Adaptable Lua filter to generate a Table of Contents

toc.lua

-- Generate a Table of Contents.
--
-- This is an adaption of the Haskell code used in pandoc. This script
-- is intended to be adapted to specific requirements before use. E.g.,
-- it can be modified to add additional elements as TOC items, to add
-- more info to each line in the TOC, or to control whether the heading
-- number is included in the link.

local PANDOC_WRITER_OPTIONS = PANDOC_WRITER_OPTIONS
_ENV = pandoc

local not_empty = function (x) return #x > 0 end


-- mutually recursive functions
local section_to_toc_item
local to_toc_item

to_toc_item = function (number, text, id, subcontents)
  -- remove notes and links from toc item text
  text = text:walk {
    Note = function (_) return {} end,
    Link = function (link) return link.content end,
  }
  -- add number to toc item
  if number then
    text = Span({Str(number),Space()} .. text, {class='toc-section-number'})
  end
  -- create link to heading if possible
  local header_link = id == '' and text or Link(text, '#' .. id)
  local subitems = subcontents:map(section_to_toc_item):filter(not_empty)
  return List{Plain{header_link}} ..
      (#subitems == 0 and {} or {BulletList(subitems)})
end

section_to_toc_item = function (div)
  -- bail if this is not a section wrapper
  if div.t ~= 'Div' or not div.content[1] or div.content[1].t ~= 'Header' then
    return {}
  end
  local heading = div.content:remove(1)
  local number = heading.attributes.number
  -- bail if this is not supposed to be included in the toc
  if not number and heading.classes:includes 'unlisted' then
    return {}
  end
  -- bail if heading is deeper than toc depth
  if heading.level > PANDOC_WRITER_OPTIONS.toc_depth then
    return {}
  end

  return to_toc_item(number, heading.content, div.identifier, div.content)
end

-- return filter
return {
  { Pandoc = function (doc)
      -- Improvement over pandoc's native TOC generator: unwrap divs to avoid
      -- problems with headings nested below divs.
      local blocks_no_divs = doc.blocks:walk{
        Div = function (x) return x.content end
      }
      local sections = utils.make_sections(true, nil, blocks_no_divs)
      local toc_items = sections:map(section_to_toc_item):filter(not_empty)
      doc.meta['table-of-contents'] = {BulletList(toc_items)}
      doc.meta.toc = doc.meta['table-of-contents'] -- backwards compatibility
      return doc
    end
  },
}