Rework xeno guaranteed slots logic and Fix thunderdome evolution slot…

… bug (#4206)

# About the pull request

This PR reworks the logic regarding guaranteed xeno slots. Before, the
issue was that there was a discrepancy between gibbing/deletion and
regular death (meaning not getting gibbed wouldn't free up a free slot).
Now that logic is unified via using hive.remove_xeno and the
used_free_slots now is just a count of slots used for a caste rather
than trying to keep the calculation always correct. This avoids the
messiness of handing the free slot usage to say the other burrower when
there are two rather than one then freeing a slot.

Additionally, I encountered a bug during testing where evolution in the
thunderdome would alter a hive's slots (and the effect would only show
in hive_status after a xeno count update on add/removal), but that is
also resolved.

See Testing section below to confirm logic, see reproduction for the
t-dome fix, and then retesting.

# Explain why it's good for the game

Should fix #398 

# Testing Photographs and Procedure
<details>
<summary>Steps 1-9:</summary>

1: Initial hive spawned in:

![1](https://github.com/cmss13-devs/cmss13/assets/76988376/fd5a453d-42a2-4ecf-8f63-c2d4dc97d7ff)

2: Evo a drone to burrower (free slot used):

![2](https://github.com/cmss13-devs/cmss13/assets/76988376/ce243339-854b-42b8-8a97-f81307548886)

3: Evo another drone to burrower (tier 2 slot used):

![3](https://github.com/cmss13-devs/cmss13/assets/76988376/cd03c6ee-8560-45a7-900b-163225806966)

4: First burrower is killed:

![4](https://github.com/cmss13-devs/cmss13/assets/76988376/0b802d07-58c0-48bd-9a55-2d2606cd6148)

5: Second burrower is gibbed:

![5](https://github.com/cmss13-devs/cmss13/assets/76988376/df0786db-a64d-46bc-abfc-18724baadd31)

6: Evo another drone to burrower (free slot used - I also tried deleting
the burrower corpse afterwards to see if it affected anything):

![6](https://github.com/cmss13-devs/cmss13/assets/76988376/ba747571-cc21-4547-a844-a675874f7ff1)

7: Evo a drone to hivelord (free slot used):

![7](https://github.com/cmss13-devs/cmss13/assets/76988376/df1c9f9e-e615-4e86-aaf0-6088d109b1a6)

8: Spawned a carrier and drone in t-dome, and evolved that drone to
carrier (no effect on hive slots):

![8](https://github.com/cmss13-devs/cmss13/assets/76988376/6a9ac50f-d31e-40fc-9aa1-22a92454dc4b)

9: Killed and gibbed the t-dome carriers (no effect on hive slots):

![9](https://github.com/cmss13-devs/cmss13/assets/76988376/b2dbccb3-2a09-4669-9593-4c3eb87214f2)

---
</details>

<details>
<summary>Steps 10-14 Before thunderdome fix:</summary>

10: !! De-evolve a drone to larva **(t2 slots went down - unintended)**:
!!

![10](https://github.com/cmss13-devs/cmss13/assets/76988376/19b2d578-8ff3-4362-811a-aec45b7d1e2d)

11: !! Evolve larva to runner **(no effect on hive slots - unintended or
should have restored to slot counts in previous step)**: !!

![11](https://github.com/cmss13-devs/cmss13/assets/76988376/c5d4cde3-c819-42bf-b016-ae66bcd3ed85)

12: Evolve runner to lurker and correctly failed attempt to evolve to
ravager (t2 slot used):

![12](https://github.com/cmss13-devs/cmss13/assets/76988376/fafe2e58-dd8c-4c27-ad56-884fff755acc)

13: Evolve another drone to burrower (last tier 2 slot used):

![13](https://github.com/cmss13-devs/cmss13/assets/76988376/84e0b82b-2962-49ba-9491-444059fa3a93)

14: Correctly failed attempt to evolve to hivelord, but successful
evolution from drone to carrier (last guaranteed slot used):

![14](https://github.com/cmss13-devs/cmss13/assets/76988376/df898858-fcad-4b2a-ab10-8e9c262ab50a)

---
</details>

<details>
<summary>Steps 10-15 After thunderdome fix:</summary>

Steps 1-9 are identical.

