# Get Python six functionality:
from __future__ import\
    absolute_import, print_function, division, unicode_literals


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__all__ = [
    "assert_lrp_epsilon_param",
    "assert_infer_lrp_alpha_beta_param"
]


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def assert_lrp_epsilon_param(epsilon, caller):
    """
        Function for asserting epsilon parameter choice
        passed to constructors inheriting from EpsilonRule
        and LRPEpsilon.
        The following conditions can not be met:

        epsilon > 1

        :param epsilon: the epsilon parameter.
        :param caller: the class instance calling this assertion function
    """

    if epsilon <= 0:
        err_head = "Constructor call to {} : ".format(caller.__class__.__name__)
        err_msg = err_head + "Parameter epsilon must be > 0 but was {}".format(epsilon)
        raise ValueError(err_msg)
    return epsilon


def assert_infer_lrp_alpha_beta_param(alpha, beta, caller):
    """
        Function for asserting parameter choices for alpha and beta
        passed to constructors inheriting from AlphaBetaRule
        and LRPAlphaBeta.

        since alpha - beta are subjected to sum to 1,
        it is sufficient for only one of the parameters to be passed
        to a corresponding class constructor.
        this method will cause an assertion error if both are None
        or the following conditions can not be met

        alpha >= 1
        beta >= 0
        alpha - beta = 1

        :param alpha: the alpha parameter.
        :param beta: the beta parameter
        :param caller: the class instance calling this assertion function
    """

    err_head = "Constructor call to {} : ".format(caller.__class__.__name__)
    if alpha is None and beta is None:
        err_msg = err_head + "Neither alpha or beta were given"
        raise ValueError(err_msg)

    #assert passed parameter choices
    if alpha is not None and alpha < 1:
        err_msg = err_head +"Passed parameter alpha invalid. Expecting alpha >= 1 but was {}".format(alpha)
        raise ValueError(err_msg)

    if beta is not None and beta < 0:
        err_msg = err_head +"Passed parameter beta invalid. Expecting beta >= 0 but was {}".format(beta)
        raise ValueError(err_msg)

    #assert inferred parameter choices
    if alpha is None:
        alpha = beta + 1
        if alpha < 1:
            err_msg = err_head +"Inferring alpha from given beta {} s.t. alpha - beta = 1, with condition alpha >= 1 not possible.".format(beta)
            raise ValueError(err_msg)


    if beta is None:
        beta = alpha - 1
        if beta < 0:
            err_msg = err_head +"Inferring beta from given alpha {} s.t. alpha - beta = 1, with condition beta >= 0 not possible.".format(alpha)
            raise ValueError(err_msg)


    #final check: alpha - beta = 1
    amb = alpha - beta
    if amb != 1:
        err_msg = err_head +"Condition alpha - beta = 1 not fulfilled. alpha={} ; beta={} -> alpha - beta = {}".format(alpha, beta, amb)
        raise ValueError(err_msg)

    #return benign values for alpha and beta
    return alpha, beta

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