# coding:utf-8
# Copyright (c) 2019  PaddlePaddle Authors. All Rights Reserved.
#
# 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.

import collections
import math
from typing import List


def _get_ngrams(segment: str, max_order: int):
    """
    Extracts all n-grams upto a given maximum order from an input segment.

    Args:
        segment: text segment from which n-grams will be extracted.
        max_order: maximum length in tokens of the n-grams returned by this
            methods.

    Returns:
        The Counter containing all n-grams upto max_order in segment
        with a count of how many times each n-gram occurred.
    """
    ngram_counts = collections.Counter()
    for order in range(1, max_order + 1):
        for i in range(0, len(segment) - order + 1):
            ngram = tuple(segment[i:i + order])
            ngram_counts[ngram] += 1
    return ngram_counts


def compute_bleu(reference_corpus: List, translation_corpus: List, max_order: int = 4, smooth: bool = False):
    '''
    Computes BLEU score of translated segments against one or more references.
    Args:
        reference_corpus: list of lists of references for each translation. Each
            reference should be tokenized into a list of tokens.
        translation_corpus: list of translations to score. Each translation
            should be tokenized into a list of tokens.
        max_order: Maximum n-gram order to use when computing BLEU score.
        smooth: Whether or not to apply Lin et al. 2004 smoothing.
    Returns:
        3-Tuple with the BLEU score, n-gram precisions, geometric mean of n-gram
        precisions and brevity penalty.
    '''
    matches_by_order = [0] * max_order
    possible_matches_by_order = [0] * max_order
    reference_length = 0
    translation_length = 0
    for (reference, translation) in zip(reference_corpus, translation_corpus):
        reference_length += len(reference)
        translation_length += len(translation)

        merged_ref_ngram_counts = collections.Counter()
        merged_ref_ngram_counts |= _get_ngrams(reference, max_order)
        translation_ngram_counts = _get_ngrams(translation, max_order)
        overlap = translation_ngram_counts & merged_ref_ngram_counts
        for ngram in overlap:
            matches_by_order[len(ngram) - 1] += overlap[ngram]
        for order in range(1, max_order + 1):
            possible_matches = len(translation) - order + 1
            if possible_matches > 0:
                possible_matches_by_order[order - 1] += possible_matches

    precisions = [0] * max_order
    for i in range(0, max_order):
        if smooth:
            precisions[i] = ((matches_by_order[i] + 1.) / (possible_matches_by_order[i] + 1.))
        else:
            if possible_matches_by_order[i] > 0:
                precisions[i] = (float(matches_by_order[i]) / possible_matches_by_order[i])
            else:
                precisions[i] = 0.0

    if min(precisions) > 0:
        p_log_sum = sum((1. / max_order) * math.log(p) for p in precisions)
        geo_mean = math.exp(p_log_sum)
    else:
        geo_mean = 0

    ratio = float(translation_length) / reference_length

    if ratio > 1.0:
        bp = 1.
    elif ratio > 0.0:
        bp = math.exp(1 - 1. / ratio)
    else:
        bp = 0

    bleu = geo_mean * bp

    return (bleu, precisions, bp, ratio, translation_length, reference_length)