# Copyright 2020 The TensorFlow Probability Authors.
#
# Licensed under the Apache License, Version 2.0 (the "License");
# you may not use this file except in compliance with the License.
# You may obtain a copy of the License at
#
#     http://www.apache.org/licenses/LICENSE-2.0
#
# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS,
# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
# See the License for the specific language governing permissions and
# limitations under the License.
# ============================================================================
"""The Truncated Cauchy distribution class."""

from __future__ import absolute_import
from __future__ import division
from __future__ import print_function

# Dependency imports
import numpy as np

import tensorflow.compat.v2 as tf

from tensorflow_probability.python.bijectors import sigmoid as sigmoid_bijector
from tensorflow_probability.python.bijectors import softplus as softplus_bijector
from tensorflow_probability.python.distributions import distribution
from tensorflow_probability.python.internal import assert_util
from tensorflow_probability.python.internal import dtype_util
from tensorflow_probability.python.internal import parameter_properties
from tensorflow_probability.python.internal import prefer_static as ps
from tensorflow_probability.python.internal import reparameterization
from tensorflow_probability.python.internal import samplers
from tensorflow_probability.python.internal import tensor_util
from tensorflow_probability.python.math import numeric
from tensorflow_probability.python.math import special as tfp_math

__all__ = [
    'TruncatedCauchy',
]


def _cauchy_cdf_diff(x, y):
  return tfp_math.atan_difference(x, y) / np.pi


class TruncatedCauchy(distribution.AutoCompositeTensorDistribution):
  """The Truncated Cauchy distribution.

  The truncated Cauchy is a Cauchy distribution bounded between `low`
  and `high` (the pdf is 0 outside these bounds and renormalized).

  Samples from this distribution are differentiable with respect to `loc`
  and `scale`, but not with respect to the bounds `low` and `high`.

  ### Mathematical Details

  The probability density function (pdf) of this distribution is:
  ```none
    pdf(x; loc, scale, low, high) =
        { 1 / (pi * scale * (1 + z**2) * A) for low <= x <= high
        { 0                                 otherwise
    z = (x - loc) / scale
    A = CauchyCDF((high - loc) / scale) - CauchyCDF((low - loc) / scale)
  ```

  where:

  * `CauchyCDF` is the cumulative density function of the Cauchy distribution
    with 0 mean and unit variance.

  This is a scalar distribution so the event shape is always scalar and the
  dimensions of the parameters define the batch_shape.

  #### Examples
  ```python

  tfd = tfp.distributions
  # Define a batch of two scalar TruncatedCauchy distributions with modes
  # at 0. and 1.0 .
  dist = tfd.TruncatedCauchy(loc=[0., 1.], scale=1.,
                             low=[-1., 0.],
                             high=[1., 1.])

  # Evaluate the pdf of the distributions at 0.5 and 0.8 respectively returning
  # a 2-vector tensor.
  dist.prob([0.5, 0.8])

  # Get 3 samples, returning a 3 x 2 tensor.
  dist.sample([3])
  ```
  """

  def __init__(self,
               loc,
               scale,
               low,
               high,
               validate_args=False,
               allow_nan_stats=True,
               name='TruncatedCauchy'):
    """Construct a TruncatedCauchy.

    All parameters of the distribution will be broadcast to the same shape,
    so the resulting distribution will have a batch_shape of the broadcast
    shape of all parameters.

    Args:
      loc: Floating point tensor; the modes of the corresponding non-truncated
        Cauchy distribution(s).
      scale: Floating point tensor; the scales of the distribution(s).
        Must contain only positive values.
      low: `float` `Tensor` representing lower bound of the distribution's
        support. Must be such that `low < high`.
      high: `float` `Tensor` representing upper bound of the distribution's
        support. Must be such that `low < high`.
      validate_args: Python `bool`, default `False`. When `True` distribution
        parameters are checked at run-time.
      allow_nan_stats: Python `bool`, default `True`. When `True`,
        statistics (e.g., mean, mode, variance) use the value '`NaN`' to
        indicate the result is undefined. When `False`, an exception is raised
        if one or more of the statistic's batch members are undefined.
      name: Python `str` name prefixed to Ops created by this class.
    """
    parameters = dict(locals())
    with tf.name_scope(name) as name:
      dtype = dtype_util.common_dtype([loc, scale, low, high], tf.float32)
      self._loc = tensor_util.convert_nonref_to_tensor(
          loc, name='loc', dtype=dtype)
      self._scale = tensor_util.convert_nonref_to_tensor(
          scale, name='scale', dtype=dtype)
      self._low = tensor_util.convert_nonref_to_tensor(
          low, name='low', dtype=dtype)
      self._high = tensor_util.convert_nonref_to_tensor(
          high, name='high', dtype=dtype)
      dtype_util.assert_same_float_dtype(
          [self._loc, self._scale, self._low, self._high])

