refactor(client): make stationary_ticks and zone_id server-authoritative (#557)

apply_snapshot() now reads stationary_ticks and zone_id directly from
the server snapshot when present (D-020 compliance). Client-side
accumulation and tile lookup retained as deprecated fallbacks until
the server populates these fields. Protocol.gd extended with decode
paths and TODO markers for the server team.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

client/scripts/autoloads/game_state.gd

@@ -109,14 +109,17 @@ var medium_sound_events: Array = []
 var close_sound_events: Array = []
 
 # D-071 (#530): Consecutive ticks without player position change.
-# Incremented per snapshot in apply_snapshot(). Reset to 0 on movement.
+# D-020: Server-authoritative — read from snapshot "stationary_ticks" field.
+# Fallback: client-side accumulation (deprecated, remove when server populates field).
 # ListeningFocus boost activates at 30+ ticks (main.gd manages the dip).
 var stationary_ticks: int = 0
+# DEPRECATED: Only used by client-side accumulation fallback. Remove with fallback.
 var _prev_player_position: Vector2 = Vector2(-1e9, -1e9)  # sentinel: no previous position
 
 # D-073 (#529): Server-authoritative zone_id from the player's current tile.
-# Extracted in apply_snapshot() — avoids O(N) tile scan in main.gd per Tyre review.
-# Empty string when zone_id field absent (server hasn't shipped OQ-09 yet).
+# D-020: Read directly from snapshot "zone_id" field.
+# Fallback: client-side tile lookup (deprecated, remove when server populates field).
+# Empty string when zone_id field absent.
 var current_zone_id: String = ""
 
 func apply_snapshot(snapshot: Dictionary) -> void:
@@ -140,12 +143,19 @@ func apply_snapshot(snapshot: Dictionary) -> void:
 			push_warning("GameState: no Player entity found in %d entities" % [
 				visible_entities.size()])
 
-	# D-071 (#530): Track consecutive stationary ticks for ListeningFocus boost.
-	# Compares current player_position against previous snapshot's position.
-	if player_position == _prev_player_position:
-		stationary_ticks += 1
+	# D-020/D-071 (#530): Server-authoritative stationary_ticks for ListeningFocus boost.
+	# Prefer server-sent value; fall back to client-side accumulation until server populates.
+	if snapshot.has("stationary_ticks") and snapshot.stationary_ticks is int:
+		# D-020: direct field assignment from server-authoritative snapshot.
+		stationary_ticks = snapshot.stationary_ticks
 	else:
-		stationary_ticks = 0
+		# DEPRECATED fallback — client-side accumulation. Remove when server sends
+		# "stationary_ticks" in ObserverSnapshot (D-020 violation: derives behavior-
+		# driving state on the client). Server tracks this in ListeningFocus component.
+		if player_position == _prev_player_position:
+			stationary_ticks += 1
+		else:
+			stationary_ticks = 0
 	_prev_player_position = player_position
 
 	# Tiles for rendering: test mode sends "tiles", live server sends tile data in "visible_tiles"
@@ -295,16 +305,24 @@ func apply_snapshot(snapshot: Dictionary) -> void:
 	if snapshot.has("player_knowledge") and snapshot.player_knowledge is Dictionary:
 		player_knowledge = snapshot.player_knowledge
 
