A Pandoc writer.

§1. Usage.

pandoc from=markdown+lists_without_preceding_blankline-smart --to=inform7.lua "examples/Cloak of Darkness.ni.md"

§2. Setup.

§3. First, we avoid some boilerplate using the scaffolding module. 

Writer = pandoc.scaffolding.Writer

§4. Then we detect whether we are writing an Inform 7 story or extension. 

for _, i in ipairs(PANDOC_STATE.input_files) do
    local ext = pandoc.List(i:gmatch "[^%.]+"):at(-2)
    if ext == "ni" or ext == "i7" then i7_story = true end
    if ext == "i7x" then i7_ext = true end
end

Writer.Extensions = {
    smart = false,
    mark = true,
    alert = true,
    lists_without_preceding_blankline = true,
}

§5. Then we come to the parsing.

§6. Plain content.What it says on the tin. 

Writer.Block.Para = function(para)
    return Writer.Inlines(para.content)
    return {Writer.Inlines(para.content)}
end

Writer.Inline.Space = function(str)
    return " "
end

Writer.Block.Plain = function(p)
    return Writer.Inlines(p.content)
end

Writer.Inline.SoftBreak = function(str)
    return "\n"
end

§7. Strings.We keep track of whether we are inside a quote, to handle bracket syntax. 

local insidequote = false

Writer.Inline.Str = function(str)
    if (str.text:find("^\"") or str.text:find("\"$")) and not str.text:find("^\"[^\"]*\"$") then
        insidequote = not insidequote
    end
    return str.text
end

§8. Quoted.And this does the same, but for quoted text in a document with smart quotes. (It's best to read in Inform 7 source with smart quotes disabled, but we support it anyway.) 

Writer.Inline.Quoted = function(str)
    insidequote = true
    local result = Writer.Inlines(str.content)
    insidequote = false
    return "\"" .. result .. "\""
end

§9. Inline code.We use inline code spans for two things: Text substitutions (inside quotes) and Inform 6 code (outside quotes). 

Writer.Inline.Code = function(code)
    if insidequote then
        return "[" .. code.text .. "]"
    else
        return "(- " .. code.text .. " -)"
    end
end

§10. Comments.Inform 7 comments are written as square brackets. We can support a few different markup elements as comments, like strikeout/strikethrough (for code that's commented out) and block quotes (for explanatory comments). 

Writer.Inline.Strikeout = function(strike)
    return "[" .. Writer.Inlines(strike.content) .. "]"
end

Writer.Block.BlockQuote = function(bq)
    return "[\n" .. Writer.Blocks(bq.content) .. "\n]"
end

§11. Code blocks.Code blocks in Inform 7 are either included Inform 6 code, or Preform grammar. 

 TODO indent each line
Writer.Block.CodeBlock = function(code)
    local after = ""
    if code.attr.classes[1] == "preform" then
        after = " in the Preform grammar"
    end

    return "Include (-\n" .. code.text .. "\n-)" .. after .. "."
end

§12. Headings.Markdown has 6 heading levels. Inform 7 has 5, but it also has a story title.

A mnemonic to remember the levels is "Very Bad People Choose Sin". 

local levels = {
    "Volume",
    "Book",
    "Part",
    "Chapter",
    "Section"
}

§13. Then we convert the headings, keeping track of how many top-level headings we have seen, to avoid converting a document with more than one story title. 

local h1 = 0

Writer.Block.Header = function(h)
    if h.level == 1 then
        h1 = h1 + 1
        if i7_story then
            if h1 == 1 then
                 TODO If title is not wrapped in quotes
                return "\"" .. pandoc.utils.stringify(h.content) .. "\""
            else
                error("Only one top-level heading allowed in Inform 7 story files (it should be used for the story title and author).")
            end
        elseif i7_ext then
            if h1 == 1 then
                return Writer.Inlines(h.content) .. " begins here."
            elseif h1 == 1 then
                return string.rep("-", 4) .." Documentation " .. string.rep("-", 4)
            else
                error("Only two top-level headings allowed in Inform 7 extensions (it should be used for the extension title and, optionally, the documentation header).")
            end
        end
    end
    return levels[h.level - 1] .. " - " .. Writer.Inlines(h.content)
end

§14. Lists.Since Markdown and Pandoc don't preserve indents, we use lists to represent indented blocks. 

local indent_level = 0

Writer.Block.BulletList = function(bl)
    local result = ""
    indent_level = indent_level + 1
    for i, item in ipairs(bl.content) do
        result = result .. string.rep("\t", indent_level) .. Writer.Blocks(item)
    end
    indent_level = indent_level - 1
    return result .. "\n"
end

Writer.Block.OrderedList = Writer.Block.BulletList

§15. And then we have to make sure that Pandoc does not insert extra blank lines before lists. 

Writer.Blocks = function(blocks, sep)
    sep = sep or pandoc.layout.blankline
    local result = ""
    for i, block in ipairs(blocks) do
        local sep_foo = sep
        if i == #blocks or ((block.tag == "Para" or block.tag == "Plain") and blocks[i+1] and (blocks[i+1].tag == "BulletList" or blocks[i+1].tag == "OrderedList")) then
            sep_foo = "\n"
        end
        result = result .. Writer.Block(block) .. sep_foo
    end
    return result
end