QuaPy/quapy/functional.py

import itertools
from collections import defaultdict
from typing import Union, Callable

import scipy
import numpy as np


def prevalence_linspace(n_prevalences=21, repeats=1, smooth_limits_epsilon=0.01):
    """
    Produces an array of uniformly separated values of prevalence.
    By default, produces an array of 21 prevalence values, with
    step 0.05 and with the limits smoothed, i.e.:
    [0.01, 0.05, 0.10, 0.15, ..., 0.90, 0.95, 0.99]

    :param n_prevalences: the number of prevalence values to sample from the [0,1] interval (default 21)
    :param repeats: number of times each prevalence is to be repeated (defaults to 1)
    :param smooth_limits_epsilon: the quantity to add and subtract to the limits 0 and 1
    :return: an array of uniformly separated prevalence values
    """
    p = np.linspace(0., 1., num=n_prevalences, endpoint=True)
    p[0] += smooth_limits_epsilon
    p[-1] -= smooth_limits_epsilon
    if p[0] > p[1]:
        raise ValueError(f'the smoothing in the limits is greater than the prevalence step')
    if repeats > 1:
        p = np.repeat(p, repeats)
    return p


def prevalence_from_labels(labels, classes):
    """
    Computed the prevalence values from a vector of labels.

    :param labels: array-like of shape `(n_instances)` with the label for each instance
    :param classes: the class labels. This is needed in order to correctly compute the prevalence vector even when
        some classes have no examples.
    :return: an ndarray of shape `(len(classes))` with the class prevalence values
    """
    if labels.ndim != 1:
        raise ValueError(f'param labels does not seem to be a ndarray of label predictions')
    unique, counts = np.unique(labels, return_counts=True)
    by_class = defaultdict(lambda:0, dict(zip(unique, counts)))
    prevalences = np.asarray([by_class[class_] for class_ in classes], dtype=float)
    prevalences /= prevalences.sum()
    return prevalences


def prevalence_from_probabilities(posteriors, binarize: bool = False):
    """
    Returns a vector of prevalence values from a matrix of posterior probabilities.

    :param posteriors: array-like of shape `(n_instances, n_classes,)` with posterior probabilities for each class
    :param binarize: set to True (default is False) for computing the prevalence values on crisp decisions (i.e.,
        converting the vectors of posterior probabilities into class indices, by taking the argmax).
    :return: array of shape `(n_classes,)` containing the prevalence values
    """
    if posteriors.ndim != 2:
        raise ValueError(f'param posteriors does not seem to be a ndarray of posteior probabilities')
    if binarize:
        predictions = np.argmax(posteriors, axis=-1)
        return prevalence_from_labels(predictions, np.arange(posteriors.shape[1]))
    else:
        prevalences = posteriors.mean(axis=0)
        prevalences /= prevalences.sum()
        return prevalences


def as_binary_prevalence(positive_prevalence: Union[float, np.ndarray], clip_if_necessary=False):
    """
    Helper that, given a float representing the prevalence for the positive class, returns a np.ndarray of two
    values representing a binary distribution.

    :param positive_prevalence: prevalence for the positive class
    :param clip_if_necessary: if True, clips the value in [0,1] in order to guarantee the resulting distribution
        is valid. If False, it then checks that the value is in the valid range, and raises an error if not.
    :return: np.ndarray of shape `(2,)`
    """
    if clip_if_necessary:
        positive_prevalence = np.clip(positive_prevalence, 0, 1)
    else:
        assert 0 <= positive_prevalence <= 1, 'the value provided is not a valid prevalence for the positive class'
    return np.asarray([1-positive_prevalence, positive_prevalence]).T



def HellingerDistance(P, Q) -> float:
    """
    Computes the Hellingher Distance (HD) between (discretized) distributions `P` and `Q`.
    The HD for two discrete distributions of `k` bins is defined as:

    .. math::
        HD(P,Q) = \\frac{ 1 }{ \\sqrt{ 2 } } \\sqrt{ \\sum_{i=1}^k ( \\sqrt{p_i} - \\sqrt{q_i} )^2 }

    :param P: real-valued array-like of shape `(k,)` representing a discrete distribution
    :param Q: real-valued array-like of shape `(k,)` representing a discrete distribution
    :return: float
    """
    return np.sqrt(np.sum((np.sqrt(P) - np.sqrt(Q))**2))


def TopsoeDistance(P, Q, epsilon=1e-20):
    """
    Topsoe distance between two (discretized) distributions `P` and `Q`.
    The Topsoe distance for two discrete distributions of `k` bins is defined as:

    .. math::
        Topsoe(P,Q) = \\sum_{i=1}^k \\left( p_i \\log\\left(\\frac{ 2 p_i + \\epsilon }{ p_i+q_i+\\epsilon }\\right) +
            q_i \\log\\left(\\frac{ 2 q_i + \\epsilon }{ p_i+q_i+\\epsilon }\\right) \\right)

    :param P: real-valued array-like of shape `(k,)` representing a discrete distribution
    :param Q: real-valued array-like of shape `(k,)` representing a discrete distribution
    :return: float
    """
    return np.sum(P*np.log((2*P+epsilon)/(P+Q+epsilon)) + Q*np.log((2*Q+epsilon)/(P+Q+epsilon)))
                  

def uniform_prevalence_sampling(n_classes, size=1):
    """
    Implements the `Kraemer algorithm <http://www.cs.cmu.edu/~nasmith/papers/smith+tromble.tr04.pdf>`_
    for sampling uniformly at random from the unit simplex. This implementation is adapted from this
    `post <https://cs.stackexchange.com/questions/3227/uniform-sampling-from-a-simplex>_`.