-	# D-073 (#529): O(1) zone_id lookup. Build coord→tile dict from member visible_tiles
-	# (populated above from either "tiles" test-mode key or "visible_tiles" live key).
-	# Must use the member var, not snapshot.visible_tiles, so test mode is covered.
-	var _tile_by_coord: Dictionary = {}
-	for vtile in visible_tiles:
-		if vtile is Dictionary and vtile.has("x") and vtile.has("y"):
-			_tile_by_coord[Vector2i(vtile.x, vtile.y)] = vtile
-	var player_pos_key := Vector2i(int(player_position.x), int(player_position.y))
-	var player_tile = _tile_by_coord.get(player_pos_key, null)
-	current_zone_id = player_tile.get("zone_id", "") if player_tile else ""
+	# D-020/D-073 (#529): Server-authoritative zone_id for zone ambient crossfade.
+	# Prefer server-sent top-level value; fall back to client-side tile lookup until
+	# server populates top-level "zone_id" in ObserverSnapshot.
+	if snapshot.has("zone_id") and snapshot.zone_id is String:
+		# D-020: direct field assignment from server-authoritative snapshot.
+		current_zone_id = snapshot.zone_id
+	else:
+		# DEPRECATED fallback — client-side tile lookup. Remove when server sends
+		# top-level "zone_id" in ObserverSnapshot (D-020 violation: derives zone
+		# identity on the client via tile iteration). Server sends zone_id per
+		# VisibleTile but not as a top-level snapshot field.
+		var _tile_by_coord: Dictionary = {}
+		for vtile in visible_tiles:
+			if vtile is Dictionary and vtile.has("x") and vtile.has("y"):
+				_tile_by_coord[Vector2i(vtile.x, vtile.y)] = vtile
+		var player_pos_key := Vector2i(int(player_position.x), int(player_position.y))
+		var player_tile = _tile_by_coord.get(player_pos_key, null)
+		current_zone_id = player_tile.get("zone_id", "") if player_tile else ""
 
 	# v2: visible_tiles with visibility sectors
 	# Derives visible_positions when not explicitly provided (real server mode)

client/scripts/protocol/protocol.gd

@@ -244,6 +244,24 @@ static func decode_snapshot(bytes: PackedByteArray) -> Variant:
 			"error": raw_save.get("error"),
 		}
 
+	# TODO(server): Send stationary_ticks in ObserverSnapshot (D-071, D-020).
+	# Server already tracks this in ListeningFocus component (server/src/simulation/listening.rs).
+	# When server populates this field, client-side accumulation fallback in game_state.gd
+	# can be removed — apply_snapshot() should contain only direct field assignments.
+	var stationary_ticks: Variant = null
+	var raw_st: Variant = raw.get("stationary_ticks")
+	if raw_st != null:
+		stationary_ticks = int(raw_st)
+
+	# TODO(server): Send top-level zone_id string in ObserverSnapshot (D-073, D-020).
+	# Server sends zone_id per VisibleTile but not as a top-level snapshot field.
+	# When server populates this, client-side tile iteration fallback in game_state.gd
+	# can be removed — apply_snapshot() should contain only direct field assignments.
+	var zone_id: Variant = null
+	var raw_zid: Variant = raw.get("zone_id")
+	if raw_zid is String:
+		zone_id = raw_zid
+
 	# v14: player_knowledge (#264, D-041) — partial KG dump for journal panel.
 	# {entities: [{entity_id, name, confidence, source, state, relationship, last_observed_tick}],
 	#  facts: [{fact_id, confidence, source, state, acquired_tick}]}
@@ -302,6 +320,8 @@ static func decode_snapshot(bytes: PackedByteArray) -> Variant:
 		"examine_result": examine_result,
 		"player_knowledge": player_knowledge,
 		"save_result": save_result,
+		"stationary_ticks": stationary_ticks,
+		"zone_id": zone_id,
 	}
 
 

client/tests/test_game_state.gd

@@ -19,6 +19,8 @@ func before_each() -> void:
 	GameState.player_stance = "Walk"
 	GameState.player_inventory = []
 	GameState.stationary_ticks = 0
+	GameState._prev_player_position = Vector2(-1e9, -1e9)
+	GameState.current_zone_id = ""
 	GameState.insert_active = true
 
 
@@ -115,19 +117,49 @@ func test_apply_snapshot_inventory_absent_clears_list() -> void:
 	assert_that(GameState.player_inventory.size()).is_equal(0)
 
 
-# -- Stationary tick counter (D-071) -------------------------------------
+# -- Stationary ticks: server-authoritative (D-020/D-071) -----------------
 
