# Natural Language Toolkit: Language Models
#
# Copyright (C) 2001-2020 NLTK Project
# Authors: Ilia Kurenkov <ilia.kurenkov@gmail.com>
# URL: <http://nltk.org/>
# For license information, see LICENSE.TXT
"""Language Model Interface."""

import random
from abc import ABCMeta, abstractmethod
from bisect import bisect


from nltk.lm.counter import NgramCounter
from nltk.lm.util import log_base2
from nltk.lm.vocabulary import Vocabulary

from itertools import accumulate


class Smoothing(metaclass=ABCMeta):
    """Ngram Smoothing Interface

    Implements Chen & Goodman 1995's idea that all smoothing algorithms have
    certain features in common. This should ideally allow smoothing algorithms to
    work both with Backoff and Interpolation.
    """

    def __init__(self, vocabulary, counter):
        """
        :param vocabulary: The Ngram vocabulary object.
        :type vocabulary: nltk.lm.vocab.Vocabulary
        :param counter: The counts of the vocabulary items.
        :type counter: nltk.lm.counter.NgramCounter
        """
        self.vocab = vocabulary
        self.counts = counter

    @abstractmethod
    def unigram_score(self, word):
        raise NotImplementedError()

    @abstractmethod
    def alpha_gamma(self, word, context):
        raise NotImplementedError()


def _mean(items):
    """Return average (aka mean) for sequence of items."""
    return sum(items) / len(items)


def _random_generator(seed_or_generator):
    if isinstance(seed_or_generator, random.Random):
        return seed_or_generator
    return random.Random(seed_or_generator)


def _weighted_choice(population, weights, random_generator=None):
    """Like random.choice, but with weights.

    Heavily inspired by python 3.6 `random.choices`.
    """
    if not population:
        raise ValueError("Can't choose from empty population")
    if len(population) != len(weights):
        raise ValueError("The number of weights does not match the population")
    cum_weights = list(accumulate(weights))
    total = cum_weights[-1]
    threshold = random_generator.random()
    return population[bisect(cum_weights, total * threshold)]


class LanguageModel(metaclass=ABCMeta):
    """ABC for Language Models.

    Cannot be directly instantiated itself.

    """

    def __init__(self, order, vocabulary=None, counter=None):
        """Creates new LanguageModel.

        :param vocabulary: If provided, this vocabulary will be used instead
        of creating a new one when training.
        :type vocabulary: `nltk.lm.Vocabulary` or None
        :param counter: If provided, use this object to count ngrams.
        :type vocabulary: `nltk.lm.NgramCounter` or None
        :param ngrams_fn: If given, defines how sentences in training text are turned to ngram
                          sequences.
        :type ngrams_fn: function or None
        :param pad_fn: If given, defines how senteces in training text are padded.
        :type pad_fn: function or None

        """
        self.order = order
        self.vocab = Vocabulary() if vocabulary is None else vocabulary
        self.counts = NgramCounter() if counter is None else counter

    def fit(self, text, vocabulary_text=None):
        """Trains the model on a text.

        :param text: Training text as a sequence of sentences.

        """
        if not self.vocab:
            if vocabulary_text is None:
                raise ValueError(
                    "Cannot fit without a vocabulary or text to create it from."
                )
            self.vocab.update(vocabulary_text)
        self.counts.update(self.vocab.lookup(sent) for sent in text)

    def score(self, word, context=None):
        """Masks out of vocab (OOV) words and computes their model score.

        For model-specific logic of calculating scores, see the `unmasked_score`
        method.
        """
        return self.unmasked_score(
            self.vocab.lookup(word), self.vocab.lookup(context) if context else None
        )

    @abstractmethod
    def unmasked_score(self, word, context=None):
        """Score a word given some optional context.

        Concrete models are expected to provide an implementation.
        Note that this method does not mask its arguments with the OOV label.
        Use the `score` method for that.

        :param str word: Word for which we want the score
        :param tuple(str) context: Context the word is in.
        If `None`, compute unigram score.
        :param context: tuple(str) or None
        :rtype: float

        """
        raise NotImplementedError()

    def logscore(self, word, context=None):
        """Evaluate the log score of this word in this context.

        The arguments are the same as for `score` and `unmasked_score`.

        """
        return log_base2(self.score(word, context))

    def context_counts(self, context):
        """Helper method for retrieving counts for a given context.

        Assumes context has been checked and oov words in it masked.
        :type context: tuple(str) or None

        """
        return (
            self.counts[len(context) + 1][context] if context else self.counts.unigrams
        )

    def entropy(self, text_ngrams):
        """Calculate cross-entropy of model for given evaluation text.

        :param Iterable(tuple(str)) text_ngrams: A sequence of ngram tuples.
        :rtype: float

        """
        return -1 * _mean(
            [self.logscore(ngram[-1], ngram[:-1]) for ngram in text_ngrams]
        )

    def perplexity(self, text_ngrams):
        """Calculates the perplexity of the given text.

        This is simply 2 ** cross-entropy for the text, so the arguments are the same.

        """
        return pow(2.0, self.entropy(text_ngrams))

    def generate(self, num_words=1, text_seed=None, random_seed=None):
        """Generate words from the model.

        :param int num_words: How many words to generate. By default 1.
        :param text_seed: Generation can be conditioned on preceding context.
        :param random_seed: A random seed or an instance of `random.Random`. If provided,
        makes the random sampling part of generation reproducible.
        :return: One (str) word or a list of words generated from model.

        Examples:

        >>> from nltk.lm import MLE
        >>> lm = MLE(2)
        >>> lm.fit([[("a", "b"), ("b", "c")]], vocabulary_text=['a', 'b', 'c'])
        >>> lm.fit([[("a",), ("b",), ("c",)]])
        >>> lm.generate(random_seed=3)
        'a'
        >>> lm.generate(text_seed=['a'])
        'b'

        """
        text_seed = [] if text_seed is None else list(text_seed)
        random_generator = _random_generator(random_seed)
        # This is the base recursion case.
        if num_words == 1:
            context = (
                text_seed[-self.order + 1 :]
                if len(text_seed) >= self.order
                else text_seed
            )
            samples = self.context_counts(self.vocab.lookup(context))
            while context and not samples:
                context = context[1:] if len(context) > 1 else []
                samples = self.context_counts(self.vocab.lookup(context))
            # Sorting samples achieves two things:
            # - reproducible randomness when sampling
            # - turns Mapping into Sequence which `_weighted_choice` expects
            samples = sorted(samples)
            return _weighted_choice(
                samples,
                tuple(self.score(w, context) for w in samples),
                random_generator,
            )
        # We build up text one word at a time using the preceding context.
        generated = []
        for _ in range(num_words):
            generated.append(
                self.generate(
                    num_words=1,
                    text_seed=text_seed + generated,
                    random_seed=random_generator,
                )
            )
        return generated
