# Sequence to sequence example in Keras (character-level).

This script demonstrates how to implement a basic character-level sequence-to-sequence model. We apply it to translating short English sentences into short French sentences, character-by-character. Note that it is fairly unusual to do character-level machine translation, as word-level models are more common in this domain.

Summary of the algorithm

- We start with input sequences from a domain (e.g. English sentences) and corresponding target sequences from another domain (e.g. French sentences).
- An encoder LSTM turns input sequences to 2 state vectors (we keep the last LSTM state and discard the outputs).
- A decoder LSTM is trained to turn the target sequences into the same sequence but offset by one timestep in the future, a training process called "teacher forcing" in this context. It uses as initial state the state vectors from the encoder. Effectively, the decoder learns to generate `targets[t+1...]` given `targets[...t]`, conditioned on the input sequence.
- In inference mode, when we want to decode unknown input sequences, we:
  - Encode the input sequence into state vectors
  - Start with a target sequence of size 1 (just the start-of-sequence character)
  - Feed the state vectors and 1-char target sequence to the decoder to produce predictions for the next character
  - Sample the next character using these predictions (we simply use argmax).
  - Append the sampled character to the target sequence
  - Repeat until we generate the end-of-sequence character or we hit the character limit.

Data download

English to French sentence pairs.

Lots of neat sentence pairs datasets.

References

- Sequence to Sequence Learning with Neural Networks
- Learning Phrase Representations using RNN Encoder-Decoder for Statistical Machine Translation

    from __future__ import print_function

    from keras.models import Model
    from keras.layers import Input, LSTM, Dense
    import numpy as np

    batch_size = 64  # Batch size for training.
    epochs = 100  # Number of epochs to train for.
    latent_dim = 256  # Latent dimensionality of the encoding space.
    num_samples = 10000  # Number of samples to train on.
    # Path to the data txt file on disk.
    data_path = 'fra-eng/fra.txt'

    # Vectorize the data.
    input_texts = []
    target_texts = []
    input_characters = set()
    target_characters = set()
    with open(data_path, 'r', encoding='utf-8') as f:
        lines = f.read().split('\n')
    for line in lines[: min(num_samples, len(lines) - 1)]:
        input_text, target_text = line.split('\t')
        # We use "tab" as the "start sequence" character
        # for the targets, and "\n" as "end sequence" character.
        target_text = '\t' + target_text + '\n'
        input_texts.append(input_text)
        target_texts.append(target_text)
        for char in input_text:
            if char not in input_characters:
                input_characters.add(char)
        for char in target_text:
            if char not in target_characters:
                target_characters.add(char)

    input_characters = sorted(list(input_characters))
    target_characters = sorted(list(target_characters))
    num_encoder_tokens = len(input_characters)
    num_decoder_tokens = len(target_characters)
    max_encoder_seq_length = max([len(txt) for txt in input_texts])
    max_decoder_seq_length = max([len(txt) for txt in target_texts])

    print('Number of samples:', len(input_texts))
    print('Number of unique input tokens:', num_encoder_tokens)
    print('Number of unique output tokens:', num_decoder_tokens)
    print('Max sequence length for inputs:', max_encoder_seq_length)
    print('Max sequence length for outputs:', max_decoder_seq_length)

    input_token_index = dict(
        [(char, i) for i, char in enumerate(input_characters)])
    target_token_index = dict(
        [(char, i) for i, char in enumerate(target_characters)])

    encoder_input_data = np.zeros(
        (len(input_texts), max_encoder_seq_length, num_encoder_tokens),
        dtype='float32')
    decoder_input_data = np.zeros(
        (len(input_texts), max_decoder_seq_length, num_decoder_tokens),
        dtype='float32')
    decoder_target_data = np.zeros(
        (len(input_texts), max_decoder_seq_length, num_decoder_tokens),
        dtype='float32')

    for i, (input_text, target_text) in enumerate(zip(input_texts, target_texts)):
        for t, char in enumerate(input_text):
            encoder_input_data[i, t, input_token_index[char]] = 1.
        encoder_input_data[i, t + 1:, input_token_index[' ']] = 1.
        for t, char in enumerate(target_text):
            # decoder_target_data is ahead of decoder_input_data by one timestep
            decoder_input_data[i, t, target_token_index[char]] = 1.
            if t > 0:
                # decoder_target_data will be ahead by one timestep
                # and will not include the start character.
                decoder_target_data[i, t - 1, target_token_index[char]] = 1.
        decoder_input_data[i, t + 1:, target_token_index[' ']] = 1.
        decoder_target_data[i, t:, target_token_index[' ']] = 1.
    # Define an input sequence and process it.
    encoder_inputs = Input(shape=(None, num_encoder_tokens))
    encoder = LSTM(latent_dim, return_state=True)
    encoder_outputs, state_h, state_c = encoder(encoder_inputs)
    # We discard `encoder_outputs` and only keep the states.
    encoder_states = [state_h, state_c]

