Towers of Hanoi

gistfile1.rb

class Tower
  attr_reader :name
  
  def initialize(name)
    @name = name
    @disks = []
  end
  
  def stack(disk_number)
    raise "Disk too big" unless can_stack?(disk_number)
    
    @disks.unshift(disk_number)
  end
  
  def pull
    raise "No disks" unless @disks.any?
    disk = @disks.shift
    disk
  end
  
  def to_s
    tower = ["|"]
    @disks.each do |length|
      tower << "-" * length
    end
    tower << self.name
    tower.join("\n")
  end
  
  def can_stack?(disk_number)
    return true if @disks.none?
    return disk_number < @disks.first
  end
  
  def top_disk?(disk_number)
    @disks.first == disk_number
  end
  
  def disk_above(disk_number)
    return nil if @disks.index(disk_number) == 0
    @disks[@disks.index(disk_number) - 1]
  end
end

class Hanoi
  def initialize(size)
    @tower1 = Tower.new(:tower1)
    @tower2 = Tower.new(:tower2)
    @tower3 = Tower.new(:tower3)
    
    size.downto(1) do |block|
      @tower1.stack(block)
    end
    
    @size = size
    @move_count = 0
  end
  
  def to_s
    "#{@tower1}\n\n#{@tower2}\n\n#{@tower3}"
  end
  
  def run
    debug
    move(@size, @tower1, @tower3, @tower2)
    @move_count
  end
  
  def self.run!(size)
    self.new(size).run
  end
  
  def move(disk_number, from, to, via)
    disk_above = from.disk_above(disk_number)
    move(disk_above, from, via, to) if disk_above
    to.stack(from.pull)
    debug
    move(disk_above, via, to, from) if disk_above
    
    @move_count += 1
  end
  
  def debug
    return unless $VERBOSE
    puts self
    puts "\n==============\n\n"
  end
end

puts Hanoi.run!(4)