# Authors: Manoj Kumar # Alexandre Gramfort # Joel Nothman # License: BSD 3 clause import warnings import numpy as np from numbers import Integral, Real from scipy import sparse from math import sqrt from ..metrics import pairwise_distances_argmin from ..metrics.pairwise import euclidean_distances from ..base import ( TransformerMixin, ClusterMixin, BaseEstimator, ClassNamePrefixFeaturesOutMixin, ) from ..utils.extmath import row_norms from ..utils._param_validation import Interval from ..utils.validation import check_is_fitted from ..exceptions import ConvergenceWarning from . import AgglomerativeClustering from .._config import config_context def _iterate_sparse_X(X): """This little hack returns a densified row when iterating over a sparse matrix, instead of constructing a sparse matrix for every row that is expensive. """ n_samples = X.shape[0] X_indices = X.indices X_data = X.data X_indptr = X.indptr for i in range(n_samples): row = np.zeros(X.shape[1]) startptr, endptr = X_indptr[i], X_indptr[i + 1] nonzero_indices = X_indices[startptr:endptr] row[nonzero_indices] = X_data[startptr:endptr] yield row def _split_node(node, threshold, branching_factor): """The node has to be split if there is no place for a new subcluster in the node. 1. Two empty nodes and two empty subclusters are initialized. 2. The pair of distant subclusters are found. 3. The properties of the empty subclusters and nodes are updated according to the nearest distance between the subclusters to the pair of distant subclusters. 4. The two nodes are set as children to the two subclusters. """ new_subcluster1 = _CFSubcluster() new_subcluster2 = _CFSubcluster() new_node1 = _CFNode( threshold=threshold, branching_factor=branching_factor, is_leaf=node.is_leaf, n_features=node.n_features, dtype=node.init_centroids_.dtype, ) new_node2 = _CFNode( threshold=threshold, branching_factor=branching_factor, is_leaf=node.is_leaf, n_features=node.n_features, dtype=node.init_centroids_.dtype, ) new_subcluster1.child_ = new_node1 new_subcluster2.child_ = new_node2 if node.is_leaf: if node.prev_leaf_ is not None: node.prev_leaf_.next_leaf_ = new_node1 new_node1.prev_leaf_ = node.prev_leaf_ new_node1.next_leaf_ = new_node2 new_node2.prev_leaf_ = new_node1 new_node2.next_leaf_ = node.next_leaf_ if node.next_leaf_ is not None: node.next_leaf_.prev_leaf_ = new_node2 dist = euclidean_distances( node.centroids_, Y_norm_squared=node.squared_norm_, squared=True ) n_clusters = dist.shape[0] farthest_idx = np.unravel_index(dist.argmax(), (n_clusters, n_clusters)) node1_dist, node2_dist = dist[(farthest_idx,)] node1_closer = node1_dist < node2_dist # make sure node1 is closest to itself even if all distances are equal. # This can only happen when all node.centroids_ are duplicates leading to all # distances between centroids being zero. node1_closer[farthest_idx[0]] = True for idx, subcluster in enumerate(node.subclusters_): if node1_closer[idx]: new_node1.append_subcluster(subcluster) new_subcluster1.update(subcluster) else: new_node2.append_subcluster(subcluster) new_subcluster2.update(subcluster) return new_subcluster1, new_subcluster2 class _CFNode: """Each node in a CFTree is called a CFNode. The CFNode can have a maximum of branching_factor number of CFSubclusters. Parameters ---------- threshold : float Threshold needed for a new subcluster to enter a CFSubcluster. branching_factor : int Maximum number of CF subclusters in each node. is_leaf : bool We need to know if the CFNode is a leaf or not, in order to retrieve the final subclusters. n_features : int The number of features. Attributes ---------- subclusters_ : list List of subclusters for a particular CFNode. prev_leaf_ : _CFNode Useful only if is_leaf is True. next_leaf_ : _CFNode next_leaf. Useful only if is_leaf is True. the final subclusters. init_centroids_ : ndarray of shape (branching_factor + 1, n_features) Manipulate ``init_centroids_`` throughout rather than centroids_ since the centroids are just a view of the ``init_centroids_`` . init_sq_norm_ : ndarray of shape (branching_factor + 1,) manipulate init_sq_norm_ throughout. similar to ``init_centroids_``. centroids_ : ndarray of shape (branching_factor + 1, n_features) View of ``init_centroids_``. squared_norm_ : ndarray of shape (branching_factor + 1,) View of ``init_sq_norm_``. """ def __init__(self, *, threshold, branching_factor, is_leaf, n_features, dtype): self.threshold = threshold self.branching_factor = branching_factor self.is_leaf = is_leaf self.n_features = n_features # The list of subclusters, centroids and squared norms # to manipulate throughout. self.subclusters_ = [] self.init_centroids_ = np.zeros((branching_factor + 1, n_features), dtype=dtype) self.init_sq_norm_ = np.zeros((branching_factor + 1), dtype) self.squared_norm_ = [] self.prev_leaf_ = None self.next_leaf_ = None def append_subcluster(self, subcluster): n_samples = len(self.subclusters_) self.subclusters_.append(subcluster) self.init_centroids_[n_samples] = subcluster.centroid_ self.init_sq_norm_[n_samples] = subcluster.sq_norm_ # Keep centroids and squared norm as views. In this way # if we change init_centroids and init_sq_norm_, it is # sufficient, self.centroids_ = self.init_centroids_[: n_samples + 1, :] self.squared_norm_ = self.init_sq_norm_[: n_samples + 1] def update_split_subclusters(self, subcluster, new_subcluster1, new_subcluster2): """Remove a subcluster from a node and update it with the split subclusters. """ ind = self.subclusters_.index(subcluster) self.subclusters_[ind] = new_subcluster1 self.init_centroids_[ind] = new_subcluster1.centroid_ self.init_sq_norm_[ind] = new_subcluster1.sq_norm_ self.append_subcluster(new_subcluster2) def insert_cf_subcluster(self, subcluster): """Insert a new subcluster into the node.""" if not self.subclusters_: self.append_subcluster(subcluster) return False threshold = self.threshold branching_factor = self.branching_factor # We need to find the closest subcluster among all the # subclusters so that we can insert our new subcluster. dist_matrix = np.dot(self.centroids_, subcluster.centroid_) dist_matrix *= -2.0 dist_matrix += self.squared_norm_ closest_index = np.argmin(dist_matrix) closest_subcluster = self.subclusters_[closest_index] # If the subcluster has a child, we need a recursive strategy. if closest_subcluster.child_ is not None: split_child = closest_subcluster.child_.insert_cf_subcluster(subcluster) if not split_child: # If it is determined that the child need not be split, we # can just update the closest_subcluster closest_subcluster.update(subcluster) self.init_centroids_[closest_index] = self.subclusters_[ closest_index ].centroid_ self.init_sq_norm_[closest_index] = self.subclusters_[ closest_index ].sq_norm_ return False # things not too good. we need to redistribute the subclusters in # our child node, and add a new subcluster in the parent # subcluster to accommodate the new child. else: new_subcluster1, new_subcluster2 = _split_node( closest_subcluster.child_, threshold, branching_factor, ) self.update_split_subclusters( closest_subcluster, new_subcluster1, new_subcluster2 ) if len(self.subclusters_) > self.branching_factor: return True return False # good to go! else: merged = closest_subcluster.merge_subcluster(subcluster, self.threshold) if merged: self.init_centroids_[closest_index] = closest_subcluster.centroid_ self.init_sq_norm_[closest_index] = closest_subcluster.sq_norm_ return False # not close to any other subclusters, and we still # have space, so add. elif len(self.subclusters_) < self.branching_factor: self.append_subcluster(subcluster) return False # We do not have enough space nor is it closer to an # other subcluster. We need to split. else: self.append_subcluster(subcluster) return True class _CFSubcluster: """Each subcluster in a CFNode is called a CFSubcluster. A CFSubcluster can have a CFNode has its child. Parameters ---------- linear_sum : ndarray of shape (n_features,), default=None Sample. This is kept optional to allow initialization of empty subclusters. Attributes ---------- n_samples_ : int Number of samples that belong to each subcluster. linear_sum_ : ndarray Linear sum of all the samples in a subcluster. Prevents holding all sample data in memory. squared_sum_ : float Sum of the squared l2 norms of all samples belonging to a subcluster. centroid_ : ndarray of shape (branching_factor + 1, n_features) Centroid of the subcluster. Prevent recomputing of centroids when ``CFNode.centroids_`` is called. child_ : _CFNode Child Node of the subcluster. Once a given _CFNode is set as the child of the _CFNode, it is set to ``self.child_``. sq_norm_ : ndarray of shape (branching_factor + 1,) Squared norm of the subcluster. Used to prevent recomputing when pairwise minimum distances are computed. """ def __init__(self, *, linear_sum=None): if linear_sum is None: self.n_samples_ = 0 self.squared_sum_ = 0.0 self.centroid_ = self.linear_sum_ = 0 else: self.n_samples_ = 1 self.centroid_ = self.linear_sum_ = linear_sum self.squared_sum_ = self.sq_norm_ = np.dot( self.linear_sum_, self.linear_sum_ ) self.child_ = None def update(self, subcluster): self.n_samples_ += subcluster.n_samples_ self.linear_sum_ += subcluster.linear_sum_ self.squared_sum_ += subcluster.squared_sum_ self.centroid_ = self.linear_sum_ / self.n_samples_ self.sq_norm_ = np.dot(self.centroid_, self.centroid_) def merge_subcluster(self, nominee_cluster, threshold): """Check if a cluster is worthy enough to be merged. If yes then merge. """ new_ss = self.squared_sum_ + nominee_cluster.squared_sum_ new_ls = self.linear_sum_ + nominee_cluster.linear_sum_ new_n = self.n_samples_ + nominee_cluster.n_samples_ new_centroid = (1 / new_n) * new_ls new_sq_norm = np.dot(new_centroid, new_centroid) # The squared radius of the cluster is defined: # r^2 = sum_i ||x_i - c||^2 / n # with x_i the n points assigned to the cluster and c its centroid: # c = sum_i x_i / n # This can be expanded to: # r^2 = sum_i ||x_i||^2 / n - 2 < sum_i x_i / n, c> + n ||c||^2 / n # and therefore simplifies to: # r^2 = sum_i ||x_i||^2 / n - ||c||^2 sq_radius = new_ss / new_n - new_sq_norm if sq_radius <= threshold**2: ( self.n_samples_, self.linear_sum_, self.squared_sum_, self.centroid_, self.sq_norm_, ) = (new_n, new_ls, new_ss, new_centroid, new_sq_norm) return True return False @property def radius(self): """Return radius of the subcluster""" # Because of numerical issues, this could become negative sq_radius = self.squared_sum_ / self.n_samples_ - self.sq_norm_ return sqrt(max(0, sq_radius)) class Birch( ClassNamePrefixFeaturesOutMixin, ClusterMixin, TransformerMixin, BaseEstimator ): """Implements the BIRCH clustering algorithm. It is a memory-efficient, online-learning algorithm provided as an alternative to :class:`MiniBatchKMeans`. It constructs a tree data structure with the cluster centroids being read off the leaf. These can be either the final cluster centroids or can be provided as input to another clustering algorithm such as :class:`AgglomerativeClustering`. Read more in the :ref:`User Guide `. .. versionadded:: 0.16 Parameters ---------- threshold : float, default=0.5 The radius of the subcluster obtained by merging a new sample and the closest subcluster should be lesser than the threshold. Otherwise a new subcluster is started. Setting this value to be very low promotes splitting and vice-versa. branching_factor : int, default=50 Maximum number of CF subclusters in each node. If a new samples enters such that the number of subclusters exceed the branching_factor then that node is split into two nodes with the subclusters redistributed in each. The parent subcluster of that node is removed and two new subclusters are added as parents of the 2 split nodes. n_clusters : int, instance of sklearn.cluster model or None, default=3 Number of clusters after the final clustering step, which treats the subclusters from the leaves as new samples. - `None` : the final clustering step is not performed and the subclusters are returned as they are. - :mod:`sklearn.cluster` Estimator : If a model is provided, the model is fit treating the subclusters as new samples and the initial data is mapped to the label of the closest subcluster. - `int` : the model fit is :class:`AgglomerativeClustering` with `n_clusters` set to be equal to the int. compute_labels : bool, default=True Whether or not to compute