    // The contents of this file are in the public domain. See LICENSE_FOR_EXAMPLE_PROGRAMS.txt
    /*
        This is an example illustrating the use of the deep learning tools from the
        dlib C++ Library.  In it, we will show how to use the loss_metric layer to do
        metric learning on images.

        The main reason you might want to use this kind of algorithm is because you
        would like to use a k-nearest neighbor classifier or similar algorithm, but
        you don't know a good way to calculate the distance between two things.  A
        popular example would be face recognition.  There are a whole lot of papers
        that train some kind of deep metric learning algorithm that embeds face
        images in some vector space where images of the same person are close to each
        other and images of different people are far apart.  Then in that vector
        space it's very easy to do face recognition with some kind of k-nearest
        neighbor classifier.

        In this example we will use a version of the ResNet network from the
        dnn_imagenet_ex.cpp example to learn to map images into some vector space where
        pictures of the same person are close and pictures of different people are far
        apart.

        You might want to read the simpler introduction to the deep metric learning
        API, dnn_metric_learning_ex.cpp, before reading this example.  You should
        also have read the examples that introduce the dlib DNN API before
        continuing.  These are dnn_introduction_ex.cpp and dnn_introduction2_ex.cpp.

    */

    #include <dlib/dnn.h>
    #include <dlib/image_io.h>
    #include <dlib/misc_api.h>

    using namespace dlib;
    using namespace std;

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

    // We will need to create some functions for loading data.  This program will
    // expect to be given a directory structured as follows:
    //    top_level_directory/
    //        person1/
    //            image1.jpg
    //            image2.jpg
    //            image3.jpg
    //        person2/
    //            image4.jpg
    //            image5.jpg
    //            image6.jpg
    //        person3/
    //            image7.jpg
    //            image8.jpg
    //            image9.jpg
    //
    // The specific folder and image names don't matter, nor does the number of folders or
    // images.  What does matter is that there is a top level folder, which contains
    // subfolders, and each subfolder contains images of a single person.

    // This function spiders the top level directory and obtains a list of all the
    // image files.
    std::vector<std::vector<string>> load_objects_list (
        const string& dir
    )
    {
        std::vector<std::vector<string>> objects;
        for (auto subdir : directory(dir).get_dirs())
        {
            std::vector<string> imgs;
            for (auto img : subdir.get_files())
                imgs.push_back(img);

            if (imgs.size() != 0)
                objects.push_back(imgs);
        }
        return objects;
    }

    // This function takes the output of load_objects_list() as input and randomly
    // selects images for training.  It should also be pointed out that it's really
    // important that each mini-batch contain multiple images of each person.  This
    // is because the metric learning algorithm needs to consider pairs of images
    // that should be close (i.e. images of the same person) as well as pairs of
    // images that should be far apart (i.e. images of different people) during each
    // training step.
    void load_mini_batch (
        const size_t num_people,     // how many different people to include
        const size_t samples_per_id, // how many images per person to select.
        dlib::rand& rnd,
        const std::vector<std::vector<string>>& objs,
        std::vector<matrix<rgb_pixel>>& images,
        std::vector<unsigned long>& labels
    )
    {
        images.clear();
        labels.clear();
        DLIB_CASSERT(num_people <= objs.size(), "The dataset doesn't have that many people in it.");

        std::vector<bool> already_selected(objs.size(), false);
        matrix<rgb_pixel> image;
        for (size_t i = 0; i < num_people; ++i)
        {
            size_t id = rnd.get_random_32bit_number()%objs.size();
            // don't pick a person we already added to the mini-batch
            while(already_selected[id])
                id = rnd.get_random_32bit_number()%objs.size();
            already_selected[id] = true;

            for (size_t j = 0; j < samples_per_id; ++j)
            {
                const auto& obj = objs[id][rnd.get_random_32bit_number()%objs[id].size()];
                load_image(image, obj);
                images.push_back(std::move(image));
                labels.push_back(id);
            }
        }

        // You might want to do some data augmentation at this point.  Here we do some simple
        // color augmentation.
        for (auto&& crop : images)
        {
            disturb_colors(crop,rnd);
            // Jitter most crops
            if (rnd.get_random_double() > 0.1)
                crop = jitter_image(crop,rnd);
        }

