Add one-slot demo snapshot save and load.

README.md

@@ -30,8 +30,11 @@ The demo is the physical substrate for a magical-law factory game, not the final
 - `WASD`: move camera
 - Mouse wheel: zoom
 - `N`: restart the demo site
+- `Esc`: pause, autosave, and open the snapshot menu
 - `F1`: toggle developer statistics
 
+One-slot snapshots live in the Godot user data folder as `mana_foundry_slot1.json`. The title screen **继续构筑** button restores construction, belts, machines, mana, items, and objective progress.
+
 ## Running
 
 Open `project.godot` with Godot 4.7.1 stable, or run the workspace-local engine after Task 1 has completed.

src/main.gd

@@ -1,6 +1,7 @@
 extends Node3D
 
 const WorldSimulation = preload("res://src/simulation/world_simulation.gd")
+const DemoSnapshot = preload("res://src/persistence/demo_snapshot.gd")
 
 const CELL_SIZE := 2.0
 const MAP_HALF_WIDTH := 10
@@ -113,6 +114,8 @@ var bridge_button: Button
 var victory_shown := false
 var selected_building_id := -1
 var charger_machine_id := -1
+var continue_button: Button
+var save_status_label: Label
 var stats_visible := true
 var stats_refresh := 0.0
 var last_tick_microseconds := 0
@@ -1068,6 +1071,7 @@ func _create_ui() -> void:
 	start_overlay = _create_start_overlay(canvas)
 	pause_overlay = _create_pause_overlay(canvas)
 	victory_overlay = _create_victory_overlay(canvas)
+	_refresh_continue_button()
 
 
 func _add_tool_button(parent: Control, text: String, kind: StringName) -> void:
@@ -1093,7 +1097,7 @@ func _create_start_overlay(canvas: CanvasLayer) -> Control:
 	center.set_anchors_preset(Control.PRESET_FULL_RECT)
 	shade.add_child(center)
 	var panel := PanelContainer.new()
-	panel.custom_minimum_size = Vector2(600.0, 370.0)
+	panel.custom_minimum_size = Vector2(600.0, 430.0)
 	panel.add_theme_stylebox_override("panel", _panel_style(Color("#0d222df2"), COLOR_CYAN, 2))
 	center.add_child(panel)
 	var box := VBoxContainer.new()
@@ -1120,8 +1124,14 @@ func _create_start_overlay(canvas: CanvasLayer) -> Control:
 	start_button.text = "开始构筑"
 	start_button.custom_minimum_size = Vector2(220.0, 48.0)
 	start_button.add_theme_font_size_override("font_size", 18)
-	start_button.pressed.connect(_start_game)
+	start_button.pressed.connect(_start_new_game)
 	box.add_child(start_button)
+	continue_button = Button.new()
+	continue_button.text = "继续构筑"
+	continue_button.custom_minimum_size = Vector2(220.0, 44.0)
+	continue_button.add_theme_font_size_override("font_size", 16)
+	continue_button.pressed.connect(_continue_game)
+	box.add_child(continue_button)
 	var note := _label("第一目标：接通魔力，再按 B 分别为两台机器指定取货与放货点", 12, COLOR_MUTED)
 	note.horizontal_alignment = HORIZONTAL_ALIGNMENT_CENTER
 	box.add_child(note)
@@ -1140,12 +1150,12 @@ func _create_pause_overlay(canvas: CanvasLayer) -> Control:
 	center.set_anchors_preset(Control.PRESET_FULL_RECT)
 	shade.add_child(center)
 	var panel := PanelContainer.new()
-	panel.custom_minimum_size = Vector2(360.0, 190.0)
+	panel.custom_minimum_size = Vector2(360.0, 320.0)
 	panel.add_theme_stylebox_override("panel", _panel_style(COLOR_PANEL, COLOR_VIOLET, 2))
 	center.add_child(panel)
 	var box := VBoxContainer.new()
 	box.alignment = BoxContainer.ALIGNMENT_CENTER
-	box.add_theme_constant_override("separation", 16)
+	box.add_theme_constant_override("separation", 10)
 	panel.add_child(box)
 	var title := _label("工坊已暂停", 26, COLOR_TEXT)
 	title.horizontal_alignment = HORIZONTAL_ALIGNMENT_CENTER
@@ -1155,6 +1165,24 @@ func _create_pause_overlay(canvas: CanvasLayer) -> Control:
 	resume.custom_minimum_size = Vector2(180.0, 42.0)
 	resume.pressed.connect(_set_paused.bind(false))
 	box.add_child(resume)
+	var save_button := Button.new()
+	save_button.text = "保存工坊"
+	save_button.custom_minimum_size = Vector2(180.0, 36.0)
+	save_button.pressed.connect(_save_demo_snapshot)
+	box.add_child(save_button)
+	var title_button := Button.new()
+	title_button.text = "回到标题"
+	title_button.custom_minimum_size = Vector2(180.0, 36.0)
+	title_button.pressed.connect(_return_to_title)
+	box.add_child(title_button)
+	var quit_button := Button.new()
+	quit_button.text = "退出"
+	quit_button.custom_minimum_size = Vector2(180.0, 36.0)
+	quit_button.pressed.connect(_quit_game)
+	box.add_child(quit_button)
+	save_status_label = _label("", 12, COLOR_MUTED)
+	save_status_label.horizontal_alignment = HORIZONTAL_ALIGNMENT_CENTER
+	box.add_child(save_status_label)
 	return shade
 
