Removes amount_list_postion from reagent containers, adds related unit test. (#76057)

We had more issues like what #76013 addressed, now they're gone.
Variable transfer amount is now explicit. 
Amount is now inferred from current value, performance concern here is
minimal. Less work and mistakes when making new types.

code/modules/reagents/reagent_containers.dm

@@ -4,12 +4,12 @@
 	icon = 'icons/obj/medical/chemical.dmi'
 	icon_state = null
 	w_class = WEIGHT_CLASS_TINY
-	/// The maximum amount of reagents per transfer that will be moved out of this reagent container. This value's position in possible_transfer_amounts should be reflected in amount_list_position.
+	/// The maximum amount of reagents per transfer that will be moved out of this reagent container.
 	var/amount_per_transfer_from_this = 5
+	/// Does this container allow changing transfer amounts at all, the container can still have only one possible transfer value in possible_transfer_amounts at some point even if this is true
+	var/has_variable_transfer_amount = TRUE
 	/// The different possible amounts of reagent to transfer out of the container
 	var/list/possible_transfer_amounts = list(5,10,15,20,25,30)
-	/// Where we are in the possible transfer amount list. Number should match the position in possible_transfer_amounts corresponding to amount_per_transfer_from_this.
-	var/amount_list_position = 1
 	/// The maximum amount of reagents this container can hold
 	var/volume = 30
 	/// Reagent flags, a few examples being if the container is open or not, if its transparent, if you can inject stuff in and out of the container, and so on
@@ -47,15 +47,15 @@
 		var/datum/disease/F = new spawned_disease()
 		var/list/data = list("viruses"= list(F))
 		reagents.add_reagent(/datum/reagent/blood, disease_amount, data)
-
 	add_initial_reagents()
 
 /obj/item/reagent_containers/examine()
 	. = ..()
-	if(possible_transfer_amounts.len > 1)
-		. += span_notice("Left-click or right-click in-hand to increase or decrease its transfer amount.")
-	else if(possible_transfer_amounts.len)
-		. += span_notice("Left-click or right-click in-hand to view its transfer amount.")
+	if(has_variable_transfer_amount)
+		if(possible_transfer_amounts.len > 1)
+			. += span_notice("Left-click or right-click in-hand to increase or decrease its transfer amount.")
+		else if(possible_transfer_amounts.len)
+			. += span_notice("Left-click or right-click in-hand to view its transfer amount.")
 
 /obj/item/reagent_containers/create_reagents(max_vol, flags)
 	. = ..()
@@ -77,10 +77,12 @@
 		reagents.add_reagent_list(list_reagents)
 
 /obj/item/reagent_containers/attack_self(mob/user)
-	change_transfer_amount(user, FORWARD)
+	if(has_variable_transfer_amount)
+		change_transfer_amount(user, FORWARD)
 
 /obj/item/reagent_containers/attack_self_secondary(mob/user)
-	change_transfer_amount(user, BACKWARD)
+	if(has_variable_transfer_amount)
+		change_transfer_amount(user, BACKWARD)
 
 /obj/item/reagent_containers/proc/mode_change_message(mob/user)
 	return
@@ -89,14 +91,15 @@
 	var/list_len = length(possible_transfer_amounts)
 	if(!list_len)
 		return
+	var/index = possible_transfer_amounts.Find(amount_per_transfer_from_this) || 1
 	switch(direction)
 		if(FORWARD)
-			amount_list_position = (amount_list_position % list_len) + 1
+			index = (index % list_len) + 1
 		if(BACKWARD)
-			amount_list_position = (amount_list_position - 1) || list_len
+			index = (index - 1) || list_len
 		else
 			CRASH("change_transfer_amount() called with invalid direction value")
-	amount_per_transfer_from_this = possible_transfer_amounts[amount_list_position]
+	amount_per_transfer_from_this = possible_transfer_amounts[index]
 	balloon_alert(user, "transferring [amount_per_transfer_from_this]u")
 	mode_change_message(user)
 

code/modules/reagents/reagent_containers/chem_pack.dm

@@ -10,7 +10,7 @@
 	resistance_flags = ACID_PROOF
 	var/sealed = FALSE
 	fill_icon_thresholds = list(10, 20, 30, 40, 50, 60, 70, 80, 90, 100)
-	possible_transfer_amounts = list()
+	has_variable_transfer_amount = FALSE
 