    :param n_classes: integer, number of classes (dimensionality of the simplex)
    :param size: number of samples to return
    :return: `np.ndarray` of shape `(size, n_classes,)` if `size>1`, or of shape `(n_classes,)` otherwise
    """
    if n_classes == 2:
        u = np.random.rand(size)
        u = np.vstack([1-u, u]).T
    else:
        u = np.random.rand(size, n_classes-1)
        u.sort(axis=-1)
        _0s = np.zeros(shape=(size, 1))
        _1s = np.ones(shape=(size, 1))
        a = np.hstack([_0s, u])
        b = np.hstack([u, _1s])
        u = b-a
    if size == 1:
        u = u.flatten()
    return u


uniform_simplex_sampling = uniform_prevalence_sampling


def strprev(prevalences, prec=3):
    """
    Returns a string representation for a prevalence vector. E.g.,

    >>> strprev([1/3, 2/3], prec=2)
    >>> '[0.33, 0.67]'

    :param prevalences: a vector of prevalence values
    :param prec: float precision
    :return: string
    """
    return '['+ ', '.join([f'{p:.{prec}f}' for p in prevalences]) + ']'


def adjusted_quantification(prevalence_estim, tpr, fpr, clip=True):
    """
    Implements the adjustment of ACC and PACC for the binary case. The adjustment for a prevalence estimate of the
    positive class `p` comes down to computing:

    .. math::
        ACC(p) = \\frac{ p - fpr }{ tpr - fpr }

    :param prevalence_estim: float, the estimated value for the positive class
    :param tpr: float, the true positive rate of the classifier
    :param fpr: float, the false positive rate of the classifier
    :param clip: set to True (default) to clip values that might exceed the range [0,1]
    :return: float, the adjusted count
    """

    den = tpr - fpr
    if den == 0:
        den += 1e-8
    adjusted = (prevalence_estim - fpr) / den
    if clip:
        adjusted = np.clip(adjusted, 0., 1.)
    return adjusted


def normalize_prevalence(prevalences):
    """
    Normalize a vector or matrix of prevalence values. The normalization consists of applying a L1 normalization in
    cases in which the prevalence values are not all-zeros, and to convert the prevalence values into `1/n_classes` in
    cases in which all values are zero.

    :param prevalences: array-like of shape `(n_classes,)` or of shape `(n_samples, n_classes,)` with prevalence values
    :return: a normalized vector or matrix of prevalence values
    """
    prevalences = np.asarray(prevalences)
    n_classes = prevalences.shape[-1]
    accum = prevalences.sum(axis=-1, keepdims=True)
    prevalences = np.true_divide(prevalences, accum, where=accum>0)
    allzeros = accum.flatten()==0
    if any(allzeros):
        if prevalences.ndim == 1:
            prevalences = np.full(shape=n_classes, fill_value=1./n_classes)
        else:
            prevalences[accum.flatten()==0] = np.full(shape=n_classes, fill_value=1./n_classes)
    return prevalences


def __num_prevalence_combinations_depr(n_prevpoints:int, n_classes:int, n_repeats:int=1):
    """
    Computes the number of prevalence combinations in the n_classes-dimensional simplex if `nprevpoints` equally distant
    prevalence values are generated and `n_repeats` repetitions are requested.

    :param n_classes: integer, number of classes
    :param n_prevpoints: integer, number of prevalence points.
    :param n_repeats: integer, number of repetitions for each prevalence combination
    :return: The number of possible combinations. For example, if n_classes=2, n_prevpoints=5, n_repeats=1, then the
        number of possible combinations are 5, i.e.: [0,1], [0.25,0.75], [0.50,0.50], [0.75,0.25], and [1.0,0.0]
    """
    __cache={}
    def __f(nc,np):
        if (nc,np) in __cache:  # cached result
            return __cache[(nc,np)]
        if nc==1:  # stop condition
            return 1
        else:  # recursive call
            x = sum([__f(nc-1, np-i) for i in range(np)])
            __cache[(nc,np)] = x
            return x
    return __f(n_classes, n_prevpoints) * n_repeats


def num_prevalence_combinations(n_prevpoints:int, n_classes:int, n_repeats:int=1):
    """
    Computes the number of valid prevalence combinations in the n_classes-dimensional simplex if `n_prevpoints` equally
    distant prevalence values are generated and `n_repeats` repetitions are requested.
    The computation comes down to calculating:

    .. math::
        \\binom{N+C-1}{C-1} \\times r

    where `N` is `n_prevpoints-1`, i.e., the number of probability mass blocks to allocate, `C` is the number of
    classes, and `r` is `n_repeats`. This solution comes from the
    `Stars and Bars <https://brilliant.org/wiki/integer-equations-star-and-bars/>`_ problem.

