class Program
  def initialize
    @instructions = []
    (2 + rand(4)).times do
      @instructions << random_instruction
    end
  end

  def initialize_copy(other)
    @instructions = Marshal.load(Marshal.dump(@instructions))
  end

  def to_s
    @instructions.map do |instruction, *params|
      case instruction
      when :load
        "load #{params.first.inspect}"
      when :output
        "output"
      end
    end.join("\n")
  end

  def execute
    output = ""
    register = "A"
    @instructions.each do |instruction, *params|
      case instruction
      when :load
        register = params.first
      when :output
        output += register
      end
    end
    output
  end

  def mutate
    case rand(4)
    when 0 # remove an instruction
      if @instructions.size > 1
        i = rand(@instructions.size)
        @instructions.delete_at(i)
      end
    when 1 # insert an instruction
      i = rand(@instructions.size + 1)
      @instructions.insert(i, random_instruction)
    when 2 # replace an instruction
      i = rand(@instructions.size)
      @instructions[i] = random_instruction
    when 3 # modify parameter
      i = rand(@instructions.size)
      if @instructions[i].first == :load
        @instructions[i][1] = mutate_param(@instructions[i][1])
      end
    end
  end

private

  def random_instruction
    case rand(2)
    when 0
      [:load, (65 + rand(26)).chr]
    when 1
      [:output]
    end
  end

  def mutate_param(old_value)
    chr = old_value.unpack("C").first
    chr = (chr + (rand(2) + 1) * [-1, 1][rand(2)]) % 26
    chr.chr
  end
end