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.CondaPkg/env/Lib/site-packages/scipy/stats/_entropy.py

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+"""
+Created on Fri Apr  2 09:06:05 2021
+
+@author: matth
+"""
+
+from __future__ import annotations
+import math
+import numpy as np
+from scipy import special
+from ._axis_nan_policy import _axis_nan_policy_factory, _broadcast_arrays
+from scipy._lib._array_api import array_namespace
+
+__all__ = ['entropy', 'differential_entropy']
+
+
+@_axis_nan_policy_factory(
+    lambda x: x,
+    n_samples=lambda kwgs: (
+        2 if ("qk" in kwgs and kwgs["qk"] is not None)
+        else 1
+    ),
+    n_outputs=1, result_to_tuple=lambda x: (x,), paired=True,
+    too_small=-1  # entropy doesn't have too small inputs
+)
+def entropy(pk: np.typing.ArrayLike,
+            qk: np.typing.ArrayLike | None = None,
+            base: float | None = None,
+            axis: int = 0
+            ) -> np.number | np.ndarray:
+    """
+    Calculate the Shannon entropy/relative entropy of given distribution(s).
+
+    If only probabilities `pk` are given, the Shannon entropy is calculated as
+    ``H = -sum(pk * log(pk))``.
+
+    If `qk` is not None, then compute the relative entropy
+    ``D = sum(pk * log(pk / qk))``. This quantity is also known
+    as the Kullback-Leibler divergence.
+
+    This routine will normalize `pk` and `qk` if they don't sum to 1.
+
+    Parameters
+    ----------
+    pk : array_like
+        Defines the (discrete) distribution. Along each axis-slice of ``pk``,
+        element ``i`` is the  (possibly unnormalized) probability of event
+        ``i``.
+    qk : array_like, optional
+        Sequence against which the relative entropy is computed. Should be in
+        the same format as `pk`.
+    base : float, optional
+        The logarithmic base to use, defaults to ``e`` (natural logarithm).
+    axis : int, optional
+        The axis along which the entropy is calculated. Default is 0.
+
+    Returns
+    -------
+    S : {float, array_like}
+        The calculated entropy.
+
+    Notes
+    -----
+    Informally, the Shannon entropy quantifies the expected uncertainty
+    inherent in the possible outcomes of a discrete random variable.
+    For example,
+    if messages consisting of sequences of symbols from a set are to be
+    encoded and transmitted over a noiseless channel, then the Shannon entropy
+    ``H(pk)`` gives a tight lower bound for the average number of units of
+    information needed per symbol if the symbols occur with frequencies
+    governed by the discrete distribution `pk` [1]_. The choice of base
+    determines the choice of units; e.g., ``e`` for nats, ``2`` for bits, etc.
+
+    The relative entropy, ``D(pk|qk)``, quantifies the increase in the average
+    number of units of information needed per symbol if the encoding is
+    optimized for the probability distribution `qk` instead of the true
+    distribution `pk`. Informally, the relative entropy quantifies the expected
+    excess in surprise experienced if one believes the true distribution is
+    `qk` when it is actually `pk`.
+
+    A related quantity, the cross entropy ``CE(pk, qk)``, satisfies the
+    equation ``CE(pk, qk) = H(pk) + D(pk|qk)`` and can also be calculated with
+    the formula ``CE = -sum(pk * log(qk))``. It gives the average
+    number of units of information needed per symbol if an encoding is
+    optimized for the probability distribution `qk` when the true distribution
+    is `pk`. It is not computed directly by `entropy`, but it can be computed
+    using two calls to the function (see Examples).
+
+    See [2]_ for more information.
+
+    References
+    ----------
+    .. [1] Shannon, C.E. (1948), A Mathematical Theory of Communication.
+           Bell System Technical Journal, 27: 379-423.
+           https://doi.org/10.1002/j.1538-7305.1948.tb01338.x
+    .. [2] Thomas M. Cover and Joy A. Thomas. 2006. Elements of Information
+           Theory (Wiley Series in Telecommunications and Signal Processing).
+           Wiley-Interscience, USA.
