-- Make a first 18x18 layout from the exit links preserved in the legacy saves.
-- Some old test rooms share the same one-way exit, so a grid cannot represent
-- every such link at once. Those rooms are still placed, but isolated, rather
-- than silently overwriting a room that already occupies that neighbour.
function seedWorldFromLegacyExits(world)
	local available = {}
	for _, name in ipairs(world:listRoomNames()) do available[name] = true end
	local legacy = {}
	for name in pairs(available) do
		local raw = readFromSource("rooms/_pre_migration_backup/" .. name .. ".sav")
		if raw then
			local ok, save = pcall(TSerial.unpack, raw)
			if ok and type(save) == "table" then legacy[name] = save.exits or {} end
		end
	end

	world.rooms, world.roomByName = {}, {}
	local root = available.start and "start" or world:listRoomNames()[1]
	if not root then return { placed = 0, links = 0, conflicts = 0 } end
	world:_putRoom(root, 9, 11)
	local queue, queued = { root }, { [root] = true }
	local directions = {
		top = { x = 0, y = -1 }, right = { x = 1, y = 0 },
		bottom = { x = 0, y = 1 }, left = { x = -1, y = 0 },
	}
	local order = { "top", "right", "bottom", "left" }
	local links, conflicts = 0, 0
	local index = 1
	while index <= #queue do
		local name = queue[index]
		local source = world:roomLocation(name)
		for _, exit in ipairs(order) do
			local target = legacy[name] and legacy[name][exit]
			target = target and tostring(target)
			local direction = directions[exit]
			if target and available[target] then
				local x, y = source.x + direction.x, source.y + direction.y
				local occupied, known = world:roomAt(x, y), world:roomLocation(target)
				if world:isInBounds(x, y) and (not occupied or occupied == target) and (not known or (known.x == x and known.y == y)) then
					if not known then world:_putRoom(target, x, y) end
					links = links + 1
					if not queued[target] then table.insert(queue, target); queued[target] = true end
				else
					conflicts = conflicts + 1
				end
			end
		end
		index = index + 1
	end

	-- Keep every migrated room visible/selectable without creating accidental
	-- exits between the disconnected legacy variants.
	for _, name in ipairs(world:listRoomNames()) do
		if not world:roomLocation(name) then
			local placed = false
			for y = 1, world.height do
				for x = 1, world.width do
					if not world:roomAt(x, y) then
						local nearby = false
						for dy = -1, 1 do for dx = -1, 1 do if world:roomAt(x + dx, y + dy) then nearby = true end end end
						if not nearby then world:_putRoom(name, x, y); placed = true; break end
					end
				end
				if placed then break end
			end
		end
	end
	world:save()
	return { placed = #world.rooms, links = links, conflicts = conflicts }
end