-func test_stationary_ticks_increments_when_player_position_unchanged() -> void:
+func test_stationary_ticks_from_server_snapshot() -> void:
+	## D-020: When server sends stationary_ticks, client reads it directly.
+	GameState.apply_snapshot({"tick": 1, "entities": [], "stationary_ticks": 42})
+	assert_int(GameState.stationary_ticks).is_equal(42)
+
+
+func test_stationary_ticks_server_value_overrides_client_accumulation() -> void:
+	## D-020: Server value takes priority — client must not accumulate on top of it.
+	var snapshot := {
+		"tick": 1,
+		"entities": [{"entity_id": 1, "x": 10.0, "y": 10.0, "z": 0, "kind": {"variant": "Player", "data": null}}],
+		"stationary_ticks": 5,
+	}
+	GameState.apply_snapshot(snapshot)
+	GameState.apply_snapshot(snapshot)  # same position, but server sends 5 again
+	assert_int(GameState.stationary_ticks).is_equal(5)  # server value, not 6
+
+
+func test_stationary_ticks_missing_field_degrades_gracefully() -> void:
+	## D-020 fallback: when server omits stationary_ticks, no crash, defaults to 0.
+	GameState.stationary_ticks = 0
+	GameState.apply_snapshot({"tick": 1, "entities": []})
+	# No crash; field retains a valid integer value.
+	assert_int(GameState.stationary_ticks).is_greater_equal(0)
+
+
+# -- Stationary ticks: deprecated client-side fallback (D-071) -----------
+
+func test_stationary_ticks_fallback_increments_when_position_unchanged() -> void:
+	## Deprecated fallback: client accumulates when server omits the field.
 	var snapshot := {
 		"tick": 1,
 		"entities": [{"entity_id": 1, "x": 10.0, "y": 10.0, "z": 0, "kind": {"variant": "Player", "data": null}}],
 	}
-	GameState.apply_snapshot(snapshot)  # first call: position changes from ZERO
+	GameState.apply_snapshot(snapshot)  # first call: position changes from sentinel
 	GameState.apply_snapshot(snapshot)  # second call: position unchanged → +1
 	assert_int(GameState.stationary_ticks).is_greater(0)
 
 
-func test_stationary_ticks_resets_on_player_movement() -> void:
+func test_stationary_ticks_fallback_resets_on_movement() -> void:
+	## Deprecated fallback: client resets on movement when server omits the field.
 	var s1 := {
 		"tick": 1,
 		"entities": [{"entity_id": 1, "x": 10.0, "y": 10.0, "z": 0, "kind": {"variant": "Player", "data": null}}],
@@ -143,6 +175,33 @@ func test_stationary_ticks_resets_on_player_movement() -> void:
 	assert_int(GameState.stationary_ticks).is_equal(0)
 
 
+# -- Zone ID: server-authoritative (D-020/D-073) -------------------------
+
+func test_zone_id_from_server_snapshot() -> void:
+	## D-020: When server sends top-level zone_id, client reads it directly.
+	GameState.apply_snapshot({"tick": 1, "entities": [], "zone_id": "zone_alpha"})
+	assert_that(GameState.current_zone_id).is_equal("zone_alpha")
+
+
+func test_zone_id_server_value_overrides_tile_derivation() -> void:
+	## D-020: Server top-level zone_id takes priority over tile-derived zone_id.
+	var snapshot := {
+		"tick": 1,
+		"entities": [{"entity_id": 1, "x": 5.0, "y": 5.0, "z": 0, "kind": {"variant": "Player", "data": null}}],
+		"tiles": [{"x": 5, "y": 5, "z": 0, "type": "floor", "zone_id": "zone_from_tile"}],
+		"zone_id": "zone_from_server",
+	}
+	GameState.apply_snapshot(snapshot)
+	assert_that(GameState.current_zone_id).is_equal("zone_from_server")
+
+
+func test_zone_id_missing_field_degrades_gracefully() -> void:
+	## D-020 fallback: when server omits zone_id and no tiles match, defaults to "".
+	GameState.current_zone_id = ""
+	GameState.apply_snapshot({"tick": 1, "entities": []})
+	assert_that(GameState.current_zone_id).is_equal("")
+
+
 # -- insert_active (OQ-07, #522) -----------------------------------------
 
