# Copyright 2020-present MongoDB, Inc.
#
# 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.

"""Utilities for caching OCSP responses."""

from collections import namedtuple
from datetime import datetime as _datetime
from threading import Lock


class _OCSPCache(object):
    """A cache for OCSP responses."""
    CACHE_KEY_TYPE = namedtuple('OcspResponseCacheKey',
                                ['hash_algorithm', 'issuer_name_hash',
                                 'issuer_key_hash', 'serial_number'])

    def __init__(self):
        self._data = {}
        # Hold this lock when accessing _data.
        self._lock = Lock()

    def _get_cache_key(self, ocsp_request):
        return self.CACHE_KEY_TYPE(
            hash_algorithm=ocsp_request.hash_algorithm.name.lower(),
            issuer_name_hash=ocsp_request.issuer_name_hash,
            issuer_key_hash=ocsp_request.issuer_key_hash,
            serial_number=ocsp_request.serial_number)

    def __setitem__(self, key, value):
        """Add/update a cache entry.

        'key' is of type cryptography.x509.ocsp.OCSPRequest
        'value' is of type cryptography.x509.ocsp.OCSPResponse

        Validity of the OCSP response must be checked by caller.
        """
        with self._lock:
            cache_key = self._get_cache_key(key)

            # As per the OCSP protocol, if the response's nextUpdate field is
            # not set, the responder is indicating that newer revocation
            # information is available all the time.
            if value.next_update is None:
                self._data.pop(cache_key, None)
                return

            # Do nothing if the response is invalid.
            if not (value.this_update <= _datetime.utcnow()
                    < value.next_update):
                return

            # Cache new response OR update cached response if new response
            # has longer validity.
            cached_value = self._data.get(cache_key, None)
            if (cached_value is None or
                    cached_value.next_update < value.next_update):
                self._data[cache_key] = value

    def __getitem__(self, item):
        """Get a cache entry if it exists.

        'item' is of type cryptography.x509.ocsp.OCSPRequest

        Raises KeyError if the item is not in the cache.
        """
        with self._lock:
            cache_key = self._get_cache_key(item)
            value = self._data[cache_key]

            # Return cached response if it is still valid.
            if (value.this_update <= _datetime.utcnow() <
                    value.next_update):
                return value

            self._data.pop(cache_key, None)
            raise KeyError(cache_key)