import argparse
import sys

from django.core.exceptions import ImproperlyConfigured
from django.core.management import BaseCommand
from django.template import engines
from django.template.backends.django import DjangoTemplates
from django.urls import resolve, reverse

from desecapi.models import User


def _get_default_template_backend():
    # Ad-hoc implementation of https://github.com/django/django/pull/15944
    for backend in engines.all():
        if isinstance(backend, DjangoTemplates):
            return backend
    raise ImproperlyConfigured("No DjangoTemplates backend is configured.")


class Command(BaseCommand):
    help = 'Reach out to users with an email. Takes email template on stdin.'

    def add_arguments(self, parser):
        parser.add_argument('email', nargs='*', help='User(s) to contact, identified by their email addresses. '
                            'Defaults to everyone with outreach_preference = True.')
        parser.add_argument('--contentfile', nargs='?', type=argparse.FileType('r'), default=sys.stdin,
                            help='File to take email content from. Defaults to stdin.')
        parser.add_argument('--reason', nargs='?', default='change-outreach-preference',
                            help='Kind of message to send. Choose from reasons given in serializers.py. Defaults to '
                                 'newsletter with unsubscribe link (reason: change-outreach-preference).')
        parser.add_argument('--subject', nargs='?', default=None, help='Subject, default according to "reason".')

    def handle(self, *args, **options):
        reason = options['reason']
        path = reverse(f'v1:confirm-{reason}', args=['code'])
        serializer_class = resolve(path).func.cls.serializer_class

        content = options['contentfile'].read().strip()
        if not content and options['contentfile'].name != '/dev/null':
            raise RuntimeError('Empty content only allowed from /dev/null')

        try:
            subject = '[deSEC] ' + options['subject']
        except TypeError:
            subject = None

        base_file = f'emails/{reason}/content.txt'
        template_code = ('{%% extends "%s" %%}' % base_file)
        if content:
            template_code += '{% block content %}' + content + '{% endblock %}'
        template = _get_default_template_backend().from_string(template_code)

        if options['email']:
            users = User.objects.filter(email__in=options['email'])
        elif content:
            users = User.objects.filter(outreach_preference=True)
        else:
            raise RuntimeError('To send default content, specify recipients explicitly.')

        for user in users:
            action = serializer_class.Meta.model(user=user)
            serializer_class(action).save(subject=subject, template=template)