# Copyright (C) 2015 Andrey Antukh <niwi@niwi.be>
# Copyright (C) 2015 Jesús Espino <jespinog@gmail.com>
# Copyright (C) 2015 David Barragán <bameda@dbarragan.com>
# This program is free software: you can redistribute it and/or modify
# it under the terms of the GNU Affero 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 Affero General Public License for more details.
#
# You should have received a copy of the GNU Affero General Public License
# along with this program.  If not, see <http://www.gnu.org/licenses/>.

# This code is partially taken from django-rest-framework:
# Copyright (c) 2011-2014, Tom Christie

"""
Provides various throttling policies.
"""

from django.core.cache import cache as default_cache
from django.core.exceptions import ImproperlyConfigured

from .settings import api_settings

import time


class BaseThrottle(object):
    """
    Rate throttling of requests.
    """
    def allow_request(self, request, view):
        """
        Return `True` if the request should be allowed, `False` otherwise.
        """
        raise NotImplementedError(".allow_request() must be overridden")

    def wait(self):
        """
        Optionally, return a recommended number of seconds to wait before
        the next request.
        """
        return None


class SimpleRateThrottle(BaseThrottle):
    """
    A simple cache implementation, that only requires `.get_cache_key()`
    to be overridden.

    The rate (requests / seconds) is set by a `throttle` attribute on the View
    class.  The attribute is a string of the form "number_of_requests/period".

    Period should be one of: ("s", "sec", "m", "min", "h", "hour", "d", "day")

    Previous request information used for throttling is stored in the cache.
    """

    cache = default_cache
    timer = time.time
    cache_format = "throtte_%(scope)s_%(ident)s"
    scope = None
    THROTTLE_RATES = api_settings.DEFAULT_THROTTLE_RATES

    def __init__(self):
        if not getattr(self, "rate", None):
            self.rate = self.get_rate()
        self.num_requests, self.duration = self.parse_rate(self.rate)

    def get_cache_key(self, request, view):
        """
        Should return a unique cache-key which can be used for throttling.
        Must be overridden.

        May return `None` if the request should not be throttled.
        """
        raise NotImplementedError(".get_cache_key() must be overridden")

    def get_rate(self):
        """
        Determine the string representation of the allowed request rate.
        """
        if not getattr(self, "scope", None):
            msg = ("You must set either `.scope` or `.rate` for \"%s\" throttle" %
                   self.__class__.__name__)
            raise ImproperlyConfigured(msg)

        try:
            return self.THROTTLE_RATES[self.scope]
        except KeyError:
            msg = "No default throttle rate set for \"%s\" scope" % self.scope
            raise ImproperlyConfigured(msg)

    def parse_rate(self, rate):
        """
        Given the request rate string, return a two tuple of:
        <allowed number of requests>, <period of time in seconds>
        """
        if rate is None:
            return (None, None)
        num, period = rate.split("/")
        num_requests = int(num)
        duration = {"s": 1, "m": 60, "h": 3600, "d": 86400}[period[0]]
        return (num_requests, duration)

    def allow_request(self, request, view):
        """
        Implement the check to see if the request should be throttled.

        On success calls `throttle_success`.
        On failure calls `throttle_failure`.
        """
        if self.rate is None:
            return True

        self.key = self.get_cache_key(request, view)
        if self.key is None:
            return True

        self.history = self.cache.get(self.key, [])
        self.now = self.timer()

        # Drop any requests from the history which have now passed the
        # throttle duration
        while self.history and self.history[-1] <= self.now - self.duration:
            self.history.pop()
        if len(self.history) >= self.num_requests:
            return self.throttle_failure()
        return self.throttle_success()

    def throttle_success(self):
        """
        Inserts the current request's timestamp along with the key
        into the cache.
        """
        self.history.insert(0, self.now)
        self.cache.set(self.key, self.history, self.duration)
        return True

    def throttle_failure(self):
        """
        Called when a request to the API has failed due to throttling.
        """
        return False

    def wait(self):
        """
        Returns the recommended next request time in seconds.
        """
        if self.history:
            remaining_duration = self.duration - (self.now - self.history[-1])
        else:
            remaining_duration = self.duration

        available_requests = self.num_requests - len(self.history) + 1
        if available_requests <= 0:
            return None

        return remaining_duration / float(available_requests)


class AnonRateThrottle(SimpleRateThrottle):
    """
    Limits the rate of API calls that may be made by a anonymous users.

    The IP address of the request will be used as the unique cache key.
    """
    scope = "anon"

    def get_cache_key(self, request, view):
        if request.user.is_authenticated():
            return None  # Only throttle unauthenticated requests.

        ident = request.META.get("HTTP_X_FORWARDED_FOR")
        if ident is None:
            ident = request.META.get("REMOTE_ADDR")

        return self.cache_format % {
            "scope": self.scope,
            "ident": ident
        }


class UserRateThrottle(SimpleRateThrottle):
    """
    Limits the rate of API calls that may be made by a given user.

    The user id will be used as a unique cache key if the user is
    authenticated.  For anonymous requests, the IP address of the request will
    be used.
    """
    scope = "user"

    def get_cache_key(self, request, view):
        if request.user.is_authenticated():
            ident = request.user.id
        else:
            ident = request.META.get("REMOTE_ADDR", None)

        return self.cache_format % {
            "scope": self.scope,
            "ident": ident
        }


class ScopedRateThrottle(SimpleRateThrottle):
    """
    Limits the rate of API calls by different amounts for various parts of
    the API.  Any view that has the `throttle_scope` property set will be
    throttled.  The unique cache key will be generated by concatenating the
    user id of the request, and the scope of the view being accessed.
    """
    scope_attr = "throttle_scope"

    def __init__(self):
        # Override the usual SimpleRateThrottle, because we can't determine
        # the rate until called by the view.
        pass

    def allow_request(self, request, view):
        # We can only determine the scope once we"re called by the view.
        self.scope = getattr(view, self.scope_attr, None)

        # If a view does not have a `throttle_scope` always allow the request
        if not self.scope:
            return True

        # Determine the allowed request rate as we normally would during
        # the `__init__` call.
        self.rate = self.get_rate()
        self.num_requests, self.duration = self.parse_rate(self.rate)

        # We can now proceed as normal.
        return super(ScopedRateThrottle, self).allow_request(request, view)

    def get_cache_key(self, request, view):
        """
        If `view.throttle_scope` is not set, don't apply this throttle.

        Otherwise generate the unique cache key by concatenating the user id
        with the ".throttle_scope` property of the view.
        """
        if request.user.is_authenticated():
            ident = request.user.id
        else:
            ident = request.META.get("REMOTE_ADDR", None)

        return self.cache_format % {
            "scope": self.scope,
            "ident": ident
        }