 
@@ -1197,12 +1225,126 @@ func _create_victory_overlay(canvas: CanvasLayer) -> Control:
 func _start_game() -> void:
 	start_overlay.visible = false
 	simulation_enabled = true
-	game_paused = false
+	_set_paused(false)
+
+
+func _start_new_game() -> void:
+	_reset_demo()
+	_start_game()
+
+
+func _continue_game() -> void:
+	if not _load_demo_snapshot():
+		_reset_demo()
+	_start_game()
 
 
 func _set_paused(value: bool) -> void:
 	game_paused = value
+	if pause_overlay != null:
 		pause_overlay.visible = value
+	if value:
+		_save_demo_snapshot()
+
+
+func _return_to_title() -> void:
+	_save_demo_snapshot()
+	simulation_enabled = false
+	_set_paused(false)
+	if start_overlay != null:
+		start_overlay.visible = true
+	_refresh_continue_button()
+
+
+func _quit_game() -> void:
+	_save_demo_snapshot()
+	get_tree().quit()
+
+
+func _refresh_continue_button() -> void:
+	if continue_button == null:
+		return
+	continue_button.disabled = not DemoSnapshot.exists()
+	continue_button.modulate = Color.WHITE if not continue_button.disabled else COLOR_MUTED
+
+
+func collect_demo_snapshot() -> Dictionary:
+	return {
+		"schema": DemoSnapshot.SCHEMA,
+		"saved_at": Time.get_datetime_string_from_system(true, true),
+		"victory_shown": victory_shown,
+		"selected_kind": String(selected_kind),
+		"selected_rotation": selected_rotation,
+		"selected_building_id": selected_building_id,
+		"anchor_machine_id": anchor_machine_id,
+		"charger_machine_id": charger_machine_id,
+		"camera_focus": {
+			"x": camera_focus.x,
+			"y": camera_focus.y,
+			"z": camera_focus.z,
+		},
+		"camera_distance": camera_distance,
+		"simulation": simulation.to_snapshot(),
+	}
+
+
+func apply_demo_snapshot(payload: Dictionary) -> bool:
+	if payload.is_empty() or int(payload.get("schema", 0)) != DemoSnapshot.SCHEMA:
+		return false
+	var simulation_payload: Dictionary = payload.get("simulation", {})
+	if not simulation.apply_snapshot(simulation_payload):
+		return false
+	victory_shown = bool(payload.get("victory_shown", false))
+	selected_kind = StringName(String(payload.get("selected_kind", String(WorldSimulation.KIND_BELT))))
+	selected_rotation = int(payload.get("selected_rotation", 0))
+	selected_building_id = int(payload.get("selected_building_id", -1))
+	anchor_machine_id = int(payload.get("anchor_machine_id", -1))
+	charger_machine_id = int(payload.get("charger_machine_id", -1))
+	var focus: Dictionary = payload.get("camera_focus", {})
+	camera_focus = Vector3(
+		float(focus.get("x", 0.0)),
+		float(focus.get("y", 0.0)),
+		float(focus.get("z", 0.0))
+	)
+	camera_distance = float(payload.get("camera_distance", 28.0))
+	_relink_locked_machines()
+	_sync_building_visuals()
+	_capture_item_positions_after_tick()
+	previous_item_positions = current_item_positions.duplicate()
+	simulation_accumulator = 0.0
+	if victory_overlay != null:
+		victory_overlay.visible = false
+	_update_camera_transform()
+	_update_tool_ui()
+	_update_objective()
+	_update_inspect_panel()
+	return true
+
+
+func _save_demo_snapshot() -> bool:
+	var ok: bool = DemoSnapshot.save_payload(collect_demo_snapshot())
+	if save_status_label != null:
+		save_status_label.text = "已写入单槽存档。" if ok else "保存失败。"
+	_refresh_continue_button()
+	return ok
+
+
+func _load_demo_snapshot() -> bool:
+	return apply_demo_snapshot(DemoSnapshot.load_payload())
+
+
+func _relink_locked_machines() -> void:
+	if simulation.buildings.has(anchor_machine_id) and simulation.buildings.has(charger_machine_id):
+		return
+	for id_variant in simulation.get_building_ids():
+		var id := int(id_variant)
+		var building: Dictionary = simulation.buildings[id]
+		if not bool(building.get("locked", false)):
+			continue
+		if building["kind"] == WorldSimulation.KIND_MACHINE:
+			anchor_machine_id = id
+		elif building["kind"] == WorldSimulation.KIND_CHARGER:
+			charger_machine_id = id
 
 
 func _select_tool(kind: StringName) -> void:

src/persistence/demo_snapshot.gd

@@ -0,0 +1,41 @@
+class_name DemoSnapshot
+extends RefCounted
+
+const SCHEMA := 1
+const DEFAULT_PATH := "user://mana_foundry_slot1.json"
+
+static var path_override := ""
+
+
+static func slot_path() -> String:
+	if not path_override.is_empty():
+		return path_override
+	return DEFAULT_PATH
+
+
+static func exists() -> bool:
+	return FileAccess.file_exists(slot_path())
+
+
+static func save_payload(payload: Dictionary) -> bool:
+	var path := slot_path()
+	var file := FileAccess.open(path, FileAccess.WRITE)
+	if file == null:
+		return false
+	file.store_string(JSON.stringify(payload, "\t"))
+	return true
+
+
+static func load_payload() -> Dictionary:
+	if not exists():
+		return {}
+	var file := FileAccess.open(slot_path(), FileAccess.READ)
+	if file == null:
+		return {}
+	var parsed: Variant = JSON.parse_string(file.get_as_text())
+	if typeof(parsed) != TYPE_DICTIONARY:
+		return {}
+	var payload: Dictionary = parsed
+	if int(payload.get("schema", 0)) != SCHEMA:
+		return {}
+	return payload

src/simulation/world_simulation.gd

@@ -462,6 +462,196 @@ func state_fingerprint() -> String:
 	return str(hash("|".join(parts)))
 
