code: def fast_combine_pairs(files, force_single, full_name, separators):
      
      files = sort_filenames(files)
      chunks = tz.sliding_window(10, files)
      pairs = [combine_pairs(chunk, force_single, full_name, separators) for chunk in chunks]
      pairs = [y for x in pairs for y in x]
      longest = defaultdict(list)
      # for each file, save the longest pair it is in
      for pair in pairs:
          for file in pair:
              if len(longest[file]) < len(pair):
                  longest[file] = pair
      # keep only unique pairs
      longest = {tuple(sort_filenames(x)) for x in longest.values()}
      # ensure filenames are R1 followed by R2
      return [sort_filenames(list(x)) for x in longest]
docs: assume files that need to be paired are within 10 entries of each other, once the list is sorted