 /obj/item/reagent_containers/chem_pack/AltClick(mob/living/user)
 	if(user.can_perform_action(src, NEED_DEXTERITY) && !sealed)

code/modules/reagents/reagent_containers/cups/_cup.dm

@@ -2,7 +2,6 @@
 	name = "open container"
 	amount_per_transfer_from_this = 10
 	possible_transfer_amounts = list(5, 10, 15, 20, 25, 30, 50)
-	amount_list_position = 2
 	volume = 50
 	reagent_flags = OPENCONTAINER | DUNKABLE
 	spillable = TRUE

code/modules/reagents/reagent_containers/cups/drinks.dm

@@ -49,7 +49,7 @@
 	throwforce = 1
 	amount_per_transfer_from_this = 5
 	custom_materials = list(/datum/material/iron=SMALL_MATERIAL_AMOUNT)
-	possible_transfer_amounts = list(5)
+	has_variable_transfer_amount = FALSE
 	volume = 5
 	flags_1 = CONDUCT_1
 	spillable = TRUE

code/modules/reagents/reagent_containers/dropper.dm

@@ -6,7 +6,6 @@
 	inhand_icon_state = "dropper"
 	worn_icon_state = "pen"
 	amount_per_transfer_from_this = 5
-	amount_list_position = 5
 	possible_transfer_amounts = list(1, 2, 3, 4, 5)
 	volume = 5
 	reagent_flags = TRANSPARENT

code/modules/reagents/reagent_containers/hypospray.dm

@@ -111,6 +111,7 @@
 	lefthand_file = 'icons/mob/inhands/equipment/medical_lefthand.dmi'
 	righthand_file = 'icons/mob/inhands/equipment/medical_righthand.dmi'
 	amount_per_transfer_from_this = 15
+	has_variable_transfer_amount = FALSE
 	volume = 15
 	ignore_flags = 1 //so you can medipen through spacesuits
 	reagent_flags = DRAWABLE

code/modules/reagents/reagent_containers/misc.dm

@@ -125,7 +125,7 @@
 	item_flags = NOBLUDGEON
 	reagent_flags = OPENCONTAINER
 	amount_per_transfer_from_this = 5
-	possible_transfer_amounts = list()
+	has_variable_transfer_amount = FALSE
 	volume = 5
 	spillable = FALSE
 

code/modules/reagents/reagent_containers/pill.dm

@@ -7,7 +7,7 @@
 	worn_icon_state = "nothing"
 	lefthand_file = 'icons/mob/inhands/equipment/medical_lefthand.dmi'
 	righthand_file = 'icons/mob/inhands/equipment/medical_righthand.dmi'
-	possible_transfer_amounts = list()
+	has_variable_transfer_amount = FALSE
 	volume = 50
 	grind_results = list()
 	var/apply_type = INGEST

code/modules/reagents/reagent_containers/spray.dm

@@ -235,7 +235,7 @@
 	lefthand_file = 'icons/mob/inhands/weapons/plants_lefthand.dmi'
 	righthand_file = 'icons/mob/inhands/weapons/plants_righthand.dmi'
 	amount_per_transfer_from_this = 1
-	possible_transfer_amounts = list(1)
+	has_variable_transfer_amount = FALSE
 	can_toggle_range = FALSE
 	current_range = 1
 	volume = 10

code/modules/reagents/reagent_containers/syringes.dm

@@ -199,6 +199,7 @@
 	name = "lethal injection syringe"
 	desc = "A syringe used for lethal injections. It can hold up to 50 units."
 	amount_per_transfer_from_this = 50
+	has_variable_transfer_amount = FALSE
 	volume = 50
 
 /obj/item/reagent_containers/syringe/lethal/choral
@@ -211,6 +212,7 @@
 	name = "Mulligan"
 	desc = "A syringe used to completely change the users identity."
 	amount_per_transfer_from_this = 1
+	has_variable_transfer_amount = FALSE
 	volume = 1
 	list_reagents = list(/datum/reagent/mulligan = 1)
 
@@ -218,6 +220,7 @@
 	name = "Gluttony's Blessing"
 	desc = "A syringe recovered from a dread place. It probably isn't wise to use."
 	amount_per_transfer_from_this = 1
+	has_variable_transfer_amount = FALSE
 	volume = 1
 	list_reagents = list(/datum/reagent/gluttonytoxin = 1)