-- Replace an empty `::: {.article-toc}` block with the current article's
-- table of contents. The block can appear anywhere in the article.

local function escape_html(text)
  return text:gsub("&", "&")
             :gsub("<", "&lt;")
             :gsub(">", "&gt;")
             :gsub('"', "&quot;")
             :gsub("'", "&#39;")
end

local function make_toc(entries)
  if #entries == 0 then
    return nil
  end

  local lines = {
    '<nav class="article-toc" aria-label="Table of contents">',
    '<h2>Contents</h2>',
    '<ul>',
  }
  for _, entry in ipairs(entries) do
    table.insert(lines, string.format(
      '<li class="toc-level-%d"><a href="#%s">%s</a></li>',
      entry.level, escape_html(entry.id), escape_html(entry.title)))
  end
  table.insert(lines, '</ul>')
  table.insert(lines, '</nav>')
  return pandoc.RawBlock('html', table.concat(lines, '\n'))
end

function Pandoc(doc)
  local output = pandoc.List()
  local article = pandoc.List()

  local function flush_article()
    if #article == 0 then
      return
    end

    local entries = pandoc.List()
    for _, block in ipairs(article) do
      if block.t == 'Header' and block.level == 2 then
        entries:insert({
          level = block.level,
          id = block.identifier,
          title = pandoc.utils.stringify(block.content),
        })
      end
    end

    for _, block in ipairs(article) do
      if block.t == 'Div' and block.classes:includes('article-toc') then
        local toc = make_toc(entries)
        if toc then
          output:insert(toc)
        end
      else
        output:insert(block)
      end
    end
  end

  for _, block in ipairs(doc.blocks) do
    if block.t == 'Header' and block.level == 1 then
      flush_article()
      article = pandoc.List()
    end
    article:insert(block)
  end
  flush_article()

  doc.blocks = output
  return doc
end