 
+func to_snapshot() -> Dictionary:
+	var building_rows: Array = []
+	for id in get_building_ids():
+		building_rows.append(_encode_value(buildings[id]))
+	var item_rows: Array = []
+	for id in get_item_ids():
+		item_rows.append(_encode_value(items[id]))
+	var wake_ids: Array = []
+	for id_variant in _wake_queue.keys():
+		wake_ids.append(int(id_variant))
+	wake_ids.sort()
+	var event_rows: Array = []
+	for event in _event_heap:
+		event_rows.append(_encode_value(event))
+	var network_rows: Array = []
+	for network in mana_networks:
+		network_rows.append(_encode_value(network))
+	var charged_ticks: Array = []
+	for tick_variant in _charged_intake_ticks:
+		charged_ticks.append(int(tick_variant))
+	return {
+		"schema": 1,
+		"tick_index": tick_index,
+		"next_building_id": next_building_id,
+		"next_item_id": next_item_id,
+		"buildings": building_rows,
+		"items": item_rows,
+		"sink_inventory": {
+			String(ITEM_RAW): int(sink_inventory.get(ITEM_RAW, 0)),
+			String(ITEM_REFINED): int(sink_inventory.get(ITEM_REFINED, 0)),
+			String(ITEM_CHARGED): int(sink_inventory.get(ITEM_CHARGED, 0)),
+		},
+		"event_heap": event_rows,
+		"wake_queue": wake_ids,
+		"charged_intake_ticks": charged_ticks,
+		"mana_networks": network_rows,
+	}
+
+
+func apply_snapshot(snapshot: Dictionary) -> bool:
+	if snapshot.is_empty() or int(snapshot.get("schema", 0)) != 1:
+		return false
+	if not snapshot.has("buildings") or not snapshot.has("items"):
+		return false
+	reset()
+	tick_index = int(snapshot.get("tick_index", 0))
+	next_building_id = int(snapshot.get("next_building_id", 1))
+	next_item_id = int(snapshot.get("next_item_id", 1))
+	var sink: Dictionary = snapshot.get("sink_inventory", {})
+	sink_inventory = {
+		ITEM_RAW: int(sink.get(String(ITEM_RAW), sink.get("raw_crystal", 0))),
+		ITEM_REFINED: int(sink.get(String(ITEM_REFINED), sink.get("refined_crystal", 0))),
+		ITEM_CHARGED: int(sink.get(String(ITEM_CHARGED), sink.get("charged_crystal", 0))),
+	}
+	for row_variant in snapshot.get("buildings", []):
+		if typeof(row_variant) != TYPE_DICTIONARY:
+			continue
+		var building: Dictionary = _decode_building(row_variant)
+		var id := int(building["id"])
+		buildings[id] = building
+		grid[Vector2i(building["cell"])] = id
+	for row_variant in snapshot.get("items", []):
+		if typeof(row_variant) != TYPE_DICTIONARY:
+			continue
+		var item: Dictionary = _decode_item(row_variant)
+		var id := int(item["id"])
+		items[id] = item
+		item_by_cell[Vector2i(item["cell"])] = id
+	for event_variant in snapshot.get("event_heap", []):
+		if typeof(event_variant) != TYPE_DICTIONARY:
+			continue
+		var event: Dictionary = event_variant
+		_push_event({
+			"tick": int(event.get("tick", 0)),
+			"type": StringName(String(event.get("type", ""))),
+			"id": int(event.get("id", 0)),
+		})
+	for id_variant in snapshot.get("wake_queue", []):
+		_wake_queue[int(id_variant)] = true
+	for tick_variant in snapshot.get("charged_intake_ticks", []):
+		_charged_intake_ticks.append(int(tick_variant))
+	for network_variant in snapshot.get("mana_networks", []):
+		if typeof(network_variant) != TYPE_DICTIONARY:
+			continue
+		mana_networks.append(_decode_mana_network(network_variant))
+	_rebuild_watchers()
+	return true
+
+
+func _rebuild_watchers() -> void:
+	_output_watchers.clear()
+	_input_watchers.clear()
+	for id_variant in get_building_ids():
+		var id := int(id_variant)
+		var kind: StringName = buildings[id]["kind"]
+		if kind == KIND_SOURCE or is_processor(kind):
+			_register_output_watcher(id)
+		if is_processor(kind):
+			_register_input_watcher(id)
