Gas burner building and the energy resource (#54)

A new building that consumes gas to produce energy, a new resource
stored and moved exactly like ore and gas.

The resource: EnergyResourceGlyph (a cell/bolt mark) + energy's own
warm yellow-green ResourceDefinition, registered in
ResourceDefinitions.ORDER/DEFINITIONS and GameState.RESOURCE_ENERGY.
Inventory/ResourceBar/ResourceTransferList needed no changes - #49
already made them resource-agnostic, confirmed structurally in the
test rather than touched.

The building: GasBurnerBuildingVisual (a hexagonal chamber with intake
vents in gas's colour and output ports in energy's, so the silhouette
itself reads "gas in, energy out"), GasBurnerBuildingDefinition
(capabilities=["gas_burner"], its own build-menu icon appended to
IconButton.Icon rather than inserted, per the ticket's own warning
about renumbering stored scene values), and the matching
UI/BuildMenu/GasBurner* nodes in Main.tscn. Gated behind a new
"Power Systems" skill-tree node, mirroring how "freight" unlocks the
transport hub.

Conversion: _advance_gas_burners(), simulated in GameState alongside
_advance_production/extractor mining so it keeps running while the
player is elsewhere. Withdraws gas and deposits energy in the
building's OWN inventory only - never the bank - so it stalls with no
local gas exactly like a lab waiting on materials.

Logistics: a gas burner's ongoing gas deficit is folded into the
existing queued-building-supply dispatch path (same "needs resource X"
loop production/research orders already use, extended with
gas_burner_gas_deficit()), and a new TO_BUILDING_PICKUP/
TO_MOTHERSHIP_FROM_BUILDING transporter state pair collects its energy
to the bank, mirroring the extractor pickup path per the ticket. The
existing f5c60f8 priority (queued deliveries outrank collection) holds:
the pickup dispatch loop runs last.

Tests: new tests/test_gas_burner.gd (resource genericity, stall
semantics, research gate, and full delivery+pickup logistics via a
real hub), run_tests transporter/resource regressions, and screenshots
of the build menu (all 7 buildings) and transfer menu (all 3
resources). Full suite: 81/81.

Two real bugs found and fixed along the way, neither gas-burner-
specific:
- _transporter_inbound_to() matched ANY non-idle transporter with a
  matching target_id, regardless of purpose. Once pickup states shared
  target_id with delivery states (both address a building, not an
  extractor), a transporter delivering gas made the SAME building look
  "busy" to the unrelated energy-pickup dispatch, starving it even
  with a second, genuinely free transporter sitting idle at the hub.
  Split into _transporter_inbound_to() (delivery states only) and a
  new _transporter_inbound_for_pickup() (pickup states only).
- The build menu's 3-row/6-slot layout (MENU_ROW_TOPS + row content
  sizing) had no room for a 7th building - despite the ticket's own
  note that no layout changes should be needed. A naive 4th row ran
  off the bottom of a 720x1280 viewport; fitting one meant both adding
  a 4th row top AND shrinking the shared button/label/cost sizing so
  each row's content no longer taller than the compressed row spacing.
  Caught only by actually screenshotting it - a real build menu, not
  headless, since --headless renders nothing.

🤖 Generated with Claude Code

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

tests/_gas_burner_build_menu_preview.gd

@@ -0,0 +1,21 @@
+extends Node2D
+
+# Preview, not a test - #54's build menu screenshot. Everything researched,
+# including power_systems, so all 7 buildings show at once: the 7th (gas
+# burner) is the first to land beyond the menu's original 3-row/6-slot
+# layout, worth checking for fit even though the ticket calls
+# BuildController._layout_build_menu() already generic enough to need no
+# changes.
+#
+#   bash tests/screenshot.sh res://tests/_gas_burner_build_menu_preview.tscn /tmp/menu.png 20 720x1280
+
+
+func _ready() -> void:
+	GameState.buildings.clear()
+	GameState.extractors.clear()
+	GameState.rocks = 500
+	GameState.unlocked = ["shipyards", "logistics", "coordination", "freight", "vessel_science", "power_systems"]
+	var main: Node = load("res://Main.tscn").instantiate()
+	add_child(main)
+	main.get_node("UI/BuildMenu").visible = true
+	main.get_node("BuildController")._refresh_build_status()

tests/_gas_burner_build_menu_preview.tscn

@@ -0,0 +1,6 @@
+[gd_scene load_steps=2 format=3]
+
+[ext_resource type="Script" path="res://tests/_gas_burner_build_menu_preview.gd" id="1"]
+
+[node name="GasBurnerBuildMenuPreview" type="Node2D"]
+script = ExtResource("1")

