// Copyright (c) 2018 GeometryFactory Sarl (France).
// All rights reserved.
//
// This file is part of CGAL (www.cgal.org).
// 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.
//
// Licensees holding a valid commercial license may use this file in
// accordance with the commercial license agreement provided with the software.
//
// This file is provided AS IS with NO WARRANTY OF ANY KIND, INCLUDING THE
// WARRANTY OF DESIGN, MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE.
//
// $URL: https://github.com/CGAL/cgal/blob/releases/CGAL-4.14.1/Classification/include/CGAL/Classification/TensorFlow/Neural_network_classifier.h $
// $Id: Neural_network_classifier.h fb63d96 %aI Simon Giraudot
// SPDX-License-Identifier: GPL-3.0+
//
// Author(s)     : Simon Giraudot

#ifndef CGAL_CLASSIFICATION_TENSORFLOW_NEURAL_NETWORK_CLASSIFIER_H
#define CGAL_CLASSIFICATION_TENSORFLOW_NEURAL_NETWORK_CLASSIFIER_H

#include <CGAL/license/Classification.h>

#include <CGAL/Classification/Feature_set.h>
#include <CGAL/Classification/Label_set.h>

#include <tensorflow/cc/client/client_session.h>
#include <tensorflow/cc/ops/standard_ops.h>
#include <tensorflow/core/framework/tensor.h>
#include <tensorflow/core/framework/types.h>
#include <tensorflow/cc/framework/gradients.h>

#include <boost/property_tree/ptree.hpp>
#include <boost/property_tree/xml_parser.hpp>
#include <boost/foreach.hpp>
#include <boost/algorithm/string/predicate.hpp>
#include <boost/algorithm/string.hpp>
#include <boost/lexical_cast.hpp>

#include <CGAL/boost/iterator/counting_iterator.hpp>

#define CGAL_NEURAL_NETWORK_VERSION "0.1"

namespace CGAL {

namespace Classification {

namespace TensorFlow {

namespace TF = tensorflow;
namespace TFops = tensorflow::ops;

/*!
  \ingroup PkgClassificationClassifiersTensorFlow

  \brief %Classifier based on the TensorFlow version of the neural
  network algorithm.

  This class provides an interface to a feature-based neural network:
  a set of features is used as an input layer followed by a
  user-specified number of hidden layers with a user-specified
  activation function. The output layer is a softmax layer providing,
  for each label, the probability that an input item belongs to it.

  \warning This feature is still experimental: it may not be stable
  and is likely to undergo substantial changes in future releases of
  \cgal. The API changes will be announced in the release notes.

  \note This class requires the \ref thirdpartyTensorFlow library.

  \tparam ActivationFunction Chosen activation function for the hidden
  layers. `Relu` is used as default. The following functions can be
  used (please refer to the documentation of \ref thirdpartyTensorFlow
  for more information):
   - `tensorflow::ops::Elu`
   - `tensorflow::ops::Relu6`
   - `tensorflow::ops::Relu`
   - `tensorflow::ops::Selu`
   - `tensorflow::ops::Sigmoid`
   - `tensorflow::ops::Tanh`

  \cgalModels `CGAL::Classification::Classifier`
*/
template <typename ActivationFunction = tensorflow::ops::Relu>
class Neural_network_classifier
{
  bool m_verbose;
  
  const Label_set& m_labels;
  const Feature_set& m_features;
  std::vector<float> m_feature_means;
  std::vector<float> m_feature_sd;

  TF::Scope* m_root;
  TFops::Placeholder* m_ph_ft;
  TFops::Placeholder* m_ph_gt;

  TFops::ReduceMean* m_loss;
  
  std::vector<std::vector<float> > m_weights;
  std::vector<std::vector<float> > m_bias;
  float m_learning_rate;
  std::vector<TF::Output> m_layers;

#define CGAL_CLASSIFICATION_TENSORFLOW_USE_ADAM
#ifdef CGAL_CLASSIFICATION_TENSORFLOW_USE_ADAM
  std::vector<TFops::ApplyAdam> m_optimizer;
#else
  std::vector<TFops::ApplyGradientDescent> m_optimizer;
#endif
  
  TF::ClientSession* m_session;
  
public:

  /// \cond SKIP_IN_MANUAL
  const Feature_set& features() const { return m_features; }
  /// \endcond
  
  /// \name Constructor
  /// @{
  
  /*!
    \brief Instantiates the classifier using the sets of `labels` and `features`.

