# Copyright (c) 2012-2014 GPy Authors
# Licensed under the BSD 3-clause license (see LICENSE.txt)
import numpy as np
from scipy import stats,special
import scipy as sp
from . import link_functions
from .likelihood import Likelihood
[docs]class Exponential(Likelihood):
"""
Expoential likelihood
Y is expected to take values in {0,1,2,...}
-----
$$
L(x) = \exp(\lambda) * \lambda**Y_i / Y_i!
$$
"""
def __init__(self,gp_link=None):
if gp_link is None:
gp_link = link_functions.Log()
super(Exponential, self).__init__(gp_link, 'ExpLikelihood')
[docs] def pdf_link(self, link_f, y, Y_metadata=None):
"""
Likelihood function given link(f)
.. math::
p(y_{i}|\\lambda(f_{i})) = \\lambda(f_{i})\\exp (-y\\lambda(f_{i}))
:param link_f: latent variables link(f)
:type link_f: Nx1 array
:param y: data
:type y: Nx1 array
:param Y_metadata: Y_metadata which is not used in exponential distribution
:returns: likelihood evaluated for this point
:rtype: float
"""
assert np.atleast_1d(link_f).shape == np.atleast_1d(y).shape
log_objective = link_f*np.exp(-y*link_f)
return np.exp(np.sum(np.log(log_objective)))
[docs] def logpdf_link(self, link_f, y, Y_metadata=None):
"""
Log Likelihood Function given link(f)
.. math::
\\ln p(y_{i}|\lambda(f_{i})) = \\ln \\lambda(f_{i}) - y_{i}\\lambda(f_{i})
:param link_f: latent variables (link(f))
:type link_f: Nx1 array
:param y: data
:type y: Nx1 array
:param Y_metadata: Y_metadata which is not used in exponential distribution
:returns: likelihood evaluated for this point
:rtype: float
"""
log_objective = np.log(link_f) - y*link_f
return log_objective
[docs] def dlogpdf_dlink(self, link_f, y, Y_metadata=None):
"""
Gradient of the log likelihood function at y, given link(f) w.r.t link(f)
.. math::
\\frac{d \\ln p(y_{i}|\lambda(f_{i}))}{d\\lambda(f)} = \\frac{1}{\\lambda(f)} - y_{i}
:param link_f: latent variables (f)
:type link_f: Nx1 array
:param y: data
:type y: Nx1 array
:param Y_metadata: Y_metadata which is not used in exponential distribution
:returns: gradient of likelihood evaluated at points
:rtype: Nx1 array
"""
grad = 1./link_f - y
#grad = y/(link_f**2) - 1./link_f
return grad
[docs] def d2logpdf_dlink2(self, link_f, y, Y_metadata=None):
"""
Hessian at y, given link(f), w.r.t link(f)
i.e. second derivative logpdf at y given link(f_i) and link(f_j) w.r.t link(f_i) and link(f_j)
The hessian will be 0 unless i == j
.. math::
\\frac{d^{2} \\ln p(y_{i}|\lambda(f_{i}))}{d^{2}\\lambda(f)} = -\\frac{1}{\\lambda(f_{i})^{2}}
:param link_f: latent variables link(f)
:type link_f: Nx1 array
:param y: data
:type y: Nx1 array
:param Y_metadata: Y_metadata which is not used in exponential distribution
:returns: Diagonal of hessian matrix (second derivative of likelihood evaluated at points f)
:rtype: Nx1 array
.. Note::
Will return diagonal of hessian, since every where else it is 0, as the likelihood factorizes over cases
(the distribution for y_i depends only on link(f_i) not on link(f_(j!=i))
"""
hess = -1./(link_f**2)
#hess = -2*y/(link_f**3) + 1/(link_f**2)
return hess
[docs] def d3logpdf_dlink3(self, link_f, y, Y_metadata=None):
"""
Third order derivative log-likelihood function at y given link(f) w.r.t link(f)
.. math::
\\frac{d^{3} \\ln p(y_{i}|\lambda(f_{i}))}{d^{3}\\lambda(f)} = \\frac{2}{\\lambda(f_{i})^{3}}
:param link_f: latent variables link(f)
:type link_f: Nx1 array
:param y: data
:type y: Nx1 array
:param Y_metadata: Y_metadata which is not used in exponential distribution
:returns: third derivative of likelihood evaluated at points f
:rtype: Nx1 array
"""
d3lik_dlink3 = 2./(link_f**3)
#d3lik_dlink3 = 6*y/(link_f**4) - 2./(link_f**3)
return d3lik_dlink3
[docs] def samples(self, gp, Y_metadata=None):
"""
Returns a set of samples of observations based on a given value of the latent variable.
:param gp: latent variable
"""
orig_shape = gp.shape
gp = gp.flatten()
Ysim = np.random.exponential(1.0/self.gp_link.transf(gp))
return Ysim.reshape(orig_shape)