from __future__ import unicode_literals

from collections import OrderedDict
import pytz

from django.conf import settings
from django.contrib.contenttypes.models import ContentType
from django.core.exceptions import ObjectDoesNotExist
from django.db.models import ManyToManyField
from django.http import Http404
from rest_framework import mixins
from rest_framework.exceptions import APIException
from rest_framework.permissions import BasePermission
from rest_framework.relations import PrimaryKeyRelatedField
from rest_framework.response import Response
from rest_framework.serializers import Field, ModelSerializer, RelatedField, ValidationError
from rest_framework.viewsets import GenericViewSet, ViewSet

from .utils import dynamic_import

WRITE_OPERATIONS = ['create', 'update', 'partial_update', 'delete']


class ServiceUnavailable(APIException):
    status_code = 503
    default_detail = "Service temporarily unavailable, please try again later."


def get_serializer_for_model(model, prefix=''):
    """
    Dynamically resolve and return the appropriate serializer for a model.
    """
    app_name, model_name = model._meta.label.split('.')
    serializer_name = '{}.api.serializers.{}{}Serializer'.format(
        app_name, prefix, model_name
    )
    try:
        return dynamic_import(serializer_name)
    except ImportError:
        return None


#
# Authentication
#

class IsAuthenticatedOrLoginNotRequired(BasePermission):
    """
    Returns True if the user is authenticated or LOGIN_REQUIRED is False.
    """
    def has_permission(self, request, view):
        if not settings.LOGIN_REQUIRED:
            return True
        return request.user.is_authenticated


#
# Fields
#

class TagField(RelatedField):
    """
    Represent a writable list of Tags associated with an object (use with many=True).
    """

    def to_internal_value(self, data):
        obj = self.parent.parent.instance
        content_type = ContentType.objects.get_for_model(obj)
        tag, _ = Tag.objects.get_or_create(content_type=content_type, object_id=obj.pk, name=data)
        return tag

    def to_representation(self, value):
        return value.name


class ChoiceFieldSerializer(Field):
    """
    Represent a ChoiceField as {'value': <DB value>, 'label': <string>}.
    """
    def __init__(self, choices, **kwargs):
        self._choices = dict()
        for k, v in choices:
            # Unpack grouped choices
            if type(v) in [list, tuple]:
                for k2, v2 in v:
                    self._choices[k2] = v2
            else:
                self._choices[k] = v
        super(ChoiceFieldSerializer, self).__init__(**kwargs)

    def to_representation(self, obj):
        return {'value': obj, 'label': self._choices[obj]}

    def to_internal_value(self, data):
        return data


class ContentTypeFieldSerializer(Field):
    """
    Represent a ContentType as '<app_label>.<model>'
    """
    def to_representation(self, obj):
        return "{}.{}".format(obj.app_label, obj.model)

    def to_internal_value(self, data):
        app_label, model = data.split('.')
        try:
            return ContentType.objects.get_by_natural_key(app_label=app_label, model=model)
        except ContentType.DoesNotExist:
            raise ValidationError("Invalid content type")


class TimeZoneField(Field):
    """
    Represent a pytz time zone.
    """
    def to_representation(self, obj):
        return obj.zone if obj else None

    def to_internal_value(self, data):
        if not data:
            return ""
        try:
            return pytz.timezone(str(data))
        except pytz.exceptions.UnknownTimeZoneError:
            raise ValidationError('Invalid time zone "{}"'.format(data))


class SerializedPKRelatedField(PrimaryKeyRelatedField):
    """
    Extends PrimaryKeyRelatedField to return a serialized object on read. This is useful for representing related
    objects in a ManyToManyField while still allowing a set of primary keys to be written.
    """
    def __init__(self, serializer, **kwargs):
        self.serializer = serializer
        self.pk_field = kwargs.pop('pk_field', None)
        super(SerializedPKRelatedField, self).__init__(**kwargs)

    def to_representation(self, value):
        return self.serializer(value, context={'request': self.context['request']}).data


#
# Serializers
#

class ValidatedModelSerializer(ModelSerializer):
    """
    Extends the built-in ModelSerializer to enforce calling clean() on the associated model during validation.
    """
    def validate(self, data):

        # Remove custom field data (if any) prior to model validation
        attrs = data.copy()
        attrs.pop('custom_fields', None)

        # Run clean() on an instance of the model
        if self.instance is None:
            model = self.Meta.model
            # Ignore ManyToManyFields for new instances (a PK is needed for validation)
            for field in model._meta.get_fields():
                if isinstance(field, ManyToManyField) and field.name in attrs:
                    attrs.pop(field.name)
            instance = self.Meta.model(**attrs)
        else:
            instance = self.instance
            for k, v in attrs.items():
                setattr(instance, k, v)
        instance.clean()

        return data


class WritableNestedSerializer(ModelSerializer):
    """
    Returns a nested representation of an object on read, but accepts only a primary key on write.
    """
    def to_internal_value(self, data):
        if data is None:
            return None
        try:
            return self.Meta.model.objects.get(pk=data)
        except ObjectDoesNotExist:
            raise ValidationError("Invalid ID")


#
# Viewsets
#

class ModelViewSet(mixins.CreateModelMixin,
                   mixins.RetrieveModelMixin,
                   mixins.UpdateModelMixin,
                   mixins.DestroyModelMixin,
                   mixins.ListModelMixin,
                   GenericViewSet):
    """
    Accept either a single object or a list of objects to create.
    """
    def get_serializer(self, *args, **kwargs):
        # If a list of objects has been provided, initialize the serializer with many=True
        if isinstance(kwargs.get('data', {}), list):
            kwargs['many'] = True
        return super(ModelViewSet, self).get_serializer(*args, **kwargs)


class FieldChoicesViewSet(ViewSet):
    """
    Expose the built-in numeric values which represent static choices for a model's field.
    """
    permission_classes = [IsAuthenticatedOrLoginNotRequired]
    fields = []

    def __init__(self, *args, **kwargs):
        super(FieldChoicesViewSet, self).__init__(*args, **kwargs)

        # Compile a dict of all fields in this view
        self._fields = OrderedDict()
        for cls, field_list in self.fields:
            for field_name in field_list:
                model_name = cls._meta.verbose_name.lower().replace(' ', '-')
                key = ':'.join([model_name, field_name])
                choices = []
                for k, v in cls._meta.get_field(field_name).choices:
                    if type(v) in [list, tuple]:
                        for k2, v2 in v:
                            choices.append({
                                'value': k2,
                                'label': v2,
                            })
                    else:
                        choices.append({
                            'value': k,
                            'label': v,
                        })
                self._fields[key] = choices

    def list(self, request):
        return Response(self._fields)

    def retrieve(self, request, pk):
        if pk not in self._fields:
            raise Http404
        return Response(self._fields[pk])

    def get_view_name(self):
        return "Field Choices"