Support gas cargo transfers

GameState.gd

@@ -42,10 +42,14 @@ var gas: int = 0
 # down and rebuilt on every trip to a planet's surface - and cargo mined ON a
 # surface would otherwise be dropped the moment the player took off.
 var fleet_cargo: int = 0
+var fleet_gas_cargo: int = 0
 
 func collect_cargo(amount: int = 1) -> void:
 	fleet_cargo += amount
 
+func collect_gas_cargo(amount: int = 1) -> void:
+	fleet_gas_cargo += amount
+
 
 # The batch size the transfer menu offers, next to "All". A batch rather
 # than only all-or-nothing: the hold is the part that can be lost, and
@@ -73,6 +77,11 @@ func building_holdings(id: int) -> int:
 		return rocks
 	return int(b.get("stock", 0))
 
+func building_gas_holdings(id: int) -> int:
+	var b: Dictionary = find_building(id)
+	if b.is_empty(): return 0
+	return gas if b.type == BUILDING_MOTHERSHIP else int(b.get("gas_stock", 0))
+
 
 func transfer_to_building(id: int, amount: int) -> int:
 	# Hold -> this building. Returns how much actually moved, which is capped
@@ -108,6 +117,24 @@ func transfer_to_fleet(id: int, amount: int) -> int:
 	fleet_cargo += moved
 	return moved
 
+func transfer_gas_to_building(id: int, amount: int) -> int:
+	var b: Dictionary = find_building(id)
+	if b.is_empty(): return 0
+	var moved := mini(amount, fleet_gas_cargo)
+	fleet_gas_cargo -= moved
+	if b.type == BUILDING_MOTHERSHIP: gas += moved
+	else: b.gas_stock = int(b.get("gas_stock", 0)) + moved
+	return moved
+
+func transfer_gas_to_fleet(id: int, amount: int) -> int:
+	var b: Dictionary = find_building(id)
+	if b.is_empty(): return 0
+	var moved := mini(amount, building_gas_holdings(id))
+	if b.type == BUILDING_MOTHERSHIP: gas -= moved
+	else: b.gas_stock = int(b.get("gas_stock", 0)) - moved
+	fleet_gas_cargo += moved
+	return moved
+
 const MOTHERSHIP_COST: int = 20
 const SHIP_CRAFTER_COST: int = 12
 const SHIP_COST: int = 6