# Django imports.
from django.core.exceptions import ValidationError
from django.core.urlresolvers import reverse
from django.db import models


class Project(models.Model):
    """
    Implements a model with information about a project. A project has some
    basic settings, and mainly serves the purpose of grouping entities for
    easier handling and administration.

    Fields:

      name - String denoting the project name.
      description - Free-form description of the project.
    """

    name = models.CharField(max_length=100, unique=True)
    description = models.TextField(blank=True)

    class Meta:
        permissions = (("view", "Can view information"),)

    def __unicode__(self):
        """
        Returns:
          String representation of a project.
        """

        return self.name

    def get_absolute_url(self):
        """
        Return absolute URL for viewing a single project.
        """

        return reverse("project", kwargs={'pk': self.pk})


class Location(models.Model):
    """
    Implements a model with information about location. Locations can further be
    assigned to entities, letting the user group different servers and equipment
    based on location.

    Locations are not tied to specific project, and they do not have to be
    actual physical locations. Such generic locations are therefore reusable
    accross multiple projects.

    For example, locations can be:

      - Main site
      - Backup site
      - Disaster recovery site
      - Belgrade
      - Stockholm

    Fields:

      name - String denoting the location name.
      description - Free-form description of a location.
    """

    name = models.CharField(max_length=100, unique=True)
    description = models.TextField(blank=True)

    def __unicode__(self):
        """
        Returns:
          String representation of a location.
        """

        return self.name


class Entity(models.Model):
    """
    Models an entity in a project. An entity can be a server, router, or any
    other piece of networking equipment that has its own IP address.

    Entities can also be used for representing subnets etc. This is useful when
    the communication restrictions need to be applied across a subnet.

    Entities are tied to specific projects and locations.

    Fields:

      name - String denoting the entity name.
      description - Free-form description of an entity.
      project - Foreign key pointing to the project to which the entity
      belongs.
      location - Foreign key pointing to the location at which the entity is
      located.
    """

    name = models.CharField(max_length=100)
    description = models.TextField(blank=True)
    project = models.ForeignKey(Project)
    location = models.ForeignKey(Location)

    class Meta:
        # Fix the plural form used by Django.
        verbose_name_plural = 'entities'
        # Enforce uniqueness of entity name in a project.
        unique_together = ("name", "project")

    def __unicode__(self):
        """
        Returns:
          String representation of an entity. This identifier contains name of
          entity, its project name, and location name.
        """

        return "%s (%s - %s)" % (self.name, self.project, self.location)

    def incoming_communications(self):
        """
        Returns:
          List of incoming communications for an entity.
        """

        communications = []

        for interface in self.interface_set.all():
            for communication in interface.destination_set.all():
                communications.append(communication)

        return communications

    def outgoing_communications(self):
        """
        Returns:
          List of outgoing communications for an entity.
        """

        communications = []

        for interface in self.interface_set.all():
            for communication in interface.source_set.all():
                communications.append(communication)

        return communications

    def get_absolute_url(self):
        """
        Return absolute URL for viewing a single entity.
        """

        return reverse("entity", kwargs={'pk': self.pk})

    def clean(self):
        """
        Performs additional validation checks on the submitted data. It will
        verify the following:

          - That entity is not linked to any other entity in case of project
            change.
        """

        # Perform the check if entity is being updated.
        if self.pk:
            # Fetch the old data from database.
            # @TODO: Is it better to do copying during __init__ instead?
            old_object = Entity.objects.get(pk=1)

            # Make sure that entity has no communications in current project if
            # moving it around.
            if self.project != old_object.project and (self.incoming_communications() or self.outgoing_communications()):
                raise ValidationError("The entity cannot be moved to different project as long as it has valid communications with entities in current project.")


class Interface(models.Model):
    """
    Models a representation of an interface on an entity. It can be used for
    representing the subnets as well.

    Each interface is coupled with a specific Entity.