  */
  Neural_network_classifier (const Label_set& labels,
                             const Feature_set& features)
    : m_verbose (true), m_labels (labels), m_features (features)
    , m_root (NULL), m_ph_ft (NULL), m_ph_gt (NULL), m_loss(NULL), m_session (NULL)
  { }
  
  /// \cond SKIP_IN_MANUAL
  ~Neural_network_classifier ()
  {
    clear();
  }

  bool initialized() const { return (m_root != NULL); }

  bool& verbose() { return m_verbose; }

  void clear()
  {
    clear (m_ph_gt);
    clear (m_ph_ft);
    clear (m_loss);
    clear (m_session);
    clear (m_root);
    m_weights.clear();
    m_bias.clear();
    m_optimizer.clear();
    m_layers.clear();
  }

  template <typename T>
  void clear (T* t)
  {
    if (t != NULL)
      delete t;
    t = NULL;
  }

  void compute_normalization_coefficients (const std::vector<std::size_t>& indices)
  {
//#define DO_NOT_NORMALIZE_FEATURES
#ifdef DO_NOT_NORMALIZE_FEATURES
    m_feature_means.clear();
    m_feature_means.resize (m_features.size(), 0.f);
    m_feature_sd.clear();
    m_feature_sd.resize (m_features.size(), 1.f);
#else
    m_feature_means.clear();
    m_feature_means.resize (m_features.size(), 0.f);
    
    for (std::size_t i = 0; i < indices.size(); ++ i)
      for (std::size_t f = 0; f < m_features.size(); ++ f)
        m_feature_means[f] += m_features[f]->value(indices[i]);
    for (std::size_t f = 0; f < m_features.size(); ++ f)
      m_feature_means[f] /= indices.size();

    m_feature_sd.clear();
    m_feature_sd.resize (m_features.size(), 0.f);

    for (std::size_t i = 0; i < indices.size(); ++ i)
      for (std::size_t f = 0; f < m_features.size(); ++ f)
        m_feature_sd[f] +=
          (m_features[f]->value(indices[i]) - m_feature_means[f]) *
          (m_features[f]->value(indices[i]) - m_feature_means[f]);
    for (std::size_t f = 0; f < m_features.size(); ++ f)
    {
      m_feature_sd[f] = std::sqrt (m_feature_sd[f] / indices.size());
      if (m_feature_sd[f] == 0.f)
        m_feature_sd[f] = 1.f;

      if (std::isnan(m_feature_means[f]))
        m_feature_means[f] = 0.f;
      if (std::isnan(m_feature_sd[f]))
        m_feature_sd[f] = 1.f;

      // if (m_verbose)
      //   std::cerr << "#" << f << ": " << m_features[f]->name() << " = "
      //             << m_feature_means[f] << " +/- " << m_feature_sd[f] << std::endl;
    }
#endif
  }
  /// \endcond
  
  /// @}

  /// \name Training
  
  /// @{
  /*!
    \brief Runs the training algorithm.

    From the set of provided ground truth, this algorithm constructs a
    neural network and applies an Adam optimizer to set up the weights
    and bias that produce the most accurate result with respect to
    this ground truth.

    \pre At least one ground truth item should be assigned to each
    label.

    \param ground_truth vector of label indices. It should contain for
    each input item, in the same order as the input set, the index of
    the corresponding label in the `Label_set` provided in the
    constructor. Input items that do not have a ground truth
    information should be given the value `-1`.

    \param restart_from_scratch should be set to `false` if the user
    wants to continue adjusting weights and bias based, and set to
    `true` if the neural network should be re-created from scratch
    (discarding all previous training results).

    \param number_of_iterations number of times the optimizer is
    called.

    \param learning_rate describes the rate at which the optimizer
    changes the weights and bias.

    \param batch_size size of the random subset of inliers used for
    optimizing at each iteration.

    \param hidden_layers vector containing the consecutive sizes
    (number of neurons) of the hidden layers of the network. If no
    vector is given, the behavior used is the following: 2 hidden
    layers are used. The first layer has as many neurons as the number
    of features; the second layer has a number of neurons equal to the
    average value between the number of features and the number of
    labels.
  */
  template <typename LabelIndexRange>
  void train (const LabelIndexRange& ground_truth,
              bool restart_from_scratch = true,
              std::size_t number_of_iterations = 5000,
              float learning_rate = 0.01,
              std::size_t batch_size = 1000,
              const std::vector<std::size_t>& hidden_layers
              = std::vector<std::size_t>())
  {
    if (restart_from_scratch)
      clear();

    std::vector<std::vector<std::size_t> > random_indices (m_labels.size());
    
    std::vector<std::size_t> indices;
    std::vector<int> raw_gt;
    for (std::size_t i = 0; i < ground_truth.size(); ++ i)
    {
      int gc = int(ground_truth[i]);
      if (gc != -1)
      {
        indices.push_back (i);
        raw_gt.push_back (gc);
        random_indices[std::size_t(gc)].push_back (indices.size() - 1);
      }
    }

    if (!initialized())
    {
      m_learning_rate = learning_rate;
      build_architecture (hidden_layers);
      if (m_verbose) std::cerr << "II - NORMALIZING FEATURES" << std::endl;
      compute_normalization_coefficients (indices);
    }
    
    if (m_verbose) std::cerr << "III - TRAINING NEURAL NETWORK" << std::endl;
    
    std::vector<TF::Output> operations;
    for (std::size_t i = 0; i < m_optimizer.size(); ++ i)
      operations.push_back (m_optimizer[i]);
    operations.push_back (m_layers.back());
    
    std::vector<TF::Tensor> outputs;

    m_weights.clear();
    m_bias.clear();
    // std::ofstream log ("loss.log");
    // log.precision(18);

    for (std::size_t i = 0; i < number_of_iterations; ++ i)
    {
      std::size_t total_batch_size = 0;
      for (std::size_t j = 0; j < random_indices.size(); ++ j)
      {
        if (random_indices[j].empty())
          continue;
        std::size_t local_batch_size = std::min (std::size_t(batch_size / m_labels.size()),
                                                 random_indices[j].size());
        random_unique (random_indices[j].begin(), random_indices[j].end(), local_batch_size);
        total_batch_size += local_batch_size;

        if (i == 0)
        {
          if (m_verbose) std::cerr << "   * Size of batch for " << m_labels[j]->name() << ": "
                                  << local_batch_size << std::endl;
        }
      }
      
