#!/usr/bin/env python3
# coding: utf-8
"""
Created on September 2018
@author: Nathan de Lara <nathan.delara@polytechnique.org>
@author: Thomas Bonald <bonald@enst.fr>
"""
from typing import Union
import numpy as np
from scipy import sparse
from sknetwork.embedding.base import BaseEmbedding
from sknetwork.linalg import LanczosEig, Laplacian, Normalizer, normalize
from sknetwork.utils.format import get_adjacency
from sknetwork.utils.check import check_format, check_adjacency_vector, check_nonnegative, check_n_components
[docs]class Spectral(BaseEmbedding):
"""Spectral embedding of graphs, based the spectral decomposition of the Laplacian matrix :math:`L = D - A`
or the transition matrix of the random walk :math:`P = D^{-1}A` (default), where :math:`D` is the
diagonal matrix of degrees.
Eigenvectors are considered in increasing order (for the Laplacian matrix :math:`L`) or decreasing order
(for the transition matrix of the random walk :math:`P`) of eigenvalues, skipping the first.
Parameters
----------
n_components : int (default = ``2``)
Dimension of the embedding space.
decomposition : str (``laplacian`` or ``rw``, default = ``rw``)
Matrix used for the spectral decomposition.
regularization : float (default = ``-1``)
Regularization factor :math:`\\alpha` so that the adjacency matrix is :math:`A + \\alpha \\frac{11^T}{n}`.
If negative, regularization is applied only if the graph is disconnected; the regularization factor
:math:`\\alpha` is then set to the absolute value of the parameter.
normalized : bool (default = ``True``)
If ``True``, normalized the embedding so that each vector has norm 1 in the embedding space, i.e.,
each vector lies on the unit sphere.
Attributes
----------
embedding_ : array, shape = (n, n_components)
Embedding of the nodes.
embedding_row_ : array, shape = (n_row, n_components)
Embedding of the rows, for bipartite graphs.
embedding_col_ : array, shape = (n_col, n_components)
Embedding of the columns, for bipartite graphs.
eigenvalues_ : array, shape = (n_components)
Eigenvalues.
eigenvectors_ : array, shape = (n, n_components)
Eigenvectors.
Example
-------
>>> from sknetwork.embedding import Spectral
>>> from sknetwork.data import karate_club
>>> spectral = Spectral(n_components=3)
>>> adjacency = karate_club()
>>> embedding = spectral.fit_transform(adjacency)
>>> embedding.shape
(34, 3)
References
----------
Belkin, M. & Niyogi, P. (2003). Laplacian Eigenmaps for Dimensionality Reduction and Data Representation,
Neural computation.
"""
def __init__(self, n_components: int = 2, decomposition: str = 'rw', regularization: float = -1,
normalized: bool = True):
super(Spectral, self).__init__()
self.embedding_ = None
self.n_components = n_components
self.decomposition = decomposition
self.regularization = regularization
self.normalized = normalized
self.bipartite = None
self.regularized = None
self.eigenvalues_ = None
self.eigenvectors_ = None
[docs] def fit(self, input_matrix: Union[sparse.csr_matrix, np.ndarray], force_bipartite: bool = False) -> 'Spectral':
"""Compute the graph embedding.
If the input matrix :math:`B` is not square (e.g., biadjacency matrix of a bipartite graph) or not symmetric
(e.g., adjacency matrix of a directed graph), use the adjacency matrix
:math:`A = \\begin{bmatrix} 0 & B \\\\ B^T & 0 \\end{bmatrix}`
and return the embedding for both rows and columns of the input matrix :math:`B`.
Parameters
----------
input_matrix :
Adjacency matrix or biadjacency matrix of the graph.
force_bipartite : bool (default = ``False``)
If ``True``, force the input matrix to be considered as a biadjacency matrix.
Returns
-------
self: :class:`Spectral`
"""
# input
input_matrix = check_format(input_matrix)
adjacency, self.bipartite = get_adjacency(input_matrix, allow_directed=False, force_bipartite=force_bipartite)
n = adjacency.shape[0]
# regularization
regularization = self._get_regularization(self.regularization, adjacency)
self.regularized = regularization > 0
# laplacian
normalized_laplacian = self.decomposition == 'rw'
laplacian = Laplacian(adjacency, regularization, normalized_laplacian)
# spectral decomposition
n_components = check_n_components(self.n_components, n - 2) + 1
solver = LanczosEig(which='SM')
solver.fit(matrix=laplacian, n_components=n_components)
index = np.argsort(solver.eigenvalues_)[1:] # increasing order, skip first
eigenvalues = solver.eigenvalues_[index]
eigenvectors = solver.eigenvectors_[:, index]
if normalized_laplacian:
eigenvectors = laplacian.norm_diag.dot(eigenvectors)
eigenvalues = 1 - eigenvalues
# embedding
embedding = eigenvectors.copy()
if self.normalized:
embedding = normalize(embedding, p=2)
# output
self.embedding_ = embedding
self.eigenvalues_ = eigenvalues
self.eigenvectors_ = eigenvectors
if self.bipartite:
self._split_vars(input_matrix.shape)
return self