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diff --git a/game/server/func_areaportalbase.h b/game/server/func_areaportalbase.h
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+++ b/game/server/func_areaportalbase.h
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+//========= Copyright Valve Corporation, All rights reserved. ============//
+//
+// Purpose: 
+//
+// $NoKeywords: $
+//=============================================================================//
+
+#ifndef FUNC_AREAPORTALBASE_H
+#define FUNC_AREAPORTALBASE_H
+#ifdef _WIN32
+#pragma once
+#endif
+
+
+#include "baseentity.h"
+#include "utllinkedlist.h"
+
+
+// Shared stuff between door portals and window portals.
+class CFuncAreaPortalBase : public CBaseEntity
+{
+	DECLARE_CLASS( CFuncAreaPortalBase, CBaseEntity );
+public:
+	DECLARE_DATADESC();
+
+					CFuncAreaPortalBase();
+	virtual			~CFuncAreaPortalBase();
+
+	// Areaportals must be placed in each map for preprocess, they can't use transitions
+	virtual int	ObjectCaps( void ) { return BaseClass::ObjectCaps() & ~FCAP_ACROSS_TRANSITION; }
+	
+	// This is called each frame for each client to all portals to close 
+	// when the viewer is far enough away, or on the backside.
+	//
+	// The default implementation closes the portal if the viewer (plus some padding)
+	// is on the backside of the portal. Return false if you close the portal.
+	//
+	// Returns whether or not the (server part of) the engine was told to shut the 
+	// portal off for purposes of flowing through portals to determine which areas to
+	// send to the client.
+	//
+	//
+	// bIsOpenOnClient is usually the same as the return value but NOT always. Here's why (explained
+	// here in depth because this case was discovered in a lengthy debugging session):
+	//
+	// - Each CFuncAreaPortalBase represents two dareaportal_t's (matched by CFuncAreaPortalBase::m_portalNumber
+	//   and dareaportal_t::m_PortalKey). Each dareaportal_t leads into one of the connected areas
+	//   and the dareaportal_t's have opposite-facing planes.
+	//
+	// - The engine's SetAreaPortalState function takes a portal key and closes BOTH dareaportal_t's associated with it.
+	//
+	// - UpdateVisibility may decide a portal leading out of the _area you're sitting in_ can be closed
+	//   for purposes of flowing through areas because you're on the backside of the dareaportal_t that 
+	//   you would flow out of. 
+	//
+	// - At the same time, you might be able to look through the other dareaportal_t attached to
+	//   that portal key (right back into the area you're standing in). 
+	//
+	// - An illustration:
+	//
+	//	 --------------------
+	//	 |					|
+	//	 |	|--------|aaaaaa|
+	//	 |	|		 |bbbbbb| <---- aaaa and bbbb area both for PortalKey1
+	//	 |**|   	 |     	|
+	//	 |	|		 | area	|
+	//	 |**|		 |	2   |
+	//	 |	|		 |------|
+	//	 |	|		 |		|
+	//	 |	---------- area |
+	//	 |				1   |
+	//	 |------------------|
+	//
+	// "aaaa" and "bbbb" each represent a different dareaportal_t, (aaa leads into area 2 and
+	// bbb leads into area 1). They both represent the same portal key though (call it PortalKey1).
+	//
+	// When standing in area 1 (where the "area 1" text is), the engine will check aaaa and it'll notice
+	// that you're on the wrong side of aaaa to be looking through it, so it'll say that PortalKey1 is closed
+	// for purposes of flowing through areas on the server (it turns out you can get to area 2 through
+	// the portal right above the "area 1" text).
+	//
+	// If you told the client that PortalKey1 was closed, then you'd get a pop when moving from area 1
+	// to area 2 because the client would think you couldn't see through PortalKey1 into area 1 UNTIL
+	// the server transmitted the new updated PortalKey bits, which can be lagged. 
+	//
+	// That's why we have bIsOpenOnClient which doesn't include this backfacing test.
+	//
+	// .. and they all lived happily ever after.
+	// The End
+	// 
+	//
+	//
+	// Note: when you're standing in the space between the **'s, then the server would stop transmitting
+	// the contents of area 2 because there would be no portal you were on the correct side of to
+	// see into area 2.
+	virtual bool	UpdateVisibility( const Vector &vOrigin, float fovDistanceAdjustFactor, bool &bIsOpenOnClient );
+
+
+public:
+
+	// This matches two dareaportal_t::m_PortalKeys.	
+	int				m_portalNumber;
+	
+	int				m_iPortalVersion;
+
+private:
+	
+	unsigned short	m_AreaPortalsElement;	// link into g_AreaPortals.
+};
+
+
+extern CUtlLinkedList<CFuncAreaPortalBase*, unsigned short> g_AreaPortals;
+
+
+
+#endif // FUNC_AREAPORTALBASE_H