O(n^2) naive implementation

gistfile1.rb

# naive implementation. We could move all the .index stuff into one loop
# and do all kinds of other terrible things
# but that's boring

class ParkingOrder
  def initialize(current)
    @state = current.split('').uniq
    @empty = @state.index ?_
  end

  def solve(final)
    @target = final.split('').uniq
    puts @state.join ' '
    unmoved = @state[0] == ?_ ? 0 : 1
    0.upto(@state.length) do |idx|
      mover = @target[@empty]
      from = @state.index mover
      from = unmoved if from == @empty # unreasonable move
      @state[from], @state[@empty] = @state[@empty], @state[from]
      @empty = from
      if from == unmoved + 1
        while @target[unmoved] == @state[unmoved]
          unmoved += 1
          return if unmoved >= @state.length
        end
      end
      puts @state.join ' '
    end
  end
end

if __FILE__ == $0
  po = ParkingOrder.new 'bedc_a'
  po.solve 'abcde_'
end