require 'nokogiri'

module GalleryItems
  JPEG_START_OF_FRAME = [0xC0, 0xC1, 0xC2, 0xC3, 0xC5, 0xC6, 0xC7,
                         0xC9, 0xCA, 0xCB, 0xCD, 0xCE, 0xCF].freeze

  # Read just the dimensions from a local JPEG. The rendered gallery needs
  # aspect ratios to size each chronological pair without cropping either photo.
  def jpeg_dimensions(path)
    File.open(path, 'rb') do |file|
      return nil unless file.read(2) == "\xFF\xD8".b

      while (prefix = file.read(1))
        next unless prefix.getbyte(0) == 0xFF

        marker = file.read(1)&.getbyte(0)
        break unless marker
        next if marker == 0xFF || marker == 0x01 || (0xD0..0xD7).cover?(marker)
        break if marker == 0xD9 || marker == 0xDA

        size = file.read(2)&.unpack1('n')
        break unless size && size >= 2
        if JPEG_START_OF_FRAME.include?(marker)
          dimensions = file.read(5)
          return nil unless dimensions&.length == 5

          height, width = dimensions.byteslice(1, 4).unpack('n2')
          return [width, height] if width.positive? && height.positive?
          return nil
        end
        file.seek(size - 2, IO::SEEK_CUR)
      end
    end
    nil
  rescue Errno::ENOENT, EOFError
    nil
  end

  def gallery_image_ratio(content)
    image = Nokogiri::HTML::DocumentFragment.parse(content).at_css('img')
    source = image&.[]('src')
    return 1.5 unless source&.start_with?('/img/arts/')

    path = File.join(File.expand_path('..', __dir__), source.delete_prefix('/'))
    width, height = jpeg_dimensions(path)
    return 1.5 unless width && height

    (width.to_f / height).round(4)
  end

  # Uploaded batches share one source document, but each photo is a gallery item.
  # Keeping this at render time also fixes batches that have already been published.
  def gallery_items(content)
    fragment = Nokogiri::HTML::DocumentFragment.parse(content)
    images = fragment.css('img')
    return [content] unless images.length > 1 && images.all? { |image|
      image['src'].to_s.start_with?('/img/arts/uploads/') && image.parent.name == 'p'
    }

    captions = fragment.css('.image-details').map(&:to_html).join
    images.each { |image| image.parent.remove }
    fragment.css('.image-details').each(&:remove)
    # Preserve the batch's existing anchor exactly once, before the first photo.
    prefix = fragment.to_html
    images.each_with_index.map do |image, index|
      "#{index.zero? ? prefix : ''}<p>#{image.to_html}</p>#{captions}"
    end
  end
end

Liquid::Template.register_filter(GalleryItems)