      super(TruncatedCauchy, self).__init__(
          dtype=dtype,
          # Samples do not have gradients with respect to `_low` and `_high`.
          # TODO(b/161297284): Implement these gradients.
          reparameterization_type=reparameterization.NOT_REPARAMETERIZED,
          validate_args=validate_args,
          allow_nan_stats=allow_nan_stats,
          parameters=parameters,
          name=name)

  def _loc_scale_low_high(self, loc=None, scale=None, low=None, high=None):
    loc = tf.convert_to_tensor(self.loc if loc is None else loc)
    scale = tf.convert_to_tensor(self.scale if scale is None else scale)
    low = tf.convert_to_tensor(self.low if low is None else low)
    high = tf.convert_to_tensor(self.high if high is None else high)
    return loc, scale, low, high

  def _standardized_low_and_high(self,
                                 loc=None,
                                 scale=None,
                                 low=None,
                                 high=None):
    loc, scale, low, high = self._loc_scale_low_high(
        loc=loc, scale=scale, low=low, high=high)
    return (low - loc) / scale, (high - loc) / scale

  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 _cauchy_cdf_diff(std_high, std_low)

  def _log_normalizer(self,
                      loc=None,
                      scale=None,
                      low=None,
                      high=None,
                      std_low=None,
                      std_high=None):
    return tf.math.log(self._normalizer(
        loc=loc,
        scale=scale,
        low=low,
        high=high,
        std_low=std_low,
        std_high=std_high))

  @classmethod
  def _parameter_properties(cls, dtype, num_classes=None):
    # pylint: disable=g-long-lambda
    return dict(
        loc=parameter_properties.ParameterProperties(),
        scale=parameter_properties.ParameterProperties(
            default_constraining_bijector_fn=(
                lambda: softplus_bijector.Softplus(low=dtype_util.eps(dtype)))),
        low=parameter_properties.ParameterProperties(),
        # TODO(b/169874884): Support decoupled parameterization.
        high=parameter_properties.ParameterProperties(
            default_constraining_bijector_fn=parameter_properties
            .BIJECTOR_NOT_IMPLEMENTED,))
    # pylint: enable=g-long-lambda

  @property
  def loc(self):
    return self._loc

  @property
  def scale(self):
    return self._scale

  @property
  def low(self):
    return self._low

  @property
  def high(self):
    return self._high

  def _event_shape(self):
    return tf.TensorShape([])

  def _sample_n(self, n, seed=None):
    loc, scale, low, high = self._loc_scale_low_high()
    batch_shape = self._batch_shape_tensor(
        loc=loc, scale=scale, low=low, high=high)
    sample_and_batch_shape = ps.concat([[n], batch_shape], axis=0)

    u = samplers.uniform(sample_and_batch_shape, dtype=self.dtype, seed=seed)
    return self._quantile(u, loc=loc, scale=scale, low=low, high=high)

  def _log_prob(self, x):
    loc, scale, low, high = self._loc_scale_low_high()
    log_prob = (
        -tf.math.log1p(tf.square((x - loc) / scale))
        - (np.log(np.pi) + tf.math.log(scale))
        - self._log_normalizer(loc=loc, scale=scale, low=low, high=high))
    # p(x) is 0 outside the bounds.
    return tf.where((x > high) | (x < low),
                    dtype_util.as_numpy_dtype(x.dtype)(-np.inf),
                    log_prob)

  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)
    return tf.clip_by_value(
        ((_cauchy_cdf_diff((x - loc) / scale, std_low))
         / self._normalizer(std_low=std_low, std_high=std_high)),
        clip_value_min=0., clip_value_max=1.)

  def _log_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)
    return (
        tf.math.log(_cauchy_cdf_diff((x - loc) / scale, std_low))
        - self._log_normalizer(std_low=std_low, std_high=std_high))

  def _mean(self):
    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)

    # Formula from David Olive, "Applied Robust Statistics" --
    # see http://parker.ad.siu.edu/Olive/ch4.pdf .
    t = (tf.math.log1p(tf.math.square(std_high))
         - tf.math.log1p(tf.math.square(std_low)))
    t = t / (2 * tfp_math.atan_difference(std_high, std_low))
    return loc + scale * t

  def _mode(self):
    # mode = { loc: for low <= loc <= high
    #          low: for loc < low
    #          high: for loc > high
    #        }
    loc = tf.convert_to_tensor(self.loc)
    low = tf.convert_to_tensor(self.low)
    high = tf.convert_to_tensor(self.high)
    shape = self._batch_shape_tensor(loc=loc, low=low, high=high)
    # We *must* broadcast with scale to get a correctly shaped output, but
    # TODO(b/141460015): we should not have to explicitly broadcast the first
    # parameter to clip_by_value to align with the second and third parameters.
    return tf.clip_by_value(tf.broadcast_to(loc, shape), low, high)

  def _variance(self):
    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)

