# Source code for macop.operators.policies.UCBPolicy

    """Policy class implementation which is used for selecting operator using Upper Confidence Bound
    """
    # main imports
    import logging
    import random
    import math
    import numpy as np

    # module imports
    from .Policy import Policy

    class UCBPolicy(Policy):
        """UCB policy class which is used for applying UCB strategy when selecting and applying operator

        Attributes:
            operators: {[Operator]} -- list of selected operators for the algorithm
            C: {float} -- tradeoff between EvE parameter for UCB
            exp_rate: {float} -- exploration rate (probability to choose randomly next operator)
            rewards: {[float]} -- list of summed rewards obtained for each operator
            occurrences: {[int]} -- number of use (selected) of each operator
        """
        def __init__(self, _operators, _C=100., _exp_rate=0.5):
            self.operators = _operators
            self.rewards = [0. for o in self.operators]
            self.occurrences = [0 for o in self.operators]
            self.C = _C
            self.exp_rate = _exp_rate

        def select(self):
            """Select randomly the next operator to use

            Returns:
                {Operator}: the selected operator
            """

            indices = [i for i, o in enumerate(self.occurrences) if o == 0]

            # random choice following exploration rate
            if np.random.uniform(0, 1) <= self.exp_rate:

                index = random.choice(range(len(self.operators)))
                return self.operators[index]

            elif len(indices) == 0:

                # if operator have at least be used one time
                ucbValues = []
                nVisits = sum(self.occurrences)

                for i in range(len(self.operators)):

                    ucbValue = self.rewards[i] + self.C * math.sqrt(
                        math.log(nVisits) / (self.occurrences[i] + 0.1))
                    ucbValues.append(ucbValue)

                return self.operators[ucbValues.index(max(ucbValues))]

            else:
                return self.operators[random.choice(indices)]

        def apply(self, _solution):
            """
            Apply specific operator chosen to create new solution, computes its fitness and returns solution

            Args:
                _solution: {Solution} -- the solution to use for generating new solution

            Returns:
                {Solution} -- new generated solution
            """

            operator = self.select()

            logging.info("---- Applying %s on %s" %
                         (type(operator).__name__, _solution))

            # apply operator on solution
            newSolution = operator.apply(_solution)

            # compute fitness of new solution
            newSolution.evaluate(self.algo.evaluator)

            # compute fitness improvment rate
            if self.algo.maximise:
                fir = (newSolution.fitness() -
                       _solution.fitness()) / _solution.fitness()
            else:
                fir = (_solution.fitness() -
                       newSolution.fitness()) / _solution.fitness()

            operator_index = self.operators.index(operator)

            if fir > 0:
                self.rewards[operator_index] += fir

            self.occurrences[operator_index] += 1

            logging.info("---- Obtaining %s" % (_solution))

            return newSolution