        // All the images going into a mini-batch have to be the same size.  And really, all
        // the images in your entire training dataset should be the same size for what we are
        // doing to make the most sense.
        DLIB_CASSERT(images.size() > 0);
        for (auto&& img : images)
        {
            DLIB_CASSERT(img.nr() == images[0].nr() && img.nc() == images[0].nc(),
                "All the images in a single mini-batch must be the same size.");
        }
    }

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

    // The next page of code defines a ResNet network.  It's basically copied
    // and pasted from the dnn_imagenet_ex.cpp example, except we replaced the loss
    // layer with loss_metric and make the network somewhat smaller.

    template <template <int,template<typename>class,int,typename> class block, int N, template<typename>class BN, typename SUBNET>
    using residual = add_prev1<block<N,BN,1,tag1<SUBNET>>>;

    template <template <int,template<typename>class,int,typename> class block, int N, template<typename>class BN, typename SUBNET>
    using residual_down = add_prev2<avg_pool<2,2,2,2,skip1<tag2<block<N,BN,2,tag1<SUBNET>>>>>>;

    template <int N, template <typename> class BN, int stride, typename SUBNET>
    using block  = BN<con<N,3,3,1,1,relu<BN<con<N,3,3,stride,stride,SUBNET>>>>>;

    template <int N, typename SUBNET> using res       = relu<residual<block,N,bn_con,SUBNET>>;
    template <int N, typename SUBNET> using ares      = relu<residual<block,N,affine,SUBNET>>;
    template <int N, typename SUBNET> using res_down  = relu<residual_down<block,N,bn_con,SUBNET>>;
    template <int N, typename SUBNET> using ares_down = relu<residual_down<block,N,affine,SUBNET>>;

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

    template <typename SUBNET> using level0 = res_down<256,SUBNET>;
    template <typename SUBNET> using level1 = res<256,res<256,res_down<256,SUBNET>>>;
    template <typename SUBNET> using level2 = res<128,res<128,res_down<128,SUBNET>>>;
    template <typename SUBNET> using level3 = res<64,res<64,res<64,res_down<64,SUBNET>>>>;
    template <typename SUBNET> using level4 = res<32,res<32,res<32,SUBNET>>>;

    template <typename SUBNET> using alevel0 = ares_down<256,SUBNET>;
    template <typename SUBNET> using alevel1 = ares<256,ares<256,ares_down<256,SUBNET>>>;
    template <typename SUBNET> using alevel2 = ares<128,ares<128,ares_down<128,SUBNET>>>;
    template <typename SUBNET> using alevel3 = ares<64,ares<64,ares<64,ares_down<64,SUBNET>>>>;
    template <typename SUBNET> using alevel4 = ares<32,ares<32,ares<32,SUBNET>>>;

    // training network type
    using net_type = loss_metric<fc_no_bias<128,avg_pool_everything<
                                level0<
                                level1<
                                level2<
                                level3<
                                level4<
                                max_pool<3,3,2,2,relu<bn_con<con<32,7,7,2,2,
                                input_rgb_image
                                >>>>>>>>>>>>;

    // testing network type (replaced batch normalization with fixed affine transforms)
    using anet_type = loss_metric<fc_no_bias<128,avg_pool_everything<
                                alevel0<
                                alevel1<
                                alevel2<
                                alevel3<
                                alevel4<
                                max_pool<3,3,2,2,relu<affine<con<32,7,7,2,2,
                                input_rgb_image
                                >>>>>>>>>>>>;

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

    int main(int argc, char** argv)
    {
        if (argc != 2)
        {
            cout << "Give a folder as input.  It should contain sub-folders of images and we will " << endl;
            cout << "learn to distinguish between these sub-folders with metric learning.  " << endl;
            cout << "For example, you can run this program on the very small examples/johns dataset" << endl;
            cout << "that comes with dlib by running this command:" << endl;
            cout << "   ./dnn_metric_learning_on_images_ex johns" << endl;
            return 1;
        }

        auto objs = load_objects_list(argv[1]);

        cout << "objs.size(): "<< objs.size() << endl;

        std::vector<matrix<rgb_pixel>> images;
        std::vector<unsigned long> labels;

        net_type net;

        dnn_trainer<net_type> trainer(net, sgd(0.0001, 0.9));
        trainer.set_learning_rate(0.1);
        trainer.be_verbose();
        trainer.set_synchronization_file("face_metric_sync", std::chrono::minutes(5));
        // I've set this to something really small to make the example terminate
        // sooner.  But when you really want to train a good model you should set
        // this to something like 10000 so training doesn't terminate too early.
        trainer.set_iterations_without_progress_threshold(300);

