# -*- coding: utf-8 -*-
# Copyright 2015 OpenMarket Ltd
#
# Licensed under the Apache License, Version 2.0 (the "License");
# you may not use this file except in compliance with the License.
# You may obtain a copy of the License at
#
#     http://www.apache.org/licenses/LICENSE-2.0
#
# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS,
# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
# See the License for the specific language governing permissions and
# limitations under the License.

from synapse.util.async import ObservableDeferred


class SnapshotCache(object):

    DURATION_MS = 5 * 60 * 1000  # Cache results for 2 minutes.

    def __init__(self):
        self.pending_result_cache = {}  # Request that haven't finished yet.
        self.prev_result_cache = {}  # The older requests that have finished.
        self.next_result_cache = {}  # The newer requests that have finished.
        self.time_last_rotated_ms = 0

    def rotate(self, time_now_ms):
        # Rotate once if the cache duration has passed since the last rotation.
        if time_now_ms - self.time_last_rotated_ms >= self.DURATION_MS:
            self.prev_result_cache = self.next_result_cache
            self.next_result_cache = {}
            self.time_last_rotated_ms += self.DURATION_MS

        # Rotate again if the cache duration has passed twice since the last
        # rotation.
        if time_now_ms - self.time_last_rotated_ms >= self.DURATION_MS:
            self.prev_result_cache = self.next_result_cache
            self.next_result_cache = {}
            self.time_last_rotated_ms = time_now_ms

    def get(self, time_now_ms, key):
        self.rotate(time_now_ms)
        # This cache is intended to deduplicate requests, so we expect it to be
        # missed most of the time. So we just lookup the key in all of the
        # dictionaries rather than trying to short circuit the lookup if the
        # key is found.
        result = self.prev_result_cache.get(key)
        result = self.next_result_cache.get(key, result)
        result = self.pending_result_cache.get(key, result)
        if result is not None:
            return result.observe()

    def set(self, time_now_ms, key, deferred):
        self.rotate(time_now_ms)

        result = ObservableDeferred(deferred)

        self.pending_result_cache[key] = result

        def shuffle_along(r):
            # When the deferred completes we shuffle it along to the first
            # generation of the result cache. So that it will eventually
            # expire from the rotation of that cache.
            self.next_result_cache[key] = result
            self.pending_result_cache.pop(key, None)

        result.observe().addBoth(shuffle_along)

        return result.observe()