def _cdf(self, x):
concentration = tf.convert_to_tensor(self.concentration)
loc = tf.convert_to_tensor(self.loc)
return (
special_math.ndtr((tf.math.rsqrt(x / concentration) * (x / loc - 1.))) +
tf.exp(2. * concentration / loc) *
special_math.ndtr(-tf.math.rsqrt(x / concentration) * (x / loc + 1)))
def _cdf(self, x):
u = self.rate * (x - self.loc)
v = self.rate * self.scale
v2 = tf.square(v)
return (
special_math.ndtr(u / v) -
tf.exp(-u + 0.5 * v2 + tf.log(special_math.ndtr((u - v2) / v))))
def _quantile(self, p):
"""See https://www.ntrand.com/truncated-normal-distribution/"""
a = (self.low - self.loc) / self.scale
b = (self.high - self.loc) / self.scale
delta = special_math.ndtr(b) - special_math.ndtr(a)
x = delta * p + special_math.ndtr(a)
return tf.math.ndtri(x) * self.scale + self.loc
def _cdf(self, x):
loc, scale, low, high = self._loc_scale_low_high()
std_low, std_high = self._standardized_low_and_high(
low=low, high=high, loc=loc, scale=scale)
cdf_in_support = ((special_math.ndtr(
(x - loc) / scale) - special_math.ndtr(std_low)) /
self._normalizer(std_low=std_low, std_high=std_high))
return tf.clip_by_value(cdf_in_support, 0., 1.)
def _normal_cdf_difference(x, y):
"""Computes ndtr(x) - ndtr(y) assuming that x >= y."""
# When x >= y >= 0, we will return ndtr(-y) - ndtr(-x)
# because ndtr does not have a good precision for large positive x, y.
is_y_positive = y >= 0
x_hat = tf.where(is_y_positive, -y, x)
y_hat = tf.where(is_y_positive, -x, y)
return special_math.ndtr(x_hat) - special_math.ndtr(y_hat)
def _cdf(self, x):
with tf.control_dependencies(self._maybe_assert_valid_sample(x)):
concentration = tf.convert_to_tensor(self.concentration)
loc = tf.convert_to_tensor(self.loc)
return (special_math.ndtr(
((concentration / x)**0.5 *
(x / loc - 1.))) + tf.exp(2. * concentration / loc) *
special_math.ndtr(-(concentration / x)**0.5 *
(x / loc + 1)))
def _normalizer(self,
loc=None,
scale=None,
low=None,
high=None,
std_low=None,
std_high=None):
if std_low is None or std_high is None:
std_low, std_high = self._standardized_low_and_high(
loc=loc, scale=scale, low=low, high=high)
return special_math.ndtr(std_high) - special_math.ndtr(std_low)
def _quantile(self, p):
# TODO(b/188413116): This implementation is analytically correct, but might
# not perform well in all cases. See
# https://en.wikipedia.org/wiki/Truncated_normal_distribution#Generating_values_from_the_truncated_normal_distribution)
# for a discussion on alternatives.
loc, scale, low, high = self._loc_scale_low_high()
std_low, std_high = self._standardized_low_and_high(
low=low, high=high, loc=loc, scale=scale)
quantile = tf.math.ndtri(
special_math.ndtr(std_low) + p *
(special_math.ndtr(std_high) - special_math.ndtr(std_low))) * scale + loc
return quantile
def _cdf(self, x):
with tf.control_dependencies([
tf.compat.v1.assert_greater(x,
tf.cast(0., x.dtype.base_dtype),
message="x must be positive.")
] if self.validate_args else []):
return (special_math.ndtr(
((self.concentration / x)**0.5 * (x / self.loc - 1.))) +
tf.exp(2. * self.concentration / self.loc) *
special_math.ndtr(-(self.concentration / x)**0.5 *
(x / self.loc + 1)))
def _cdf(self, x):
with tf.control_dependencies([
assert_util.assert_greater(x,
dtype_util.as_numpy_dtype(x.dtype)
(0),
message="x must be positive.")
] if self.validate_args else []):
return (special_math.ndtr(
((self.concentration / x)**0.5 * (x / self.loc - 1.))) +
tf.exp(2. * self.concentration / self.loc) *
special_math.ndtr(-(self.concentration / x)**0.5 *
(x / self.loc + 1)))
def test_chandrupatla_scalar_inverse_gaussian_cdf(self):
true_x = 3.14159
u = special_math.ndtr(true_x)
roots, value_at_roots, _ = tfp.math.find_root_chandrupatla(
objective_fn=lambda x: special_math.ndtr(x) - u,
low=-100.,
high=100.,
position_tolerance=1e-8)
self.assertAllClose(value_at_roots, tf.zeros_like(value_at_roots))
# The normal CDF function is not precise enough to be inverted to a
# position tolerance of 1e-8 (the objective goes to zero relatively
# far from the expected point), so check it at a lower tolerance.
self.assertAllClose(roots, true_x, atol=1e-4)
def _test_grid_no_log(self, dtype, grid_spec, error_spec):
if not special:
return
grid = _make_grid(dtype, grid_spec)
actual = self.evaluate(special_math.ndtr(grid))
