'''Transfer learning toy example:

1- Train a simple convnet on the MNIST dataset the first 5 digits [0..4].
2- Freeze convolutional layers and fine-tune dense layers
   for the classification of digits [5..9].

Run on GPU: THEANO_FLAGS=mode=FAST_RUN,device=gpu,floatX=float32 python mnist_cnn.py

Get to 99.8% test accuracy after 5 epochs
for the first five digits classifier
and 99.2% for the last five digits after transfer + fine-tuning.
'''

from __future__ import print_function
import numpy as np
import datetime

np.random.seed(1337)  # for reproducibility

from keras.datasets import mnist
from keras.models import Sequential
from keras.layers.core import Dense, Dropout, Activation, Flatten
from keras.layers.convolutional import Convolution2D, MaxPooling2D
from keras.utils import np_utils


now = datetime.datetime.now

batch_size = 128
nb_classes = 5
nb_epoch = 5

# input image dimensions
img_rows, img_cols = 28, 28
# number of convolutional filters to use
nb_filters = 32
# size of pooling area for max pooling
nb_pool = 2
# convolution kernel size
nb_conv = 3


def train_model(model, train, test, nb_classes):
    X_train = train[0].reshape(train[0].shape[0], 1, img_rows, img_cols)
    X_test = test[0].reshape(test[0].shape[0], 1, img_rows, img_cols)
    X_train = X_train.astype('float32')
    X_test = X_test.astype('float32')
    X_train /= 255
    X_test /= 255
    print('X_train shape:', X_train.shape)
    print(X_train.shape[0], 'train samples')
    print(X_test.shape[0], 'test samples')

    # convert class vectors to binary class matrices
    Y_train = np_utils.to_categorical(train[1], nb_classes)
    Y_test = np_utils.to_categorical(test[1], nb_classes)

    model.compile(loss='categorical_crossentropy', optimizer='adadelta')

    t = now()
    model.fit(X_train, Y_train,
              batch_size=batch_size, nb_epoch=nb_epoch,
              show_accuracy=True, verbose=1,
              validation_data=(X_test, Y_test))
    print('Training time: %s' % (now() - t))
    score = model.evaluate(X_test, Y_test, show_accuracy=True, verbose=0)
    print('Test score:', score[0])
    print('Test accuracy:', score[1])


# the data, shuffled and split between train and test sets
(X_train, y_train), (X_test, y_test) = mnist.load_data()

# create two datasets one with digits below 5 and one with 5 and above
X_train_lt5 = X_train[y_train < 5]
y_train_lt5 = y_train[y_train < 5]
X_test_lt5 = X_test[y_test < 5]
y_test_lt5 = y_test[y_test < 5]

X_train_gte5 = X_train[y_train >= 5]
y_train_gte5 = y_train[y_train >= 5] - 5  # make classes start at 0 for
X_test_gte5 = X_test[y_test >= 5]         # np_utils.to_categorical
y_test_gte5 = y_test[y_test >= 5] - 5

# define two groups of layers: feature (convolutions) and classification (dense)
feature_layers = [
    Convolution2D(nb_filters, nb_conv, nb_conv,
                  border_mode='valid',
                  input_shape=(1, img_rows, img_cols)),
    Activation('relu'),
    Convolution2D(nb_filters, nb_conv, nb_conv),
    Activation('relu'),
    MaxPooling2D(pool_size=(nb_pool, nb_pool)),
    Dropout(0.25),
    Flatten(),
]
classification_layers = [
    Dense(128),
    Activation('relu'),
    Dropout(0.5),
    Dense(nb_classes),
    Activation('softmax')
]

# create complete model
model = Sequential()
for l in feature_layers + classification_layers:
    model.add(l)

# train model for 5-digit classification [0..4]
train_model(model,
            (X_train_lt5, y_train_lt5),
            (X_test_lt5, y_test_lt5), nb_classes)

# freeze feature layers and rebuild model
for l in feature_layers:
    l.trainable = False

# transfer: train dense layers for new classification task [5..9]
train_model(model,
            (X_train_gte5, y_train_gte5),
            (X_test_gte5, y_test_gte5), nb_classes)