module Img2Zpl
  class Image < MiniMagick::Image

    def to_zpl(black_threshold: 0.5, invert: false, compress: true)
      bytes_per_row = (width % 8).positive? ? (width / 8) + 1 : (width / 8)
      byte_count = bytes_per_row * height
      data, line, previous_line, byte = '', '', '', ''

      get_pixels.each do |row|
        row.each_with_index do |column, i|
          r, g, b = column.map(&:to_i)
          b_dot, w_dot = invert ? %w(1 0) : %w(0 1)
          byte << ((r + g + b) > (black_threshold * 765) ? b_dot : w_dot)
          if (i % 8).zero?
            line << byte.to_i(2).to_s(16).upcase.rjust(2, '0')
            byte = ''
          end
        end

        data << if compress
          (line == previous_line ? ':' : line.gsub(/0+$/, ',').gsub(/F+$/, '!'))
        else
          line
        end

        previous_line = line
        line = ''
      end

      _compress(data) if compress
      "^GFA,#{byte_count},#{byte_count},#{bytes_per_row},#{data}^FS"
    end

    private

    def _compression_map
      map = {}
      start = 'G'.ord
      19.times { |i| map[i + 1] = (start+i).chr }
      start = 'g'.ord
      (20..400).step(20).each_with_index { |i, j| map[i] = (start+j).chr }
      map
    end

    def _reduce(n)
      str = ''
      counts = _compression_map.keys.sort.reverse
      counts.each do |c|
        if c <= n
          str << (_compression_map[c])
          n -= c
        end
      end
      str
    end

    def _compress(data)
      data.gsub!(/([\da-zA-Z])(\1+)/) do |m|
        "#{_reduce(m.length)}#{$1}"
      end
    end

  end
end