+
+
+func _encode_value(value: Variant) -> Variant:
+	match typeof(value):
+		TYPE_VECTOR2I:
+			var cell := Vector2i(value)
+			return {"x": cell.x, "y": cell.y}
+		TYPE_STRING_NAME:
+			return String(value)
+		TYPE_ARRAY:
+			var encoded_array: Array = []
+			for entry in value:
+				encoded_array.append(_encode_value(entry))
+			return encoded_array
+		TYPE_DICTIONARY:
+			var encoded: Dictionary = {}
+			for key in value.keys():
+				encoded[str(key)] = _encode_value(value[key])
+			return encoded
+	return value
+
+
+func _decode_building(row: Dictionary) -> Dictionary:
+	var building := row.duplicate(true)
+	building["id"] = int(row.get("id", 0))
+	building["kind"] = StringName(String(row.get("kind", "")))
+	building["cell"] = _decode_cell(row.get("cell", {}))
+	building["rotation"] = int(row.get("rotation", 0))
+	building["locked"] = bool(row.get("locked", false))
+	building["status"] = StringName(String(row.get("status", "idle")))
+	if is_processor(building["kind"]):
+		building["recipe"] = StringName(String(row.get("recipe", String(RECIPE_REFINE))))
+		building["input_kind"] = StringName(String(row.get("input_kind", String(ITEM_RAW))))
+		building["output_kind"] = StringName(String(row.get("output_kind", String(ITEM_REFINED))))
+		building["input_count"] = int(row.get("input_count", 0))
+		building["output_buffer"] = int(row.get("output_buffer", 0))
+		building["completion_tick"] = int(row.get("completion_tick", -1))
+		building["remaining_ticks"] = int(row.get("remaining_ticks", 0))
+		building["completed"] = int(row.get("completed", 0))
+		building["mana_powered"] = bool(row.get("mana_powered", false))
+		building["mana_network"] = int(row.get("mana_network", -1))
+		building["input_cell"] = _decode_cell(row.get("input_cell", {}))
+		building["output_cell"] = _decode_cell(row.get("output_cell", {}))
+		building["bridge_configured"] = bool(row.get("bridge_configured", false))
+	if building["kind"] == KIND_SOURCE:
+		building["next_ready_tick"] = int(row.get("next_ready_tick", 0))
+		building["scheduled_tick"] = int(row.get("scheduled_tick", -1))
+		building["produced"] = int(row.get("produced", 0))
+	if building["kind"] == KIND_SINK:
+		building["accepted"] = int(row.get("accepted", 0))
+	if building["kind"] == KIND_MANA_SOURCE:
+		building["capacity"] = int(row.get("capacity", MANA_SOURCE_CAPACITY))
+		building["mana_network"] = int(row.get("mana_network", -1))
+	if building["kind"] == KIND_MANA_PIPE:
+		building["mana_network"] = int(row.get("mana_network", -1))
+	return building
+
+
+func _decode_item(row: Dictionary) -> Dictionary:
+	return {
+		"id": int(row.get("id", 0)),
+		"kind": StringName(String(row.get("kind", String(ITEM_RAW)))),
+		"cell": _decode_cell(row.get("cell", {})),
+		"progress": float(row.get("progress", 0.0)),
+	}
+
+
+func _decode_mana_network(row: Dictionary) -> Dictionary:
+	var infrastructure: Array = []
+	for id_variant in row.get("infrastructure", []):
+		infrastructure.append(int(id_variant))
+	var machines: Array = []
+	for id_variant in row.get("machines", []):
+		machines.append(int(id_variant))
+	return {
+		"id": int(row.get("id", 0)),
+		"capacity": int(row.get("capacity", 0)),
+		"used": int(row.get("used", 0)),
+		"infrastructure": infrastructure,
+		"machines": machines,
+	}
+
+
+func _decode_cell(value: Variant) -> Vector2i:
+	if value is Vector2i:
+		return value
+	if typeof(value) == TYPE_DICTIONARY:
+		return Vector2i(int(value.get("x", 0)), int(value.get("y", 0)))
+	return Vector2i.ZERO
+
+
 func _move_items() -> void:
 	var ids := get_item_ids()
 	for id_variant in ids:

