!
                ensures
                    - returns the number of nodes in the first hidden layer.  This is
                      the hidden layer that is directly connected to the input layer.
            !*/

            long second_hidden_layer_nodes (
            ) const;
            /*!
                ensures
                    - if (this network has a second hidden layer) then
                        - returns the number of nodes in the second hidden layer.  This is
                          the hidden layer that is directly connected to the output layer.
                    - else
                        - returns 0
            !*/

            long output_layer_nodes (
            ) const;
            /*!
                ensures
                    - returns the number of nodes in the output layer
            !*/

            double get_alpha (
            ) const;
            /*!
                ensures
                    - returns the back propagation learning rate used by this object.
            !*/

            double get_momentum (
            ) const;
            /*!
                ensures
                    - returns the momentum term used by this object during back propagation
                      training.  The momentum is is the fraction of a previous update to
                      carry forward to the next call to train()
            !*/

            template <typename EXP>
            const matrix<double> operator() (
                const matrix_exp<EXP>& in
            ) const;
            /*!
                requires
                    - in.nr() == input_layer_nodes()
                    - in.nc() == 1
                ensures
                    - returns the output of the network when it is given the
                      input in.  The output's elements are always in the range
                      of 0.0 to 1.0
            !*/

            template <typename EXP1, typename EXP2>
            void train (
                const matrix_exp<EXP1>& example_in,
                const matrix_exp<EXP2>& example_out
            );
            /*!
                requires
                    - example_in.nr() == input_layer_nodes()
                    - example_in.nc() == 1
                    - example_out.nr() == output_layer_nodes()
                    - example_out.nc() == 1
                    - max(example_out) <= 1.0 && min(example_out) >= 0.0
                ensures
                    - trains the network that the correct output when given example_in
                      should be example_out.
            !*/

            template <typename EXP>
            void train (
                const matrix_exp<EXP>& example_in,
                double example_out
            );
            /*!
                requires
                    - example_in.nr() == input_layer_nodes()
                    - example_in.nc() == 1
                    - output_layer_nodes() == 1
                    - example_out <= 1.0 && example_out >= 0.0
                ensures
                    - trains the network that the correct output when given example_in
                      should be example_out.
            !*/

            double get_average_change (
            ) const;
            /*!
                ensures
                    - returns the average change in the node weights in the
                      neural network during the last call to train()
            !*/

            void swap (
                mlp& item
            );
            /*!
                ensures
                    - swaps *this and item
            !*/

        };

        inline void swap (
            mlp& a,
            mlp& b
        ) { a.swap(b); }
        /*!
            provides a global swap function
        !*/

        void serialize (
            const mlp& item,
            std::ostream& out
        );
        /*!
            provides serialization support
        !*/

        void deserialize (
            mlp& item,
            std::istream& in
        );
        /*!
            provides deserialization support
        !*/

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

    }

    #endif // DLIB_MLp_ABSTRACT_