220 lines
8.0 KiB
Python
220 lines
8.0 KiB
Python
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# Natural Language Toolkit: Expectation Maximization Clusterer
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#
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# Copyright (C) 2001-2023 NLTK Project
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# Author: Trevor Cohn <tacohn@cs.mu.oz.au>
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# URL: <https://www.nltk.org/>
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# For license information, see LICENSE.TXT
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try:
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import numpy
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except ImportError:
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pass
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from nltk.cluster.util import VectorSpaceClusterer
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class EMClusterer(VectorSpaceClusterer):
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"""
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The Gaussian EM clusterer models the vectors as being produced by
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a mixture of k Gaussian sources. The parameters of these sources
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(prior probability, mean and covariance matrix) are then found to
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maximise the likelihood of the given data. This is done with the
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expectation maximisation algorithm. It starts with k arbitrarily
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chosen means, priors and covariance matrices. It then calculates
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the membership probabilities for each vector in each of the
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clusters; this is the 'E' step. The cluster parameters are then
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updated in the 'M' step using the maximum likelihood estimate from
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the cluster membership probabilities. This process continues until
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the likelihood of the data does not significantly increase.
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"""
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def __init__(
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self,
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initial_means,
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priors=None,
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covariance_matrices=None,
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conv_threshold=1e-6,
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bias=0.1,
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normalise=False,
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svd_dimensions=None,
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):
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"""
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Creates an EM clusterer with the given starting parameters,
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convergence threshold and vector mangling parameters.
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:param initial_means: the means of the gaussian cluster centers
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:type initial_means: [seq of] numpy array or seq of SparseArray
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:param priors: the prior probability for each cluster
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:type priors: numpy array or seq of float
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:param covariance_matrices: the covariance matrix for each cluster
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:type covariance_matrices: [seq of] numpy array
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:param conv_threshold: maximum change in likelihood before deemed
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convergent
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:type conv_threshold: int or float
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:param bias: variance bias used to ensure non-singular covariance
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matrices
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:type bias: float
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:param normalise: should vectors be normalised to length 1
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:type normalise: boolean
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:param svd_dimensions: number of dimensions to use in reducing vector
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dimensionsionality with SVD
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:type svd_dimensions: int
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"""
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VectorSpaceClusterer.__init__(self, normalise, svd_dimensions)
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self._means = numpy.array(initial_means, numpy.float64)
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self._num_clusters = len(initial_means)
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self._conv_threshold = conv_threshold
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self._covariance_matrices = covariance_matrices
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self._priors = priors
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self._bias = bias
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def num_clusters(self):
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return self._num_clusters
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def cluster_vectorspace(self, vectors, trace=False):
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assert len(vectors) > 0
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# set the parameters to initial values
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dimensions = len(vectors[0])
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means = self._means
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priors = self._priors
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if not priors:
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priors = self._priors = (
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numpy.ones(self._num_clusters, numpy.float64) / self._num_clusters
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)
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covariances = self._covariance_matrices
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if not covariances:
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covariances = self._covariance_matrices = [
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numpy.identity(dimensions, numpy.float64)
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for i in range(self._num_clusters)
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]
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# do the E and M steps until the likelihood plateaus
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lastl = self._loglikelihood(vectors, priors, means, covariances)
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converged = False
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while not converged:
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if trace:
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print("iteration; loglikelihood", lastl)
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# E-step, calculate hidden variables, h[i,j]
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h = numpy.zeros((len(vectors), self._num_clusters), numpy.float64)
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for i in range(len(vectors)):
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for j in range(self._num_clusters):
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h[i, j] = priors[j] * self._gaussian(
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means[j], covariances[j], vectors[i]
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)
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h[i, :] /= sum(h[i, :])
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# M-step, update parameters - cvm, p, mean
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for j in range(self._num_clusters):
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covariance_before = covariances[j]
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new_covariance = numpy.zeros((dimensions, dimensions), numpy.float64)
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new_mean = numpy.zeros(dimensions, numpy.float64)
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sum_hj = 0.0
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for i in range(len(vectors)):
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delta = vectors[i] - means[j]
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new_covariance += h[i, j] * numpy.multiply.outer(delta, delta)
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sum_hj += h[i, j]
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new_mean += h[i, j] * vectors[i]
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covariances[j] = new_covariance / sum_hj
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means[j] = new_mean / sum_hj
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priors[j] = sum_hj / len(vectors)
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# bias term to stop covariance matrix being singular
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covariances[j] += self._bias * numpy.identity(dimensions, numpy.float64)
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# calculate likelihood - FIXME: may be broken
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l = self._loglikelihood(vectors, priors, means, covariances)
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# check for convergence
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if abs(lastl - l) < self._conv_threshold:
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converged = True
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lastl = l
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def classify_vectorspace(self, vector):
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best = None
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for j in range(self._num_clusters):
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p = self._priors[j] * self._gaussian(
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self._means[j], self._covariance_matrices[j], vector
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)
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if not best or p > best[0]:
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best = (p, j)
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return best[1]
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def likelihood_vectorspace(self, vector, cluster):
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cid = self.cluster_names().index(cluster)
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return self._priors[cluster] * self._gaussian(
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self._means[cluster], self._covariance_matrices[cluster], vector
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)
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def _gaussian(self, mean, cvm, x):
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m = len(mean)
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assert cvm.shape == (m, m), "bad sized covariance matrix, %s" % str(cvm.shape)
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try:
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det = numpy.linalg.det(cvm)
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inv = numpy.linalg.inv(cvm)
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a = det**-0.5 * (2 * numpy.pi) ** (-m / 2.0)
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dx = x - mean
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print(dx, inv)
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b = -0.5 * numpy.dot(numpy.dot(dx, inv), dx)
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return a * numpy.exp(b)
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except OverflowError:
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# happens when the exponent is negative infinity - i.e. b = 0
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# i.e. the inverse of cvm is huge (cvm is almost zero)
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return 0
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def _loglikelihood(self, vectors, priors, means, covariances):
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llh = 0.0
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for vector in vectors:
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p = 0
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for j in range(len(priors)):
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p += priors[j] * self._gaussian(means[j], covariances[j], vector)
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llh += numpy.log(p)
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return llh
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def __repr__(self):
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return "<EMClusterer means=%s>" % list(self._means)
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def demo():
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"""
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Non-interactive demonstration of the clusterers with simple 2-D data.
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"""
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from nltk import cluster
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# example from figure 14.10, page 519, Manning and Schutze
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vectors = [numpy.array(f) for f in [[0.5, 0.5], [1.5, 0.5], [1, 3]]]
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means = [[4, 2], [4, 2.01]]
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clusterer = cluster.EMClusterer(means, bias=0.1)
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clusters = clusterer.cluster(vectors, True, trace=True)
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print("Clustered:", vectors)
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print("As: ", clusters)
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print()
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for c in range(2):
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print("Cluster:", c)
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print("Prior: ", clusterer._priors[c])
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print("Mean: ", clusterer._means[c])
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print("Covar: ", clusterer._covariance_matrices[c])
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print()
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# classify a new vector
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vector = numpy.array([2, 2])
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print("classify(%s):" % vector, end=" ")
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print(clusterer.classify(vector))
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# show the classification probabilities
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vector = numpy.array([2, 2])
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print("classification_probdist(%s):" % vector)
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pdist = clusterer.classification_probdist(vector)
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for sample in pdist.samples():
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print(f"{sample} => {pdist.prob(sample) * 100:.0f}%")
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if __name__ == "__main__":
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demo()
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