tests/_gas_burner_transfer_preview.gd

@@ -0,0 +1,31 @@
+extends Node2D
+
+# Preview, not a test - #54's "transfer menu with three resources laid out
+# cleanly" screenshot. A gas burner holding all three resources at once
+# (ore incidentally, gas as its fuel, energy as its output) exercises the
+# transfer list at its widest.
+#
+#   bash tests/screenshot.sh res://tests/_gas_burner_transfer_preview.tscn /tmp/m.png 20 720x1280
+
+
+func _ready() -> void:
+	GameState.buildings.clear()
+	GameState.extractors.clear()
+	GameState.transporters.clear()
+	GameState.rocks = 180
+	GameState.gas = 90
+	GameState.fleet_cargo = 24
+	GameState.fleet_gas_cargo = 12
+	GameState.unlocked = ["shipyards", "logistics", "freight", "power_systems"]
+	var main: Node = load("res://Main.tscn").instantiate()
+	add_child(main)
+	main.get_node("BuildingField").place(GameState.BUILDING_GAS_BURNER, Vector2(400.0, 0.0))
+	var burner_id: int = int(GameState.buildings[-1].id)
+	GameState.deposit_resource_to_building(burner_id, GameState.RESOURCE_ORE, 14)
+	GameState.deposit_resource_to_building(burner_id, GameState.RESOURCE_GAS, 22)
+	GameState.deposit_resource_to_building(burner_id, GameState.RESOURCE_ENERGY, 36)
+	var bc: Node = main.get_node("BuildController")
+	bc.transfer_building_id = burner_id
+	main.get_node("UI/TransferMenu").visible = true
+	bc._refresh_transfer()
+	bc.transfer_status.text = "Moved 25 ore into this building"

tests/_gas_burner_transfer_preview.tscn

@@ -0,0 +1,6 @@
+[gd_scene load_steps=2 format=3]
+
+[ext_resource type="Script" path="res://tests/_gas_burner_transfer_preview.gd" id="1"]
+
+[node name="GasBurnerTransferPreview" type="Node2D"]
+script = ExtResource("1")