tasks/plan.md

@@ -83,7 +83,7 @@ stress scene + release check
 
 ### Phase 3: Presentation and scale guardrails
 
-- [ ] Task 9: Add menu, save snapshot, audio hooks, and visual polish
+- [x] Task 9: Add menu, save snapshot, audio hooks, and visual polish
 - [ ] Task 10: Add deterministic stress generation and metrics
 - [ ] Task 11: Package a Windows demo build
 

tasks/todo.md

@@ -142,12 +142,12 @@
 **Description:** Add title/pause screens, coherent materials and mana effects, audio hooks, and one-slot world snapshot persistence.
 
 **Acceptance criteria:**
-- [ ] New game, continue, pause, restart, and quit flows work.
-- [ ] Snapshot restores construction, transport, production, and objectives.
-- [ ] Industrial assets read as magical machinery through a unified material pass.
+- [x] New game, continue, pause, restart, and quit flows work.
+- [x] Snapshot restores construction, transport, production, and objectives.
+- [x] Industrial assets read as magical machinery through a unified material pass.
 
 **Verification:**
-- [ ] Save-then-run and run-from-save produce matching state hashes.
+- [x] Save-then-run and run-from-save produce matching state hashes.
 - [ ] Manual UI flow works at 1080p and a smaller window.
 