      TF::Tensor ft
        (TF::DataTypeToEnum<float>::v(), 
         TF::TensorShape {(long long)(total_batch_size), (long long)(m_features.size())});
      TF::Tensor gt
        (TF::DataTypeToEnum<float>::v(), 
         TF::TensorShape {(long long)(total_batch_size), (long long)(m_labels.size())});

      float* ft_data = ft.flat<float>().data();
      float* gt_data = gt.flat<float>().data();

      // Fill input tensors
      std::size_t current_i = 0;
      for (std::size_t j = 0; j < random_indices.size(); ++ j)
      {
        if (random_indices[j].empty())
          continue;
        std::size_t local_batch_size = std::min (std::size_t(batch_size / m_labels.size()),
                                                 random_indices[j].size());
        for (std::size_t k = 0; k < local_batch_size; ++ k)
        {
          std::size_t idx = indices[random_indices[j][k]];
          int g = raw_gt[random_indices[j][k]];

          for (std::size_t f = 0; f < m_features.size(); ++ f)
            ft_data[current_i * m_features.size() + f]
              = (m_features[f]->value(idx) - m_feature_means[f]) / m_feature_sd[f];

          for (std::size_t l = 0; l < m_labels.size(); ++ l)
            if (std::size_t(g) == l)
              gt_data[current_i * m_labels.size() + l] = 1.f;
            else
              gt_data[current_i * m_labels.size() + l] = 0.f;

          ++ current_i;
        }
      }
      
      TF_CHECK_OK (m_session->Run ({{*m_ph_ft, ft}, {*m_ph_gt, gt}}, {*m_loss}, &outputs));

      if ((i+1) % (number_of_iterations / 20) == 0)
      {
        if (m_verbose) std::cerr << "   * Step " << i+1 << "/" << number_of_iterations << ": loss = "
                                << outputs[0].scalar<float>() << std::endl;
      }
      if (!std::isfinite(*outputs[0].scalar<float>().data()))
      {
        if (m_verbose)
          std::cerr << "Loss is " << outputs[0].scalar<float>() << ", aborting" << std::endl;
        return;
      }

      if (i == number_of_iterations - 1)
      {
        TF_CHECK_OK(m_session->Run({{*m_ph_ft, ft}, {*m_ph_gt, gt}}, operations, &outputs));
        for (std::size_t o = 0; o < outputs.size() - 1; ++ o)
        {
          std::size_t size = outputs[o].dim_size(0) * outputs[o].dim_size(1);
          if (o < outputs.size() / 2)
          {
            m_weights.push_back (std::vector<float>(size));
            float* weights_data = outputs[o].flat<float>().data();
            std::copy_n (weights_data, size, m_weights.back().begin());
          }
          else
          {
            m_bias.push_back (std::vector<float>(size));
            float* bias_data = outputs[o].flat<float>().data();
            std::copy_n (bias_data, size, m_bias.back().begin());
          }
        }
      }
      else
        TF_CHECK_OK(m_session->Run({{*m_ph_ft, ft}, {*m_ph_gt, gt}}, operations, nullptr));
    }

  }

  /// \cond SKIP_IN_MANUAL
  void operator() (std::size_t item_index, std::vector<float>& out) const
  {
    out.resize (m_labels.size(), 0.);
    
    TF::Tensor ft
      (TF::DataTypeToEnum<float>::v(), 
       TF::TensorShape {(long long)(1), (long long)(m_features.size())});

    float* ft_data = ft.flat<float>().data();

    // Fill input tensor
    for (std::size_t f = 0; f < m_features.size(); ++ f)
      ft_data[f] = (m_features[f]->value(item_index) - m_feature_means[f]) / m_feature_sd[f];
    
    std::vector<TF::Tensor> outputs;
    TF_CHECK_OK(m_session->Run({{*m_ph_ft, ft}}, {m_layers.back()}, &outputs));

    float* output_data = outputs[0].flat<float>().data();

    for (std::size_t i = 0; i < m_labels.size(); ++ i)
      out[i] = output_data[i];
  }

  
  void operator() (const std::vector<std::size_t>& item_indices,
                   std::vector<std::vector<float> >& out) const
  {
    out.resize (item_indices.size(), std::vector<float>(m_labels.size(), 0.));
    
    TF::Tensor ft
      (TF::DataTypeToEnum<float>::v(), 
       TF::TensorShape {(long long)(item_indices.size()), (long long)(m_features.size())});

    float* ft_data = ft.flat<float>().data();

    // Fill input tensor
    for (std::size_t i = 0; i < item_indices.size(); ++ i)
      for (std::size_t f = 0; f < m_features.size(); ++ f)
        ft_data[i * m_features.size() + f]
          = (m_features[f]->value(item_indices[i]) - m_feature_means[f]) / m_feature_sd[f];
    
    std::vector<TF::Tensor> outputs;
    TF_CHECK_OK(m_session->Run({{*m_ph_ft, ft}}, {m_layers.back()}, &outputs));

    float* output_data = outputs[0].flat<float>().data();

    for (std::size_t i = 0; i < item_indices.size(); ++ i)
      for (std::size_t l = 0; l < m_labels.size(); ++ l)
        out[i][l] = output_data[i * m_labels.size() + l];
  }
  /// \endcond
  
  /// @}

  /// \name Input/Output
  /// @{
  
  /*!
    \brief Saves the current configuration in the stream `output`.

