Fixes robot stacks transferability

Cleans up transferring between stacks and makes robot stacks
transferable with regular ones.

code/game/objects/items/stacks/sheets/glass.dm

@@ -28,6 +28,7 @@
 	icon_state = "sheet-glass"
 	matter = null
 	created_window = /obj/structure/window/basic
+	stacktype = /obj/item/stack/sheet/glass
 
 /obj/item/stack/sheet/glass/attack_self(mob/user as mob)
 	construct_window(user)

code/game/objects/items/stacks/sheets/mineral.dm

@@ -140,6 +140,7 @@ obj/item/stack/sheet/mineral/iron/New()
 	name = "plastic sheets"
 	icon_state = "sheet-plastic"
 	perunit = 2000
+	stacktype = /obj/item/stack/sheet/mineral/plastic
 
 /obj/item/stack/sheet/mineral/gold
 	name = "gold"

code/game/objects/items/stacks/sheets/sheet_types.dm

@@ -92,6 +92,7 @@ var/global/list/datum/stack_recipe/metal_recipes = list ( \
 	icon_state = "sheet-metal"
 	throwforce = 14.0
 	flags = FPRINT | TABLEPASS | CONDUCT
+	stacktype = /obj/item/stack/sheet/metal
 
 /obj/item/stack/sheet/metal/New(var/loc, var/amount=null)
 	recipes = metal_recipes
@@ -152,6 +153,7 @@ var/global/list/datum/stack_recipe/wood_recipes = list ( \
 	desc = "One can only guess that this is a bunch of wood."
 	singular_name = "wood plank"
 	icon_state = "sheet-wood"
+	stacktype = /obj/item/stack/sheet/wood
 
 /obj/item/stack/sheet/wood/New(var/loc, var/amount=null)
 	recipes = wood_recipes

code/game/objects/items/stacks/stack.dm

@@ -16,9 +16,12 @@
 	var/singular_name
 	var/amount = 1
 	var/max_amount //also see stack recipes initialisation, param "max_res_amount" must be equal to this max_amount
+	var/stacktype //determines whether different stack types can merge
 
 /obj/item/stack/New(var/loc, var/amount=null)
 	..()
+	if (!stacktype)
+		stacktype = type
 	if (amount)
 		src.amount = amount
 	return
@@ -184,7 +187,7 @@
 	for (var/obj/item/stack/item in usr.loc)
 		if (item==oldsrc)
 			continue
-		if (!istype(item, oldsrc.type))
+		if (item.stacktype != oldsrc.stacktype)
 			continue
 		if (item.amount>=item.max_amount)
 			continue
@@ -193,6 +196,23 @@
 		if(!oldsrc)
 			break
 
+//attempts to transfer amount to S, and returns the amount actually transferred
+/obj/item/stack/proc/transfer_to(obj/item/stack/S, var/tamount=null)
+	if (stacktype != S.stacktype)
+		return 0
+	if (isnull(tamount))
+		tamount = src.amount
+	
+	var/transfer = min(tamount, src.amount, (S.max_amount - S.amount))
+
+	if (transfer)
+		if (prob(transfer/src.amount *100))
+			S.copy_evidences(src) //have to do this before use() is called, unfortunately
+		if (oldsrc.use(transfer))
+			S.add(transfer)
+	
+	return transfer
+
 /obj/item/stack/attack_hand(mob/user as mob)
 	if (user.get_inactive_hand() == src)
 		var/obj/item/stack/F = new src.type(user, 1)
@@ -209,28 +229,27 @@
 
 /obj/item/stack/attackby(obj/item/W as obj, mob/user as mob)
 	..()
-	if (istype(W, src.type))
+	if (istype(W, /var/obj/item/stack))
 		var/obj/item/stack/S = W
-		if (S.amount >= max_amount)
-			return 1
-		var/to_transfer as num
-		if (user.get_inactive_hand()==src)
-			to_transfer = 1
+		
+		var/obj/item/stack/oldsrc = src
+		src = null
+		if (user.get_inactive_hand()==oldsrc)
+			oldsrc.transfer_to(S, 1)
 		else
-			to_transfer = min(src.amount, S.max_amount-S.amount)
-		S.add(to_transfer)
+			oldsrc.transfer_to(S)
+		
 		if (S && usr.machine==S)
 			spawn(0) S.interact(usr)
-		src.use(to_transfer)
-		if (src && usr.machine==src)
-			spawn(0) src.interact(usr)
+		if (oldsrc && usr.machine==oldsrc)
+			spawn(0) oldsrc.interact(usr)
 	else return ..()
 
 /obj/item/stack/proc/copy_evidences(obj/item/stack/from as obj)
-	src.blood_DNA = from.blood_DNA
-	src.fingerprints  = from.fingerprints
-	src.fingerprintshidden  = from.fingerprintshidden
-	src.fingerprintslast  = from.fingerprintslast
+	src.blood_DNA |= from.blood_DNA
+	src.fingerprints  |= from.fingerprints
+	src.fingerprintshidden |= from.fingerprintshidden
+	src.fingerprintslast = from.fingerprintslast
 	//TODO bloody overlay
 
 /*