# This program is free software: you can redistribute it and/or modify
# it under the terms of the GNU General Public License as published by
# the Free Software Foundation, either version 3 of the License, or
# (at your option) any later version.
#
# This program is distributed in the hope that it will be useful,
# but WITHOUT ANY WARRANTY; without even the implied warranty of
# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
# GNU General Public License for more details.
#
# You should have received a copy of the GNU General Public License
# along with this program.  If not, see <http://www.gnu.org/licenses/>.
#
# Copyright(C) 2013-2018 Max-Planck-Society
#
# NIFTy is being developed at the Max-Planck-Institut fuer Astrophysik
# and financially supported by the Studienstiftung des deutschen Volkes.

from __future__ import absolute_import, division, print_function

import abc

from ..compat import *
from ..utilities import NiftyMetaBase


class IterationController(NiftyMetaBase()):
    """The abstract base class for all iteration controllers.
    An iteration controller is an object that monitors the progress of a
    minimization iteration. At the begin of the minimization, its start()
    method is called with the energy object at the initial position.
    Afterwards, its check() method is called during every iteration step with
    the energy object describing the current position.
    Based on that information, the iteration controller has to decide whether
    iteration needs to progress further (in this case it returns CONTINUE), or
    if sufficient convergence has been reached (in this case it returns
    CONVERGED), or if some error has been detected (then it returns ERROR).

    The concrete convergence criteria can be chosen by inheriting from this
    class; the implementer has full flexibility to use whichever criteria are
    appropriate for a particular problem - as long as they can be computed from
    the information passed to the controller during the iteration process.
    """

    CONVERGED, CONTINUE, ERROR = list(range(3))

    @abc.abstractmethod
    def start(self, energy):
        """Starts the iteration.

        Parameters
        ----------
        energy : Energy object
           Energy object at the start of the iteration

        Returns
        -------
        status : integer status, can be CONVERGED, CONTINUE or ERROR
        """
        raise NotImplementedError

    @abc.abstractmethod
    def check(self, energy):
        """Checks the state of the iteration. Called after every step.

        Parameters
        ----------
        energy : Energy object
           Energy object at the start of the iteration

        Returns
        -------
        status : integer status, can be CONVERGED, CONTINUE or ERROR
        """
        raise NotImplementedError