"""Class to perform under-sampling based on one-sided selection method."""

# Authors: Guillaume Lemaitre <g.lemaitre58@gmail.com>
#          Christos Aridas
# License: MIT

from collections import Counter

import numpy as np

from sklearn.base import clone
from sklearn.neighbors import KNeighborsClassifier
from sklearn.utils import check_random_state, _safe_indexing

from ..base import BaseCleaningSampler
from ._tomek_links import TomekLinks
from ...utils import Substitution
from ...utils._docstring import _n_jobs_docstring
from ...utils._docstring import _random_state_docstring
from ...utils._validation import _deprecate_positional_args


@Substitution(
    sampling_strategy=BaseCleaningSampler._sampling_strategy_docstring,
    n_jobs=_n_jobs_docstring,
    random_state=_random_state_docstring,
)
class OneSidedSelection(BaseCleaningSampler):
    """Class to perform under-sampling based on one-sided selection method.

    Read more in the :ref:`User Guide <condensed_nearest_neighbors>`.

    Parameters
    ----------
    {sampling_strategy}

    {random_state}

    n_neighbors : int or estimator object, default=None
        If ``int``, size of the neighbourhood to consider to compute the
        nearest neighbors. If object, an estimator that inherits from
        :class:`~sklearn.neighbors.base.KNeighborsMixin` that will be used to
        find the nearest-neighbors. If `None`, a
        :class:`~sklearn.neighbors.KNeighborsClassifier` with a 1-NN rules will
        be used.

    n_seeds_S : int, default=1
        Number of samples to extract in order to build the set S.

    {n_jobs}

    Attributes
    ----------
    sampling_strategy_ : dict
        Dictionary containing the information to sample the dataset. The keys
        corresponds to the class labels from which to sample and the values
        are the number of samples to sample.

    estimator_ : estimator object
        Validated K-nearest neighbors estimator created from parameter `n_neighbors`.

    sample_indices_ : ndarray of shape (n_new_samples,)
        Indices of the samples selected.

        .. versionadded:: 0.4

    n_features_in_ : int
        Number of features in the input dataset.

        .. versionadded:: 0.9

    See Also
    --------
    EditedNearestNeighbours : Undersample by editing noisy samples.

    Notes
    -----
    The method is based on [1]_.

    Supports multi-class resampling. A one-vs.-one scheme is used when sampling
    a class as proposed in [1]_. For each class to be sampled, all samples of
    this class and the minority class are used during the sampling procedure.

    References
    ----------
    .. [1] M. Kubat, S. Matwin, "Addressing the curse of imbalanced training
       sets: one-sided selection," In ICML, vol. 97, pp. 179-186, 1997.

    Examples
    --------

    >>> from collections import Counter
    >>> from sklearn.datasets import make_classification
    >>> from imblearn.under_sampling import \
    OneSidedSelection # doctest: +NORMALIZE_WHITESPACE
    >>> X, y = make_classification(n_classes=2, class_sep=2,
    ... weights=[0.1, 0.9], n_informative=3, n_redundant=1, flip_y=0,
    ... n_features=20, n_clusters_per_class=1, n_samples=1000, random_state=10)
    >>> print('Original dataset shape %s' % Counter(y))
    Original dataset shape Counter({{1: 900, 0: 100}})
    >>> oss = OneSidedSelection(random_state=42)
    >>> X_res, y_res = oss.fit_resample(X, y)
    >>> print('Resampled dataset shape %s' % Counter(y_res))
    Resampled dataset shape Counter({{1: 496, 0: 100}})
    """

    @_deprecate_positional_args
    def __init__(
        self,
        *,
        sampling_strategy="auto",
        random_state=None,
        n_neighbors=None,
        n_seeds_S=1,
        n_jobs=None,
    ):
        super().__init__(sampling_strategy=sampling_strategy)
        self.random_state = random_state
        self.n_neighbors = n_neighbors
        self.n_seeds_S = n_seeds_S
        self.n_jobs = n_jobs

    def _validate_estimator(self):
        """Private function to create the NN estimator"""
        if self.n_neighbors is None:
            self.estimator_ = KNeighborsClassifier(n_neighbors=1, n_jobs=self.n_jobs)
        elif isinstance(self.n_neighbors, int):
            self.estimator_ = KNeighborsClassifier(
                n_neighbors=self.n_neighbors, n_jobs=self.n_jobs
            )
        elif isinstance(self.n_neighbors, KNeighborsClassifier):
            self.estimator_ = clone(self.n_neighbors)
        else:
            raise ValueError(
                f"`n_neighbors` has to be a int or an object"
                f" inherited from KNeighborsClassifier."
                f" Got {type(self.n_neighbors)} instead."
            )

    def _fit_resample(self, X, y):
        self._validate_estimator()

        random_state = check_random_state(self.random_state)
        target_stats = Counter(y)
        class_minority = min(target_stats, key=target_stats.get)

        idx_under = np.empty((0,), dtype=int)

        for target_class in np.unique(y):
            if target_class in self.sampling_strategy_.keys():
                # select a sample from the current class
                idx_maj = np.flatnonzero(y == target_class)
                sel_idx_maj = random_state.randint(
                    low=0, high=target_stats[target_class], size=self.n_seeds_S
                )
                idx_maj_sample = idx_maj[sel_idx_maj]

                minority_class_indices = np.flatnonzero(y == class_minority)
                C_indices = np.append(minority_class_indices, idx_maj_sample)

                # create the set composed of all minority samples and one
                # sample from the current class.
                C_x = _safe_indexing(X, C_indices)
                C_y = _safe_indexing(y, C_indices)

                # create the set S with removing the seed from S
                # since that it will be added anyway
                idx_maj_extracted = np.delete(idx_maj, sel_idx_maj, axis=0)
                S_x = _safe_indexing(X, idx_maj_extracted)
                S_y = _safe_indexing(y, idx_maj_extracted)
                self.estimator_.fit(C_x, C_y)
                pred_S_y = self.estimator_.predict(S_x)

                S_misclassified_indices = np.flatnonzero(pred_S_y != S_y)
                idx_tmp = idx_maj_extracted[S_misclassified_indices]
                idx_under = np.concatenate((idx_under, idx_maj_sample, idx_tmp), axis=0)
            else:
                idx_under = np.concatenate(
                    (idx_under, np.flatnonzero(y == target_class)), axis=0
                )

        X_resampled = _safe_indexing(X, idx_under)
        y_resampled = _safe_indexing(y, idx_under)

        # apply Tomek cleaning
        tl = TomekLinks(sampling_strategy=list(self.sampling_strategy_.keys()))
        X_cleaned, y_cleaned = tl.fit_resample(X_resampled, y_resampled)

        self.sample_indices_ = _safe_indexing(idx_under, tl.sample_indices_)

        return X_cleaned, y_cleaned

    def _more_tags(self):
        return {"sample_indices": True}