 **Dependencies:** Task 8

tests/snapshot_smoke.gd

@@ -0,0 +1,126 @@
+extends SceneTree
+
+const WorldSimulation = preload("res://src/simulation/world_simulation.gd")
+const DemoSnapshot = preload("res://src/persistence/demo_snapshot.gd")
+const TEST_PATH := "user://mana_foundry_snapshot_test.json"
+
+
+func _initialize() -> void:
+	DemoSnapshot.path_override = TEST_PATH
+	if FileAccess.file_exists(TEST_PATH):
+		DirAccess.remove_absolute(ProjectSettings.globalize_path(TEST_PATH))
+
+	var failures: Array[String] = []
+	var live := _make_two_stage()
+	for _tick in range(400):
+		live.step()
+	var before := live.state_fingerprint()
+	var charged_before := live.get_charged_count()
+	var snapshot := live.to_snapshot()
+
+	var restored := WorldSimulation.new()
+	if not restored.apply_snapshot(snapshot):
+		failures.append("apply_snapshot rejected a live two-stage snapshot.")
+	elif restored.state_fingerprint() != before:
+		failures.append("Restored simulation fingerprint did not match the live factory.")
+	elif restored.get_charged_count() != charged_before:
+		failures.append("Restored charged count did not match.")
+
+	if failures.is_empty():
+		for _tick in range(200):
+			live.step()
+			restored.step()
+		if live.state_fingerprint() != restored.state_fingerprint():
+			failures.append("Live and restored factories diverged after more ticks.")
+
+	if not DemoSnapshot.save_payload({"schema": DemoSnapshot.SCHEMA, "simulation": snapshot}):
+		failures.append("DemoSnapshot could not write the test slot.")
+	else:
+		var loaded := DemoSnapshot.load_payload()
+		var from_disk := WorldSimulation.new()
+		if not from_disk.apply_snapshot(loaded.get("simulation", {})):
+			failures.append("Disk snapshot failed to apply.")
+		elif from_disk.state_fingerprint() != before:
+			failures.append("Disk snapshot fingerprint did not match the pre-save factory.")
+
+	var empty := WorldSimulation.new()
+	if empty.is_demo_complete():
+		failures.append("Empty factory reported demo complete.")
+	var first_item := _make_two_stage()
+	var saw_item := false
+	for _tick in range(2000):
+		first_item.step()
+		if first_item.get_charged_count() >= 1:
+			saw_item = true
+			break
+	if not saw_item:
+		failures.append("Two-stage factory never produced a charged crystal for the one-item check.")
+	elif first_item.is_demo_complete():
+		failures.append("Demo completed after a single charged crystal.")
+	else:
+		var mid := WorldSimulation.new()
+		if not mid.apply_snapshot(first_item.to_snapshot()):
+			failures.append("Could not restore the one-item factory.")
+		elif mid.is_demo_complete():
+			failures.append("Loaded one-item factory reported complete.")
+
+	var packed := load("res://scenes/main.tscn") as PackedScene
+	var demo := packed.instantiate()