10: De-evolve a drone to larva (correctly didn't change slot values
because of tdome evolution):

![new10](https://github.com/cmss13-devs/cmss13/assets/76988376/a51083f4-7ac7-47b1-8710-b9d740ea1f9e)

11: Evolve larva to runner (no effect on slots):

![new11](https://github.com/cmss13-devs/cmss13/assets/76988376/ae547ef6-a846-4936-97d8-e116d284c45c)

12: Evolve runner to lurker and correctly failed attempt to evolve to
ravager (t2 slot used):

![new12](https://github.com/cmss13-devs/cmss13/assets/76988376/b858823e-8611-4db9-ab76-60fece28e46b)

13: Evolve another drone to burrower (tier 2 slot used):

![new13](https://github.com/cmss13-devs/cmss13/assets/76988376/f1044827-df38-4d40-9d82-2737a09e5ffc)

14: Evolve another drone to burrower (last tier 2 slot used):

![new14](https://github.com/cmss13-devs/cmss13/assets/76988376/e1ea916c-b001-4d0e-973c-fc49de7ec419)

15: Correctly failed attempt to evolve to hivelord, but successful
evolution from drone to carrier (last guaranteed slot used):

![new15](https://github.com/cmss13-devs/cmss13/assets/76988376/4214ace4-8202-4b68-b526-f1ac5aae1e24)

</details>


# Changelog
:cl: Drathek
fix: Fix/Rework logic regarding xeno guaranteed slots
fix: Fix evolution in thunder dome affecting hive slots
/:cl:

code/modules/mob/living/carbon/xenomorph/Evolution.dm

@@ -134,11 +134,13 @@
 			qdel(new_xeno)
 		return
 
-	switch(new_xeno.tier) //They have evolved, add them to the slot count
-		if(2)
-			hive.tier_2_xenos |= new_xeno
-		if(3)
-			hive.tier_3_xenos |= new_xeno
+	var/area/xeno_area = get_area(new_xeno)
+	if(!is_admin_level(new_xeno.z) || (xeno_area.flags_atom & AREA_ALLOW_XENO_JOIN))
+		switch(new_xeno.tier) //They have evolved, add them to the slot count IF they are in regular game space
+			if(2)
+				hive.tier_2_xenos |= new_xeno
+			if(3)
+				hive.tier_3_xenos |= new_xeno
 
 	log_game("EVOLVE: [key_name(src)] evolved into [new_xeno].")
 	if(mind)
@@ -355,24 +357,27 @@
 
 /mob/living/carbon/xenomorph/proc/can_evolve(castepick, potential_queens)
 	var/selected_caste = GLOB.xeno_datum_list[castepick]?.type
-	var/free_slots = LAZYACCESS(hive.free_slots, selected_caste)
-	if(free_slots)
+	var/free_slot = LAZYACCESS(hive.free_slots, selected_caste)
+	var/used_slot = LAZYACCESS(hive.used_slots, selected_caste)
+	if(free_slot > used_slot)
 		return TRUE
 
-	var/burrowed_factor = min(hive.stored_larva, sqrt(4*hive.stored_larva))
-	burrowed_factor = round(burrowed_factor)
-
 	var/used_tier_2_slots = length(hive.tier_2_xenos)
 	var/used_tier_3_slots = length(hive.tier_3_xenos)
-	for(var/caste_path in hive.used_free_slots)
-		if(!hive.used_free_slots[caste_path])
+	for(var/caste_path in hive.free_slots)
+		var/slots_free = hive.free_slots[caste_path]
+		var/slots_used = hive.used_slots[caste_path]
+		if(!slots_used)
 			continue
-		var/datum/caste_datum/C = caste_path
-		switch(initial(C.tier))
-			if(2) used_tier_2_slots--
-			if(3) used_tier_3_slots--
+		var/datum/caste_datum/current_caste = caste_path
+		switch(initial(current_caste.tier))
+			if(2)
+				used_tier_2_slots -= min(slots_used, slots_free)
+			if(3)
+				used_tier_3_slots -= min(slots_used, slots_free)
 
-	var/totalXenos = burrowed_factor
+	var/burrowed_factor = min(hive.stored_larva, sqrt(4*hive.stored_larva))
+	var/totalXenos = round(burrowed_factor)
 	for(var/mob/living/carbon/xenomorph/xeno as anything in hive.totalXenos)
 		if(xeno.counts_for_slots)
 			totalXenos++

code/modules/mob/living/carbon/xenomorph/Xenomorph.dm

@@ -499,11 +499,7 @@
 	// Only handle free slots if the xeno is not in tdome
 	if(!is_admin_level(z))
 		var/selected_caste = GLOB.xeno_datum_list[caste_type]?.type
-		var/free_slots = LAZYACCESS(hive.free_slots, selected_caste)
-		if(free_slots)
-			hive.free_slots[selected_caste]--
-			var/new_val = LAZYACCESS(hive.used_free_slots, selected_caste) + 1
-			LAZYSET(hive.used_free_slots, selected_caste, new_val)
+		hive.used_slots[selected_caste]++
 
 	if(round_statistics && !statistic_exempt)
 		round_statistics.track_new_participant(faction, 1)
@@ -706,16 +702,7 @@
 		hive.remove_hive_leader(src, light_mode = TRUE)
 	SStracking.stop_tracking("hive_[hivenumber]", src)
 