    This allows to easily save and recover a specific classification
    configuration, that is to say:

    - The statistics of features (mean and standard deviation)
    - The number of hiddens layers and their respectives sizes
    - The weights and bias of the neurons

    The output file is written in an XML format that is readable by
    the `load_configuration()` method.
  */
  void save_configuration (std::ostream& output)
  {
    boost::property_tree::ptree tree;

    {
      boost::property_tree::ptree ptr;
      ptr.put("classifier_version", CGAL_NEURAL_NETWORK_VERSION);
      ptr.put("number_of_features", m_features.size());
      ptr.put("number_of_labels", m_labels.size());
      ptr.put("number_of_hidden_layers", m_layers.size() - 1);
      ptr.put("learning_rate", m_learning_rate);
      tree.add_child("classification.metadata", ptr);
    }
    
    for (std::size_t i = 0; i < m_features.size(); ++ i)
    {
      boost::property_tree::ptree ptr;
      ptr.put("name", m_features[i]->name());
      ptr.put("mean", m_feature_means[i]);
      ptr.put("standard_deviation", m_feature_sd[i]);
      tree.add_child("classification.features.feature", ptr);
    }

    for (std::size_t i = 0; i < m_labels.size(); ++ i)
    {
      boost::property_tree::ptree ptr;
      ptr.put("name", m_labels[i]->name());
      tree.add_child("classification.labels.label", ptr);
    }

    for (std::size_t i = 0; i < m_weights.size(); ++ i)
    {
      boost::property_tree::ptree ptr;
      ptr.put("height", (m_weights[i].size() / m_bias[i].size()));
      ptr.put("width", m_bias[i].size());

      std::ostringstream oss;
      oss.precision(std::numeric_limits<float>::digits10 + 2);
      for (std::size_t j = 0; j < m_weights[i].size(); ++ j)
      {
        oss << m_weights[i][j];
        if (j != m_weights[i].size() - 1)
          oss << " ";
      }
      ptr.put("weights", oss.str());
      
      oss = std::ostringstream();
      oss.precision(std::numeric_limits<float>::digits10 + 2);
      for (std::size_t j = 0; j < m_bias[i].size(); ++ j)
      {
        oss << m_bias[i][j];
        if (j != m_bias[i].size() - 1)
          oss << " ";
      }
      ptr.put("bias", oss.str());
      
      tree.add_child("classification.layers.layer", ptr);
    }
    
    // Write property tree to XML file
    boost::property_tree::write_xml(output, tree,
#if BOOST_VERSION >= 105600
                                    boost::property_tree::xml_writer_make_settings<std::string>(' ', 3));
#else
    boost::property_tree::xml_writer_make_settings<char>(' ', 3));
#endif
  }

  /*!
    \brief Loads a configuration from the stream `input`.

    The input file should be in the XML format written by the
    `save_configuration()` method. The feature set of the classifier
    should contain the exact same features in the exact same order as
    the ones present when the file was generated using
    `save_configuration()`.
  */
  bool load_configuration (std::istream& input, bool verbose = false)
  {
    clear();
    
    bool out = true;

    boost::property_tree::ptree tree;
    boost::property_tree::read_xml(input, tree);

    std::string version = tree.get<std::string>("classification.metadata.classifier_version");
    if (version != CGAL_NEURAL_NETWORK_VERSION)
    {
      if (verbose)
        std::cerr << "Error: CGAL Neural Network version mismatch " << version << "/" << CGAL_NEURAL_NETWORK_VERSION << std::endl;
      return false;
    }
    std::size_t nb_features = std::size_t(tree.get<int>("classification.metadata.number_of_features"));
    if (nb_features != m_features.size())
    {
      if (verbose)
        std::cerr << "Error: number of features mismatch " << nb_features << "/" << m_features.size() << std::endl;
      return false;
    }

    std::size_t nb_labels = std::size_t(tree.get<int>("classification.metadata.number_of_labels"));
    if (nb_labels != m_labels.size())
    {
      if (verbose)
        std::cerr << "Error: number of labels mismatch " << nb_labels << "/" << m_labels.size() << std::endl;
      return false;
    }

    std::size_t nb_layers = std::size_t(tree.get<int>("classification.metadata.number_of_hidden_layers")) + 1;
    m_learning_rate = tree.get<float>("classification.metadata.learning_rate");

    std::size_t idx = 0;
    BOOST_FOREACH(boost::property_tree::ptree::value_type &v, tree.get_child("classification.features"))
    {
      std::string name = v.second.get<std::string>("name");

      if (name != m_features[idx]->name())
      {
        if (verbose)
          std::cerr << "Warning: feature mismatch " << name << "/" << m_features[idx]->name() << std::endl;
        out = false;
      }

      m_feature_means.push_back(v.second.get<float>("mean"));
      m_feature_sd.push_back(v.second.get<float>("standard_deviation"));
      
