The classic problem of the Towers of Hanoi.

towers.rb

require 'minitest/autorun'

class Towers
  class CantPlaceError < StandardError; end

  attr_accessor :from, :to

  def initialize(from, to)
    @from   = from
    @to     = to
    @buffer = Array.new(from.size)
  end

  def move
    move_disks(@from.size, @from, @to, @buffer)
  end

  private

  def move_disks(n, from, to, buffer)
    return if n <= 0
    move_disks(n - 1, from, buffer, to)
    move_top(from, to)
    move_disks(n - 1, buffer, to, from)
  end

  def move_top(from, to)
    if to.any? && from.last > to.last
      raise CantPlaceError.new("Can't place #{from.last} onto #{to.last}")
    end
    to.push(from.pop)
  end
end

class TestTowers < MiniTest::Unit::TestCase
  def test_move_disks
    towers = Towers.new([3, 2, 1], [])
    assert_equal towers.from, [3, 2, 1]
    assert_equal towers.to,   []

    towers.move
    assert_equal towers.from, []
    assert_equal towers.to,   [3, 2, 1]
  end

  def test_raise_cant_place
    proc {
      Towers.new([2, 3, 1], []).move
    }.must_raise(Towers::CantPlaceError)
  end
end