Add directional sticky and skewer box variants

movement.lua

@@ -0,0 +1,83 @@
+-- Finds the complete set of blocks that must move together for one grid step.
+-- A block joins the set when it is in another block's destination (ordinary
+-- pushing), is behind a directional sticky puller, or touches a skewer block.
+local Movement = {}
+
+local adjacentDirections = {
+	{x = 1, y = 0},
+	{x = -1, y = 0},
+	{x = 0, y = 1},
+	{x = 0, y = -1},
+}
+
+local function isSticky(entity)
+	return entity.isSticky and entity:isSticky()
+end
+
+local function isPassable(entity)
+	return entity.canPassOver and entity:canPassOver()
+end
+
+local function isSkewer(entity)
+	return entity.isSkewer and entity:isSkewer()
+end
+
+function Movement.findMovableGroup(startCells, direction, entityAt, isInBounds)
+	local entities = {}
+	local pendingCells = {}
+	for _, cell in ipairs(startCells) do
+		table.insert(pendingCells, cell)
+	end
+
+	local nextCell = 1
+	while nextCell <= #pendingCells do
+		local cell = pendingCells[nextCell]
+		nextCell = nextCell + 1
+
+		if not isInBounds(cell) then
+			return false, entities
+		end
+
+		local entity = entityAt(cell)
+		if entity and not isPassable(entity) and not entities[entity] then
+			if not entity:canMove() then
+				return false, entities
+		end
+
+			entities[entity] = entity
+
+			-- Anything occupying this block's destination must be pushed too.
+			for _, projectedCell in ipairs(entity:getOccupiedCells(direction)) do
+				table.insert(pendingCells, projectedCell)
+			end
+
+			-- Sticky blocks pull only from the trailing face, like the player. A
+			-- lateral move therefore shears the contact. Skewers retain the older
+			-- all-edge behaviour and make everything they touch one rigid group.
+			for _, occupiedCell in ipairs(entity:getOccupiedCells()) do
+				for _, adjacentDirection in ipairs(adjacentDirections) do
+					local adjacentCell = {
+						x = occupiedCell.x + adjacentDirection.x,
+						y = occupiedCell.y + adjacentDirection.y,
+					}
+					if isInBounds(adjacentCell) then
+						local adjacentEntity = entityAt(adjacentCell)
+						local trailing = adjacentDirection.x == -direction.x and
+							adjacentDirection.y == -direction.y
+						local attached = adjacentEntity and (
+							isSkewer(entity) or isSkewer(adjacentEntity) or
+							(trailing and (isSticky(entity) or isSticky(adjacentEntity)))
+						)
+						if attached and adjacentEntity ~= entity and adjacentEntity:canMove() then
+							table.insert(pendingCells, adjacentCell)
+						end
+					end
+				end
+			end
+		end
+	end
+
+	return true, entities
+end
+
+return Movement