Makes tear in the fabric of reality inaccessible until a boh bomb is detonated (#38852)

* a commit message

* reviews addressed?

code/__DEFINES/turf_flags.dm

@@ -1,4 +1,5 @@
 #define CHANGETURF_DEFER_CHANGE		1
-#define CHANGETURF_IGNORE_AIR		2
+#define CHANGETURF_IGNORE_AIR		2 // This flag prevents changeturf from gathering air from nearby turfs to fill the new turf with an approximation of local air
 #define CHANGETURF_FORCEOP			4
 #define CHANGETURF_SKIP				8 // A flag for PlaceOnTop to just instance the new turf instead of calling ChangeTurf. Used for uninitialized turfs NOTHING ELSE
+#define CHANGETURF_INHERIT_AIR 16 // Inherit air from previous turf. Implies CHANGETURF_IGNORE_AIR

code/datums/components/storage/concrete/bag_of_holding.dm

@@ -23,6 +23,8 @@
 					M.visible_message("<span class='danger'>The bluespace collapse crushes the air towards it, pulling [M] towards the ground...</span>")
 					M.Knockdown(5, TRUE, TRUE)		//Overrides stun absorbs.
 			T.TerraformTurf(/turf/open/chasm/magic, /turf/open/chasm/magic)
+		for (var/obj/structure/ladder/unbreakable/binary/ladder in GLOB.ladders)
+			ladder.ActivateAlmonds()
 		message_admins("[ADMIN_LOOKUPFLW(user)] detonated a bag of holding at [ADMIN_VERBOSEJMP(loccheck)].")
 		log_game("[key_name(user)] detonated a bag of holding at [AREACOORD(loccheck)].")
 		qdel(A)

code/game/machinery/navbeacon.dm

@@ -37,10 +37,18 @@
 		GLOB.deliverybeacontags += location
 
 /obj/machinery/navbeacon/Destroy()
-	GLOB.navbeacons["[z]"] -= src //Remove from beacon list, if in one.
+	if (GLOB.navbeacons["[z]"])
+		GLOB.navbeacons["[z]"] -= src //Remove from beacon list, if in one.
 	GLOB.deliverybeacons -= src
 	return ..()
 
+/obj/machinery/navbeacon/onTransitZ(old_z, new_z)
+	if (GLOB.navbeacons["[old_z]"])
+		GLOB.navbeacons["[old_z]"] -= src
+	if (GLOB.navbeacons["[new_z]"])
+		GLOB.navbeacons["[new_z]"] += src
+	..()
+
 // set the transponder codes assoc list from codes_txt
 /obj/machinery/navbeacon/proc/set_codes()
 	if(!codes_txt)

code/game/objects/structures/ladders.dm

@@ -182,31 +182,50 @@
 
 	update_icon()
 
-
 /obj/structure/ladder/unbreakable/binary
 	name = "mysterious ladder"
 	desc = "Where does it go?"
 	height = 0
 	id = "lavaland_binary"
 	var/area_to_place = /area/lavaland/surface/outdoors
+	var/active = FALSE
 
-/obj/structure/ladder/unbreakable/binary/Initialize()
-	if(area_to_place)
-		var/turf/T = safepick(get_area_turfs(area_to_place))
+/obj/structure/ladder/unbreakable/binary/proc/ActivateAlmonds()
+	if(area_to_place && !active)
+		var/turf/T = getTargetTurf()
 		if(T)
 			var/obj/structure/ladder/unbreakable/U = new (T)
 			U.id = id
 			U.height = height+1
+			LateInitialize() // LateInit both of these to build the links. It's fine.
+			U.LateInitialize()
 			for(var/turf/TT in range(2,U))
-				TT.TerraformTurf(/turf/open/indestructible/binary, /turf/open/indestructible/binary)
-	return ..()
+				TT.TerraformTurf(/turf/open/indestructible/binary, /turf/open/indestructible/binary, CHANGETURF_INHERIT_AIR)
+		active = TRUE
 
+/obj/structure/ladder/unbreakable/binary/proc/getTargetTurf()
+	var/list/turfList = get_area_turfs(area_to_place)
+	while (turfList.len && !.)
+		var/i = rand(1, turfList.len)
+		var/turf/potentialTurf = turfList[i]
+		if (is_centcom_level(potentialTurf.z)) // These ladders don't lead to centcom.
+			turfList.Cut(i,i+1)
+			continue
+		if(!istype(potentialTurf, /turf/open/lava) && !potentialTurf.density)			// Or inside dense turfs or lava
+			var/clear = TRUE
+			for(var/obj/O in potentialTurf) // Let's not place these on dense objects either. Might be funny though.
+				if(O.density)
+					clear = FALSE
+					break
+			if(clear)
+				. = potentialTurf
+		if (!.)
+			turfList.Cut(i,i+1)
 
 /obj/structure/ladder/unbreakable/binary/space
 	id = "space_binary"
 	area_to_place = /area/space
 
-
 /obj/structure/ladder/unbreakable/binary/unlinked //Crew gets to complete one
 	id = "unlinked_binary"
 	area_to_place = null

code/game/turfs/change_turf.dm

@@ -114,6 +114,23 @@ GLOBAL_LIST_INIT(blacklisted_automated_baseturfs, typecacheof(list(
 
 	return W
 
+/turf/open/ChangeTurf(path, list/new_baseturfs, flags)
+	if ((flags & CHANGETURF_INHERIT_AIR) && ispath(path, /turf/open))
+		SSair.remove_from_active(src)
+		var/stashed_air = air
+		air = null // so that it doesn't get deleted
+		. = ..()
+		if (!. || . == src) // changeturf failed or didn't do anything
+			air = stashed_air
+			return
+		var/turf/open/newTurf = .
+		if (!istype(newTurf.air, /datum/gas_mixture/immutable/space))
+			QDEL_NULL(newTurf.air)
+			newTurf.air = stashed_air
+		SSair.add_to_active(newTurf)
+	else
+		return ..()
+
 // Take off the top layer turf and replace it with the next baseturf down
 /turf/proc/ScrapeAway(amount=1, flags)
 	if(!amount)
@@ -251,7 +268,7 @@ GLOBAL_LIST_INIT(blacklisted_automated_baseturfs, typecacheof(list(
 /turf/open/AfterChange(flags)
 	..()
 	RemoveLattice()
-	if(!(flags & CHANGETURF_IGNORE_AIR))
+	if(!(flags & (CHANGETURF_IGNORE_AIR | CHANGETURF_INHERIT_AIR)))
 		Assimilate_Air()
 
 //////Assimilate Air//////