def run!(&block)
      raise RuntimeError, "unknown layout #{layout_name}"     if !Layout.respond_to?(layout_name)
      raise RuntimeError, "unknown style #{style_name}"       if !Style.respond_to?(style_name)
      raise RuntimeError, "unknown library #{library_name}"   if !Library.respond_to?(library_name)

      raise RuntimeError, "input must be a single directory"  if input.nil?  || input.to_s.empty? || !File.directory?(input)
      raise RuntimeError, "no image files found"              if image_files.empty?
      raise RuntimeError, "no output file specified"          if output.to_s.empty?
      raise RuntimeError, "no output image file specified"    if output_image_file.to_s.empty?
      raise RuntimeError, "no output style file specified"    if output_style_file.to_s.empty?

      raise RuntimeError, "set :width for fixed width, or :hpadding for horizontal padding, but not both." if width  && !hpadding.zero?
      raise RuntimeError, "set :height for fixed height, or :vpadding for vertical padding, but not both." if height && !vpadding.zero?
      raise RuntimeError, "set :width for fixed width, or :hmargin for horizontal margin, but not both." if width  && !hmargin.zero?
      raise RuntimeError, "set :height for fixed height, or :vmargin for vertical margin, but not both." if height && !hmargin.zero?

      raise RuntimeError, "The legacy :csspath attribute is no longer supported, please use :cssurl instead (see README)" unless @config[:csspath].nil?

      images = load_images
      max    = layout_images(images)
      header = summary(images, max)

      report(header)

      css = []
      css << style_comment(header) unless nocomments?                       # header comment
      css << style(selector, css_url, images, &block)                       # generated styles
      css << IO.read(custom_style_file) if File.exists?(custom_style_file)  # custom styles
      css = css.join("\n")

      create_sprite(images, max[:width], max[:height])

      unless nocss?
        css_file = File.open(output_style_file, "w+")
        css_file.puts css
        css_file.close
      end

      if config[:return] == :images
        images # if caller explicitly asked for detailed images hash instead of generated CSS
      else
        css    # otherwise, default is to return the generated CSS to caller in string form
      end

    end
def run!(&block)
      raise RuntimeError, "unknown layout #{layout_name}"     if !Layout.respond_to?(layout_name)
      raise RuntimeError, "unknown style #{style_name}"       if !Style.respond_to?(style_name)
      raise RuntimeError, "unknown library #{library_name}"   if !Library.respond_to?(library_name)

      raise RuntimeError, "input must be a single directory"  if input.nil?  || input.to_s.empty? || !File.directory?(input)
      raise RuntimeError, "no image files found"              if image_files.empty?
      raise RuntimeError, "no output file specified"          if output.to_s.empty?
      raise RuntimeError, "no output image file specified"    if output_image_file.to_s.empty?
      raise RuntimeError, "no output style file specified"    if output_style_file.to_s.empty?

      raise RuntimeError, "set :width for fixed width, or :hpadding for horizontal padding, but not both." if width  && !hpadding.zero?
      raise RuntimeError, "set :height for fixed height, or :vpadding for vertical padding, but not both." if height && !vpadding.zero?
      raise RuntimeError, "set :width for fixed width, or :hmargin for horizontal margin, but not both." if width  && !hmargin.zero?
      raise RuntimeError, "set :height for fixed height, or :vmargin for vertical margin, but not both." if height && !hmargin.zero?

      raise RuntimeError, "The legacy :csspath attribute is no longer supported, please use :cssurl instead (see README)" unless @config[:csspath].nil?

      images = load_images
      max    = layout_images(images)
      header = summary(images, max)

      report(header)

      css = []
      css << style_comment(header) unless nocomments?                       # header comment
      css << style(selector, css_url, images, &block)                       # generated styles
      css << IO.read(custom_style_file) if File.exists?(custom_style_file)  # custom styles
      css = css.join("\n")

      create_sprite(images, max[:width], max[:height])

      unless nocss?
        css_file = File.open(output_style_file, "w+")
        css_file.puts css
        css_file.close
      end

      if config[:return] == :images
        images # if caller explicitly asked for detailed images hash instead of generated CSS
      else
        css    # otherwise, default is to return the generated CSS to caller in string form
      end

    end
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