+
+
+    Examples
+    --------
+    The outcome of a fair coin is the most uncertain:
+
+    >>> import numpy as np
+    >>> from scipy.stats import entropy
+    >>> base = 2  # work in units of bits
+    >>> pk = np.array([1/2, 1/2])  # fair coin
+    >>> H = entropy(pk, base=base)
+    >>> H
+    1.0
+    >>> H == -np.sum(pk * np.log(pk)) / np.log(base)
+    True
+
+    The outcome of a biased coin is less uncertain:
+
+    >>> qk = np.array([9/10, 1/10])  # biased coin
+    >>> entropy(qk, base=base)
+    0.46899559358928117
+
+    The relative entropy between the fair coin and biased coin is calculated
+    as:
+
+    >>> D = entropy(pk, qk, base=base)
+    >>> D
+    0.7369655941662062
+    >>> D == np.sum(pk * np.log(pk/qk)) / np.log(base)
+    True
+
+    The cross entropy can be calculated as the sum of the entropy and
+    relative entropy`:
+
+    >>> CE = entropy(pk, base=base) + entropy(pk, qk, base=base)
+    >>> CE
+    1.736965594166206
+    >>> CE == -np.sum(pk * np.log(qk)) / np.log(base)
+    True
+
+    """
+    if base is not None and base <= 0:
+        raise ValueError("`base` must be a positive number or `None`.")
+
+    xp = array_namespace(pk) if qk is None else array_namespace(pk, qk)
+
+    pk = xp.asarray(pk)
+    with np.errstate(invalid='ignore'):
+        pk = 1.0*pk / xp.sum(pk, axis=axis, keepdims=True)  # type: ignore[operator]
+    if qk is None:
+        vec = special.entr(pk)
+    else:
+        qk = xp.asarray(qk)
+        pk, qk = _broadcast_arrays((pk, qk), axis=None, xp=xp)  # don't ignore any axes
+        sum_kwargs = dict(axis=axis, keepdims=True)
+        qk = 1.0*qk / xp.sum(qk, **sum_kwargs)  # type: ignore[operator, call-overload]
+        vec = special.rel_entr(pk, qk)
+    S = xp.sum(vec, axis=axis)
+    if base is not None:
+        S /= math.log(base)
+    return S
+
+
+def _differential_entropy_is_too_small(samples, kwargs, axis=-1):
+    values = samples[0]
+    n = values.shape[axis]
+    window_length = kwargs.get("window_length",
+                               math.floor(math.sqrt(n) + 0.5))
+    if not 2 <= 2 * window_length < n:
+        return True
+    return False
+
+
+@_axis_nan_policy_factory(
+    lambda x: x, n_outputs=1, result_to_tuple=lambda x: (x,),
+    too_small=_differential_entropy_is_too_small
+)
+def differential_entropy(
+    values: np.typing.ArrayLike,
+    *,
+    window_length: int | None = None,
+    base: float | None = None,
+    axis: int = 0,
+    method: str = "auto",
+) -> np.number | np.ndarray:
+    r"""Given a sample of a distribution, estimate the differential entropy.
+
+    Several estimation methods are available using the `method` parameter. By
+    default, a method is selected based the size of the sample.
+
+    Parameters
+    ----------
+    values : sequence
+        Sample from a continuous distribution.
+    window_length : int, optional
+        Window length for computing Vasicek estimate. Must be an integer
+        between 1 and half of the sample size. If ``None`` (the default), it
+        uses the heuristic value
+
+        .. math::
+            \left \lfloor \sqrt{n} + 0.5 \right \rfloor
+
+        where :math:`n` is the sample size. This heuristic was originally
+        proposed in [2]_ and has become common in the literature.
+    base : float, optional
+        The logarithmic base to use, defaults to ``e`` (natural logarithm).
+    axis : int, optional
+        The axis along which the differential entropy is calculated.
+        Default is 0.
+    method : {'vasicek', 'van es', 'ebrahimi', 'correa', 'auto'}, optional
+        The method used to estimate the differential entropy from the sample.
+        Default is ``'auto'``.  See Notes for more information.
+
+    Returns
+    -------
+    entropy : float
+        The calculated differential entropy.
+
+    Notes
+    -----
+    This function will converge to the true differential entropy in the limit
+
+    .. math::
+        n \to \infty, \quad m \to \infty, \quad \frac{m}{n} \to 0
+
+    The optimal choice of ``window_length`` for a given sample size depends on
+    the (unknown) distribution. Typically, the smoother the density of the
+    distribution, the larger the optimal value of ``window_length`` [1]_.
