-- center-v.lua
-- Pandoc Lua filter for converting ~V~ section break symbols
-- into properly centered paragraphs in EPUB/DOCX/HTML output.
--
-- Handles multiple input patterns that Obsidian might produce:
--   1. <center>\n~V~\n</center>  (HTML blocks wrapping subscript)
--   2. Raw ~V~ as a paragraph
--   3. The tilde-based subscript interpretation (~V~ → subscript V)

local function make_centered()
  if FORMAT == "epub" or FORMAT == "epub3" or FORMAT == "html" or FORMAT == "html5" then
    return pandoc.RawBlock("html", '<p style="text-align: center;">~V~</p>')
  elseif FORMAT == "docx" or FORMAT == "openxml" then
    return pandoc.RawBlock("openxml", [[
<w:p>
  <w:pPr><w:jc w:val="center"/></w:pPr>
  <w:r><w:t xml:space="preserve">~V~</w:t></w:r>
</w:p>
]])
  else
    return pandoc.Para({pandoc.Str("~V~")})
  end
end

function Pandoc(doc)
  local newblocks = {}
  local i = 1
  while i <= #doc.blocks do
    local b1 = doc.blocks[i]
    local b2 = doc.blocks[i+1]
    local b3 = doc.blocks[i+2]

    -- Pattern 1: <center> block + subscript V + </center> block
    if b1 and b2 and b3
       and b1.t == "RawBlock" and b1.format == "html" and b1.text:match("^<center>%s*$")
       and b2.t == "Plain"
       and #b2.content == 1
       and b2.content[1].t == "Subscript"
       and #b2.content[1].content == 1
       and b2.content[1].content[1].t == "Str"
       and b2.content[1].content[1].text == "V"
       and b3.t == "RawBlock" and b3.format == "html" and b3.text:match("^</center>%s*$")
    then
      table.insert(newblocks, make_centered())
      i = i + 3

    -- Pattern 2: Single paragraph containing just "~V~"
    elseif b1 and b1.t == "Para"
       and #b1.content == 1
       and b1.content[1].t == "Str"
       and b1.content[1].text == "~V~"
    then
      table.insert(newblocks, make_centered())
      i = i + 1

    -- Pattern 3: Plain block with subscript V (standalone, no center tags)
    elseif b1 and b1.t == "Plain"
       and #b1.content == 1
       and b1.content[1].t == "Subscript"
       and #b1.content[1].content == 1
       and b1.content[1].content[1].t == "Str"
       and b1.content[1].content[1].text == "V"
    then
      table.insert(newblocks, make_centered())
      i = i + 1

    -- Pattern 4: Para with Str "~" + Subscript "V" + Str "~" (tilde interpreted separately)
    elseif b1 and b1.t == "Para"
       and #b1.content == 3
       and b1.content[1].t == "Str" and b1.content[1].text == "~"
       and b1.content[2].t == "Subscript"
       and #b1.content[2].content == 1
       and b1.content[2].content[1].t == "Str"
       and b1.content[2].content[1].text == "V"
       and b1.content[3].t == "Str" and b1.content[3].text == "~"
    then
      table.insert(newblocks, make_centered())
      i = i + 1

    else
      table.insert(newblocks, b1)
      i = i + 1
    end
  end
  doc.blocks = newblocks
  return doc
end