local p = {}

local function trim(s)
	if s == nil then return nil end
	return mw.text.trim(tostring(s))
end

local function is_nonempty(s)
	return s ~= nil and s ~= ''
end

-- Fetches P3837 ("US public law number") from Wikidata for a QID.
-- Returns the raw string value (e.g., "117-1") or nil if missing/unreadable.
local function get_p3837(qid)
	if not is_nonempty(qid) then return nil end
	if not mw.wikibase or not mw.wikibase.entityExists(qid) then return nil end

	local stmts = mw.wikibase.getBestStatements(qid, 'P3837')
	if not stmts or not stmts[1] then return nil end

	local snak = stmts[1].mainsnak
	if not snak or snak.snaktype ~= 'value' then return nil end

	local dv = snak.datavalue
	if not dv or dv.type ~= 'string' then return nil end

	return trim(dv.value)
end

-- Parses a public-law number string in the form "<congress>-<law>" (allowing whitespace).
-- Returns congress, law as strings (digits only), or nil on failure.
local function parse_plaw_number(plaw)
	if not is_nonempty(plaw) then return nil end
	local a, b = tostring(plaw):match('^%s*(%d+)%s*%-%s*(%d+)%s*$')
	if not a or not b then return nil end
	return a, b
end

-- Builds a govinfo "link-docs" URL. For our context we treat plaw/statute as sharing the same path shape,
-- differing only by the collection segment, per user requirement.
local function build_url(collection, congress, law, linktype)
	collection = trim(collection) or 'plaw'
	linktype = trim(linktype) or 'uslm'

	local base = 'https://www.govinfo.gov/link/' .. collection .. '/' .. congress .. '/public/' .. law

	-- Govinfo defaults to PDF if link-type is omitted, but this template defaults to USLM.
	-- If caller explicitly blanks |type=, we omit link-type= entirely (and the result becomes govinfo's default PDF).
	local url
	if is_nonempty(linktype) then
		url = base .. '?link-type=' .. mw.uri.encode(linktype, 'PATH')
	else
		url = base
	end

	-- MediaWiki filetype hinting: for PDFs, append "&.pdf" (govinfo ignores it, MW uses it).
	-- Only do this when link-type is exactly "pdf".
	if linktype == 'pdf' then
		if url:find('?', 1, true) then
			url = url .. '&.pdf'
		else
			url = url .. '?.pdf'
		end
	end

	return url
end

local function default_label(congress, law)
	-- Keep label identical regardless of collection.
	-- Use an en dash in the rendered label.
	return 'Pub. L. ' .. congress .. '–' .. law
end

function p.main(frame)
	local args = frame.args or {}
	local collection = trim(args.collection) or 'plaw'
	local qid = trim(args.qid)
	local id_override = trim(args.id)
	local linktype = args.type
	if linktype ~= nil then linktype = trim(linktype) end -- allow blanking via |type=

	-- Default to the page's Wikidata item when no QID is supplied.
	if not is_nonempty(qid)
		and mw.wikibase
		and mw.wikibase.getEntityIdForCurrentPage
	then
		qid = mw.wikibase.getEntityIdForCurrentPage()
	end

	local plaw_raw = nil
	if is_nonempty(id_override) then
		plaw_raw = id_override
	else
		plaw_raw = get_p3837(qid)
	end

	local congress, law = parse_plaw_number(plaw_raw)
	if not congress or not law then
		-- Fail closed: emit a small, explicit error instead of malformed output.
		-- No extra whitespace/newlines.
		return '<span class="error">Govinfo: missing/invalid P3837 (expected “&lt;congress&gt;-&lt;law&gt;”).</span>'
	end

	local url = build_url(collection, congress, law, linktype)

	local label = trim(args.label)
	if not is_nonempty(label) then
		label = default_label(congress, law)
	end

	return '[' .. url .. ' ' .. label .. ']'
end

return p