#! /usr/bin/env python

# Copyright (c) 2019 Victor Terron. All rights reserved.
#
# This file is part of LEMON.
#
# LEMON 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/>.


import multiprocessing
import multiprocessing.queues


class SharedCounter(object):
    """A synchronized shared counter.

    The locking done by multiprocessing.Value ensures that only a single
    process or thread may read or write the in-memory ctypes object. However,
    in order to do n += 1, Python performs a read followed by a write, so a
    second process may read the old value before the new one is written by the
    first process. The solution is to use a multiprocessing.Lock to guarantee
    the atomicity of the modifications to Value.

    This class comes almost entirely from Eli Bendersky's blog:
    http://eli.thegreenplace.net/2012/01/04/shared-counter-with-pythons-multiprocessing/

    """

    def __init__(self, n=0):
        self.count = multiprocessing.Value("i", n)

    def increment(self, n=1):
        """ Increment the counter by n (default = 1) """
        with self.count.get_lock():
            self.count.value += n

    @property
    def value(self):
        """ Return the value of the counter """
        return self.count.value


class Queue(multiprocessing.queues.Queue):
    """A portable implementation of multiprocessing.Queue.

    Because of multithreading / multiprocessing semantics, Queue.qsize() may
    raise the NotImplementedError exception on Unix platforms like Mac OS X
    where sem_getvalue() is not implemented. This subclass addresses this
    problem by using a synchronized shared counter (initialized to zero) and
    increasing / decreasing its value every time the put() and get() methods
    are called, respectively. This not only prevents NotImplementedError from
    being raised, but also allows us to implement a reliable version of both
    qsize() and empty().

    """

    def __init__(self, *args, **kwargs):
        super(Queue, self).__init__(*args, **kwargs)
        self._size = SharedCounter(0)

    def put(self, *args, **kwargs):
        super(Queue, self).put(*args, **kwargs)
        self._size.increment(1)

    def get(self, *args, **kwargs):
        item = super(Queue, self).get(*args, **kwargs)
        self._size.increment(-1)
        return item

    def qsize(self):
        """ Reliable implementation of multiprocessing.Queue.qsize() """
        return self._size.value

    def empty(self):
        """ Reliable implementation of multiprocessing.Queue.empty() """
        return not self.qsize()

    def clear(self):
        """ Remove all elements from the Queue. """
        while not self.empty():
            self.get()