      ++ idx;
    }

    if (idx != nb_features)
    {
      if (verbose)
        std::cerr << "Error: number of features mismatch " << nb_features << "/" << idx << std::endl;
      return false;
    }

    idx = 0;
    BOOST_FOREACH(boost::property_tree::ptree::value_type &v, tree.get_child("classification.labels"))
    {
      std::string name = v.second.get<std::string>("name");
      if (name != m_labels[idx]->name())
      {
        if (verbose)
          std::cerr << "Warning: label mismatch " << name << "/" << m_labels[idx]->name() << std::endl;
        out = false;
      }
      ++ idx;
    }
    
    if (idx != nb_labels)
    {
      if (verbose)
        std::cerr << "Error: number of labels mismatch " << nb_labels << "/" << idx << std::endl;
      return false;
    }

    idx = 0;
    m_weights.resize (nb_layers);
    m_bias.resize (nb_layers);

    std::vector<std::size_t> hidden_layers;
    BOOST_FOREACH(boost::property_tree::ptree::value_type &v, tree.get_child("classification.layers"))
    {
      if (idx >= nb_layers)
      {
        if (verbose)
          std::cerr << "Error: number of layers mismatch " << nb_layers << "/" << idx << std::endl;
        return false;
      }
      std::size_t height = std::size_t(v.second.get<int>("height"));
      std::size_t width = std::size_t(v.second.get<int>("width"));
      std::size_t size = height * width;
      m_weights[idx].reserve (size);
      m_bias[idx].reserve (width);

      std::string weights = v.second.get<std::string>("weights");
      std::istringstream iss (weights);
      float f;
      while (iss >> f)
        m_weights[idx].push_back (f);

      std::string bias = v.second.get<std::string>("bias");
      iss = std::istringstream (bias);
      while (iss >> f)
        m_bias[idx].push_back (f);

      if (idx != nb_layers - 1)
        hidden_layers.push_back (width);
      
      ++ idx;
    }
    
    if (idx != nb_layers)
    {
      if (verbose)
        std::cerr << "Error: number of layers mismatch " << nb_layers << "/" << idx << std::endl;
      return false;
    }

    build_architecture(hidden_layers);

    return out;
  }

private:

  template<class BidiIter >
  BidiIter random_unique(BidiIter begin, BidiIter end, size_t num_random) {
    size_t left = std::distance(begin, end);
    while (num_random--) {
      BidiIter r = begin;
      std::advance(r, rand()%left);
      std::swap(*begin, *r);
      ++begin;
      --left;
    }
    return begin;
  }

  void build_architecture (const std::vector<std::size_t>& hidden_layers)
  {
    m_root = new TF::Scope (TF::Scope::NewRootScope());

    
    if (m_verbose) std::cerr << "I - BUILDING NEURAL NETWORK ARCHITECTURE" << std::endl;
    
    // Get layers and sizes or init with default values
    std::vector<std::size_t> hl = hidden_layers;
    if (hl.empty())
    {
      hl.push_back (m_features.size());
      hl.push_back ((m_features.size() + m_labels.size()) / 2);
    }
    
    if (m_verbose) std::cerr << " 1) Initializing architecture:" << std::endl
                            << "   * Layer 0: " << m_features.size() << " neuron(s) (input features)" << std::endl;
      
    if (m_verbose)
      for (std::size_t i = 0; i < hl.size(); ++ i)
        std::cerr << "   * Layer " << i+1 << ": " << hl[i] << " neuron(s)" << std::endl;
    
    if (m_verbose) std::cerr << "   * Layer " << hl.size() + 1 << ": "
                            << m_labels.size() << " neuron(s) (output labels)" << std::endl;


    
    m_ph_ft = new TFops::Placeholder (*m_root, TF::DT_FLOAT);
    m_ph_gt = new TFops::Placeholder (*m_root, TF::DT_FLOAT);
    
    if (m_verbose) std::cerr << " 2) Creating weight matrices and bias" << std::endl;
    
    // create weight matrices and bias for each layer transition
    std::vector<TFops::Variable> weights;
    std::vector<TFops::Variable> bias;
    std::vector<TFops::Assign> assign_weights;
    std::vector<TFops::Assign> assign_bias;

    for (std::size_t i = 0; i <= hl.size(); ++ i)
    {
      long long size_from = 0;
      long long size_to = 0;
      if (i == 0)
      {
        size_from = m_features.size();
        size_to = hl[0];
      }
      else if (i == hl.size())
      {
        size_from = hl.back();
        size_to = m_labels.size();
      }
      else
      {
        size_from = hl[i-1];
        size_to = hl[i];
      }

      if (m_verbose) std::cerr << "   * Weight matrix " << i << " [" << size_from << ";" << size_to << "]" << std::endl;
      weights.push_back (TFops::Variable (*m_root, { size_from, size_to }, TF::DT_FLOAT));