    :param n_classes: integer, number of classes
    :param n_prevpoints: integer, number of prevalence points.
    :param n_repeats: integer, number of repetitions for each prevalence combination
    :return: The number of possible combinations. For example, if n_classes=2, n_prevpoints=5, n_repeats=1, then the
        number of possible combinations are 5, i.e.: [0,1], [0.25,0.75], [0.50,0.50], [0.75,0.25], and [1.0,0.0]
    """
    N = n_prevpoints-1
    C = n_classes
    r = n_repeats
    return int(scipy.special.binom(N + C - 1, C - 1) * r)


def get_nprevpoints_approximation(combinations_budget:int, n_classes:int, n_repeats:int=1):
    """
    Searches for the largest number of (equidistant) prevalence points to define for each of the `n_classes` classes so
    that the number of valid prevalence values generated as combinations of prevalence points (points in a
    `n_classes`-dimensional simplex) do not exceed combinations_budget.

    :param combinations_budget: integer, maximum number of combinations allowed
    :param n_classes: integer, number of classes
    :param n_repeats: integer, number of repetitions for each prevalence combination
    :return: the largest number of prevalence points that generate less than combinations_budget valid prevalences
    """
    assert n_classes > 0 and n_repeats > 0 and combinations_budget > 0, 'parameters must be positive integers'
    n_prevpoints = 1
    while True:
        combinations = num_prevalence_combinations(n_prevpoints, n_classes, n_repeats)
        if combinations > combinations_budget:
            return n_prevpoints-1
        else:
            n_prevpoints += 1


def check_prevalence_vector(p, raise_exception=False, toleranze=1e-08):
    """
    Checks that p is a valid prevalence vector, i.e., that it contains values in [0,1] and that the values sum up to 1.

    :param p: the prevalence vector to check
    :return: True if `p` is valid, False otherwise
    """
    p = np.asarray(p)
    if not all(p>=0):
        if raise_exception:
            raise ValueError('the prevalence vector contains negative numbers')
        return False
    if not all(p<=1):
        if raise_exception:
            raise ValueError('the prevalence vector contains values >1')
        return False
    if not np.isclose(p.sum(), 1, atol=toleranze):
        if raise_exception:
            raise ValueError('the prevalence vector does not sum up to 1')
        return False
    return True


def get_divergence(divergence: Union[str, Callable]):
    if isinstance(divergence, str):
        if divergence=='HD':
            return HellingerDistance
        elif divergence=='topsoe':
            return TopsoeDistance
        else:
            raise ValueError(f'unknown divergence {divergence}')
    elif callable(divergence):
        return divergence
    else:
        raise ValueError(f'argument "divergence" not understood; use a str or a callable function')


def argmin_prevalence(loss, n_classes, method='optim_minimize'):
    if method == 'optim_minimize':
        return optim_minimize(loss, n_classes)
    elif method == 'linear_search':
        return linear_search(loss, n_classes)
    elif method == 'ternary_search':
        raise NotImplementedError()
    else:
        raise NotImplementedError()


def optim_minimize(loss, n_classes):
    """
    Searches for the optimal prevalence values, i.e., an `n_classes`-dimensional vector of the (`n_classes`-1)-simplex
    that yields the smallest lost. This optimization is carried out by means of a constrained search using scipy's
    SLSQP routine.

    :param loss: (callable) the function to minimize
    :param n_classes: (int) the number of classes, i.e., the dimensionality of the prevalence vector
    :return: (ndarray) the best prevalence vector found
    """
    from scipy import optimize

    # the initial point is set as the uniform distribution
    uniform_distribution = np.full(fill_value=1 / n_classes, shape=(n_classes,))

    # solutions are bounded to those contained in the unit-simplex
    bounds = tuple((0, 1) for _ in range(n_classes))  # values in [0,1]
    constraints = ({'type': 'eq', 'fun': lambda x: 1 - sum(x)})  # values summing up to 1
    r = optimize.minimize(loss, x0=uniform_distribution, method='SLSQP', bounds=bounds, constraints=constraints)
    return r.x


def linear_search(loss, n_classes):
    """
    Performs a linear search for the best prevalence value in binary problems. The search is carried out by exploring
    the range [0,1] stepping by 0.01. This search is inefficient, and is added only for completeness (some of the
    early methods in quantification literature used it, e.g., HDy). A most powerful alternative is `optim_minimize`.

    :param loss: (callable) the function to minimize
    :param n_classes: (int) the number of classes, i.e., the dimensionality of the prevalence vector
    :return: (ndarray) the best prevalence vector found
    """
    assert n_classes==2, 'linear search is only available for binary problems'

    prev_selected, min_score = None, None
    for prev in prevalence_linspace(n_prevalences=100, repeats=1, smooth_limits_epsilon=0.0):
        score = loss(np.asarray([1 - prev, prev]))
        if min_score is None or score < min_score:
            prev_selected, min_score = prev, score

    return np.asarray([1 - prev_selected, prev_selected])