System.Net.HttpListener.SaveDigestContext C# (CSharp) Method

        private void SaveDigestContext(NTAuthentication digestContext)
        {
            if (_savedDigests == null)
            {
                Interlocked.CompareExchange<DigestContext[]>(ref _savedDigests, new DigestContext[MaximumDigests], null);
            }

            // We want to actually close the contexts outside the lock.
            NTAuthentication oldContext = null;
            ArrayList digestsToClose = null;
            lock (_savedDigests)
            {
                // If we're stopped, just throw it away.
                if (!IsListening)
                {
                    digestContext.CloseContext();
                    return;
                }

                int now = ((now = Environment.TickCount) == 0 ? 1 : now);

                _newestContext = (_newestContext + 1) & (MaximumDigests - 1);

                int oldTimestamp = _savedDigests[_newestContext].timestamp;
                oldContext = _savedDigests[_newestContext].context;
                _savedDigests[_newestContext].timestamp = now;
                _savedDigests[_newestContext].context = digestContext;

                // May need to move this up.
                if (_oldestContext == _newestContext)
                {
                    _oldestContext = (_newestContext + 1) & (MaximumDigests - 1);
                }

                // Delete additional contexts older than five minutes.
                while (unchecked(now - _savedDigests[_oldestContext].timestamp) >= DigestLifetimeSeconds && _savedDigests[_oldestContext].context != null)
                {
                    if (digestsToClose == null)
                    {
                        digestsToClose = new ArrayList();
                    }
                    digestsToClose.Add(_savedDigests[_oldestContext].context);
                    _savedDigests[_oldestContext].context = null;
                    _oldestContext = (_oldestContext + 1) & (MaximumDigests - 1);
                }

                // If the old context is younger than 10 seconds, put it in the backup pile.
                if (oldContext != null && unchecked(now - oldTimestamp) <= MinimumDigestLifetimeSeconds * 1000)
                {
                    // Use a two-tier ArrayList system to guarantee each entry lives at least 10 seconds.
                    if (_extraSavedDigests == null ||
                        unchecked(now - _extraSavedDigestsTimestamp) > MinimumDigestLifetimeSeconds * 1000)
                    {
                        digestsToClose = _extraSavedDigestsBaking;
                        _extraSavedDigestsBaking = _extraSavedDigests;
                        _extraSavedDigestsTimestamp = now;
                        _extraSavedDigests = new ArrayList();
                    }
                    _extraSavedDigests.Add(oldContext);
                    oldContext = null;
                }
            }

            if (oldContext != null)
            {
                oldContext.CloseContext();
            }
            if (digestsToClose != null)
            {
                for (int i = 0; i < digestsToClose.Count; i++)
                {
                    ((NTAuthentication)digestsToClose[i]).CloseContext();
                }
            }
        }