def build_language_model(self):
"""
creates the language model for our target language and saves it in a class
field
"""
all_data = []
for e_sentence,_ in self.aligned_sentences:
all_data.append(e_sentence.data)
self.language_model = BigramModel(all_data, self.e_alphabet)
self.language_model.train()
class TranslationModel:
def __init__(self, aligned_sentences, max_iterations=-1, eta=None):
"""
:param aligned_sentences: a list of tuples of aligned sentences
:param max_iterations: the number of iterations to run EM
:param eta: the value that the delta of the EM probabilities must fall below to be considered converged
"""
self.aligned_sentences = aligned_sentences
self.e_alphabet = Alphabet()
self.f_alphabet = Alphabet()
if eta is None:
# very simple heuristic
self.eta = len(aligned_sentences)/100.
else:
self.eta = eta
self.max_iterations = max_iterations
if max_iterations == -1:
self.do_more = self.has_converged
else:
self.do_more = self.stop_iterations
def convert_to_vector(self, raw_data, lang="e", training=True):
"""
:param raw_data: a tokenized sentence
:param lang: whether it's source or target
:param training: whether this is during training or testing
:return: numpy array of the integers corresponding to words
"""
if lang == "e":
alphabet = self.e_alphabet
else:
alphabet = self.f_alphabet
if training:
return numpy.array(map(alphabet.get_index, raw_data), dtype=int)
else:
vector = numpy.zeros(len(raw_data), dtype=int)
for i,word in enumerate(raw_data):
try:
vector[i] = alphabet.get_index(word)
except KeyError:
continue #ignoring OOV words
return vector
def populate_alphabets(self):
"""
Populates the alphabets so that the tokens can have an integer
representation. Also converts the sentences into this format.
"""
for e_instance,f_instance in self.aligned_sentences:
for i,token in enumerate(e_instance.raw_data):
self.e_alphabet.add(token)
for i,token in enumerate(f_instance.raw_data):
self.f_alphabet.add(token)
e_instance.data = self.convert_to_vector(e_instance.raw_data, "e")
f_instance.data = self.convert_to_vector(f_instance.raw_data, "f")
def init_translation_table(self):
"""
Sets up the class field of the translation table and the cache of
the previous table in order to do the initial delta.
Initializes the probability of everything at 0.25
"""
self.t_table = numpy.zeros([self.e_alphabet.size(), self.f_alphabet.size()])
self.previous_t_table = numpy.zeros([self.e_alphabet.size(), self.f_alphabet.size()])
self.t_table.fill(.25)
def expectation_maximization(self):
"""
runs the EM algorithm for a specific number of iterations or until
it has converged.
"""
i = 0
while not self.do_more(i):
time1 = time.time()
print "iteration {:d}".format(i+1),
# initialize
self.total = numpy.zeros(self.f_alphabet.size())
self.counts = numpy.zeros([self.e_alphabet.size(), self.f_alphabet.size()])
for e_instance,f_instance in self.aligned_sentences:
self.s_total = numpy.zeros(self.e_alphabet.size())
# compute normalization
for e_word in e_instance.data:
self.s_total[e_word] += numpy.sum(self.t_table[e_word, f_instance.data])
# collect counts
for e in e_instance.data:
tmp = self.t_table[e,f_instance.data]/self.s_total[e]
self.counts[e,f_instance.data] += tmp
self.total[f_instance.data] += tmp
# estimate probabilities
self.t_table = self.counts/self.total
i += 1
print "\t{:.3f} seconds".format(time.time()-time1),
def has_converged(self, i):
"""
calculates the delta, sees if it is lower than eta
@param i: only used so this method can have the same signature as stop_iterations
@return: a boolean whether the EM iterations need to stop
"""
delta = numpy.sum(numpy.abs(self.t_table - self.previous_t_table))
self.previous_t_table = numpy.copy(self.t_table)
if i != 0:
print "\tdelta: {:.3f}".format(delta)
if delta < self.eta:
return True
return False
def stop_iterations(self, i):
"""
@param i: current iteration nubmer
@return: boolean whether EM need to stop iterating
"""
print
return i >= self.max_iterations
def train(self):
"""
does all tasks necessary to train our model
"""
self.populate_alphabets()
self.init_translation_table()
self.expectation_maximization()
self.build_language_model()
def evaluate(self, candidate_data):
"""
given candidate translations, this will select the best according to
our translation table and language model. prints the source sentence
and best candidate, which is argmax(p(t|s))
:param candidate_data: a list with a source sentence and translation candidates
"""
for (source, source_sent_tokenized), candidates in candidate_data:
candidate_scores = numpy.zeros(len(candidates))
for i,(c_sent, c) in enumerate(candidates):
candidate_scores[i] = self.translation_log_prob(c, source_sent_tokenized)
print u"source sentence: {:s}".format(source)
print u"best translation: {:s}".format(candidates[numpy.argmax(candidate_scores)][0])
print
def t_table_log_prob(self, e_sentence, f_sentence):
"""
gives the log(p(s|t))
:param e_sentence: tokenized target sentence
:param f_sentence: tokenized candidate sentence
:return: the log probability of a sentence translating to a candidate
"""
e_sentence = self.convert_to_vector(e_sentence, lang="e", training=False)
f_sentence = self.convert_to_vector(f_sentence, lang="f", training=False)
product = 1.
for e in e_sentence:
product *= numpy.sum(self.t_table[e,f_sentence])
return numpy.log(product/(len(f_sentence)**len(e_sentence)))
def build_language_model(self):
"""
creates the language model for our target language and saves it in a class
field
"""
all_data = []
for e_sentence,_ in self.aligned_sentences:
all_data.append(e_sentence.data)
self.language_model = BigramModel(all_data, self.e_alphabet)
self.language_model.train()
def translation_log_prob(self, e_sentence, f_sentence):
"""
:param e_sentence: tokenized target sentence
:param f_sentence: tokenized source sentence
:return: the log(p(s|t)*p())
"""
return self.t_table_log_prob(e_sentence, f_sentence) + self.language_model.log_prob(e_sentence)
def save(self):
raise NotImplementedError
def load(self, data):
raise NotImplementedError