 func test_apply_snapshot_insert_active_false() -> void:

client/tests/test_game_state_sprint20.gd

@@ -0,0 +1,164 @@
+## Sprint 20 #557: GameState.apply_snapshot() refactor tests.
+##
+## Verifies D-020 compliance: apply_snapshot() reads server-authoritative values
+## for stationary_ticks and zone_id directly from the snapshot when present,
+## and degrades gracefully when the server has not yet added these fields.
+##
+## Does NOT replace test_game_state.gd or test_snapshot_zone_id.gd — those cover
+## existing behaviour. This file covers the new code paths added in Sprint 20.
+##
+## D-030: fixture-based, server-free, no subprocess required.
+class_name TestGameStateSprint20
+extends GdUnitTestSuite
+
+
+func before_each() -> void:
+	GameState.stationary_ticks = 0
+	GameState._prev_player_position = Vector2(-1e9, -1e9)
+	GameState.current_zone_id = ""
+	GameState.player_position = Vector2.ZERO
+	GameState.visible_tiles = []
+	GameState.visible_positions = {}
+	GameState.visibility_sectors = {}
+
+
+# -- stationary_ticks: server-authoritative path (D-020) ----------------------
+
+func test_stationary_ticks_reads_server_value_when_present() -> void:
+	# When snapshot includes stationary_ticks, apply_snapshot() must use the
+	# server value directly without client-side accumulation (D-020).
+	var snapshot := {
+		"tick": 5,
+		"entities": [
+			{"entity_id": 1, "x": 10.0, "y": 10.0, "z": 0, "kind": {"variant": "Player", "data": null}},
+		],
+		"stationary_ticks": 42,
+	}
+	GameState.apply_snapshot(snapshot)
+	assert_int(GameState.stationary_ticks).override_failure_message(
+		"apply_snapshot() must read stationary_ticks=42 from snapshot (D-020)"
+	).is_equal(42)
+
+
+func test_stationary_ticks_server_value_does_not_accumulate() -> void:
+	# Server-sent value must be assigned directly — NOT added to existing value.
+	# Two calls with stationary_ticks=10 must yield 10, not 20.
+	var snapshot := {
+		"tick": 1,
+		"entities": [
+			{"entity_id": 1, "x": 5.0, "y": 5.0, "z": 0, "kind": {"variant": "Player", "data": null}},
+		],
+		"stationary_ticks": 10,
+	}
+	GameState.apply_snapshot(snapshot)
+	GameState.apply_snapshot(snapshot)
+	assert_int(GameState.stationary_ticks).override_failure_message(
+		"Server value must be assigned directly, not accumulated (D-020)"
+	).is_equal(10)
+
+
+func test_stationary_ticks_server_can_reset_to_zero() -> void:
+	# Server sends 0 when player moves — client must accept this reset.
+	GameState.stationary_ticks = 50
+	var snapshot := {
+		"tick": 2,
+		"entities": [
+			{"entity_id": 1, "x": 5.0, "y": 5.0, "z": 0, "kind": {"variant": "Player", "data": null}},
+		],
+		"stationary_ticks": 0,
+	}
+	GameState.apply_snapshot(snapshot)
+	assert_int(GameState.stationary_ticks).override_failure_message(
+		"Server reset to 0 must override client-held value"
+	).is_equal(0)
+
+
+# -- stationary_ticks: DEPRECATED fallback (graceful degradation) --------------
+
+func test_stationary_ticks_fallback_when_field_absent() -> void:
+	# When snapshot has no stationary_ticks, client-side accumulation must still
+	# run (backward compat until server ships field). No crash.
+	var snapshot := {
+		"tick": 1,
+		"entities": [