+
+    The following options are available for the `method` parameter.
+
+    * ``'vasicek'`` uses the estimator presented in [1]_. This is
+      one of the first and most influential estimators of differential entropy.
+    * ``'van es'`` uses the bias-corrected estimator presented in [3]_, which
+      is not only consistent but, under some conditions, asymptotically normal.
+    * ``'ebrahimi'`` uses an estimator presented in [4]_, which was shown
+      in simulation to have smaller bias and mean squared error than
+      the Vasicek estimator.
+    * ``'correa'`` uses the estimator presented in [5]_ based on local linear
+      regression. In a simulation study, it had consistently smaller mean
+      square error than the Vasiceck estimator, but it is more expensive to
+      compute.
+    * ``'auto'`` selects the method automatically (default). Currently,
+      this selects ``'van es'`` for very small samples (<10), ``'ebrahimi'``
+      for moderate sample sizes (11-1000), and ``'vasicek'`` for larger
+      samples, but this behavior is subject to change in future versions.
+
+    All estimators are implemented as described in [6]_.
+
+    References
+    ----------
+    .. [1] Vasicek, O. (1976). A test for normality based on sample entropy.
+           Journal of the Royal Statistical Society:
+           Series B (Methodological), 38(1), 54-59.
+    .. [2] Crzcgorzewski, P., & Wirczorkowski, R. (1999). Entropy-based
+           goodness-of-fit test for exponentiality. Communications in
+           Statistics-Theory and Methods, 28(5), 1183-1202.
+    .. [3] Van Es, B. (1992). Estimating functionals related to a density by a
+           class of statistics based on spacings. Scandinavian Journal of
+           Statistics, 61-72.
+    .. [4] Ebrahimi, N., Pflughoeft, K., & Soofi, E. S. (1994). Two measures
+           of sample entropy. Statistics & Probability Letters, 20(3), 225-234.
+    .. [5] Correa, J. C. (1995). A new estimator of entropy. Communications
+           in Statistics-Theory and Methods, 24(10), 2439-2449.
+    .. [6] Noughabi, H. A. (2015). Entropy Estimation Using Numerical Methods.
+           Annals of Data Science, 2(2), 231-241.
+           https://link.springer.com/article/10.1007/s40745-015-0045-9
+
+    Examples
+    --------
+    >>> import numpy as np
+    >>> from scipy.stats import differential_entropy, norm
+
+    Entropy of a standard normal distribution:
+
+    >>> rng = np.random.default_rng()
+    >>> values = rng.standard_normal(100)
+    >>> differential_entropy(values)
+    1.3407817436640392
+
+    Compare with the true entropy:
+
+    >>> float(norm.entropy())
+    1.4189385332046727
+
+    For several sample sizes between 5 and 1000, compare the accuracy of
+    the ``'vasicek'``, ``'van es'``, and ``'ebrahimi'`` methods. Specifically,
+    compare the root mean squared error (over 1000 trials) between the estimate
+    and the true differential entropy of the distribution.
+
+    >>> from scipy import stats
+    >>> import matplotlib.pyplot as plt
+    >>>
+    >>>
+    >>> def rmse(res, expected):
+    ...     '''Root mean squared error'''
+    ...     return np.sqrt(np.mean((res - expected)**2))
+    >>>
+    >>>
+    >>> a, b = np.log10(5), np.log10(1000)
+    >>> ns = np.round(np.logspace(a, b, 10)).astype(int)
+    >>> reps = 1000  # number of repetitions for each sample size
+    >>> expected = stats.expon.entropy()
+    >>>
+    >>> method_errors = {'vasicek': [], 'van es': [], 'ebrahimi': []}
+    >>> for method in method_errors:
+    ...     for n in ns:
+    ...        rvs = stats.expon.rvs(size=(reps, n), random_state=rng)
+    ...        res = stats.differential_entropy(rvs, method=method, axis=-1)
+    ...        error = rmse(res, expected)
+    ...        method_errors[method].append(error)
+    >>>
+    >>> for method, errors in method_errors.items():
+    ...     plt.loglog(ns, errors, label=method)
+    >>>
+    >>> plt.legend()
+    >>> plt.xlabel('sample size')
+    >>> plt.ylabel('RMSE (1000 trials)')
+    >>> plt.title('Entropy Estimator Error (Exponential Distribution)')
+
+    """
+    values = np.asarray(values)
+    values = np.moveaxis(values, axis, -1)
+    n = values.shape[-1]  # number of observations
+
+    if window_length is None:
+        window_length = math.floor(math.sqrt(n) + 0.5)
+
+    if not 2 <= 2 * window_length < n:
+        raise ValueError(
+            f"Window length ({window_length}) must be positive and less "
+            f"than half the sample size ({n}).",
+        )
+
+    if base is not None and base <= 0:
+        raise ValueError("`base` must be a positive number or `None`.")
