from datetime import datetime

from abc import ABCMeta, abstractmethod
from six import add_metaclass

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

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(rebuilder=lambda: {})

  def retrieve(self, cache_key, loader, should_cache=is_not_none):
    not_found = [None]
    result = self.cache.get(cache_key.key, default_value=not_found)
    if result != not_found:
      return result

    result = loader()
    if should_cache(result):
      expires = convert_to_timedelta(cache_key.expiration) + datetime.now()
      self.cache.set(cache_key.key, result, expires=expires)

    return result