      if (m_weights.empty())
      {
        if (boost::is_same<ActivationFunction, TFops::Tanh>::value)  // Weights initialized by Xavier method
          assign_weights.push_back (TFops::Assign (*m_root, weights.back(),
                                                   TFops::Mul (*m_root, TFops::Const(*m_root,
                                                                                     std::sqrt (1.f / (float)(size_from))),
                                                               TFops::RandomNormal (*m_root, { size_from, size_to}, TF::DT_FLOAT))));
        else if (boost::is_same<ActivationFunction, TFops::Relu>::value ||
                 boost::is_same<ActivationFunction, TFops::Relu6>::value)  // Weights initialized by He method
          assign_weights.push_back (TFops::Assign (*m_root, weights.back(),
                                                   TFops::Mul (*m_root, TFops::Const(*m_root,
                                                                                     std::sqrt (2.f / (float)(size_from))),
                                                               TFops::RandomNormal (*m_root, { size_from, size_to}, TF::DT_FLOAT))));
        else // Default: weights truncated normal
          assign_weights.push_back (TFops::Assign (*m_root, weights.back(),
                                                   TFops::TruncatedNormal (*m_root, { size_from, size_to}, TF::DT_FLOAT)));
      }
      else
      {
        TF::Tensor init_weight
          (TF::DataTypeToEnum<float>::v(), 
           TF::TensorShape {(long long)(size_from), (long long)(size_to)});
        float* init_weight_data = init_weight.flat<float>().data();
        std::copy_n (m_weights[i].begin(), size_from * size_to, init_weight_data);

        assign_weights.push_back (TFops::Assign (*m_root, weights.back(), init_weight));
      }
      
      if (m_verbose) std::cerr << "   * Bias " << i << " [" << size_to << "]" << std::endl;
      bias.push_back (TFops::Variable (*m_root, { (long long)(1), size_to }, TF::DT_FLOAT));

      TF::Tensor init_bias
        (TF::DataTypeToEnum<float>::v(), 
         TF::TensorShape {(long long)(1), (long long)(size_to)});
      float* init_bias_data = init_bias.flat<float>().data();
      if (m_bias.empty())
        for (std::size_t s = 0; s < size_to; ++ s)
          init_bias_data[s] = 0.f;
      else
        std::copy_n (m_bias[i].begin(), size_to, init_bias_data);
      
      assign_bias.push_back (TFops::Assign (*m_root, bias.back(), init_bias));
    }

    if (!m_root->status().ok())
    {
      if (m_verbose) std::cerr << "Error: " << m_root->status().ToString() << std::endl;
      return;
    }
    
    if (m_verbose) std::cerr << " 3) Creating layers" << std::endl;
    
    for (std::size_t i = 0; i < weights.size(); ++ i)
    {
      if (i == 0)
        m_layers.push_back (ActivationFunction (*m_root, TFops::Add
                                                (*m_root, TFops::MatMul (*m_root, *m_ph_ft, weights[0]),
                                                 bias[0])));
      else if (i == weights.size() - 1)
        m_layers.push_back (TFops::Softmax (*m_root, TFops::Add
                                            (*m_root, TFops::MatMul (*m_root, m_layers.back(), weights[i]),
                                             bias[i])));
      else
        m_layers.push_back (ActivationFunction (*m_root, TFops::Add
                                                (*m_root, TFops::MatMul (*m_root, m_layers.back(), weights[i]),
                                                 bias[i])));
    }

    if (!m_root->status().ok())
    {
      if (m_verbose) std::cerr << "Error: " << m_root->status().ToString() << std::endl;
      return;
    }

    if (m_verbose) std::cerr << " 4) Setting up loss calculation" << std::endl;
    
    // loss calculation based on cross-entropy
    TFops::Maximum truncated_ypred = TFops::Maximum (*m_root, TFops::Const(*m_root, 0.0001f), m_layers.back());
    TFops::Log log_ypred = TFops::Log (*m_root, truncated_ypred);
    TFops::Mul ygt_times_log_ypred = TFops::Mul (*m_root, *m_ph_gt, log_ypred);
    TFops::ReduceSum sum_ygt_times_log_ypred = TFops::ReduceSum(*m_root, ygt_times_log_ypred, {1});
    TFops::Mul minus_sum_ygt_times_log_ypred = TFops::Mul (*m_root, TFops::Const(*m_root, -1.f), sum_ygt_times_log_ypred);

    m_loss = new TFops::ReduceMean (*m_root, minus_sum_ygt_times_log_ypred, {0});
    
    if (!m_root->status().ok())
    {
      if (m_verbose) std::cerr << "Error: " << m_root->status().ToString() << std::endl;
      return;
    }

    if (m_verbose) std::cerr << " 5) Setting up gradient descent" << std::endl;
    
    std::vector<TF::Output> weights_and_bias;
    for (std::size_t i = 0; i < weights.size(); ++ i)
      weights_and_bias.push_back (TF::Output(weights[i]));
    for (std::size_t i = 0; i < bias.size(); ++ i)
      weights_and_bias.push_back (TF::Output(bias[i]));
    
    std::vector<TF::Output> gradients;