+
+    sorted_data = np.sort(values, axis=-1)
+
+    methods = {"vasicek": _vasicek_entropy,
+               "van es": _van_es_entropy,
+               "correa": _correa_entropy,
+               "ebrahimi": _ebrahimi_entropy,
+               "auto": _vasicek_entropy}
+    method = method.lower()
+    if method not in methods:
+        message = f"`method` must be one of {set(methods)}"
+        raise ValueError(message)
+
+    if method == "auto":
+        if n <= 10:
+            method = 'van es'
+        elif n <= 1000:
+            method = 'ebrahimi'
+        else:
+            method = 'vasicek'
+
+    res = methods[method](sorted_data, window_length)
+
+    if base is not None:
+        res /= np.log(base)
+
+    return res
+
+
+def _pad_along_last_axis(X, m):
+    """Pad the data for computing the rolling window difference."""
+    # scales a  bit better than method in _vasicek_like_entropy
+    shape = np.array(X.shape)
+    shape[-1] = m
+    Xl = np.broadcast_to(X[..., [0]], shape)  # [0] vs 0 to maintain shape
+    Xr = np.broadcast_to(X[..., [-1]], shape)
+    return np.concatenate((Xl, X, Xr), axis=-1)
+
+
+def _vasicek_entropy(X, m):
+    """Compute the Vasicek estimator as described in [6] Eq. 1.3."""
+    n = X.shape[-1]
+    X = _pad_along_last_axis(X, m)
+    differences = X[..., 2 * m:] - X[..., : -2 * m:]
+    logs = np.log(n/(2*m) * differences)
+    return np.mean(logs, axis=-1)
+
+
+def _van_es_entropy(X, m):
+    """Compute the van Es estimator as described in [6]."""
+    # No equation number, but referred to as HVE_mn.
+    # Typo: there should be a log within the summation.
+    n = X.shape[-1]
+    difference = X[..., m:] - X[..., :-m]
+    term1 = 1/(n-m) * np.sum(np.log((n+1)/m * difference), axis=-1)
+    k = np.arange(m, n+1)
+    return term1 + np.sum(1/k) + np.log(m) - np.log(n+1)
+
+
+def _ebrahimi_entropy(X, m):
+    """Compute the Ebrahimi estimator as described in [6]."""
+    # No equation number, but referred to as HE_mn
+    n = X.shape[-1]
+    X = _pad_along_last_axis(X, m)
+
+    differences = X[..., 2 * m:] - X[..., : -2 * m:]
+
+    i = np.arange(1, n+1).astype(float)
+    ci = np.ones_like(i)*2
+    ci[i <= m] = 1 + (i[i <= m] - 1)/m
+    ci[i >= n - m + 1] = 1 + (n - i[i >= n-m+1])/m
+
+    logs = np.log(n * differences / (ci * m))
+    return np.mean(logs, axis=-1)
+
+
+def _correa_entropy(X, m):
+    """Compute the Correa estimator as described in [6]."""
+    # No equation number, but referred to as HC_mn
+    n = X.shape[-1]
+    X = _pad_along_last_axis(X, m)
+
+    i = np.arange(1, n+1)
+    dj = np.arange(-m, m+1)[:, None]
+    j = i + dj
+    j0 = j + m - 1  # 0-indexed version of j
+
+    Xibar = np.mean(X[..., j0], axis=-2, keepdims=True)
+    difference = X[..., j0] - Xibar
+    num = np.sum(difference*dj, axis=-2)  # dj is d-i
+    den = n*np.sum(difference**2, axis=-2)
+    return -np.mean(np.log(num/den), axis=-1)