labels for each fit. copy : bool, default=True Whether or not to make a copy of the given data. If set to False, the initial data will be overwritten. Attributes ---------- root_ : _CFNode Root of the CFTree. dummy_leaf_ : _CFNode Start pointer to all the leaves. subcluster_centers_ : ndarray Centroids of all subclusters read directly from the leaves. subcluster_labels_ : ndarray Labels assigned to the centroids of the subclusters after they are clustered globally. labels_ : ndarray of shape (n_samples,) Array of labels assigned to the input data. if partial_fit is used instead of fit, they are assigned to the last batch of data. 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 See Also -------- MiniBatchKMeans : Alternative implementation that does incremental updates of the centers' positions using mini-batches. Notes ----- The tree data structure consists of nodes with each node consisting of a number of subclusters. The maximum number of subclusters in a node is determined by the branching factor. Each subcluster maintains a linear sum, squared sum and the number of samples in that subcluster. In addition, each subcluster can also have a node as its child, if the subcluster is not a member of a leaf node. For a new point entering the root, it is merged with the subcluster closest to it and the linear sum, squared sum and the number of samples of that subcluster are updated. This is done recursively till the properties of the leaf node are updated. References ---------- * Tian Zhang, Raghu Ramakrishnan, Maron Livny BIRCH: An efficient data clustering method for large databases. https://www.cs.sfu.ca/CourseCentral/459/han/papers/zhang96.pdf * Roberto Perdisci JBirch - Java implementation of BIRCH clustering algorithm https://code.google.com/archive/p/jbirch Examples -------- >>> from sklearn.cluster import Birch >>> X = [[0, 1], [0.3, 1], [-0.3, 1], [0, -1], [0.3, -1], [-0.3, -1]] >>> brc = Birch(n_clusters=None) >>> brc.fit(X) Birch(n_clusters=None) >>> brc.predict(X) array([0, 0, 0, 1, 1, 1]) """ _parameter_constraints: dict = { "threshold": [Interval(Real, 0.0, None, closed="neither")], "branching_factor": [Interval(Integral, 1, None, closed="neither")], "n_clusters": [None, ClusterMixin, Interval(Integral, 1, None, closed="left")], "compute_labels": ["boolean"], "copy": ["boolean"], } def __init__( self, *, threshold=0.5, branching_factor=50, n_clusters=3, compute_labels=True, copy=True, ): self.threshold = threshold self.branching_factor = branching_factor self.n_clusters = n_clusters self.compute_labels = compute_labels self.copy = copy def fit(self, X, y=None): """ Build a CF Tree for the input data. Parameters ---------- X : {array-like, sparse matrix} of shape (n_samples, n_features) Input data. y : Ignored Not used, present here for API consistency by convention. Returns ------- self Fitted estimator. """ self._validate_params() return self._fit(X, partial=False) def _fit(self, X, partial): has_root = getattr(self, "root_", None) first_call = not (partial and has_root) X = self._validate_data( X, accept_sparse="csr", copy=self.copy, reset=first_call, dtype=[np.float64, np.float32], ) threshold = self.threshold branching_factor = self.branching_factor n_samples, n_features = X.shape # If partial_fit is called for the first time or fit is called, we # start a new tree. if first_call: # The first root is the leaf. Manipulate this object throughout. self.root_ = _CFNode( threshold=threshold, branching_factor=branching_factor, is_leaf=True, n_features=n_features, dtype=X.dtype, ) # To enable getting back subclusters. self.dummy_leaf_ = _CFNode( threshold=threshold, branching_factor=branching_factor, is_leaf=True, n_features=n_features, dtype=X.dtype, ) self.dummy_leaf_.next_leaf_ = self.root_ self.root_.prev_leaf_ = self.dummy_leaf_ # Cannot vectorize. Enough to convince to use cython. if not sparse.issparse(X): iter_func = iter else: iter_func = _iterate_sparse_X for sample in iter_func(X): subcluster = _CFSubcluster(linear_sum=sample) split = self.root_.insert_cf_subcluster(subcluster) if split: new_subcluster1, new_subcluster2 = _split_node( self.root_, threshold, branching_factor ) del self.root_ self.root_ = _CFNode( threshold=threshold, branching_factor=branching_factor, is_leaf=False, n_features=n_features, dtype=X.dtype, ) self.root_.append_subcluster(new_subcluster1) self.root_.append_subcluster(new_subcluster2) centroids = np.concatenate([leaf.centroids_ for leaf in self._get_leaves()]) self.subcluster_centers_ = centroids self._n_features_out = self.subcluster_centers_.shape[0] self._global_clustering(X) return self def _get_leaves(self): """ Retrieve the leaves of the CF Node. Returns ------- leaves : list of shape (n_leaves,) List of the leaf nodes. """ leaf_ptr = self.dummy_leaf_.next_leaf_ leaves = [] while leaf_ptr is not None: leaves.append(leaf_ptr) leaf_ptr = leaf_ptr.next_leaf_ return leaves def partial_fit(self, X=None, y=None): """ Online learning. Prevents rebuilding of CFTree from scratch. Parameters ---------- X : {array-like, sparse matrix} of shape (n_samples, n_features), \ default=None Input data. If X is not provided, only the global clustering step is done. y : Ignored Not used, present here for API consistency by convention. Returns ------- self Fitted estimator. """ self._validate_params() if X is None: # Perform just the final global clustering step. self._global_clustering() return self else: return self._fit(X, partial=True) def _check_fit(self, X): check_is_fitted(self) if ( hasattr(self, "subcluster_centers_") and X.shape[1] != self.subcluster_centers_.shape[1] ): raise ValueError( "Training data and predicted data do not have same number of features." ) def predict(self, X): """ Predict data using the ``centroids_`` of subclusters. Avoid computation of the row norms of X. Parameters ---------- X : {array-like, sparse matrix} of shape (n_samples, n_features) Input data. Returns ------- labels : ndarray of shape(n_samples,) Labelled data. """ check_is_fitted(self) X = self._validate_data(X, accept_sparse="csr", reset=False) return self._predict(X) def _predict(self, X): """Predict data using the ``centroids_`` of subclusters.""" kwargs = {"Y_norm_squared": self._subcluster_norms} with config_context(assume_finite=True): argmin = pairwise_distances_argmin( X, self.subcluster_centers_, metric_kwargs=kwargs ) return self.subcluster_labels_[argmin] def transform(self, X): """ Transform X into subcluster centroids dimension. Each dimension represents the distance from the sample point to each cluster centroid. Parameters ---------- X : {array-like, sparse matrix} of shape (n_samples, n_features) Input data. Returns ------- X_trans : {array-like, sparse matrix} of shape (n_samples, n_clusters) Transformed data. """ check_is_fitted(self) X = self._validate_data(X, accept_sparse="csr", reset=False) with config_context(assume_finite=True): return euclidean_distances(X, self.subcluster_centers_) def _global_clustering(self, X=None): """ Global clustering for the subclusters obtained after fitting """ clusterer = self.n_clusters centroids = self.subcluster_centers_ compute_labels = (X is not None) and self.compute_labels # Preprocessing for the global clustering. not_enough_centroids = False if isinstance(clusterer, Integral): clusterer = AgglomerativeClustering(n_clusters=self.n_clusters) # There is no need to perform the global clustering step. if len(centroids) < self.n_clusters: not_enough_centroids = True # To use in predict to avoid recalculation. self._subcluster_norms = row_norms(self.subcluster_centers_, squared=True) if clusterer is None or not_enough_centroids: self.subcluster_labels_ = np.arange(len(centroids)) if not_enough_centroids: warnings.warn( "Number of subclusters found (%d) by BIRCH is less " "than (%d). Decrease the threshold." % (len(centroids), self.n_clusters), ConvergenceWarning, ) else: # The global clustering step that clusters the subclusters of # the leaves. It assumes the centroids of the subclusters as # samples and finds the final centroids. self.subcluster_labels_ = clusterer.fit_predict(self.subcluster_centers_) if compute_labels: self.labels_ = self._predict(X) def _more_tags(self): return {"preserves_dtype": [np.float64, np.float32]}