Mediator

# The Mediator: towers and enemies never talk to each other directly.
# The battlefield decides who gets hit and what happens when an enemy falls.
class Battlefield
  attr_reader :gold, :log

  def initialize
    @towers = []
    @enemies = []
    @gold = 0
    @log = []
  end

  def deploy(tower) = @towers << tower
  def spawn(enemy) = @enemies << enemy

  def tick
    @towers.each(&:fire)
  end

  # Called by a tower: aim at the enemy in range that is furthest along.
  def fire(tower)
    in_range = @enemies.select { |enemy| (enemy.position - tower.position).abs <= tower.range }
    target = in_range.max_by(&:position) or return

    @log << "#{tower.name} hit #{target.name}"
    target.hit(tower.damage)
  end

  # Called by an enemy when its health runs out.
  def defeated(enemy)
    @enemies.delete(enemy)
    @gold += enemy.bounty
    @log << "#{enemy.name} defeated, +#{enemy.bounty}g"
  end
end

# Colleagues: each one knows only the battlefield.
class Tower
  attr_reader :name, :position, :range, :damage

  def initialize(name, battlefield, position:, range:, damage:)
    @name = name
    @battlefield = battlefield
    @position = position
    @range = range
    @damage = damage
    battlefield.deploy(self)
  end

  def fire = @battlefield.fire(self)
end

class Enemy
  attr_reader :name, :position, :hp, :bounty

  def initialize(name, battlefield, position:, hp:, bounty:)
    @name = name
    @battlefield = battlefield
    @position = position
    @hp = hp
    @bounty = bounty
    battlefield.spawn(self)
  end

  def hit(damage)
    @hp -= damage
    @battlefield.defeated(self) unless @hp.positive?
  end
end