Module:Auto compact TOC

require('strict')
local p = {}
local horizontal = require('Module:List').horizontal
local start = [[
__NOTOC__<div id="toc" class="toc plainlinks" role="navigation" aria-labelledby="mw-toc-heading">
<div class="toctitle"><h2 id="mw-toc-heading">Contents</h2></div>
]]

local close = [[</div>]]

local function make_TOC_item(anchor, link_text)
	link_text = link_text or anchor
	return ("[[#%s|%s]]"):format(anchor, link_text)
end

local Array_mt = { __index = table }
local function Array()
	return setmetatable({}, Array_mt)
end

function p.make_TOC(frame)
	local content = mw.title.getCurrentTitle():getContent()
	
	if not content then
		error "The current page has no content"
	end
	
	local letters = Array()
	-- Find uppermost headers containing a single ASCII letter.
	for letter in content:gmatch "%f[^\n%z]==%s*(%a)%s*==%f[^=]" do
		letter = letter:upper()
		letters:insert(make_TOC_item(letter))
	end
	
	local yesno = require('Module:Yesno')
	local rest = Array()
	local other_headers = require('Module:TableTools').listToSet{
		"See also", "References", "Notes", "Further reading", "External links",
	}
	for header in content:gmatch "%f[^\n%z]==%s*(..-)%s*==%f[^=]" do
		if other_headers[header] then
			rest:insert(make_TOC_item(header))
		end
	end
	
	return start .. horizontal(letters) .. horizontal(rest) .. close
end

return p