from django.db import models
from django.contrib.auth.models import Group as DjangoGroup
from django.contrib.auth import get_user_model
from django.db.models.signals import post_save, m2m_changed
from django.dispatch import receiver


class WikiGroup(models.Model):
    """ A structured user group, used to grant permissions on sub-wikis

    This model contains a structured group of users, in the sense that a group contains
    both users and other groups, allowing a DAG group structure.

    To make it work with django-wiki, we then flatten the set of users in the group
    into a native Django group. This might seem a bit inelegant, but it has been
    discussed in
      https://git.eleves.ens.fr/klub-dev-ens/WikiENS/issues/5
    and seemed the best way at the time, in the sense that it seems stable and reliable
    wrt. internal behaviour and upstream changes in django-wiki.
    """

    class CyclicStructureException(Exception):
        """ Exception raised when a new edge introduces a cycle in the groups'
        structure """

        def __init__(self, from_group, to_group):
            self.from_group = from_group
            self.to_group = to_group

        def __str__(self):
            return (
                "The new relation {} --> {} introduces a cycle in the groups' graph"
            ).format(self.from_group, self.to_group)

    django_group = models.OneToOneField(DjangoGroup, on_delete=models.CASCADE)
    includes_groups = models.ManyToManyField(
        "self",
        symmetrical=False,  # without this Django assumes that (a -> b) => (b -> a) on
        # a many-to-many on self
        related_name="included_in_groups",
        blank=True,
    )
    users = models.ManyToManyField(get_user_model(), blank=True)

    def __str__(self):
        return str(self.django_group)

    def get_all_users(self):
        """ Get the queryset of all the users in this group, including recursively
        included users """
        users_set = self.users.all()
        for subgroup in self.includes_groups.all():
            users_set = users_set.union(subgroup.get_all_users())
        return users_set

    def propagate_update(self, already_notified=None):
        """ Commits itself to the Django group, and calls this method on every group in
        `included_in_groups` """

        # Check that we did not already propagate the update signal to this group
        if already_notified is None:
            already_notified = set()
        elif self.pk in already_notified:
            return
        already_notified.add(self.pk)

        self.commit_to_django_group()
        for metagroup in self.included_in_groups.all():
            metagroup.propagate_update(already_notified=already_notified)

    def commit_to_django_group(self):
        """ Writes this model's data to the related Django group """

        self.django_group.user_set.set(self.get_all_users())

    def group_in_cycle(self, with_children):
        """ Checks whether this group would be in a group cycle if it had
        `with_children` as child nodes. This assumes that the graph currently stored is
        acyclic.

        Returns `None` if no cycle is found, else retuns a child from `with_children`
        causing the cycle to appear. """

        def do_dfs(cur_node, visited_nodes):
            """ DFS to check whether we find `self` again """
            if cur_node.pk in visited_nodes:
                return False
            if cur_node.pk == self.pk:
                return True
            visited_nodes.add(cur_node.pk)

            for child in cur_node.includes_groups.all():
                if do_dfs(child, visited_nodes):
                    return True
            return False

        visited = set()
        for child in with_children:
            if do_dfs(child, visited):
                return child
        return None


@receiver(post_save, sender=WikiGroup, dispatch_uid="on_wiki_group_changed")
def on_wiki_group_changed(sender, instance, **kwargs):
    """ Commit the related WikiGroups to Django Group upon model change """
    instance.propagate_update()


@receiver(
    m2m_changed,
    sender=WikiGroup.includes_groups.through,
    dispatch_uid="on_wiki_group_includes_changed",
)
def on_wiki_group_includes_changed(sender, instance, action, **kwargs):
    """ Commit the related WikiGroups to Django Group upon change of the set of
    included other groups """
    if action in ["post_add", "post_remove", "post_clear"]:
        instance.propagate_update()


@receiver(
    m2m_changed,
    sender=WikiGroup.users.through,
    dispatch_uid="on_wiki_group_users_changed",
)
def on_wiki_group_users_changed(sender, instance, action, **kwargs):
    """ Commit the related WikiGroups to Django Group upon change of included users """
    if action in ["post_add", "post_remove", "post_clear"]:
        instance.propagate_update()


@receiver(
    m2m_changed,
    sender=WikiGroup.includes_groups.through,
    dispatch_uid="on_wiki_group_includes_check_acyclic",
)
def on_wiki_group_includes_check_acyclic(sender, instance, action, pk_set, **kwargs):
    """ Checks the acyclicity of the groups' graph before committing new edges.

    PLEASE NOTE that this check is only a fallback, and that forms should validate
    the acyclicity before committing anything.
    """
    if action == "pre_add":
        children = [WikiGroup.objects.get(pk=cur_pk) for cur_pk in pk_set]
        cycle_child = instance.group_in_cycle(children)
        if cycle_child:
            raise WikiGroup.CyclicStructureException(instance, cycle_child)