# Copyright 2021 Huawei Technologies Co., Ltd
#
# Licensed under the Apache License, Version 2.0 (the "License");
# you may not use this file except in compliance with the License.
# You may obtain a copy of the License at
#
# http://www.apache.org/licenses/LICENSE-2.0
#
# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS,
# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
# See the License for the specific language governing permissions and
# limitations under the License.
# ============================================================================
"""BleuScore."""
from collections import Counter
import numpy as np
from mindspore._checkparam import Validator as validator
from .metric import Metric, rearrange_inputs
[docs]class BleuScore(Metric):
"""
Calculates BLEU score of machine translated text with one or more references.
Args:
n_gram (int): The n_gram value ranged from 1 to 4. Default: 4.
smooth (bool): Whether or not to apply smoothing. Default: False.
Raises:
ValueError: If the value range of n_gram is not 1 to 4.
Supported Platforms:
``Ascend`` ``GPU`` ``CPU``
Example:
>>> candidate_corpus = [['i', 'have', 'a', 'pen', 'on', 'my', 'desk']]
>>> reference_corpus = [[['i', 'have', 'a', 'pen', 'in', 'my', 'desk'],
... ['there', 'is', 'a', 'pen', 'on', 'the', 'desk']]]
>>> metric = BleuScore()
>>> metric.clear()
>>> metric.update(candidate_corpus, reference_corpus)
>>> bleu_score = metric.eval()
>>> print(bleu_score)
0.5946035575013605
"""
def __init__(self, n_gram=4, smooth=False):
super().__init__()
self.n_gram = validator.check_value_type("n_gram", n_gram, [int])
if self.n_gram > 4 or self.n_gram < 1:
raise ValueError('The n_gram value ranged from 1 to 4, but got {}'.format(n_gram))
self.smooth = validator.check_value_type("smooth", smooth, [bool])
self.clear()
[docs] def clear(self):
"""Clear the internal evaluation result."""
self._numerator = np.zeros(self.n_gram)
self._denominator = np.zeros(self.n_gram)
self._precision_scores = np.zeros(self.n_gram)
self._c = 0.0
self._r = 0.0
self._trans_len = 0
self._ref_len = 0
self._is_update = False
def _count_ngram(self, ngram_input_list, n_gram):
"""
Counting how many times each word appears in a given text with ngram.
Args:
ngram_input_list (list): A list of translated text or reference texts.
n_gram (int): gram value ranged 1 to 4.
Return:
ngram_counter: a collections.Counter object of ngram.
"""
ngram_counter = Counter()
for i in range(1, n_gram + 1):
for j in range(len(ngram_input_list) - i + 1):
ngram_key = tuple(ngram_input_list[j:(i + j)])
ngram_counter[ngram_key] += 1
return ngram_counter
[docs] @rearrange_inputs
def update(self, *inputs):
"""
Updates the internal evaluation result with `candidate_corpus` and `reference_corpus`.
Args:
inputs: Input `candidate_corpus` and `reference_corpus`. `candidate_corpus` and `reference_corpus` are a
list. The `candidate_corpus` is an iterable of machine translated corpus. The `reference_corpus` is
an iterable of iterables of reference corpus.
Raises:
ValueError: If the number of input is not 2.
"""
if len(inputs) != 2:
raise ValueError('The bleu_score need 2 inputs (candidate_corpus, reference_corpus), '
'but got {}'.format(len(inputs)))
candidate_corpus = inputs[0]
reference_corpus = inputs[1]
if len(candidate_corpus) != len(reference_corpus):
raise ValueError('translate_corpus and reference_corpus should be equal in length, '
'but got {} {}'.format(len(candidate_corpus), len(reference_corpus)))
for (candidate, references) in zip(candidate_corpus, reference_corpus):
self._c += len(candidate)
ref_len_list = [len(ref) for ref in references]
ref_len_diff = [abs(len(candidate) - x) for x in ref_len_list]
self._r += ref_len_list[ref_len_diff.index(min(ref_len_diff))]
translation_counter = self._count_ngram(candidate, self.n_gram)
reference_counter = Counter()
for ref in references:
reference_counter |= self._count_ngram(ref, self.n_gram)
ngram_counter_clip = translation_counter & reference_counter
for counter_clip in ngram_counter_clip:
self._numerator[len(counter_clip) - 1] += ngram_counter_clip[counter_clip]
for counter in translation_counter:
self._denominator[len(counter) - 1] += translation_counter[counter]
self._trans_len = np.array(self._c)
self._ref_len = np.array(self._r)
self._is_update = True
[docs] def eval(self):
"""
Computes the bleu score.
Returns:
A numpy with bleu score.
Raises:
RuntimeError: If the update method is not called first, an error will be reported.
"""
if self._is_update is False:
raise RuntimeError('Call the update method before calling eval.')
if min(self._numerator) == 0.0:
return np.array(0.0)
if self.smooth:
precision_scores = np.add(self._numerator, np.ones(self.n_gram)) / np.add(self._denominator,
np.ones(self.n_gram))
else:
precision_scores = self._numerator / self._denominator
log_precision_scores = np.array([1.0 / self.n_gram] * self.n_gram) * np.log(precision_scores)
geometric_mean = np.exp(np.sum(log_precision_scores))
brevity_penalty = np.array(1.0) if self._c > self._r else np.exp(1 - (self._ref_len / self._trans_len))
bleu = brevity_penalty * geometric_mean
return bleu