"""Nearest Neighbor Regression."""

# Authors: The scikit-learn developers
# SPDX-License-Identifier: BSD-3-Clause

import warnings

import numpy as np

from ..base import RegressorMixin, _fit_context
from ..metrics import DistanceMetric
from ..utils._param_validation import StrOptions
from ._base import KNeighborsMixin, NeighborsBase, RadiusNeighborsMixin, _get_weights


class KNeighborsRegressor(KNeighborsMixin, RegressorMixin, NeighborsBase):
    """Regression based on k-nearest neighbors.

    The target is predicted by local interpolation of the targets
    associated of the nearest neighbors in the training set.

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

    .. versionadded:: 0.9

    Parameters
    ----------
    n_neighbors : int, default=5
        Number of neighbors to use by default for :meth:`kneighbors` queries.

    weights : {'uniform', 'distance'}, callable or None, default='uniform'
        Weight function used in prediction.  Possible values:

        - 'uniform' : uniform weights.  All points in each neighborhood
          are weighted equally.
        - 'distance' : weight points by the inverse of their distance.
          in this case, closer neighbors of a query point will have a
          greater influence than neighbors which are further away.
        - [callable] : a user-defined function which accepts an
          array of distances, and returns an array of the same shape
          containing the weights.

        Uniform weights are used by default.

        See the following example for a demonstration of the impact of
        different weighting schemes on predictions:
        :ref:`sphx_glr_auto_examples_neighbors_plot_regression.py`.

    algorithm : {'auto', 'ball_tree', 'kd_tree', 'brute'}, default='auto'
        Algorithm used to compute the nearest neighbors:

        - 'ball_tree' will use :class:`BallTree`
        - 'kd_tree' will use :class:`KDTree`
        - 'brute' will use a brute-force search.
        - 'auto' will attempt to decide the most appropriate algorithm
          based on the values passed to :meth:`fit` method.

        Note: fitting on sparse input will override the setting of
        this parameter, using brute force.

    leaf_size : int, default=30
        Leaf size passed to BallTree or KDTree.  This can affect the
        speed of the construction and query, as well as the memory
        required to store the tree.  The optimal value depends on the
        nature of the problem.

    p : float, default=2
        Power parameter for the Minkowski metric. When p = 1, this is
        equivalent to using manhattan_distance (l1), and euclidean_distance
        (l2) for p = 2. For arbitrary p, minkowski_distance (l_p) is used.

    metric : str, DistanceMetric object or callable, default='minkowski'
        Metric to use for distance computation. Default is "minkowski", which
        results in the standard Euclidean distance when p = 2. See the
        documentation of `scipy.spatial.distance
        <https://docs.scipy.org/doc/scipy/reference/spatial.distance.html>`_ and
        the metrics listed in
        :class:`~sklearn.metrics.pairwise.distance_metrics` for valid metric
        values.

        If metric is "precomputed", X is assumed to be a distance matrix and
        must be square during fit. X may be a :term:`sparse graph`, in which
        case only "nonzero" elements may be considered neighbors.

        If metric is a callable function, it takes two arrays representing 1D
        vectors as inputs and must return one value indicating the distance
        between those vectors. This works for Scipy's metrics, but is less
        efficient than passing the metric name as a string.

        If metric is a DistanceMetric object, it will be passed directly to
        the underlying computation routines.

    metric_params : dict, default=None
        Additional keyword arguments for the metric function.

    n_jobs : int, default=None
        The number of parallel jobs to run for neighbors search.
        ``None`` means 1 unless in a :obj:`joblib.parallel_backend` context.
        ``-1`` means using all processors. See :term:`Glossary <n_jobs>`
        for more details.
        Doesn't affect :meth:`fit` method.

    Attributes
    ----------
    effective_metric_ : str or callable
        The distance metric to use. It will be same as the `metric` parameter
        or a synonym of it, e.g. 'euclidean' if the `metric` parameter set to
        'minkowski' and `p` parameter set to 2.

    effective_metric_params_ : dict
        Additional keyword arguments for the metric function. For most metrics
        will be same with `metric_params` parameter, but may also contain the
        `p` parameter value if the `effective_metric_` attribute is set to
        'minkowski'.

    n_features_in_ : int
        Number of features seen during :term:`fit`.

