import base64
import cPickle as pickle

from django.db import models
from django.utils.translation import ugettext_lazy as _
from django.conf import settings
from django.utils.hashcompat import md5_constructor


class SessionManager(models.Manager):
    def encode(self, session_dict):
        """
        Returns the given session dictionary pickled and encoded as a string.
        """
        pickled = pickle.dumps(session_dict)
        pickled_md5 = md5_constructor(pickled + settings.SECRET_KEY).hexdigest()
        return base64.encodestring(pickled + pickled_md5)

    def save(self, session_key, session_dict, expire_date):
        s = self.model(session_key, self.encode(session_dict), expire_date)
        if session_dict:
            s.save()
        else:
            s.delete() # Clear sessions with no data.
        return s


class Session(models.Model):
    """
    Django provides full support for anonymous sessions. The session
    framework lets you store and retrieve arbitrary data on a
    per-site-visitor basis. It stores data on the server side and
    abstracts the sending and receiving of cookies. Cookies contain a
    session ID -- not the data itself.

    The Django sessions framework is entirely cookie-based. It does
    not fall back to putting session IDs in URLs. This is an intentional
    design decision. Not only does that behavior make URLs ugly, it makes
    your site vulnerable to session-ID theft via the "Referer" header.

    For complete documentation on using Sessions in your code, consult
    the sessions documentation that is shipped with Django (also available
    on the Django Web site).
    """
    session_key = models.CharField(_('session key'), max_length=40,
                                   primary_key=True)
    session_data = models.TextField(_('session data'))
    expire_date = models.DateTimeField(_('expire date'), db_index=True)
    objects = SessionManager()

    class Meta:
        db_table = 'django_session'
        verbose_name = _('session')
        verbose_name_plural = _('sessions')

    def get_decoded(self):
        encoded_data = base64.decodestring(self.session_data)
        pickled, tamper_check = encoded_data[:-32], encoded_data[-32:]
        if md5_constructor(pickled + settings.SECRET_KEY).hexdigest() != tamper_check:
            from django.core.exceptions import SuspiciousOperation
            raise SuspiciousOperation("User tampered with session cookie.")
        try:
            return pickle.loads(pickled)
        # Unpickling can cause a variety of exceptions. If something happens,
        # just return an empty dictionary (an empty session).
        except:
            return {}
