Facade

# A subsystem of small objects, each with its own interface.
class Wallet
  attr_reader :gold

  def initialize(gold)
    @gold = gold
  end

  def afford?(amount) = amount <= gold

  def spend(amount)
    @gold -= amount
  end
end

class PriceList
  PRICES = { arrow: 50, frost: 70, cannon: 90 }.freeze

  def price(kind) = PRICES.fetch(kind)
end

class Grid
  def initialize(path:)
    @path = path
    @towers = {}
  end

  def free?(cell) = !@path.include?(cell) && !@towers.key?(cell)

  def place(kind, cell)
    @towers[cell] = kind
  end

  def tower_at(cell) = @towers[cell]
end

# The Facade: one simple shop counter in front of the whole subsystem.
# The UI asks for a tower; the shop knows which objects to consult.
class Shop
  def initialize(wallet:, grid:, prices: PriceList.new)
    @wallet = wallet
    @grid = grid
    @prices = prices
  end

  def buy(kind, cell)
    price = @prices.price(kind)
    return "Can't build there" unless @grid.free?(cell)
    return "Not enough gold (need #{price})" unless @wallet.afford?(price)

    @wallet.spend(price)
    @grid.place(kind, cell)
    "Built #{kind} tower for #{price}g"
  end
end