        .. versionadded:: 0.24

    feature_names_in_ : ndarray of shape (`n_features_in_`,)
        Names of features seen during :term:`fit`. Defined only when `X`
        has feature names that are all strings.

        .. versionadded:: 1.0

    n_samples_fit_ : int
        Number of samples in the fitted data.

    See Also
    --------
    NearestNeighbors : Unsupervised learner for implementing neighbor searches.
    RadiusNeighborsRegressor : Regression based on neighbors within a fixed radius.
    KNeighborsClassifier : Classifier implementing the k-nearest neighbors vote.
    RadiusNeighborsClassifier : Classifier implementing
        a vote among neighbors within a given radius.

    Notes
    -----
    See :ref:`Nearest Neighbors <neighbors>` in the online documentation
    for a discussion of the choice of ``algorithm`` and ``leaf_size``.

    .. warning::

       Regarding the Nearest Neighbors algorithms, if it is found that two
       neighbors, neighbor `k+1` and `k`, have identical distances but
       different labels, the results will depend on the ordering of the
       training data.

    https://en.wikipedia.org/wiki/K-nearest_neighbors_algorithm

    Examples
    --------
    >>> X = [[0], [1], [2], [3]]
    >>> y = [0, 0, 1, 1]
    >>> from sklearn.neighbors import KNeighborsRegressor
    >>> neigh = KNeighborsRegressor(n_neighbors=2)
    >>> neigh.fit(X, y)
    KNeighborsRegressor(...)
    >>> print(neigh.predict([[1.5]]))
    [0.5]
    """

    _parameter_constraints: dict = {
        **NeighborsBase._parameter_constraints,
        "weights": [StrOptions({"uniform", "distance"}), callable, None],
    }
    _parameter_constraints["metric"].append(DistanceMetric)
    _parameter_constraints.pop("radius")

    def __init__(
        self,
        n_neighbors=5,
        *,
        weights="uniform",
        algorithm="auto",
        leaf_size=30,
        p=2,
        metric="minkowski",
        metric_params=None,
        n_jobs=None,
    ):
        super().__init__(
            n_neighbors=n_neighbors,
            algorithm=algorithm,
            leaf_size=leaf_size,
            metric=metric,
            p=p,
            metric_params=metric_params,
            n_jobs=n_jobs,
        )
        self.weights = weights

    def __sklearn_tags__(self):
        tags = super().__sklearn_tags__()
        # For cross-validation routines to split data correctly
        tags.input_tags.pairwise = self.metric == "precomputed"
        return tags

    @_fit_context(
        # KNeighborsRegressor.metric is not validated yet
        prefer_skip_nested_validation=False
    )
    def fit(self, X, y):
        """Fit the k-nearest neighbors regressor from the training dataset.

        Parameters
        ----------
        X : {array-like, sparse matrix} of shape (n_samples, n_features) or \
                (n_samples, n_samples) if metric='precomputed'
            Training data.

        y : {array-like, sparse matrix} of shape (n_samples,) or \
                (n_samples, n_outputs)
            Target values.

        Returns
        -------
        self : KNeighborsRegressor
            The fitted k-nearest neighbors regressor.
        """
        return self._fit(X, y)

    def predict(self, X):
        """Predict the target for the provided data.

        Parameters
        ----------
        X : {array-like, sparse matrix} of shape (n_queries, n_features), \
                or (n_queries, n_indexed) if metric == 'precomputed', or None
            Test samples. If `None`, predictions for all indexed points are
            returned; in this case, points are not considered their own
            neighbors.

