class ImageString
  attr_reader :columns, :rows

  def initialize(filename)
    File.open(filename, "rb") do |file|
      (columns_str, rows_str) = file.gets.split(" ")
      @columns = columns_str.to_i
      @rows = rows_str.to_i
      @image_string = file.read
    end
  end

  def pixel_at(row, column)
    location = column + (row * @columns)
    if location < 0 or location >= @image_string.length
      return nil
    end
    single = @image_string[location, 1]
    return (single.unpack("C"))[0]
  end

  def set_pixel(row, column, value)
    # TODO exception - bad position and bad value
    location = column + (row * @columns)
    # usage of pack here is to avoid differences in string[x] behavior in 1.8 and 1.9
    raw_value = [value].pack("C")
    @image_string[location] = raw_value
  end

  def dump_to_file(filename)
    File.open(filename, "wb") do |out|
      out.puts(@columns.to_s + " " + @rows.to_s)
      out.puts(@image_string)
    end
  end
end