import logging
import json

from datetime import datetime

from abc import ABCMeta, abstractmethod
from six import add_metaclass

from pymemcache.client.base import Client

from util.expiresdict import ExpiresDict
from util.timedeltastring import convert_to_timedelta

logger = logging.getLogger(__name__)


def is_not_none(value):
  return value is not None


@add_metaclass(ABCMeta)
class DataModelCache(object):
  """ Defines an interface for cache storing and returning tuple data model objects. """

  @abstractmethod
  def retrieve(self, cache_key, loader, should_cache=is_not_none):
    """ Checks the cache for the specified cache key and returns the value found (if any). If none
        found, the loader is called to get a result and populate the cache.
    """
    pass


class NoopDataModelCache(DataModelCache):
  """ Implementation of the data model cache which does nothing. """

  def retrieve(self, cache_key, loader, should_cache=is_not_none):
    return loader()


class InMemoryDataModelCache(DataModelCache):
  """ Implementation of the data model cache backed by an in-memory dictionary. """
  def __init__(self):
    self.cache = ExpiresDict()

  def retrieve(self, cache_key, loader, should_cache=is_not_none):
    not_found = [None]
    logger.debug('Checking cache for key %s', cache_key.key)
    result = self.cache.get(cache_key.key, default_value=not_found)
    if result != not_found:
      logger.debug('Found result in cache for key %s: %s', cache_key.key, result)
      return json.loads(result)

    logger.debug('Found no result in cache for key %s; calling loader', cache_key.key)
    result = loader()
    logger.debug('Got loaded result for key %s: %s', cache_key.key, result)
    if should_cache(result):
      logger.debug('Caching loaded result for key %s with expiration %s: %s', cache_key.key,
                   result, cache_key.expiration)
      expires = convert_to_timedelta(cache_key.expiration) + datetime.now()
      self.cache.set(cache_key.key, json.dumps(result), expires=expires)
      logger.debug('Cached loaded result for key %s with expiration %s: %s', cache_key.key,
                   result, cache_key.expiration)
    else:
      logger.debug('Not caching loaded result for key %s: %s', cache_key.key, result)

    return result


_DEFAULT_MEMCACHE_TIMEOUT = 1 # second
_DEFAULT_MEMCACHE_CONNECT_TIMEOUT = 1 # second

_STRING_TYPE = 1
_JSON_TYPE = 2

class MemcachedModelCache(DataModelCache):
  """ Implementation of the data model cache backed by a memcached. """
  def __init__(self, endpoint, timeout=_DEFAULT_MEMCACHE_TIMEOUT,
               connect_timeout=_DEFAULT_MEMCACHE_CONNECT_TIMEOUT):
    self.endpoint = endpoint
    self.timeout = timeout
    self.connect_timeout = connect_timeout
    self.client = None

  def _get_client(self):
    client = self.client
    if client is not None:
      return client

    try:
      # Copied from the doc comment for Client.
      def serialize_json(key, value):
        if type(value) == str:
          return value, _STRING_TYPE

        return json.dumps(value), _JSON_TYPE

      def deserialize_json(key, value, flags):
        if flags == _STRING_TYPE:
          return value

        if flags == _JSON_TYPE:
          return json.loads(value)

        raise Exception("Unknown flags for value: {1}".format(flags))

      self.client = Client(self.endpoint, no_delay=True, timeout=self.timeout,
                           connect_timeout=self.connect_timeout,
                           key_prefix='data_model_cache__',
                           serializer=serialize_json,
                           deserializer=deserialize_json,
                           ignore_exc=True)
      return self.client
    except:
      logger.exception('Got exception when creating memcached client to %s', self.endpoint)
      return None

  def retrieve(self, cache_key, loader, should_cache=is_not_none):
    not_found = [None]
    client = self._get_client()
    if client is not None:
      logger.debug('Checking cache for key %s', cache_key.key)
      try:
        result = client.get(cache_key.key, default=not_found)
        if result != not_found:
          logger.debug('Found result in cache for key %s: %s', cache_key.key, result)
          return result
      except:
       logger.exception('Got exception when trying to retrieve key %s', cache_key.key)

    logger.debug('Found no result in cache for key %s; calling loader', cache_key.key)
    result = loader()
    logger.debug('Got loaded result for key %s: %s', cache_key.key, result)
    if client is not None and should_cache(result):
      try:
        logger.debug('Caching loaded result for key %s with expiration %s: %s', cache_key.key,
                      result, cache_key.expiration)
        expires = convert_to_timedelta(cache_key.expiration) if cache_key.expiration else None
        client.set(cache_key.key, result, expire=int(expires.total_seconds()) if expires else None)
        logger.debug('Cached loaded result for key %s with expiration %s: %s', cache_key.key,
                      result, cache_key.expiration)
      except:
        logger.exception('Got exception when trying to set key %s to %s', cache_key.key, result)
    else:
      logger.debug('Not caching loaded result for key %s: %s', cache_key.key, result)

    return result