from base64 import b64decode, b64encode
from copy import deepcopy
from pickle import dumps, loads
from zlib import compress, decompress

from django.conf import settings
from django.core import checks
from django.db import models
from django.utils.encoding import force_str

from .constants import DEFAULT_PROTOCOL


class PickledObject(str):
    """
    A subclass of string so it can be told whether a string is a pickled
    object or not (if the object is an instance of this class then it must
    [well, should] be a pickled one).

    Only really useful for passing pre-encoded values to ``default``
    with ``dbsafe_encode``, not that doing so is necessary. If you
    remove PickledObject and its references, you won't be able to pass
    in pre-encoded values anymore, but you can always just pass in the
    python objects themselves.
    """


class _ObjectWrapper:
    """
    A class used to wrap object that have properties that may clash with the
    ORM internals.

    For example, objects with the `prepare_database_save` property such as
    `django.db.Model` subclasses won't work under certain conditions and the
    same apply for trying to retrieve any `callable` object.
    """
    __slots__ = ('_obj',)

    def __init__(self, obj):
        self._obj = obj


def wrap_conflictual_object(obj):
    if hasattr(obj, 'prepare_database_save') or callable(obj):
        obj = _ObjectWrapper(obj)
    return obj


def get_default_protocol():
    return getattr(settings, 'PICKLEFIELD_DEFAULT_PROTOCOL', DEFAULT_PROTOCOL)


def dbsafe_encode(value, compress_object=False, pickle_protocol=None, copy=True):
    # We use deepcopy() here to avoid a problem with cPickle, where dumps
    # can generate different character streams for same lookup value if
    # they are referenced differently.
    # The reason this is important is because we do all of our lookups as
    # simple string matches, thus the character streams must be the same
    # for the lookups to work properly. See tests.py for more information.
    if pickle_protocol is None:
        pickle_protocol = get_default_protocol()
    if copy:
        # Copy can be very expensive if users aren't going to perform lookups
        # on the value anyway.
        value = deepcopy(value)
    value = dumps(value, protocol=pickle_protocol)
    if compress_object:
        value = compress(value)
    value = b64encode(value).decode()  # decode bytes to str
    return PickledObject(value)


def dbsafe_decode(value, compress_object=False):
    value = value.encode()  # encode str to bytes
    value = b64decode(value)
    if compress_object:
        value = decompress(value)
    return loads(value)


class PickledObjectField(models.Field):
    """
    A field that will accept *any* python object and store it in the
    database. PickledObjectField will optionally compress its values if
    declared with the keyword argument ``compress=True``.

    Does not actually encode and compress ``None`` objects (although you
    can still do lookups using None). This way, it is still possible to
    use the ``isnull`` lookup type correctly.
    """
    empty_strings_allowed = False

    def __init__(self, *args, **kwargs):
        self.compress = kwargs.pop('compress', False)
        protocol = kwargs.pop('protocol', None)
        if protocol is None:
            protocol = get_default_protocol()
        self.protocol = protocol
        self.copy = kwargs.pop('copy', True)
        kwargs.setdefault('editable', False)
        super().__init__(*args, **kwargs)

    def get_default(self):
        """
        Returns the default value for this field.

        The default implementation on models.Field calls force_unicode
        on the default, which means you can't set arbitrary Python
        objects as the default. To fix this, we just return the value
        without calling force_unicode on it. Note that if you set a
        callable as a default, the field will still call it. It will
        *not* try to pickle and encode it.

        """
        if self.has_default():
            if callable(self.default):
                return self.default()
            return self.default
        # If the field doesn't have a default, then we punt to models.Field.
        return super().get_default()

    def _check_default(self):
        if self.has_default() and isinstance(self.default, (list, dict, set)):
            return [
                checks.Warning(
                    "%s default should be a callable instead of a mutable instance so "
                    "that it's not shared between all field instances." % (
                        self.__class__.__name__,
                    ),
                    hint=(
                        'Use a callable instead, e.g., use `%s` instead of '
                        '`%r`.' % (
                            type(self.default).__name__,
                            self.default,
                        )
                    ),
                    obj=self,
                    id='picklefield.E001',
                )
            ]
        else:
            return []

    def check(self, **kwargs):
        errors = super().check(**kwargs)
        errors.extend(self._check_default())
        return errors

    def deconstruct(self):
        name, path, args, kwargs = super().deconstruct()
        if self.compress:
            kwargs['compress'] = True
        if self.protocol != get_default_protocol():
            kwargs['protocol'] = self.protocol
        return name, path, args, kwargs

    def to_python(self, value):
        """
        B64decode and unpickle the object, optionally decompressing it.

        If an error is raised in de-pickling and we're sure the value is
        a definite pickle, the error is allowed to propagate. If we
        aren't sure if the value is a pickle or not, then we catch the
        error and return the original value instead.

        """
        if value is not None:
            try:
                value = dbsafe_decode(value, self.compress)
            except Exception:
                # If the value is a definite pickle; and an error is raised in
                # de-pickling it should be allowed to propagate.
                if isinstance(value, PickledObject):
                    raise
            else:
                if isinstance(value, _ObjectWrapper):
                    return value._obj
        return value

    def pre_save(self, model_instance, add):
        value = super().pre_save(model_instance, add)
        return wrap_conflictual_object(value)

    def from_db_value(self, value, expression, connection):
        return self.to_python(value)

    def get_db_prep_value(self, value, connection=None, prepared=False):
        """
        Pickle and b64encode the object, optionally compressing it.

        The pickling protocol is specified explicitly (by default 2),
        rather than as -1 or HIGHEST_PROTOCOL, because we don't want the
        protocol to change over time. If it did, ``exact`` and ``in``
        lookups would likely fail, since pickle would now be generating
        a different string.

        """
        if value is not None and not isinstance(value, PickledObject):
            # We call force_str here explicitly, so that the encoded string
            # isn't rejected by the postgresql_psycopg2 backend. Alternatively,
            # we could have just registered PickledObject with the psycopg
            # marshaller (telling it to store it like it would a string), but
            # since both of these methods result in the same value being stored,
            # doing things this way is much easier.
            value = force_str(dbsafe_encode(value, self.compress, self.protocol, self.copy))
        return value

    def value_to_string(self, obj):
        value = self.value_from_object(obj)
        return self.get_db_prep_value(value)

    def get_internal_type(self):
        return 'TextField'

    def get_lookup(self, lookup_name):
        """
        We need to limit the lookup types.
        """
        if lookup_name not in ['exact', 'in', 'isnull']:
            raise TypeError('Lookup type %s is not supported.' % lookup_name)
        return super().get_lookup(lookup_name)