    TF_CHECK_OK(TF::AddSymbolicGradients(*m_root, {*m_loss},
                                         weights_and_bias,
                                         &gradients));
  
    if (!m_root->status().ok())
    {
      if (m_verbose) std::cerr << "Error: " << m_root->status().ToString() << std::endl;
      return;
    }
  
    std::vector<TF::Output> assigners;
  
#ifdef CGAL_CLASSIFICATION_TENSORFLOW_USE_ADAM
    for (std::size_t i = 0; i < weights.size(); ++ i)
    {
      long long size_from = 0;
      long long size_to = 0;
      if (i == 0)
      {
        size_from = m_features.size();
        size_to = hl[0];
      }
      else if (i == weights.size() - 1)
      {
        size_from = hl.back();
        size_to = m_labels.size();
      }
      else
      {
        size_from = hl[i-1];
        size_to = hl[i];
      }

      TFops::Variable m (*m_root, { size_from, size_to}, TF::DT_FLOAT);
      TF::Tensor init_m
        (TF::DataTypeToEnum<float>::v(), 
         TF::TensorShape {(long long)(size_from), (long long)(size_to)});
      float* init_m_data = init_m.flat<float>().data();
      for (std::size_t s = 0; s < size_from; ++ s)
        for (std::size_t ss = 0; ss < size_to; ++ ss)
          init_m_data[ss * size_from + s] = 0.f;
      assigners.push_back (TFops::Assign (*m_root, m, init_m));

      TFops::Variable v (*m_root, { size_from, size_to}, TF::DT_FLOAT);
      TF::Tensor init_v
        (TF::DataTypeToEnum<float>::v(), 
         TF::TensorShape {(long long)(size_from), (long long)(size_to)});
      float* init_v_data = init_v.flat<float>().data();
      for (std::size_t s = 0; s < size_from; ++ s)
        for (std::size_t ss = 0; ss < size_to; ++ ss)
          init_v_data[ss * size_from + s] = 0.f;
      assigners.push_back (TFops::Assign (*m_root, v, init_v));

      m_optimizer.push_back (TFops::ApplyAdam
                                    (*m_root,
                                     weights[i],
                                     m,
                                     v,
                                     TFops::Cast(*m_root, 0.9, TF::DT_FLOAT),
                                     TFops::Cast(*m_root, 0.999, TF::DT_FLOAT),
                                     TFops::Cast(*m_root, m_learning_rate, TF::DT_FLOAT),
                                     TFops::Cast(*m_root, 0.9, TF::DT_FLOAT),
                                     TFops::Cast(*m_root, 0.999, TF::DT_FLOAT),
                                     TFops::Cast(*m_root, 1e-8, TF::DT_FLOAT),
                                     {gradients[i]}));
    }
    
    for (std::size_t i = 0; i < bias.size(); ++ i)
    {
      long long size_from = 0;
      long long size_to = 0;
      if (i == 0)
        size_to = hl[0];
      else if (i == bias.size() - 1)
        size_to = m_labels.size();
      else
        size_to = hl[i];

      TFops::Variable m (*m_root, { 1, size_to}, TF::DT_FLOAT);
      TF::Tensor init_m
        (TF::DataTypeToEnum<float>::v(), 
         TF::TensorShape {(long long)(1), (long long)(size_to)});
      float* init_m_data = init_m.flat<float>().data();
      for (std::size_t s = 0; s < size_to; ++ s)
        init_m_data[s] = 0.f;
      assigners.push_back (TFops::Assign (*m_root, m, init_m));

      TFops::Variable v (*m_root, { 1, size_to}, TF::DT_FLOAT);
      TF::Tensor init_v
        (TF::DataTypeToEnum<float>::v(), 
         TF::TensorShape {(long long)(1), (long long)(size_to)});
      float* init_v_data = init_v.flat<float>().data();
      for (std::size_t s = 0; s < size_to; ++ s)
        init_v_data[s] = 0.f;
      assigners.push_back (TFops::Assign (*m_root, v, init_v));

      m_optimizer.push_back (TFops::ApplyAdam
                                    (*m_root,
                                     bias[i],
                                     m,
                                     v,
                                     TFops::Cast(*m_root, 0.9, TF::DT_FLOAT),
                                     TFops::Cast(*m_root, 0.999, TF::DT_FLOAT),
                                     TFops::Cast(*m_root, m_learning_rate, TF::DT_FLOAT),
                                     TFops::Cast(*m_root, 0.9, TF::DT_FLOAT),
                                     TFops::Cast(*m_root, 0.999, TF::DT_FLOAT),
                                     TFops::Cast(*m_root, 1e-8, TF::DT_FLOAT),
                                     {gradients[weights.size() + i]}));
    }
    
#else
    for (std::size_t i = 0; i < weights.size(); ++ i)
      m_optimizer.push_back (TFops::ApplyGradientDescent
                                    (*m_root, weights[i],
                                     TFops::Cast(*m_root, m_learning_rate, TF::DT_FLOAT), {gradients[i]}));
    for (std::size_t i = 0; i < bias.size(); ++ i)
      m_optimizer.push_back (TFops::ApplyGradientDescent
                                    (*m_root, bias[i],
                                     TFops::Cast(*m_root, m_learning_rate, TF::DT_FLOAT), {gradients[weights.size() + i]}));
#endif
    if (!m_root->status().ok())
    {
      if (m_verbose) std::cerr << "Error: " << m_root->status().ToString() << std::endl;
      return;
    }

    if (m_verbose) std::cerr << " 6) Starting session" << std::endl;
    
