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Source code for torch.distributions.von_mises

import math

import torch
import torch.jit
from torch.distributions import constraints
from torch.distributions.distribution import Distribution
from torch.distributions.utils import broadcast_all, lazy_property

__all__ = ["VonMises"]


def _eval_poly(y, coef):
    coef = list(coef)
    result = coef.pop()
    while coef:
        result = coef.pop() + y * result
    return result


_I0_COEF_SMALL = [
    1.0,
    3.5156229,
    3.0899424,
    1.2067492,
    0.2659732,
    0.360768e-1,
    0.45813e-2,
]
_I0_COEF_LARGE = [
    0.39894228,
    0.1328592e-1,
    0.225319e-2,
    -0.157565e-2,
    0.916281e-2,
    -0.2057706e-1,
    0.2635537e-1,
    -0.1647633e-1,
    0.392377e-2,
]
_I1_COEF_SMALL = [
    0.5,
    0.87890594,
    0.51498869,
    0.15084934,
    0.2658733e-1,
    0.301532e-2,
    0.32411e-3,
]
_I1_COEF_LARGE = [
    0.39894228,
    -0.3988024e-1,
    -0.362018e-2,
    0.163801e-2,
    -0.1031555e-1,
    0.2282967e-1,
    -0.2895312e-1,
    0.1787654e-1,
    -0.420059e-2,
]

_COEF_SMALL = [_I0_COEF_SMALL, _I1_COEF_SMALL]
_COEF_LARGE = [_I0_COEF_LARGE, _I1_COEF_LARGE]


def _log_modified_bessel_fn(x, order=0):
    """
    Returns ``log(I_order(x))`` for ``x > 0``,
    where `order` is either 0 or 1.
    """
    assert order == 0 or order == 1

    # compute small solution
    y = x / 3.75
    y = y * y
    small = _eval_poly(y, _COEF_SMALL[order])
    if order == 1:
        small = x.abs() * small
    small = small.log()

    # compute large solution
    y = 3.75 / x
    large = x - 0.5 * x.log() + _eval_poly(y, _COEF_LARGE[order]).log()

    result = torch.where(x < 3.75, small, large)
    return result


@torch.jit.script_if_tracing
def _rejection_sample(loc, concentration, proposal_r, x):
    done = torch.zeros(x.shape, dtype=torch.bool, device=loc.device)
    while not done.all():
        u = torch.rand((3,) + x.shape, dtype=loc.dtype, device=loc.device)
        u1, u2, u3 = u.unbind()
        z = torch.cos(math.pi * u1)
        f = (1 + proposal_r * z) / (proposal_r + z)
        c = concentration * (proposal_r - f)
        accept = ((c * (2 - c) - u2) > 0) | ((c / u2).log() + 1 - c >= 0)
        if accept.any():
            x = torch.where(accept, (u3 - 0.5).sign() * f.acos(), x)
            done = done | accept
    return (x + math.pi + loc) % (2 * math.pi) - math.pi


[docs]class VonMises(Distribution): """ A circular von Mises distribution. This implementation uses polar coordinates. The ``loc`` and ``value`` args can be any real number (to facilitate unconstrained optimization), but are interpreted as angles modulo 2 pi. Example:: >>> # xdoctest: +IGNORE_WANT("non-deterinistic") >>> m = VonMises(torch.tensor([1.0]), torch.tensor([1.0])) >>> m.sample() # von Mises distributed with loc=1 and concentration=1 tensor([1.9777]) :param torch.Tensor loc: an angle in radians. :param torch.Tensor concentration: concentration parameter """ arg_constraints = {"loc": constraints.real, "concentration": constraints.positive} support = constraints.real has_rsample = False def __init__(self, loc, concentration, validate_args=None): self.loc, self.concentration = broadcast_all(loc, concentration) batch_shape = self.loc.shape event_shape = torch.Size() # Parameters for sampling tau = 1 + (1 + 4 * self.concentration**2).sqrt() rho = (tau - (2 * tau).sqrt()) / (2 * self.concentration) self._proposal_r = (1 + rho**2) / (2 * rho) super().__init__(batch_shape, event_shape, validate_args)
[docs] def log_prob(self, value): if self._validate_args: self._validate_sample(value) log_prob = self.concentration * torch.cos(value - self.loc) log_prob = ( log_prob - math.log(2 * math.pi) - _log_modified_bessel_fn(self.concentration, order=0) ) return log_prob
[docs] @torch.no_grad() def sample(self, sample_shape=torch.Size()): """ The sampling algorithm for the von Mises distribution is based on the following paper: Best, D. J., and Nicholas I. Fisher. "Efficient simulation of the von Mises distribution." Applied Statistics (1979): 152-157. """ shape = self._extended_shape(sample_shape) x = torch.empty(shape, dtype=self.loc.dtype, device=self.loc.device) return _rejection_sample(self.loc, self.concentration, self._proposal_r, x)
[docs] def expand(self, batch_shape): try: return super().expand(batch_shape) except NotImplementedError: validate_args = self.__dict__.get("_validate_args") loc = self.loc.expand(batch_shape) concentration = self.concentration.expand(batch_shape) return type(self)(loc, concentration, validate_args=validate_args)
@property def mean(self): """ The provided mean is the circular one. """ return self.loc @property def mode(self): return self.loc @lazy_property def variance(self): """ The provided variance is the circular one. """ return ( 1 - ( _log_modified_bessel_fn(self.concentration, order=1) - _log_modified_bessel_fn(self.concentration, order=0) ).exp() )

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