        Returns
        -------
        y : ndarray of shape (n_queries,) or (n_queries, n_outputs), dtype=int
            Target values.
        """
        if self.weights == "uniform":
            # In that case, we do not need the distances to perform
            # the weighting so we do not compute them.
            neigh_ind = self.kneighbors(X, return_distance=False)
            neigh_dist = None
        else:
            neigh_dist, neigh_ind = self.kneighbors(X)

        weights = _get_weights(neigh_dist, self.weights)

        _y = self._y
        if _y.ndim == 1:
            _y = _y.reshape((-1, 1))

        if weights is None:
            y_pred = np.mean(_y[neigh_ind], axis=1)
        else:
            y_pred = np.empty((neigh_dist.shape[0], _y.shape[1]), dtype=np.float64)
            denom = np.sum(weights, axis=1)

            for j in range(_y.shape[1]):
                num = np.sum(_y[neigh_ind, j] * weights, axis=1)
                y_pred[:, j] = num / denom

        if self._y.ndim == 1:
            y_pred = y_pred.ravel()

        return y_pred


class RadiusNeighborsRegressor(RadiusNeighborsMixin, RegressorMixin, NeighborsBase):
    """Regression based on neighbors within a fixed radius.

    The target is predicted by local interpolation of the targets
    associated of the nearest neighbors in the training set.

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

    .. versionadded:: 0.9

    Parameters
    ----------
    radius : float, default=1.0
        Range of parameter space to use by default for :meth:`radius_neighbors`
        queries.

    weights : {'uniform', 'distance'}, callable or None, default='uniform'
        Weight function used in prediction.  Possible values:

        - 'uniform' : uniform weights.  All points in each neighborhood
          are weighted equally.
        - 'distance' : weight points by the inverse of their distance.
          in this case, closer neighbors of a query point will have a
          greater influence than neighbors which are further away.
        - [callable] : a user-defined function which accepts an
          array of distances, and returns an array of the same shape
          containing the weights.

        Uniform weights are used by default.

    algorithm : {'auto', 'ball_tree', 'kd_tree', 'brute'}, default='auto'
        Algorithm used to compute the nearest neighbors:

        - 'ball_tree' will use :class:`BallTree`
        - 'kd_tree' will use :class:`KDTree`
        - 'brute' will use a brute-force search.
        - 'auto' will attempt to decide the most appropriate algorithm
          based on the values passed to :meth:`fit` method.

        Note: fitting on sparse input will override the setting of
        this parameter, using brute force.

    leaf_size : int, default=30
        Leaf size passed to BallTree or KDTree.  This can affect the
        speed of the construction and query, as well as the memory
        required to store the tree.  The optimal value depends on the
        nature of the problem.

    p : float, default=2
        Power parameter for the Minkowski metric. When p = 1, this is
        equivalent to using manhattan_distance (l1), and euclidean_distance
        (l2) for p = 2. For arbitrary p, minkowski_distance (l_p) is used.

    metric : str or callable, default='minkowski'
        Metric to use for distance computation. Default is "minkowski", which
        results in the standard Euclidean distance when p = 2. See the
        documentation of `scipy.spatial.distance
        <https://docs.scipy.org/doc/scipy/reference/spatial.distance.html>`_ and
        the metrics listed in
        :class:`~sklearn.metrics.pairwise.distance_metrics` for valid metric
        values.

        If metric is "precomputed", X is assumed to be a distance matrix and
        must be square during fit. X may be a :term:`sparse graph`, in which
        case only "nonzero" elements may be considered neighbors.

        If metric is a callable function, it takes two arrays representing 1D
        vectors as inputs and must return one value indicating the distance
        between those vectors. This works for Scipy's metrics, but is less
        efficient than passing the metric name as a string.

    metric_params : dict, default=None
        Additional keyword arguments for the metric function.

    n_jobs : int, default=None
        The number of parallel jobs to run for neighbors search.
        ``None`` means 1 unless in a :obj:`joblib.parallel_backend` context.
        ``-1`` means using all processors. See :term:`Glossary <n_jobs>`
        for more details.

    Attributes
    ----------
    effective_metric_ : str or callable
        The distance metric to use. It will be same as the `metric` parameter
        or a synonym of it, e.g. 'euclidean' if the `metric` parameter set to
        'minkowski' and `p` parameter set to 2.

    effective_metric_params_ : dict
        Additional keyword arguments for the metric function. For most metrics
        will be same with `metric_params` parameter, but may also contain the
        `p` parameter value if the `effective_metric_` attribute is set to
        'minkowski'.

    n_features_in_ : int
        Number of features seen during :term:`fit`.