    # Set up the decoder, using `encoder_states` as initial state.
    decoder_inputs = Input(shape=(None, num_decoder_tokens))
    # We set up our decoder to return full output sequences,
    # and to return internal states as well. We don't use the
    # return states in the training model, but we will use them in inference.
    decoder_lstm = LSTM(latent_dim, return_sequences=True, return_state=True)
    decoder_outputs, _, _ = decoder_lstm(decoder_inputs,
                                         initial_state=encoder_states)
    decoder_dense = Dense(num_decoder_tokens, activation='softmax')
    decoder_outputs = decoder_dense(decoder_outputs)

    # Define the model that will turn
    # `encoder_input_data` & `decoder_input_data` into `decoder_target_data`
    model = Model([encoder_inputs, decoder_inputs], decoder_outputs)

    # Run training
    model.compile(optimizer='rmsprop', loss='categorical_crossentropy',
                  metrics=['accuracy'])
    model.fit([encoder_input_data, decoder_input_data], decoder_target_data,
              batch_size=batch_size,
              epochs=epochs,
              validation_split=0.2)
    # Save model
    model.save('s2s.h5')

    # Next: inference mode (sampling).
    # Here's the drill:
    # 1) encode input and retrieve initial decoder state
    # 2) run one step of decoder with this initial state
    # and a "start of sequence" token as target.
    # Output will be the next target token
    # 3) Repeat with the current target token and current states

    # Define sampling models
    encoder_model = Model(encoder_inputs, encoder_states)

    decoder_state_input_h = Input(shape=(latent_dim,))
    decoder_state_input_c = Input(shape=(latent_dim,))
    decoder_states_inputs = [decoder_state_input_h, decoder_state_input_c]
    decoder_outputs, state_h, state_c = decoder_lstm(
        decoder_inputs, initial_state=decoder_states_inputs)
    decoder_states = [state_h, state_c]
    decoder_outputs = decoder_dense(decoder_outputs)
    decoder_model = Model(
        [decoder_inputs] + decoder_states_inputs,
        [decoder_outputs] + decoder_states)

    # Reverse-lookup token index to decode sequences back to
    # something readable.
    reverse_input_char_index = dict(
        (i, char) for char, i in input_token_index.items())
    reverse_target_char_index = dict(
        (i, char) for char, i in target_token_index.items())

    def decode_sequence(input_seq):
        # Encode the input as state vectors.
        states_value = encoder_model.predict(input_seq)

        # Generate empty target sequence of length 1.
        target_seq = np.zeros((1, 1, num_decoder_tokens))
        # Populate the first character of target sequence with the start character.
        target_seq[0, 0, target_token_index['\t']] = 1.

        # Sampling loop for a batch of sequences
        # (to simplify, here we assume a batch of size 1).
        stop_condition = False
        decoded_sentence = ''
        while not stop_condition:
            output_tokens, h, c = decoder_model.predict(
                [target_seq] + states_value)

            # Sample a token
            sampled_token_index = np.argmax(output_tokens[0, -1, :])
            sampled_char = reverse_target_char_index[sampled_token_index]
            decoded_sentence += sampled_char

            # Exit condition: either hit max length
            # or find stop character.
            if (sampled_char == '\n' or
               len(decoded_sentence) > max_decoder_seq_length):
                stop_condition = True

            # Update the target sequence (of length 1).
            target_seq = np.zeros((1, 1, num_decoder_tokens))
            target_seq[0, 0, sampled_token_index] = 1.

            # Update states
            states_value = [h, c]

        return decoded_sentence

    for seq_index in range(100):
        # Take one sequence (part of the training set)
        # for trying out decoding.
        input_seq = encoder_input_data[seq_index: seq_index + 1]
        decoded_sentence = decode_sequence(input_seq)
        print('-')
        print('Input sentence:', input_texts[seq_index])
        print('Decoded sentence:', decoded_sentence)

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