        // If you have a lot of data then it might not be reasonable to load it all
        // into RAM.  So you will need to be sure you are decompressing your images
        // and loading them fast enough to keep the GPU occupied.  I like to do this
        // using the following coding pattern: create a bunch of threads that dump
        // mini-batches into dlib::pipes.
        dlib::pipe<std::vector<matrix<rgb_pixel>>> qimages(4);
        dlib::pipe<std::vector<unsigned long>> qlabels(4);
        auto data_loader = [&qimages, &qlabels, &objs](time_t seed)
        {
            dlib::rand rnd(time(0)+seed);
            std::vector<matrix<rgb_pixel>> images;
            std::vector<unsigned long> labels;
            while(qimages.is_enabled())
            {
                try
                {
                    load_mini_batch(5, 5, rnd, objs, images, labels);
                    qimages.enqueue(images);
                    qlabels.enqueue(labels);
                }
                catch(std::exception& e)
                {
                    cout << "EXCEPTION IN LOADING DATA" << endl;
                    cout << e.what() << endl;
                }
            }
        };
        // Run the data_loader from 5 threads.  You should set the number of threads
        // relative to the number of CPU cores you have.
        std::thread data_loader1([data_loader](){ data_loader(1); });
        std::thread data_loader2([data_loader](){ data_loader(2); });
        std::thread data_loader3([data_loader](){ data_loader(3); });
        std::thread data_loader4([data_loader](){ data_loader(4); });
        std::thread data_loader5([data_loader](){ data_loader(5); });

        // Here we do the training.  We keep passing mini-batches to the trainer until the
        // learning rate has dropped low enough.
        while(trainer.get_learning_rate() >= 1e-4)
        {
            qimages.dequeue(images);
            qlabels.dequeue(labels);
            trainer.train_one_step(images, labels);
        }

        // Wait for training threads to stop
        trainer.get_net();
        cout << "done training" << endl;

        // Save the network to disk
        net.clean();
        serialize("metric_network_renset.dat") << net;

        // stop all the data loading threads and wait for them to terminate.
        qimages.disable();
        qlabels.disable();
        data_loader1.join();
        data_loader2.join();
        data_loader3.join();
        data_loader4.join();
        data_loader5.join();

        // Now, just to show an example of how you would use the network, let's check how well
        // it performs on the training data.
        dlib::rand rnd(time(0));
        load_mini_batch(5, 5, rnd, objs, images, labels);

        // Normally you would use the non-batch-normalized version of the network to do
        // testing, which is what we do here.
        anet_type testing_net = net;

        // Run all the images through the network to get their vector embeddings.
        std::vector<matrix<float,0,1>> embedded = testing_net(images);

        // Now, check if the embedding puts images with the same labels near each other and
        // images with different labels far apart.
        int num_right = 0;
        int num_wrong = 0;
        for (size_t i = 0; i < embedded.size(); ++i)
        {
            for (size_t j = i+1; j < embedded.size(); ++j)
            {
                if (labels[i] == labels[j])
                {
                    // The loss_metric layer will cause images with the same label to be less
                    // than net.loss_details().get_distance_threshold() distance from each
                    // other.  So we can use that distance value as our testing threshold.
                    if (length(embedded[i]-embedded[j]) < testing_net.loss_details().get_distance_threshold())
                        ++num_right;
                    else
                        ++num_wrong;
                }
                else
                {
                    if (length(embedded[i]-embedded[j]) >= testing_net.loss_details().get_distance_threshold())
                        ++num_right;
                    else
                        ++num_wrong;
                }
            }
        }

        cout << "num_right: "<< num_right << endl;
        cout << "num_wrong: "<< num_wrong << endl;

    }