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bleu.py
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bleu.py
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# Copyright The Lightning team.
#
# 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.
# referenced from
# Library Name: torchtext
# Authors: torchtext authors and @sluks
# Date: 2020-07-18
# Link: https://pytorch.org/text/_modules/torchtext/data/metrics.html#bleu_score
from collections import Counter
from typing import Callable, Optional, Sequence, Tuple, Union
import torch
from torch import Tensor, tensor
def _count_ngram(ngram_input_list: Sequence[str], n_gram: int) -> Counter:
"""Count how many times each word appears in a given text with ngram.
Args:
ngram_input_list: A list of translated text or reference texts
n_gram: gram value ranged 1 to 4
Return:
ngram_counter: a collections.Counter object of ngram
"""
ngram_counter: 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
def _tokenize_fn(sentence: str) -> Sequence[str]:
"""Tokenizes sentence into list of words.
Args:
sentence: A sentence separated by white space.
Return:
List of words
"""
return sentence.split()
def _bleu_score_update(
preds: Sequence[str],
target: Sequence[Sequence[str]],
numerator: Tensor,
denominator: Tensor,
preds_len: Tensor,
target_len: Tensor,
n_gram: int = 4,
tokenizer: Callable[[str], Sequence[str]] = _tokenize_fn,
) -> Tuple[Tensor, Tensor]:
"""Update and returns variables required to compute the BLEU score.
Args:
preds: An iterable of machine translated corpus
target: An iterable of iterables of reference corpus
numerator: Numerator of precision score (true positives)
denominator: Denominator of precision score (true positives + false positives)
preds_len: count of words in a candidate prediction
target_len: count of words in a reference translation
target: count of words in a reference translation
n_gram: gram value ranged 1 to 4
tokenizer: A function that turns sentence into list of words
"""
target_: Sequence[Sequence[Sequence[str]]] = [[tokenizer(line) if line else [] for line in t] for t in target]
preds_: Sequence[Sequence[str]] = [tokenizer(line) if line else [] for line in preds]
for pred, targets in zip(preds_, target_):
preds_len += len(pred)
target_len_list = [len(tgt) for tgt in targets]
target_len_diff = [abs(len(pred) - x) for x in target_len_list]
target_len += target_len_list[target_len_diff.index(min(target_len_diff))]
preds_counter: Counter = _count_ngram(pred, n_gram)
target_counter: Counter = Counter()
for tgt in targets:
target_counter |= _count_ngram(tgt, n_gram)
ngram_counter_clip = preds_counter & target_counter
for counter_clip in ngram_counter_clip:
numerator[len(counter_clip) - 1] += ngram_counter_clip[counter_clip]
for counter in preds_counter:
denominator[len(counter) - 1] += preds_counter[counter]
return preds_len, target_len
def _bleu_score_compute(
preds_len: Tensor,
target_len: Tensor,
numerator: Tensor,
denominator: Tensor,
n_gram: int,
weights: Sequence[float],
smooth: bool,
) -> Tensor:
"""Compute the BLEU score.
Args:
preds_len: count of words in a candidate translation
target_len: count of words in a reference translation
numerator: Numerator of precision score (true positives)
denominator: Denominator of precision score (true positives + false positives)
n_gram: gram value ranged 1 to 4
weights: Weights used for unigrams, bigrams, etc. to calculate BLEU score.
smooth: Whether to apply smoothing
"""
device = numerator.device
if min(numerator) == 0.0:
return tensor(0.0, device=device)
if smooth:
precision_scores = torch.div(
torch.add(numerator, torch.ones(n_gram, device=device)),
torch.add(denominator, torch.ones(n_gram, device=device)),
)
precision_scores[0] = numerator[0] / denominator[0]
else:
precision_scores = numerator / denominator
log_precision_scores = tensor(weights, device=device) * torch.log(precision_scores)
geometric_mean = torch.exp(torch.sum(log_precision_scores))
brevity_penalty = tensor(1.0, device=device) if preds_len > target_len else torch.exp(1 - (target_len / preds_len))
return brevity_penalty * geometric_mean
def bleu_score(
preds: Union[str, Sequence[str]],
target: Sequence[Union[str, Sequence[str]]],
n_gram: int = 4,
smooth: bool = False,
weights: Optional[Sequence[float]] = None,
) -> Tensor:
"""Calculate `BLEU score`_ of machine translated text with one or more references.
Args:
preds: An iterable of machine translated corpus
target: An iterable of iterables of reference corpus
n_gram: Gram value ranged from 1 to 4
smooth: Whether to apply smoothing - see [2]
weights:
Weights used for unigrams, bigrams, etc. to calculate BLEU score.
If not provided, uniform weights are used.
Return:
Tensor with BLEU Score
Raises:
ValueError: If ``preds`` and ``target`` corpus have different lengths.
ValueError: If a length of a list of weights is not ``None`` and not equal to ``n_gram``.
Example:
>>> from torchmetrics.functional.text import bleu_score
>>> preds = ['the cat is on the mat']
>>> target = [['there is a cat on the mat', 'a cat is on the mat']]
>>> bleu_score(preds, target)
tensor(0.7598)
References:
[1] BLEU: a Method for Automatic Evaluation of Machine Translation by Papineni,
Kishore, Salim Roukos, Todd Ward, and Wei-Jing Zhu `BLEU`_
[2] Automatic Evaluation of Machine Translation Quality Using Longest Common Subsequence
and Skip-Bigram Statistics by Chin-Yew Lin and Franz Josef Och `Machine Translation Evolution`_
"""
preds_ = [preds] if isinstance(preds, str) else preds
target_ = [[tgt] if isinstance(tgt, str) else tgt for tgt in target]
if len(preds_) != len(target_):
raise ValueError(f"Corpus has different size {len(preds_)} != {len(target_)}")
if weights is not None and len(weights) != n_gram:
raise ValueError(f"List of weights has different weights than `n_gram`: {len(weights)} != {n_gram}")
if weights is None:
weights = [1.0 / n_gram] * n_gram
numerator = torch.zeros(n_gram)
denominator = torch.zeros(n_gram)
preds_len = tensor(0.0)
target_len = tensor(0.0)
preds_len, target_len = _bleu_score_update(
preds_, target_, numerator, denominator, preds_len, target_len, n_gram, _tokenize_fn
)
return _bleu_score_compute(preds_len, target_len, numerator, denominator, n_gram, weights, smooth)