+			{"entity_id": 1, "x": 10.0, "y": 10.0, "z": 0, "kind": {"variant": "Player", "data": null}},
+		],
+	}
+	GameState.apply_snapshot(snapshot)  # position changes from ZERO → resets to 0
+	GameState.apply_snapshot(snapshot)  # position unchanged → increments
+	assert_int(GameState.stationary_ticks).override_failure_message(
+		"Client-side fallback must increment stationary_ticks when field absent"
+	).is_greater(0)
+
+
+func test_apply_snapshot_missing_stationary_ticks_no_crash() -> void:
+	# Snapshots lacking stationary_ticks must not crash apply_snapshot().
+	var snapshot := {"tick": 1, "entities": []}
+	# No assertion needed beyond confirming no exception is raised.
+	GameState.apply_snapshot(snapshot)
+	assert_bool(true).is_true()
+
+
+# -- zone_id: server-authoritative path (D-020/D-073) -------------------------
+
+func test_current_zone_id_reads_server_value_when_present() -> void:
+	# When snapshot includes top-level zone_id, apply_snapshot() must use it
+	# directly without tile lookup (D-020).
+	var snapshot := {
+		"tick": 1,
+		"entities": [
+			{"entity_id": 1, "x": 5.0, "y": 5.0, "z": 0, "kind": {"variant": "Player", "data": null}},
+		],
+		"zone_id": "zone_server_direct",
+	}
+	GameState.apply_snapshot(snapshot)
+	assert_str(GameState.current_zone_id).override_failure_message(
+		"apply_snapshot() must read zone_id='zone_server_direct' from snapshot (D-020)"
+	).is_equal("zone_server_direct")
+
+
+func test_current_zone_id_server_value_overrides_tile_data() -> void:
+	# When snapshot has both zone_id and visible_tiles with a different zone,
+	# the top-level zone_id field takes priority.
+	var snapshot := {
+		"tick": 1,
+		"entities": [
+			{"entity_id": 1, "x": 5.0, "y": 5.0, "z": 0, "kind": {"variant": "Player", "data": null}},
+		],
+		"zone_id": "zone_from_server",
+		"visible_tiles": [
+			{"x": 5, "y": 5, "z": 0, "type": "floor", "zone_id": "zone_from_tile"},
+		],
+	}
+	GameState.apply_snapshot(snapshot)
+	assert_str(GameState.current_zone_id).override_failure_message(
+		"Top-level zone_id must override tile-derived zone when both present"
+	).is_equal("zone_from_server")
+
+
+# -- zone_id: DEPRECATED fallback (graceful degradation) ----------------------
+
+func test_current_zone_id_fallback_to_tile_lookup_when_absent() -> void:
+	# When snapshot lacks top-level zone_id, tile lookup fallback must run.
+	# Existing test_snapshot_zone_id.gd covers detailed scenarios; this is a
+	# smoke test confirming the fallback path still works after Sprint 20 refactor.
+	var snapshot := {
+		"tick": 1,
+		"entities": [
+			{"entity_id": 1, "x": 3.0, "y": 3.0, "z": 0, "kind": {"variant": "Player", "data": null}},
+		],
+		"visible_tiles": [
+			{"x": 3, "y": 3, "z": 0, "type": "floor", "zone_id": "zone_tile_fallback"},
+		],
+	}
+	GameState.apply_snapshot(snapshot)
+	assert_str(GameState.current_zone_id).override_failure_message(
+		"Tile lookup fallback must populate zone_id when top-level field absent"
+	).is_equal("zone_tile_fallback")
+
+
+func test_apply_snapshot_missing_zone_id_no_crash() -> void:
+	# Snapshots lacking both zone_id and visible_tiles must not crash.
+	var snapshot := {"tick": 1, "entities": []}
+	GameState.apply_snapshot(snapshot)
+	assert_str(GameState.current_zone_id).is_equal("")