-	// Only handle free slots if the xeno is not in tdome
-	if(!is_admin_level(z))
-		var/selected_caste = GLOB.xeno_datum_list[caste_type]?.type
-		var/used_slots = LAZYACCESS(hive.used_free_slots, selected_caste)
-		if(used_slots)
-			hive.used_free_slots[selected_caste]--
-			var/new_val = LAZYACCESS(hive.free_slots, selected_caste) + 1
-			LAZYSET(hive.free_slots, selected_caste, new_val)
-
-	hive.remove_xeno(src)
+	hive?.remove_xeno(src)
 	remove_from_all_mob_huds()
 
 	observed_xeno = null

code/modules/mob/living/carbon/xenomorph/xeno_defines.dm

@@ -264,8 +264,8 @@
 		/datum/caste_datum/hivelord = 1,
 		/datum/caste_datum/carrier = 1
 	)
-	/// Assoc list of free slots currently used by specific castes
-	var/list/used_free_slots
+	/// Assoc list of slots currently used by specific castes (for calculating free_slot usage)
+	var/list/used_slots = list()
 	/// list of living tier2 xenos
 	var/list/tier_2_xenos = list()
 	/// list of living tier3 xenos
@@ -479,6 +479,12 @@
 	else if(xeno.tier == 3)
 		tier_3_xenos -= xeno
 
+	// Only handle free slots if the xeno is not in tdome
+	if(!is_admin_level(xeno.z))
+		var/selected_caste = GLOB.xeno_datum_list[xeno.caste_type]?.type
+		if(used_slots[selected_caste])
+			used_slots[selected_caste]--
+
 	if(!light_mode)
 		hive_ui.update_xeno_counts()
 		hive_ui.xeno_removed(xeno)
@@ -789,30 +795,31 @@
 		),
 	)
 
-	var/burrowed_factor = min(stored_larva, sqrt(4*stored_larva))
-	burrowed_factor = round(burrowed_factor)
-
 	var/used_tier_2_slots = length(tier_2_xenos)
 	var/used_tier_3_slots = length(tier_3_xenos)
-	for(var/caste_path in used_free_slots)
-		var/used_count = used_free_slots[caste_path]
-		if(!used_count)
-			continue
-		var/datum/caste_datum/C = caste_path
-		switch(initial(C.tier))
-			if(2) used_tier_2_slots -= used_count
-			if(3) used_tier_3_slots -= used_count
 
 	for(var/caste_path in free_slots)
-		var/slot_count = free_slots[caste_path]
-		if(!slot_count)
+		var/slots_free = free_slots[caste_path]
+		var/slots_used = used_slots[caste_path]
+		var/datum/caste_datum/current_caste = caste_path
+		if(slots_used)
+			// Don't count any free slots in use
+			switch(initial(current_caste.tier))
+				if(2)
+					used_tier_2_slots -= min(slots_used, slots_free)
+				if(3)
+					used_tier_3_slots -= min(slots_used, slots_free)
+		if(slots_free <= slots_used)
 			continue
-		var/datum/caste_datum/C = caste_path
-		switch(initial(C.tier))
-			if(2) slots[TIER_2][GUARANTEED_SLOTS][initial(C.caste_type)] = slot_count
-			if(3) slots[TIER_3][GUARANTEED_SLOTS][initial(C.caste_type)] = slot_count
+		// Display any free slots available
+		switch(initial(current_caste.tier))
+			if(2)
+				slots[TIER_2][GUARANTEED_SLOTS][initial(current_caste.caste_type)] = slots_free - slots_used
+			if(3)
+				slots[TIER_3][GUARANTEED_SLOTS][initial(current_caste.caste_type)] = slots_free - slots_used
 
-	var/effective_total = burrowed_factor
+	var/burrowed_factor = min(stored_larva, sqrt(4*stored_larva))
+	var/effective_total = round(burrowed_factor)
 	for(var/mob/living/carbon/xenomorph/xeno as anything in totalXenos)
 		if(xeno.counts_for_slots)
 			effective_total++