# Basic tests.
# isfinite checks for NaN and Inf.
self.assertTrue(np.isfinite(actual).all())
# On the grid, 0 < cdf(x) < 1. The grid cannot contain everything due
# to numerical limitations of cdf.
self.assertTrue((actual > 0).all())
self.assertTrue((actual < 1).all())
_check_strictly_increasing(actual)
# Versus scipy.
expected = special.ndtr(grid)
# Scipy prematurely goes to zero at some places that we don't. So don't
# include these in the comparison.
self.assertAllClose(
expected.astype(np.float64)[expected < 0],
actual.astype(np.float64)[expected < 0],
rtol=error_spec.rtol,
atol=error_spec.atol)
def _cdf(self, x):
rate = tf.convert_to_tensor(self.rate)
x_centralized = x - self.loc
u = rate * x_centralized
v = rate * self.scale
vsquared = tf.square(v)
return special_math.ndtr(x_centralized / self.scale) - tf.exp(
-u + vsquared / 2. + special_math.log_ndtr((u - vsquared) / v))
def grad(dy):
"""Computes a derivative for the min and max parameters.
This function implements the derivative wrt the truncation bounds, which
get blocked by the sampler. We use a custom expression for numerical
stability instead of automatic differentiation on CDF for implicit
gradients.
Args:
dy: output gradients
Returns:
The standard normal samples and the gradients wrt the upper
bound and lower bound.
"""
# std_samples has an extra dimension (the sample dimension), expand
# lower and upper so they broadcast along this dimension.
# See note above regarding parameterized_truncated_normal, the sample
# dimension is the final dimension.
lower_broadcast = lower[..., tf.newaxis]
upper_broadcast = upper[..., tf.newaxis]
cdf_samples = ((special_math.ndtr(std_samples) -
special_math.ndtr(lower_broadcast)) /
(special_math.ndtr(upper_broadcast)
- special_math.ndtr(lower_broadcast)))
# tiny, eps are tolerance parameters to ensure we stay away from giving
# a zero arg to the log CDF expression.
tiny = np.finfo(self.dtype.as_numpy_dtype).tiny
eps = np.finfo(self.dtype.as_numpy_dtype).eps
cdf_samples = tf.clip_by_value(cdf_samples, tiny, 1 - eps)
du = tf.exp(0.5 * (std_samples**2 - upper_broadcast**2) +
tf.math.log(cdf_samples))
dl = tf.exp(0.5 * (std_samples**2 - lower_broadcast**2) +
tf.math.log1p(-cdf_samples))
# Reduce the gradient across the samples
grad_u = tf.reduce_sum(input_tensor=dy * du, axis=-1)
grad_l = tf.reduce_sum(input_tensor=dy * dl, axis=-1)
return [grad_l, grad_u]
def _test_grad_finite(self, dtype):
x = tf.constant([-100., 0., 100.], dtype=dtype)
output = (special_math.log_ndtr(x) if self._use_log
else special_math.ndtr(x))
fn = special_math.log_ndtr if self._use_log else special_math.ndtr
# Not having the lambda sanitzer means we'd get an `IndexError` whenever
# the user supplied function has default args.
output, grad_output = value_and_gradient(fn, x)
# isfinite checks for NaN and Inf.
output_, grad_output_ = self.evaluate([output, grad_output])
self.assert_all_true(np.isfinite(output_))
self.assert_all_true(np.isfinite(grad_output_[0]))
def _survival_function(self, x): return special_math.ndtr(-self._z(x))
def _cdf(self, x):
cdf_in_support = ((special_math.ndtr((x - self.loc) / self.scale)
- special_math.ndtr(self._standardized_low))
/ self._normalizer)
return tf.clip_by_value(cdf_in_support, 0., 1.)
def _normalizer(self):
return (special_math.ndtr(self._standardized_high) -
special_math.ndtr(self._standardized_low))
def _forward(self, x): return special_math.ndtr(x)
def _survival_function(self, x):
return special_math.ndtr(-self._z(x))
def _cdf(self, x):
return special_math.ndtr(self._z(x))
def mean(self, name='mean', **kwargs):
u = self.loc
s = self.scale
return s * np.sqrt(2 / np.pi) * tf.math.exp(
-0.5 * (u / s)**2.) + u * (1. - 2. * special_math.ndtr(-u / s))
def _cdf(self, x): return special_math.ndtr(self._z(x))
def _probs_parameter_no_checks(self):
if self._probits is None:
return tf.identity(self._probs)
return special_math.ndtr(self._probits)
def norm_cdf(x, approx=False):
return abrahamowitz_stegun_cdf(x) if approx else special_math.ndtr(x)
def _probs_parameter_no_checks(self):
if self._probits is None:
return tensor_util.identity_as_tensor(self._probs)
return special_math.ndtr(self._probits)
