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import torch
from torch._six import nan
from torch.distributions import constraints
from torch.distributions.uniform import Uniform
from torch.distributions.transformed_distribution import TransformedDistribution
from torch.distributions.transforms import AffineTransform, PowerTransform
from torch.distributions.utils import broadcast_all, euler_constant
__all__ = ['Kumaraswamy']
def _moments(a, b, n):
"""
Computes nth moment of Kumaraswamy using using torch.lgamma
"""
arg1 = 1 + n / a
log_value = torch.lgamma(arg1) + torch.lgamma(b) - torch.lgamma(arg1 + b)
return b * torch.exp(log_value)
class Kumaraswamy(TransformedDistribution):
r"""
Samples from a Kumaraswamy distribution.
Example::
>>> # xdoctest: +IGNORE_WANT("non-deterinistic")
>>> m = Kumaraswamy(torch.tensor([1.0]), torch.tensor([1.0]))
>>> m.sample() # sample from a Kumaraswamy distribution with concentration alpha=1 and beta=1
tensor([ 0.1729])
Args:
concentration1 (float or Tensor): 1st concentration parameter of the distribution
(often referred to as alpha)
concentration0 (float or Tensor): 2nd concentration parameter of the distribution
(often referred to as beta)
"""
arg_constraints = {'concentration1': constraints.positive, 'concentration0': constraints.positive}
support = constraints.unit_interval
has_rsample = True
def __init__(self, concentration1, concentration0, validate_args=None):
self.concentration1, self.concentration0 = broadcast_all(concentration1, concentration0)
finfo = torch.finfo(self.concentration0.dtype)
base_dist = Uniform(torch.full_like(self.concentration0, 0),
torch.full_like(self.concentration0, 1),
validate_args=validate_args)
transforms = [PowerTransform(exponent=self.concentration0.reciprocal()),
AffineTransform(loc=1., scale=-1.),
PowerTransform(exponent=self.concentration1.reciprocal())]
super(Kumaraswamy, self).__init__(base_dist, transforms, validate_args=validate_args)
def expand(self, batch_shape, _instance=None):
new = self._get_checked_instance(Kumaraswamy, _instance)
new.concentration1 = self.concentration1.expand(batch_shape)
new.concentration0 = self.concentration0.expand(batch_shape)
return super(Kumaraswamy, self).expand(batch_shape, _instance=new)
@property
def mean(self):
return _moments(self.concentration1, self.concentration0, 1)
@property
def mode(self):
# Evaluate in log-space for numerical stability.
log_mode = self.concentration0.reciprocal() * \
(-self.concentration0).log1p() - (-self.concentration0 * self.concentration1).log1p()
log_mode[(self.concentration0 < 1) | (self.concentration1 < 1)] = nan
return log_mode.exp()
@property
def variance(self):
return _moments(self.concentration1, self.concentration0, 2) - torch.pow(self.mean, 2)
def entropy(self):
t1 = (1 - self.concentration1.reciprocal())
t0 = (1 - self.concentration0.reciprocal())
H0 = torch.digamma(self.concentration0 + 1) + euler_constant
return t0 + t1 * H0 - torch.log(self.concentration1) - torch.log(self.concentration0)
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