from django.conf import settings
from django.contrib.sites.models import Site
from django.contrib.redirects.models import Redirect
from random import choice, seed
from os import urandom

SHORTURL_CHARS = getattr(settings, "SHORTURL_CHARS", "bcdfghjklmnpqrstvwxyz2346789")
SHORTURL_CHAR_NO = getattr(settings, "SHORTURL_CHAR_NO", 5)
SHORTURL_APPEND_SLASH = getattr(settings, "SHORTURL_APPEND_SLASH", True)

class ShortURLException: pass

class ShortURL(object):
    """
    A mixin that sets up short url redirects for models that have a get_absolute_url 
    method.  Requires django.contrib.redirects to be installed to create redirects, and
    django.contrib.redirects.middleware.RedirectFallbackMiddleware to use them. 
    """
    def __init__(self, *args, **kwargs):
        """
        Seeds randomiser 
        """
        seed(urandom(256))
        super(ShortURL, self, *args, **kwargs)
        
    def get_short_url(self, *args, **kwargs):
        """
        Finds the short url for the object's absolute url in the Redirects model objects.  
        If it doesn't exist, generate a short url and create a new Redirect object.
        """
        if not hasattr(self, 'get_absolute_url'):
            return None
        else:
            currenturl = self.get_absolute_url()
            site = Site.objects.get(id=settings.SITE_ID)
            redirects = Redirect.objects.filter(site=site, new_path=currenturl)
            
            for url in redirects:
                if len(url.old_path) <= SHORTURL_CHAR_NO + 1: #allow for leading slash
                    shorturl = url.old_path
                    break
            else: 
                shorturl = None
                
            if not shorturl:    
                # Check we've got at least a 9 in ten chance of not colliding or throw an exception
                if Redirect.objects.count() > (len(SHORTURL_CHARS) ** SHORTURL_CHAR_NO) / 10:
                    raise ShortURLException
                while True:
                    shorturl = '/'+''.join([choice(SHORTURL_CHARS) for char in range(SHORTURL_CHAR_NO)]) 
                    if not Redirect.objects.filter(site=site, old_path=shorturl):
                        # save shorturl without trailing slash so redirect middleware will find both forms
                        r = Redirect(site=site, old_path=shorturl, new_path=currenturl)
                        r.save()
                        break
            
            shorturl += '/' if SHORTURL_APPEND_SLASH else ''
            return shorturl