    # Formula from David Olive, "Applied Robust Statistics" --
    # see http://parker.ad.siu.edu/Olive/ch4.pdf .
    atan_diff = tfp_math.atan_difference(std_high, std_low)
    t = (std_high - std_low - atan_diff) / atan_diff
    std_mean = ((tf.math.log1p(tf.math.square(std_high))
                 - tf.math.log1p(tf.math.square(std_low))) / (2 * atan_diff))
    return tf.math.square(scale) * (t - tf.math.square(std_mean))

  def _quantile(self, p, loc=None, scale=None, low=None, high=None):
    loc, scale, low, high = self._loc_scale_low_high(loc, scale, low, high)
    std_low, std_high = self._standardized_low_and_high(
        low=low, high=high, loc=loc, scale=scale)
    # Use the sum of tangents formula.
    # First, the quantile of the cauchy distribution is tan(pi * (x - 0.5)).
    # and the cdf of the cauchy distribution is 0.5 + arctan(x) / np.pi
    # WLOG, we will assume loc = 0 , scale = 1 (these can be taken in to account
    # by rescaling and shifting low and high, and then scaling the output).
    # We would like to compute quantile(p * (cdf(high) - cdf(low)) + cdf(low))
    # This is the same as:
    # tan(pi * (cdf(low) + (cdf(high) - cdf(low)) * p - 0.5))
    # Let a = pi * (cdf(low) - 0.5), b = pi * (cdf(high) - cdf(low)) * u
    # By using the formula for the cdf we have:
    # a = arctan(low), b = arctan_difference(high, low) * u
    # Thus the quantile is now tan(a + b).
    # By appealing to the sum of tangents formula we have:
    # tan(a + b) = (tan(a) + tan(b)) / (1 - tan(a) * tan(b)) =
    # (low + tan(b)) / (1 - low * tan(b))
    # Thus for a 'standard' truncated cauchy we have the quantile as:
    # quantile(p) = (low + tan(b)) / (1 - low * tan(b)) where
    # b = arctan_difference(high, low) * p.

    tanb = tf.math.tan(tfp_math.atan_difference(std_high, std_low) * p)
    x = (std_low + tanb) / (1 - std_low * tanb)
    # Clip the answer to prevent it from falling numerically outside
    # the support.
    return numeric.clip_by_value_preserve_gradient(
        x * scale + loc, clip_value_min=low, clip_value_max=high)

  def _default_event_space_bijector(self):
    return sigmoid_bijector.Sigmoid(
        low=self.low, high=self.high, validate_args=self.validate_args)

  def _parameter_control_dependencies(self, is_init):
    if not self.validate_args:
      return []
    assertions = []
    low = None
    high = None
    if is_init != tensor_util.is_ref(self.low):
      low = tf.convert_to_tensor(self.low)
      assertions.append(
          assert_util.assert_finite(low, message='`low` is not finite'))
    if is_init != tensor_util.is_ref(self.high):
      high = tf.convert_to_tensor(self.high)
      assertions.append(
          assert_util.assert_finite(high, message='`high` is not finite'))
    if is_init != tensor_util.is_ref(self.loc):
      assertions.append(
          assert_util.assert_finite(self.loc, message='`loc` is not finite'))
    if is_init != tensor_util.is_ref(self.scale):
      scale = tf.convert_to_tensor(self.scale)
      assertions.extend([
          assert_util.assert_positive(
              scale, message='`scale` must be positive'),
          assert_util.assert_finite(scale, message='`scale` is not finite'),
      ])
    if (is_init != tensor_util.is_ref(self.low) or
        is_init != tensor_util.is_ref(self.high)):
      low = tf.convert_to_tensor(self.low) if low is None else low
      high = tf.convert_to_tensor(self.high) if high is None else high
      assertions.append(
          assert_util.assert_greater(
              high,
              low,
              message='TruncatedCauchy not defined when `low >= high`.'))
    return assertions

  def _sample_control_dependencies(self, x):
    assertions = []
    if not self.validate_args:
      return assertions
    assertions.append(assert_util.assert_greater_equal(
        x, self.low, message='Sample must be greater than or equal to `low`.'))
    assertions.append(assert_util.assert_less_equal(
        x, self.high, message='Sample must be less than or equal to `high`.'))
    return assertions
