from django.conf import settings
from django.core.cache import cache

from collections import deque

class HammeringMiddleware(object):
    """
    A middleware which will protect from page hammering using flexible spanning time windows using the cache backend.
    One IP is allowed to do SPAM_REQUEST_WINDOW_PER_IP requests per SPAM_TIME_WINDOW seconds.
    If more requests are done, the middleware will simply redirect to a warning page specified in SPAM_WINDOW_EXCEEDED_VIEW and block
    requests to pages other than SPAM_WINDOW_EXCEEDED_VIEW for HAMMER_WINDOW_EXCEEDED_BLOCK seconds.
    
    The last SPAM_REQUEST_WINDOW_PER_IP request timestamps will be logged in the cache in a list (deque). This cache key
    by default lasts for a maximum of SPAM_REQUEST_WINDOW_PER_IP * SPAM_TIME_WINDOW and can be configured
    using SPAM_CACHE_TIMEOUT.
    
    Notes on Ajax requests: If you have a website that does many subrequests per page view, such as websites with much Ajax-based
          content, you may have to consider either lowering HAMMER_TIME_WINDOW or increasing HAMMER_REQUEST_WINDOW_PER_IP. The time window
          threshold may hit often on those mentioned sites and users within a network (with the same IP).
          You also have the option to turn HammeringMiddleware off for Ajax-based requests by enabling HAMMER_EXEMPT_AJAX in settings. This
          is not recommended, though.
          
    Note on django-newcache: If you are using newcache as your backend, which supports thundering herd mitigation, set HAMMER_CACHE_HAS_HERDING
          to True. This will append the kwarg herd=False to cache.set calls.
    
    Notes: 
          - HammeringMiddleware should be the very first entry in your MIDDLEWARE_CLASSES tuple
          - This middleware should be used with a memory-based (locmem/memcached) cache backend but works well with others
    """
    
    maxreq = getattr(settings, "HAMMER_REQUEST_WINDOW_PER_IP", 25)
    window = getattr(settings, "HAMMER_TIME_WINDOW", 5)
    ctimeout = getattr(settings, "HAMMER_CACHE_TIMEOUT", window * maxreq)
    blockFor = getattr(settings, "HAMMER_WINDOW_EXCEEDED_BLOCK", window)
    tooFastView = getattr(settings, "HAMMER_WINDOW_EXCEEDED_VIEW", "/hammering/")
    
    exemptAjax = getattr(settings, "HAMMER_EXEMPT_AJAX", False)
    skipHerd = getattr(settings, "HAMMER_CACHE_HAS_HERDING", False)
    
    def reject(self):
        return HttpResponseRedirect(self.tooFastView)
    
    def setCache(self, key, value, timeout):
        if self.skipHerd:
            cache.set(key, value, timeout, herd=False)
        else:
            cache.set(key, value, timeout)

    def process_request(self, request):        
        # get ident from forwarded IP, else REMOTE_ADDR
        addr = request.META.get("HTTP_X_FORWARDED_FOR", request.META["REMOTE_ADDR"])
        ident, identBlocked = "rsm_%s" % addr, "rsmb_%s" % addr
        
        # we don't want to hammer ourselves with our HttpResponseRedirect, if we are on the warning page.
        # if settings tells us we should exempt ajax requests, don't go futher, either.
        if (request.is_ajax() and self.exemptAjax) or request.path.startswith(self.tooFastView):
            return
        
        # user is blocked? don't handle the request
        if cache.get(identBlocked, False):
            return self.reject()
        
        reqinfo = cache.get(ident, deque([], self.maxreq+1))
        cTime = time.time()
        
        # append the current timestamp to the list - we are using deque here, with maxitems set, so it will be capped at all times.
        reqinfo.append(cTime)
        
        # remove all timestamps from our list that are older than our time window
        for stamp in list(reqinfo):
            if cTime - stamp > self.window:
                reqinfo.popleft()

        self.setCache(ident, reqinfo, self.ctimeout)
               
        itemcount = len(reqinfo)
        if itemcount == self.maxreq and itemcount > 1:
            if reqinfo[-1] - reqinfo[0] < self.window:
                # if we hit the threshold, redirect (the first item in the list, which is the oldest timestamp, 
                # compared to the newest timestamp in the list, exceeds the time window)
                
                # block him also
                self.setCache(identBlocked, 1, self.blockFor)
                return self.reject()