    TF::SessionOptions options = TF::SessionOptions();
    
    options.config.mutable_gpu_options()->set_allow_growth(true);
    options.config.mutable_gpu_options()->set_per_process_gpu_memory_fraction(0.8);
    options.config.mutable_gpu_options()->set_force_gpu_compatible(true);

    m_session = new TF::ClientSession (*m_root, options);
    if (!m_root->status().ok())
    {
      if (m_verbose) std::cerr << "Error: " << m_root->status().ToString() << std::endl;
      return;
    }

    std::vector<TF::Tensor> outputs;

    for (std::size_t i = 0; i < assign_weights.size(); ++ i)
      assigners.push_back (TF::Output(assign_weights[i]));
    for (std::size_t i = 0; i < assign_bias.size(); ++ i)
      assigners.push_back (TF::Output(assign_bias[i]));
    TF_CHECK_OK (m_session->Run(assigners, nullptr));

    if (!m_root->status().ok())
    {
      if (m_verbose) std::cerr << "Error: " << m_root->status().ToString() << std::endl;
      return;
    }
  }
};


/// \cond SKIP_IN_MANUAL
#ifdef CGAL_CLASSIFICATION_TENSORFLOW_ENABLE_GPU_SPECIALIZATION
// Specialization to use GPU parallelization
template <typename ConcurrencyTag,
          typename ItemRange,
          typename LabelIndexRange,
          typename ProbabilitiesRanges>
void classify (const ItemRange& input,
               const Label_set& labels,
               const TensorFlow::Neural_network_classifier& classifier,
               LabelIndexRange& output,
               ProbabilitiesRanges& probabilities)
{
  std::cerr << "Classify with TensorFlow classifier" << std::endl;
  
  output.resize(input.size());
  probabilities.resize (labels.size());
  for (std::size_t i = 0; i < probabilities.size(); ++ i)
    probabilities[i].resize (input.size());

  const std::size_t mem_allocated = sizeof(float) * input.size() * (labels.size() + classifier.features().size());
  const std::size_t size_max = 512 * 1024 * 1024;
  const std::size_t nb_subdivisions = (mem_allocated / size_max) + 1;
  std::cerr << nb_subdivisions << " subdivision(s) for GPU processing" << std::endl;

  std::size_t idx = 0;
  for (std::size_t n = 0; n < nb_subdivisions; ++ n)
  {
    std::vector<std::size_t> indices;
    indices.reserve (input.size() / nb_subdivisions);
    for (std::size_t i = 0; i < input.size() / nb_subdivisions && idx < input.size(); ++ i)
      indices.push_back(idx ++);
    
    std::vector<std::vector<float> > values;
    classifier (indices, values);
    for(std::size_t i = 0; i < indices.size(); ++ i)
    {
      std::size_t nb_class_best = 0;
      float val_class_best = 0.f;

      for (std::size_t j = 0; j < labels.size(); ++ j)
      {
        probabilities[j][indices[i]] = values[i][j];
        
        if(val_class_best < values[i][j])
        {
          val_class_best = values[i][j];
          nb_class_best = j;
        }
      }

      output[indices[i]] = nb_class_best;
    }
  }
}

// Specialization to use GPU parallelization
template <typename ConcurrencyTag,
          typename ItemRange,
          typename LabelIndexRange>
void classify (const ItemRange& input,
               const Label_set& labels,
               const TensorFlow::Neural_network_classifier& classifier,
               LabelIndexRange& output)
{
  std::cerr << "Classify with TensorFlow classifier" << std::endl;
  
  output.resize(input.size());

  const std::size_t mem_allocated = sizeof(float) * input.size() * (labels.size() + classifier.features().size());
  const std::size_t size_max = 512 * 1024 * 1024;
  const std::size_t nb_subdivisions = (mem_allocated / size_max) + 1;
  std::cerr << nb_subdivisions << " subdivision(s) for GPU processing" << std::endl;

  std::size_t idx = 0;
  for (std::size_t n = 0; n < nb_subdivisions; ++ n)
  {
    std::vector<std::size_t> indices;
    indices.reserve (input.size() / nb_subdivisions);
    for (std::size_t i = 0; i < input.size() / nb_subdivisions && idx < input.size(); ++ i)
      indices.push_back(idx ++);
    
    std::vector<std::vector<float> > values;
    classifier (indices, values);

    for(std::size_t i = 0; i < indices.size(); ++ i)
    {
      std::size_t nb_class_best = 0;
      float val_class_best = 0.f;

      for (std::size_t j = 0; j < labels.size(); ++ j)
      {
        if(val_class_best < values[i][j])
        {
          val_class_best = values[i][j];
          nb_class_best = j;
        }
      }

      output[indices[i]] = nb_class_best;
    }
  }
}
#endif  
/// \endcond

}

}

}

#endif // CGAL_CLASSIFICATION_TENSORFLOW_NEURAL_NETWORK_CLASSIFIER_H
