ghostscript_service.rb

# [putting-together-pdf-files](https://www.linux.com/news/putting-together-pdf-files)

gs -dBATCH -dNOPAUSE -q -sDEVICE=pdfwrite -sOutputFile=finished.pdffile1.pdf file2.pdf

# \t
# gs -- starts the Ghostscript program\t
# -dBATCH -- once Ghostscript processes the PDF files, it should exit. If you don't include this option, Ghostscript will just keep running\t
# -dNOPAUSE -- forces Ghostscript to process each page without pausing for user interaction\t
# -q -- stops Ghostscript from displaying messages while it works\t
# -sDEVICE=pdfwrite -- tells Ghostscript to use its built-in PDF writer to process the files\t
# -sOutputFile=finished.pdf -- tells Ghostscript to save the combined PDF file with the name that you specified

# When using Ghostscript to combine PDF files, you can add any PDF-relatedoption to the command line.
# For example, you can compress the file, target it to an eBook reader, or encrypt it. 
# See the Ghostscript documentation for more information.
## https://www.ghostscript.com/Documentation.html

 gs \
  -o book.pdf \
  -sDEVICE=pdfwrite \
  -dColorConversionStrategy=/LeaveColorUnchanged \
  -dDownsampleMonoImages=false \
  -dDownsampleGrayImages=false \
  -dDownsampleColorImages=false \
  -dAutoFilterColorImages=false \
  -dAutoFilterGrayImages=false \
  -dColorImageFilter=/FlateEncode \
  -dGrayImageFilter=/FlateEncode \
   title.pdf \
   content.pdf

def combine_files(output: "book.pdf", directory: nil, files: [], options: ['-dColorConversionStrategy=/LeaveColorUnchanged', '-dDownsampleMonoImages=false', '-dDownsampleGrayImages=false', '-dDownsampleColorImages=false', '-dAutoFilterColorImages=false', '-dAutoFilterGrayImages=false', '-dColorImageFilter=/FlateEncode', '-dGrayImageFilter=/FlateEncode'])
  # output = "/Users/me/Downloads/whatever.pdf" 
  if directory
   files = Dir[directory + "/*.pdf"]
   # and maybe
   # files = files.sort_by{|f| f.split('_').last.split('.').first.to_i}
  end
  files = files.map{|f| Shellwords.escape(f)}
  output = Shellwords.escape(output)
  command = ['gs', '-o', output, '-sDEVICE=pdfwrite', options, files].flatten.join(' ') # add '-' as the last argument to print results inline
  `#{command}`
end

def alt_combine_files(output: "book.pdf", directory: nil, files: [], options: ['-dColorConversionStrategy=/LeaveColorUnchanged', '-dDownsampleMonoImages=false', '-dDownsampleGrayImages=false', '-dDownsampleColorImages=false', '-dAutoFilterColorImages=false', '-dAutoFilterGrayImages=false', '-dColorImageFilter=/FlateEncode', '-dGrayImageFilter=/FlateEncode'])
  if directory
   files = Dir[directory + "/*.pdf"]
   # and maybe
   files = files.sort_by{|f| f.split('_').last.split('.').first.to_i}
  end
  files = files.map{|f| Shellwords.escape(f)}
  output = Shellwords.escape(output)
  command = ['gs', '-o', output, '-sDEVICE=pdfwrite', options, files].flatten.join(' ') # add '-' as the last argument to print results inline
  `#{command}`
end