lstm_seq2seq.py

'''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 correspding 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.
    Is 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.
http://www.manythings.org/anki/fra-eng.zip
Lots of neat sentence pairs datasets can be found at:
http://www.manythings.org/anki/
# References
- Sequence to Sequence Learning with Neural Networks
    https://arxiv.org/abs/1409.3215
- Learning Phrase Representations using
    RNN Encoder-Decoder for Statistical Machine Translation
    https://arxiv.org/abs/1406.1078
'''

from __future__ import print_function

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

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'
data_path = 'fra.txt'          # what is "fra.txt"!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

# Vectorize the data.
input_texts = []
target_texts = []
input_characters = set()
target_characters = set()
with io.open(data_path, 'r', encoding='utf-8') as f:
# with open(data_path, 'r') 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.
#     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.

# # Define an input sequence and process it.
# encoder_inputs = Input(shape=(None, num_encoder_tokens)) #tensor
# 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')    #loss, optimization
# model.fit([encoder_input_data, decoder_input_data], decoder_target_data, #training
#           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)