add empty cell remapping; add some comments

addons/wfc/problems/2d/preconditions/precondition_2d_remap.gd

@@ -5,19 +5,21 @@ class_name WFC2DPreconditionRemap
 var _node: Node
 var _mapper: WFCMapper2D
 var _domains_map: Array[WFCBitSet]
+var _default_domain: WFCBitSet
 
-func _init(node: Node, mapper: WFCMapper2D, domains_map: Array[WFCBitSet]) -> void:
+func _init(node: Node, mapper: WFCMapper2D, domains_map: Array[WFCBitSet], default_domain: WFCBitSet = null) -> void:
 	assert(mapper.supports_map(node))
 	assert(mapper.size() == domains_map.size())
 
 	_node = node
 	_mapper = mapper
 	_domains_map = domains_map
+	_default_domain = default_domain
 
 func read_domain(coords: Vector2i) -> WFCBitSet:
 	var read := _mapper.read_cell(_node, coords)
-	
+
 	if read < 0:
-		return null
-	
+		return _default_domain
+
 	return _domains_map[read]

addons/wfc/problems/2d/preconditions/precondition_2d_remap_by_map.gd

@@ -2,9 +2,23 @@ extends WFC2DPrecondition2DNullSettings
 
 class_name WFC2DPrecondition2DRemapByMapSettings
 
+## A map that defines possible tile replacements.
+##
+## The tiles placed in the target map may be replaced by any tiles that are present in same rows of
+## this class map.
+##
+## E.g. if there are tiles A B C D in the tileset, and the class map contains a row of A B and A C,
+## then A tile may be replaced by either A, B or C; B - by A or B; C - by A or C; and the tile D
+## will always remain as is.
 @export_node_path
 var class_map: NodePath
 
+## Remap empty cells of target map to tiles from first row of class map.
+##
+## If not set, empty cells may be filled with any tile.
+@export
+var remap_empty: bool = false
+
 func _learn_classes(mapper: WFCMapper2D, map: Node) -> Array[WFCBitSet]:
 	var res: Array[WFCBitSet] = []
 
@@ -31,9 +45,16 @@ func create_precondition(parameters: CreationParameters) -> WFC2DPrecondition:
 	assert(class_map != null)
 	var class_map_node := parameters.generator_node.get_node(class_map)
 	assert(parameters.problem_settings.rules.mapper.supports_map(class_map_node))
+
+	var classes := _learn_classes(parameters.problem_settings.rules.mapper, class_map_node)
+	var empty_domain: WFCBitSet = null
+
+	if remap_empty:
+		empty_domain = classes[0]
 
 	return WFC2DPreconditionRemap.new(
 		parameters.target_node,
 		parameters.problem_settings.rules.mapper,
-		_learn_classes(parameters.problem_settings.rules.mapper, class_map_node)
+		classes,
+		empty_domain,
 	)