Extract a Table of Contents from a Markdown document

[RobertDober]

Synopsis

As Extractly is used by Earmark and potentially by EarmarkParser parsing the markdown is not an option (and probably total overkill anyway).

Therefore scanning for lines matching ~r{\A \s{0,3} (#{1,7}) \s+ (.*)}x shall deliver the necessary information.

Sketch

With that in mind such a list shall be obtained by the following pseudo code

     lines
     |> Enum.map(&Regex.run(~r{\A \s{0,3} (#{1,7}) \s+ (.*)}x, &1)
     |> Enum.filter(& &1)
     |> Enum.map(fn [_, header, title] -> {String.length(header), title} end) 

yielding, e.g. ...

   [{1, "Main"}, {2, "Sub 1"}, {2, "Sub 2"}, {3, "SubSub"}, {1, "Epilogue"}]

which than, depending on some options shall yield markdown, e.g.

1. Main
   1. Sub 1
   1. Sub 2
      1. SubSub
1. Epilogue

Options

• numbered: 1 value for starting position, nil for unnumbered lists
• levels: 7 maximum level of headers taken into account
• output: :markdown alternatives like :ast, :html or :json might be a nice enhancement

Limitations

Setext headers are not taken into account, as I do not have a use case for now