from django.core.exceptions import ValidationError
from django.db import models
from django.utils.encoding import force_str

from timezone_field.backends import TimeZoneNotFoundError, get_tz_backend
from timezone_field.choices import standard, with_gmt_offset
from timezone_field.utils import AutoDeserializedAttribute


class TimeZoneField(models.Field):
    """
    Provides database store for pytz timezone objects.

    Valid inputs:
        * use_pytz=True:
            * any instance of pytz.tzinfo.DstTzInfo or pytz.tzinfo.StaticTzInfo
            * the pytz.UTC singleton
            * any string that validates against pytz.common_timezones. pytz will
              be used to build a timezone object from the string.
        * use_pytz=False:
            * any instance of zoneinfo.ZoneInfo
            * any string that validates against zoneinfo.available_timezones().
        * None and the empty string both represent 'no timezone'

    Valid outputs:
        * None
        * use_pytz=True: instances of pytz.tzinfo.DstTzInfo,
          pytz.tzinfo.StaticTzInfo and the pytz.UTC singleton
        * use_pytz=False: instances of zoneinfo.ZoneInfo

    Blank values are stored in the DB as the empty string. Timezones are stored
    in their string representation.

    The `choices` kwarg can be specified as a list of either
    [<timezone object>, <str>] or [<str>, <str>]. Internally in memory, it is
    stored as [<timezone object>, <str>].
    """

    descriptor_class = AutoDeserializedAttribute

    description = "A timezone object"

    # NOTE: these defaults are excluded from migrations. If these are changed,
    #       existing migration files will need to be accomodated.
    default_max_length = 63

    def __init__(self, *args, **kwargs):
        # allow some use of positional args up until the args we customize
        # https://github.com/mfogel/django-timezone-field/issues/42
        # https://github.com/django/django/blob/1.11.11/django/db/models/fields/__init__.py#L145
        if len(args) > 3:
            raise ValueError("Cannot specify max_length by positional arg")
        kwargs.setdefault("max_length", self.default_max_length)

        self.use_pytz = kwargs.pop("use_pytz", None)
        self.tz_backend = get_tz_backend(self.use_pytz)
        self.default_tzs = [self.tz_backend.to_tzobj(v) for v in self.tz_backend.base_tzstrs]

        if "choices" in kwargs:
            values, displays = zip(*kwargs["choices"])
            # Choices can be specified in two forms: either
            # [<timezone object>, <str>] or [<str>, <str>]
            #
            # The [<timezone object>, <str>] format is the one we actually
            # store the choices in memory because of
            # https://github.com/mfogel/django-timezone-field/issues/24
            #
            # The [<str>, <str>] format is supported because since django
            # can't deconstruct pytz.timezone objects, migration files must
            # use an alternate format. Representing the timezones as strings
            # is the obvious choice.
            if not self.tz_backend.is_tzobj(values[0]):
                # using force_str b/c of https://github.com/mfogel/django-timezone-field/issues/38
                values = [self.tz_backend.to_tzobj(force_str(v)) for v in values]
        else:
            values = self.default_tzs
            displays = None

        self.choices_display = kwargs.pop("choices_display", None)
        if self.choices_display == "WITH_GMT_OFFSET":
            choices = with_gmt_offset(values, use_pytz=self.use_pytz)
        elif self.choices_display == "STANDARD":
            choices = standard(values)
        elif self.choices_display is None:
            choices = zip(values, displays) if displays else standard(values)
        else:
            raise ValueError(f"Unrecognized value for kwarg 'choices_display' of '{self.choices_display}'")

        kwargs["choices"] = choices
        super().__init__(*args, **kwargs)

    def validate(self, value, model_instance):
        if not self.tz_backend.is_tzobj(value):
            raise ValidationError(f"'{value}' is not a pytz timezone object")
        super().validate(value, model_instance)

    def deconstruct(self):
        name, path, args, kwargs = super().deconstruct()
        if kwargs.get("max_length") == self.default_max_length:
            del kwargs["max_length"]

        if self.use_pytz is not None:
            kwargs["use_pytz"] = self.use_pytz

        if self.choices_display is not None:
            kwargs["choices_display"] = self.choices_display

        # don't assume super().deconstruct() will pass us back our kwargs["choices"]
        # https://github.com/mfogel/django-timezone-field/issues/96
        if "choices" in kwargs:
            if self.choices_display is None:
                if kwargs["choices"] == standard(self.default_tzs):
                    kwargs.pop("choices")
            else:
                values, _ = zip(*kwargs["choices"])
                if sorted(values, key=str) == sorted(self.default_tzs, key=str):
                    kwargs.pop("choices")
                else:
                    kwargs["choices"] = [(value, "") for value in values]

        # django can't decontruct pytz objects, so transform choices
        # to [<str>, <str>] format for writing out to the migration
        if "choices" in kwargs:
            kwargs["choices"] = [(str(tz), n) for tz, n in kwargs["choices"]]

        return name, path, args, kwargs

    def get_internal_type(self):
        return "CharField"

    def get_default(self):
        # allow defaults to be still specified as strings. Allows for easy
        # serialization into migration files
        value = super().get_default()
        return self._get_python_and_db_repr(value)[0]

    def from_db_value(self, value, *_args):
        "Convert to pytz timezone object"
        return self._get_python_and_db_repr(value)[0]

    def to_python(self, value):
        "Convert to pytz timezone object"
        return self._get_python_and_db_repr(value)[0]

    def get_prep_value(self, value):
        "Convert to string describing a valid pytz timezone object"
        return self._get_python_and_db_repr(value)[1]

    def _get_python_and_db_repr(self, value):
        "Returns a tuple of (python representation, db representation)"
        if value is None or value == "":
            return (None, "")
        if self.tz_backend.is_tzobj(value):
            return (value, str(value))
        try:
            return (self.tz_backend.to_tzobj(force_str(value)), force_str(value))
        except TimeZoneNotFoundError as err:
            raise ValidationError(f"Invalid timezone '{value}'") from err