tests/test_gas_burner.gd

@@ -0,0 +1,163 @@
+extends SceneTree
+
+# Headless gameplay test: run with
+#   godot --headless -s res://tests/test_gas_burner.gd
+# Forgejo #54: gas burner building + the energy resource, stored/moved
+# exactly like ore and gas.
+#
+# Modeled on tests/test_extractor_income.gd's shape: pure GameState state,
+# no scene loaded, poking buildings/inventories directly and waiting out
+# real production/transporter timing with generous margins rather than
+# fixed frame counts (SceneTree physics can run ahead of _process(), trap 5
+# in the repo notes).
+#
+# Covers:
+#   A. energy is fully generic - in resource_types(), has its own
+#      ResourceDefinition/glyph/colour - with no per-resource code anywhere
+#      (ResourceBar/ResourceTransferList already iterate resource_types())
+#   B. a burner with local gas converts it to energy, into its OWN
+#      inventory, at the configured rate; with no gas it stalls and never
+#      touches the bank
+#   C. the building is gated behind its own research node
+#   D. a hub transporter both delivers gas to a burner and collects its
+#      energy, following the existing supply/pickup dispatch paths
+
+var game_state: Node
+var stage: int = 0
+var elapsed: float = 0.0
+var mothership_id: int = 0
+var hub_id: int = 0
+var burner_id: int = 0
+var load_limit: int = 0
+var energy_before_stall: int = 0
+
+const MOTHERSHIP_AT := Vector2.ZERO
+const HUB_AT := Vector2(150.0, 0.0)
+const BURNER_AT := Vector2(400.0, 0.0)
+
+
+func _initialize() -> void:
+	game_state = root.get_node("GameState")
+	game_state.rocks = 0
+	game_state.gas = 0
+	game_state.buildings.clear()
+	game_state.extractors.clear()
+	game_state.transporters.clear()
+	game_state.reset_progression()
+
+	mothership_id = game_state.add_building(GameState.BUILDING_MOTHERSHIP, MOTHERSHIP_AT)
+	burner_id = game_state.add_building(GameState.BUILDING_GAS_BURNER, BURNER_AT)
+	game_state.deposit_resource_to_building(burner_id, GameState.RESOURCE_GAS, 20)
+
+
+func _process(delta: float) -> bool:
+	elapsed += delta
+	match stage:
+		0:
+			# --- A. energy is fully generic -------------------------------
+			if not game_state.resource_types().has(GameState.RESOURCE_ENERGY):
+				return _fail("energy is not in resource_types() - the HUD/transfer list would never show it")
+			var definition: Resource = game_state.resource_definition(GameState.RESOURCE_ENERGY)
+			if definition == null or definition.glyph_scene == null:
+				return _fail("energy has no ResourceDefinition or glyph_scene")
+			var energy_color: Color = game_state.resource_color(GameState.RESOURCE_ENERGY)
+			if energy_color == game_state.resource_color(GameState.RESOURCE_ORE) \
+					or energy_color == game_state.resource_color(GameState.RESOURCE_GAS):
+				return _fail("energy shares a colour with ore or gas")
+			print("energy is generic: in resource_types(), own definition, own colour %s" % energy_color)
+
+			# --- B. produces energy from local gas at the configured rate -
+			if elapsed < 2.5:
+				return false
+			var gas_left: int = game_state.building_resource(burner_id, GameState.RESOURCE_GAS)
+			var energy_made: int = game_state.building_resource(burner_id, GameState.RESOURCE_ENERGY)
+			var gas_burned: int = 20 - gas_left
+			if gas_burned < 2:
+				return _fail("burner only burned %d gas in 2.5s at rate %.1f/s" % [gas_burned, GameState.GAS_BURN_RATE])
+			if energy_made != gas_burned * GameState.ENERGY_PER_GAS:
+				return _fail("burner made %d energy for %d gas burned, expected %dx" % [
+					energy_made, gas_burned, GameState.ENERGY_PER_GAS
+				])
+			if game_state.rocks != 0 or game_state.gas != 0:
+				return _fail("conversion touched the bank (rocks=%d gas=%d) - it must only use local stock" % [
+					game_state.rocks, game_state.gas
+				])
+			print("burner converted %d gas -> %d energy locally, bank untouched" % [gas_burned, energy_made])
+
+			# --- stall with no gas -----------------------------------------
+			var burner_inventory: RefCounted = game_state.building_inventory(burner_id)
+			burner_inventory.withdraw(GameState.RESOURCE_GAS, burner_inventory.amount(GameState.RESOURCE_GAS))
+			energy_before_stall = game_state.building_resource(burner_id, GameState.RESOURCE_ENERGY)
+			elapsed = 0.0
+			stage = 1
+			return false
+		1:
+			if elapsed < 1.5:
+				return false
+			var energy_now: int = game_state.building_resource(burner_id, GameState.RESOURCE_ENERGY)
+			if energy_now != energy_before_stall:
+				return _fail("burner made energy with no local gas (energy %d -> %d) - it must stall" % [
+					energy_before_stall, energy_now
+				])
+			print("stalled with no gas: energy held at %d" % energy_now)
+
+			# --- C. gated behind research -----------------------------------
+			if game_state.is_building_unlocked(GameState.BUILDING_GAS_BURNER):
+				return _fail("gas_burner is buildable with no research at all")
+			var gate: String = game_state.research_for_building(GameState.BUILDING_GAS_BURNER)
+			if gate == "":
+				return _fail("no research node gates gas_burner")
+			game_state.unlocked = ["shipyards", "logistics", "freight", "power_systems"]
+			if not game_state.is_building_unlocked(GameState.BUILDING_GAS_BURNER):
+				return _fail("still locked after researching '%s'" % gate)
+			print("locked behind research '%s', unlockable" % gate)
+
+			# --- D. logistics: reset for a fresh delivery/pickup test -----
+			game_state.buildings.clear()
+			game_state.transporters.clear()
+			game_state.gas = 200
+			mothership_id = game_state.add_building(GameState.BUILDING_MOTHERSHIP, MOTHERSHIP_AT)
+			hub_id = game_state.add_building(GameState.BUILDING_TRANSPORT_HUB, HUB_AT)
+			burner_id = game_state.add_building(GameState.BUILDING_GAS_BURNER, BURNER_AT)
+			if game_state.transporters.size() != game_state.hub_capacity():
+				return _fail("hub did not provision its transporters")
+			load_limit = game_state.transporter_load_limit()
+			elapsed = 0.0
+			stage = 2
+			return false
+		2:
+			# --- a transporter delivers gas to the burner ------------------
+			if game_state.building_resource(burner_id, GameState.RESOURCE_GAS) > 0:
+				print("transporter delivered gas to the burner: %d" % [
+					game_state.building_resource(burner_id, GameState.RESOURCE_GAS)
+				])
+				# Force the burner's local energy up to a full load so pickup
+				# dispatch fires immediately, rather than waiting out the
+				# genuinely slow conversion rate for a second, separate check.
+				game_state.building_inventory(burner_id).set_amount(GameState.RESOURCE_ENERGY, load_limit)
+				elapsed = 0.0
+				stage = 3
+				return false
+			if elapsed > 10.0:
+				return _fail("no gas was ever delivered to the burner")
+			return false
+		3:
+			# --- a transporter collects the burner's energy to the bank ---
+			if game_state.banked_resource(GameState.RESOURCE_ENERGY) >= load_limit:
+				print("transporter collected %d energy to the bank" % game_state.banked_resource(GameState.RESOURCE_ENERGY))
+				print("TEST_GAS_BURNER: PASS")
+				quit(0)
+				return true
+			if elapsed > 10.0:
+				return _fail("burner's energy was never collected (bank=%d burner=%d)" % [
+					game_state.banked_resource(GameState.RESOURCE_ENERGY),
+					game_state.building_resource(burner_id, GameState.RESOURCE_ENERGY),
+				])
+			return false
+	return false
+
+
+func _fail(message: String) -> bool:
+	print("TEST_GAS_BURNER: FAIL (%s)" % message)
+	quit(1)
+	return true