    Fields:
      name - String denoting the interface name. For example 'eth0', 'eth1'
      etc.
      description - Free-form description of an interface.
      entity - Foreign key pointing to the entity to which the interface
      belongs.
      address - IP address of an interface. It's possible to store network
      address in it as well.
      netmask - Netmask of the interface. By default this is /32
      (255.255.255.255), but in case of subnet entities this can be used for
      denoting the network netmask.
    """

    name = models.CharField(max_length=100, default='eth0')
    description = models.TextField(blank=True, default='Main network interface.')
    entity = models.ForeignKey(Entity)
    address = models.IPAddressField()
    netmask = models.IPAddressField(default='255.255.255.255')

    class Meta:
        # Enforce uniqueness of interface name in an entity. Enforce uniqueness
        # of IP address in a subnet for an entity.
        unique_together = (("name", "entity"),
                           ("entity", "address", "netmask"),)

    def __unicode__(self):
        """
        Returns:
          String representation of an interface. In case of single IP this will
          simply be the interface name and IP address. In case of subnet it will
          include the netmask as well.
        """

        if self.netmask == '255.255.255.255':
            return '%s (%s)' % (self.entity.name, self.address)
        else:
            return '%s (%s/%s)' % (self.entity.name, self.address, self.netmask)


class Communication(models.Model):
    """
    Models a representation of allowed network communication. This lets the user
    display the possible network connections that should be allowed. Information
    from the communication instances is also used for generating the iptables
    rules for the entities.

    Communication instances allow the user to specify one of the three possible
    protocols and related information:

      - TCP, along with the TCP port.
      - UDP, along with the UDP port.
      - ICMP, along with the ICMP type.

    Allowed communication is always represented as combination of source
    interface, destination interface, protocol, and port/ICMP type.

    Fields:
      source - Foreign key to the source (originating) interface. The
      communication is expected to come _from_ the source.
      destination - Foreign key to the destination interface. The destination
      interface is expected to be able to accept incoming connections
      (i.e. entity's servers are listening on those).
      protocol - Textual field denoting the protocol that is used for
      communication. This can be 'TCP', 'UDP', or 'ICMP'.
      port - Port number used by the protocol. In case of ICMP, this is an ICMP
      type (in numeric form).
      description - Free-form text that can be used to describe the
      communication. This is also used when generating the iptables rules for
      documenting the rules.
    """

    PROTOCOL_CHOICES = (
        ('TCP', 'TCP'),
        ('UDP', 'UDP'),
        ('ICMP', 'ICMP'),
        )

    source = models.ForeignKey(Interface, related_name='source_set')
    destination = models.ForeignKey(Interface, related_name='destination_set')
    protocol = models.CharField(max_length=10, choices=PROTOCOL_CHOICES)
    port = models.IntegerField(default=0)
    description = models.TextField(blank=True)

    class Meta:
        # Enforce uniqueness of communication.
        unique_together = ("source", "destination", "protocol", "port")

    def __unicode__(self):
        """
        Returns:
          String representation of an interface. This involves showing the
          source and destination _entity_ name, protocol, and port.
        """

        return "%s -> %s (%s:%s)" % (self.source.entity.name, self.destination.entity.name, self.protocol, self.port)

    def clean(self):
        """
        Performs additional validation checks on the submitted data. It will
        verify the following:

          - That source and destination interface belongs to distinct entities.
          - That the specified protocol is supported.
        """

        if self.source.entity == self.destination.entity:
            raise ValidationError('Source and destination entities are identical.')

        if self.source.entity.project != self.destination.entity.project:
            raise ValidationError('Source and destination entities do not belong to the same project')

        if (self.protocol.upper(), self.protocol.upper()) not in self.PROTOCOL_CHOICES:
            raise ValidationError('%s is not a supported protocol.' % self.protocol)

    def edit_link(self):
        """
        This method is used for providing an additional 'Edit' link in the admin
        site for the communication instances (for the display_list).

        This provides ability to let all of the other fields of a communication
        instance to be editable.
        """

        return "Edit"

    def source_representation(self):
        """
        Produces string representation of communication that includes only the
        source interface information.

        The method is useful where the destination context is well known.

        Returns:
            Communication representation that includes only information about
            the source interface.
        """

        return "%s - %s: %d" % (self.source, self.protocol, self.port)

    def destination_representation(self):
        """
        Produces string representation of communication that includes only the
        destination interface information.

        The method is useful where the source context is well known.

        Returns:
            Communication representation that includes only information about
            the destination interface.
        """

        return "%s - %s: %d" % (self.destination, self.protocol, self.port)