text: # -*- coding: utf-8 -*-
  """
  Nearest Centroid Classification
  """
  
  # Author: Robert Layton <robertlayton@gmail.com>
  #         Olivier Grisel <olivier.grisel@ensta.org>
  #
  # License: BSD 3 clause
  
  import warnings
  import numpy as np
  from scipy import sparse as sp
  
  from ..base import BaseEstimator, ClassifierMixin
  from ..metrics.pairwise import pairwise_distances
  from ..preprocessing import LabelEncoder
  from ..utils.validation import check_array, check_X_y, check_is_fitted
  from ..utils.sparsefuncs import csc_median_axis_0
  from ..utils.multiclass import check_classification_targets
  
  
  class NearestCentroid(ClassifierMixin, BaseEstimator):
      """Nearest centroid classifier.
  
      Each class is represented by its centroid, with test samples classified to
      the class with the nearest centroid.
  
      Read more in the :ref:`User Guide <nearest_centroid_classifier>`.
  
      Parameters
      ----------
      metric : string, or callable
          The metric to use when calculating distance between instances in a
          feature array. If metric is a string or callable, it must be one of
          the options allowed by metrics.pairwise.pairwise_distances for its
          metric parameter.
          The centroids for the samples corresponding to each class is the point
          from which the sum of the distances (according to the metric) of all
          samples that belong to that particular class are minimized.
          If the "manhattan" metric is provided, this centroid is the median and
          for all other metrics, the centroid is now set to be the mean.
  
      shrink_threshold : float, optional (default = None)
          Threshold for shrinking centroids to remove features.
  
      Attributes
      ----------
      centroids_ : array-like of shape (n_classes, n_features)
          Centroid of each class.
  
      classes_ : array of shape (n_classes,)
          The unique classes labels.
  
      Examples
      --------
      >>> from sklearn.neighbors import NearestCentroid
      >>> import numpy as np
      >>> X = np.array([[-1, -1], [-2, -1], [-3, -2], [1, 1], [2, 1], [3, 2]])
      >>> y = np.array([1, 1, 1, 2, 2, 2])
      >>> clf = NearestCentroid()
      >>> clf.fit(X, y)
      NearestCentroid()
      >>> print(clf.predict([[-0.8, -1]]))
      [1]
  
      See also
      --------
      sklearn.neighbors.KNeighborsClassifier: nearest neighbors classifier
  
      Notes
      -----
      When used for text classification with tf-idf vectors, this classifier is
      also known as the Rocchio classifier.
  
      References
      ----------
      Tibshirani, R., Hastie, T., Narasimhan, B., & Chu, G. (2002). Diagnosis of
      multiple cancer types by shrunken centroids of gene expression. Proceedings
      of the National Academy of Sciences of the United States of America,
      99(10), 6567-6572. The National Academy of Sciences.
  
      """
  
      def __init__(self, metric='euclidean', shrink_threshold=None):
          self.metric = metric
          self.shrink_threshold = shrink_threshold
  
      def fit(self, X, y):
          """
          Fit the NearestCentroid model according to the given training data.
  
          Parameters
          ----------
          X : {array-like, sparse matrix} of shape (n_samples, n_features)
              Training vector, where n_samples is the number of samples and
              n_features is the number of features.
              Note that centroid shrinking cannot be used with sparse matrices.
          y : array, shape = [n_samples]
              Target values (integers)
          """
          if self.metric == 'precomputed':
              raise ValueError("Precomputed is not supported.")
          # If X is sparse and the metric is "manhattan", store it in a csc
          # format is easier to calculate the median.
          if self.metric == 'manhattan':
              X, y = check_X_y(X, y, ['csc'])
          else:
              X, y = check_X_y(X, y, ['csr', 'csc'])
          is_X_sparse = sp.issparse(X)
          if is_X_sparse and self.shrink_threshold:
              raise ValueError("threshold shrinking not supported"
                               " for sparse input")
          check_classification_targets(y)
  
          n_samples, n_features = X.shape
          le = LabelEncoder()
          y_ind = le.fit_transform(y)
          self.classes_ = classes = le.classes_
          n_classes = classes.size
          if n_classes < 2:
              raise ValueError('The number of classes has to be greater than'
                               ' one; got %d class' % (n_classes))
  
          # Mask mapping each class to its members.
          self.centroids_ = np.empty((n_classes, n_features), dtype=np.float64)
          # Number of clusters in each class.
          nk = np.zeros(n_classes)
  
          for cur_class in range(n_classes):
              center_mask = y_ind == cur_class
              nk[cur_class] = np.sum(center_mask)
              if is_X_sparse:
                  center_mask = np.where(center_mask)[0]
  
              # XXX: Update other averaging methods according to the metrics.
              if self.metric == "manhattan":
                  # NumPy does not calculate median of sparse matrices.
                  if not is_X_sparse:
                      self.centroids_[cur_class] = np.median(X[center_mask], axis=0)
                  else:
                      self.centroids_[cur_class] = csc_median_axis_0(X[center_mask])
              else:
                  if self.metric != 'euclidean':
                      warnings.warn("Averaging for metrics other than "
                                    "euclidean and manhattan not supported. "
                                    "The average is set to be the mean."
                                    )
                  self.centroids_[cur_class] = X[center_mask].mean(axis=0)
  
          if self.shrink_threshold:
              dataset_centroid_ = np.mean(X, axis=0)
  
              # m parameter for determining deviation
              m = np.sqrt((1. / nk) - (1. / n_samples))
              # Calculate deviation using the standard deviation of centroids.
              variance = (X - self.centroids_[y_ind]) ** 2
              variance = variance.sum(axis=0)
              s = np.sqrt(variance / (n_samples - n_classes))
              s += np.median(s)  # To deter outliers from affecting the results.
              mm = m.reshape(len(m), 1)  # Reshape to allow broadcasting.
              ms = mm * s
              deviation = ((self.centroids_ - dataset_centroid_) / ms)
              # Soft thresholding: if the deviation crosses 0 during shrinking,
              # it becomes zero.
              signs = np.sign(deviation)
              deviation = (np.abs(deviation) - self.shrink_threshold)
              np.clip(deviation, 0, None, out=deviation)
              deviation *= signs
              # Now adjust the centroids using the deviation
              msd = ms * deviation
              self.centroids_ = dataset_centroid_[np.newaxis, :] + msd
          return self
  
      def predict(self, X):
          """Perform classification on an array of test vectors X.
  
          The predicted class C for each sample in X is returned.
  
          Parameters
          ----------
          X : array-like of shape (n_samples, n_features)
  
          Returns
          -------
          C : ndarray of shape (n_samples,)
  
          Notes
          -----
          If the metric constructor parameter is "precomputed", X is assumed to
          be the distance matrix between the data to be predicted and
          ``self.centroids_``.
          """
          check_is_fitted(self)
  
          X = check_array(X, accept_sparse='csr')
          return self.classes_[pairwise_distances(
              X, self.centroids_, metric=self.metric).argmin(axis=1)]
  