        .. versionadded:: 0.24

    feature_names_in_ : ndarray of shape (`n_features_in_`,)
        Names of features seen during :term:`fit`. Defined only when `X`
        has feature names that are all strings.

        .. versionadded:: 1.0

    n_samples_fit_ : int
        Number of samples in the fitted data.

    See Also
    --------
    NearestNeighbors : Unsupervised learner for implementing neighbor searches.
    KNeighborsRegressor : Regression based on k-nearest neighbors.
    KNeighborsClassifier : Classifier based on the k-nearest neighbors.
    RadiusNeighborsClassifier : Classifier based on neighbors within a given radius.

    Notes
    -----
    See :ref:`Nearest Neighbors <neighbors>` in the online documentation
    for a discussion of the choice of ``algorithm`` and ``leaf_size``.

    https://en.wikipedia.org/wiki/K-nearest_neighbor_algorithm

    Examples
    --------
    >>> X = [[0], [1], [2], [3]]
    >>> y = [0, 0, 1, 1]
    >>> from sklearn.neighbors import RadiusNeighborsRegressor
    >>> neigh = RadiusNeighborsRegressor(radius=1.0)
    >>> neigh.fit(X, y)
    RadiusNeighborsRegressor(...)
    >>> print(neigh.predict([[1.5]]))
    [0.5]
    """

    _parameter_constraints: dict = {
        **NeighborsBase._parameter_constraints,
        "weights": [StrOptions({"uniform", "distance"}), callable, None],
    }
    _parameter_constraints.pop("n_neighbors")

    def __init__(
        self,
        radius=1.0,
        *,
        weights="uniform",
        algorithm="auto",
        leaf_size=30,
        p=2,
        metric="minkowski",
        metric_params=None,
        n_jobs=None,
    ):
        super().__init__(
            radius=radius,
            algorithm=algorithm,
            leaf_size=leaf_size,
            p=p,
            metric=metric,
            metric_params=metric_params,
            n_jobs=n_jobs,
        )
        self.weights = weights

    @_fit_context(
        # RadiusNeighborsRegressor.metric is not validated yet
        prefer_skip_nested_validation=False
    )
    def fit(self, X, y):
        """Fit the radius neighbors regressor from the training dataset.

        Parameters
        ----------
        X : {array-like, sparse matrix} of shape (n_samples, n_features) or \
                (n_samples, n_samples) if metric='precomputed'
            Training data.

        y : {array-like, sparse matrix} of shape (n_samples,) or \
                (n_samples, n_outputs)
            Target values.

        Returns
        -------
        self : RadiusNeighborsRegressor
            The fitted radius neighbors regressor.
        """
        return self._fit(X, y)

    def predict(self, X):
        """Predict the target for the provided data.

        Parameters
        ----------
        X : {array-like, sparse matrix} of shape (n_queries, n_features), \
                or (n_queries, n_indexed) if metric == 'precomputed', or None
            Test samples. If `None`, predictions for all indexed points are
            returned; in this case, points are not considered their own
            neighbors.

        Returns
        -------
        y : ndarray of shape (n_queries,) or (n_queries, n_outputs), \
                dtype=double
            Target values.
        """
        neigh_dist, neigh_ind = self.radius_neighbors(X)

        weights = _get_weights(neigh_dist, self.weights)

        _y = self._y
        if _y.ndim == 1:
            _y = _y.reshape((-1, 1))

        empty_obs = np.full_like(_y[0], np.nan)

        if weights is None:
            y_pred = np.array(
                [
                    np.mean(_y[ind, :], axis=0) if len(ind) else empty_obs
                    for (i, ind) in enumerate(neigh_ind)
                ]
            )

        else:
            y_pred = np.array(
                [
                    (
                        np.average(_y[ind, :], axis=0, weights=weights[i])
                        if len(ind)
                        else empty_obs
                    )
                    for (i, ind) in enumerate(neigh_ind)
                ]
            )

        if np.any(np.isnan(y_pred)):
            empty_warning_msg = (
                "One or more samples have no neighbors "
                "within specified radius; predicting NaN."
            )
            warnings.warn(empty_warning_msg)

        if self._y.ndim == 1:
            y_pred = y_pred.ravel()

        return y_pred