+	root.add_child.call_deferred(demo)
+	await process_frame
+	await process_frame
+	demo.call("_start_new_game")
+	var sim: WorldSimulation = demo.get("simulation")
+	for x in range(-5, -1):
+		sim.place_building(WorldSimulation.KIND_BELT, Vector2i(x, 0), 0)
+	sim.place_building(WorldSimulation.KIND_BELT, Vector2i(0, 0), 0)
+	for x in range(2, 6):
+		sim.place_building(WorldSimulation.KIND_BELT, Vector2i(x, 0), 0)
+	for z in range(1, 5):
+		sim.place_building(WorldSimulation.KIND_MANA_PIPE, Vector2i(0, z), 1)
+	sim.configure_machine_bridge(int(demo.get("anchor_machine_id")), Vector2i(-2, 0), Vector2i(0, 0))
+	sim.configure_machine_bridge(int(demo.get("charger_machine_id")), Vector2i(0, 0), Vector2i(2, 0))
+	for _tick in range(240):
+		sim.step()
+	var scene_before := sim.state_fingerprint()
+	var payload: Dictionary = demo.call("collect_demo_snapshot")
+	if not bool(demo.call("_save_demo_snapshot")):
+		failures.append("Scene save helper failed.")
+	demo.call("_reset_demo")
+	if sim.state_fingerprint() == scene_before:
+		failures.append("Reset after save did not change simulation state.")
+	if not bool(demo.call("_load_demo_snapshot")):
+		failures.append("Scene load helper failed.")
+	elif sim.state_fingerprint() != scene_before:
+		failures.append("Scene save/load did not restore the simulation fingerprint.")
+
+	if FileAccess.file_exists(TEST_PATH):
+		DirAccess.remove_absolute(ProjectSettings.globalize_path(TEST_PATH))
+	DemoSnapshot.path_override = ""
+
+	if failures.is_empty():
+		print("SNAPSHOT_SMOKE_OK charged=%d fingerprint=%s" % [charged_before, before])
+		quit(0)
+	else:
+		for failure in failures:
+			push_error(failure)
+		quit(1)
+
+
+func _make_two_stage() -> WorldSimulation:
+	var simulation := WorldSimulation.new()
+	simulation.place_building(WorldSimulation.KIND_SOURCE, Vector2i(-6, 0), 0)
+	for x in range(-5, -1):
+		simulation.place_building(WorldSimulation.KIND_BELT, Vector2i(x, 0), 0)
+	simulation.place_building(WorldSimulation.KIND_BELT, Vector2i(0, 0), 0)
+	for x in range(2, 6):
+		simulation.place_building(WorldSimulation.KIND_BELT, Vector2i(x, 0), 0)
+	simulation.place_building(WorldSimulation.KIND_SINK, Vector2i(6, 0), 0)
+	var refine_id := simulation.place_building(WorldSimulation.KIND_MACHINE, Vector2i(-1, 1), 0)
+	var charger_id := simulation.place_building(WorldSimulation.KIND_CHARGER, Vector2i(1, 1), 0)
+	simulation.place_building(WorldSimulation.KIND_MANA_SOURCE, Vector2i(0, 5), 1)
+	for z in range(1, 5):
+		simulation.place_building(WorldSimulation.KIND_MANA_PIPE, Vector2i(0, z), 1)
+	simulation.configure_machine_bridge(refine_id, Vector2i(-2, 0), Vector2i(0, 0))
+	simulation.configure_machine_bridge(charger_id, Vector2i(0, 0), Vector2i(2, 0))
+	return simulation