from __future__ import unicode_literals
from collections import OrderedDict

from django.conf import settings
from django.contrib.contenttypes.models import ContentType
from django.http import Http404

from rest_framework.exceptions import APIException
from rest_framework.permissions import BasePermission
from rest_framework.response import Response
from rest_framework.serializers import Field, ModelSerializer, ValidationError
from rest_framework.viewsets import ViewSet


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


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


#
# 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()


#
# 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:
            instance = self.Meta.model(**attrs)
        else:
            instance = self.instance
            for k, v in attrs.items():
                setattr(instance, k, v)
        instance.clean()

        return data


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 self._choices.get(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")


#
# Views
#

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"


#
# Mixins
#

class WritableSerializerMixin(object):
    """
    Allow for the use of an alternate, writable serializer class for write operations (e.g. POST, PUT).
    """
    def get_serializer_class(self):
        if self.action in WRITE_OPERATIONS and hasattr(self, 'write_serializer_class'):
            return self.write_serializer_class
        return self.serializer_class
