-- Render content below header in three columns

local multicol_begin = pandoc.RawBlock('latex', [[
\begin{multicols}{3}\raggedcolumns
]])
local multicol_end = pandoc.RawBlock('latex', [[
\end{multicols}
]])

local ad_template = [[
\begin{minipage}[%s][%s\textheight][c]{\textwidth}
\centering
\includegraphics[width=\linewidth,height=%s\textheight,keepaspectratio]{%s}
\end{minipage}
]]

local function has_class(elem, class)
  local classes = elem.classes
  if not classes then
    return false
  end
  for _, c in ipairs(classes) do
    if c == class then
      return true
    end
  end
  return false
end

local function is_columnar_header(elem)
  return elem.t == "Header" and elem.level == 1
end

local function is_multicols_begin(elem)
  return elem.t == "RawBlock" and elem.text == "\\begin{multicols}{3}\\raggedcolumns"
end

local function is_multicols_end(elem)
  return elem.t == "RawBlock" and elem.text == "\\end{multicols}"
end

-- a standalone image with optional caption that has class .wide
-- TODO: detect pre-quarto-filter pattern too
local function is_wide(elem)
  return ((elem.t == "Para" and has_class(elem.content[1], "wide"))
  or (elem.t == "Figure" and has_class(elem.content[1].content[1], "wide")))
end

-- TODO: detect pre-quarto-filter pattern too
local function is_newpage_command(elem)
  return (elem.t == "RawBlock" and elem.text == "\\newpage{}")
    or (elem.t == "Para" and #elem.content == 5 and elem.content[3] == "pagebreak")
end

-- a paragraph where initial element has class .ad is an advertising
local function is_advertising(elem)
  if elem.t == "Para" then
    local elem1 = elem.content[1]
    if elem1.t == "Image" and has_class(elem1, "ad") then
      return true
    end
  end
  return false
end

-- wrap images with class .ad to span half or full page
local function ad(elem)
  local amount = #elem.content

  -- 1 ad, spanning full page except with class .top or .bottom
  if amount == 1 then
    local elem1 = elem.content[1]
    if elem1.t ~= "Image" or not elem1.src then
      error("failed to parse advertisement")
      return elem
    end
    if has_class(elem1, "top") then
      return pandoc.RawBlock('latex', "\\noindent\n"
        .. string.format(ad_template, "t", "0.5", "0.5", elem1.src))
    elseif has_class(elem1, "bottom") then
      return pandoc.RawBlock('latex', "\\vspace*{\\fill}\\noindent\n"
        .. string.format(ad_template, "b", "0.5", "0.5", elem1.src))
    end
    return pandoc.RawBlock('latex',"\\noindent\n"
      .. string.format(ad_template, "t", "1", "1", elem1.src))
  end

  -- 2 ads spanning full page
  if amount == 3 then
    local elem1 = elem.content[1]
    local elem2 = elem.content[3]
    if elem1.t ~= "Image" or not elem1.src
      or elem2.t ~= "Image" or not elem2.src
    then
      error("failed to parse 2-element advertisement")
      return elem
    end
    return pandoc.RawBlock('latex', "\\noindent\n"
      .. string.format(ad_template, "t", "0.5", "0.5", elem1.src)
      .. string.format(ad_template, "b", "0.5", "0.5", elem2.src))
  end

  error("failed to parse advertisement, wrong amount of elements: "
    .. amount)
  return elem
end

function Pandoc(doc)
  if FORMAT:match 'latex' then

    -- locate topmost-only elements where multicol state should change
    local multicol_now = false
    local multicol_places = {}
    for i, elem in ipairs(doc.blocks) do
      if is_columnar_header(elem) then
        multicol_places[i] = multicol_now and "renew" or "begin"
        multicol_now = true
      elseif is_wide(elem) then
        if multicol_now then
          multicol_places[i] = "renew"
        end
      elseif is_advertising(elem) then
        doc.blocks[i] = ad(elem)
        if multicol_now then
          multicol_places[i] = "renew"
        end
      elseif is_newpage_command(elem) then
        if multicol_now then
          multicol_places[i] = "end"
          multicol_now = false
        end
      elseif elem.t == "HorizontalRule" then
        if multicol_now then
          multicol_places[i] = "end"
          multicol_now = false
        end
      elseif is_multicols_begin(elem) then
        multicol_now = true
      elseif is_multicols_end(elem) then
        multicol_now = false
      end
    end
    if multicol_now then
      multicol_places[#doc.blocks] = "end"
    end

    -- apply multicol, in reverse order since it shifts later indices
    for i = #doc.blocks, 1, -1 do
      local state = multicol_places[i]
      if state ~= nil then
        if state == "begin" then
          doc.blocks:insert(i + 1, multicol_begin)
        elseif state == "renew" then
          doc.blocks:insert(i + 1, multicol_begin)
          doc.blocks:insert(i, multicol_end)
        elseif state == "end" then
          doc.blocks:insert(i, multicol_end)
        end
      end
    end

    if #multicol_places >= 1 then
      return doc
    end
  end
end