local p = {}

local u = mw.ustring

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

local function strip_outer_braces(s)
	s = trim(s or "")
	-- remove one layer of surrounding { ... } if it matches as a whole
	if s:match("^%b{}$") then
		return s:sub(2, -2)
	end
	return s
end

local function strip_outer_quotes(s)
	s = trim(s or "")
	if s:match('^".*"$') then
		return s:sub(2, -2)
	end
	return s
end

local function normalize_value(s)
	s = strip_outer_quotes(strip_outer_braces(s))
	s = s:gsub("\\&", "&")
	s = s:gsub("~", " ")
	s = s:gsub("%s+", " ")
	return trim(s)
end

local function normalize_suffix(s)
	s = trim(s or "")
	if s == "" then return nil end
	s = s:gsub("%.$","") -- drop trailing dot for matching
	if s == "" then return nil end --in case it was just a dot
	local low = mw.ustring.lower(s)
	if low == "jr" then return "Jr." end -- Do we use . any more?
	if low == "sr" then return "Sr." end
	-- Roman numerals (II, III, IV, …)
	if s:match("^[IVXLCDM]+$") then return s end -- assuming caps
	return nil
end

local function normalize_pages(pages)
	pages = trim(pages or "")
	if pages == "" then return nil end
	-- TeX conventions: -- en dash, --- em dash
	pages = pages:gsub("%-%-%-", "—")
	pages = pages:gsub("%-%-", "–")
	return pages
end

-- Very small TeX->Unicode helper for common BibTeX name/title encodings.
local function tex_to_unicode(s)
	if not s or s == "" then return s end

	local accents = {
		["'"] = {a="á",e="é",i="í",o="ó",u="ú",y="ý",A="Á",E="É",I="Í",O="Ó",U="Ú",Y="Ý",c="ć",C="Ć",n="ń",N="Ń",s="ś",S="Ś",z="ź",Z="Ź"},
		["`"] = {a="à",e="è",i="ì",o="ò",u="ù",A="À",E="È",I="Ì",O="Ò",U="Ù"},
		['"'] = {a="ä",e="ë",i="ï",o="ö",u="ü",y="ÿ",A="Ä",E="Ë",I="Ï",O="Ö",U="Ü"},
		["^"] = {a="â",e="ê",i="î",o="ô",u="û",A="Â",E="Ê",I="Î",O="Ô",U="Û"},
		["~"] = {a="ã",n="ñ",o="õ",A="Ã",N="Ñ",O="Õ"},
		["c"] = {c="ç",C="Ç"},
		["v"] = {s="š",S="Š",z="ž",Z="Ž",c="č",C="Č",r="ř",R="Ř",n="ň",N="Ň"},
		["u"] = {a="ă",A="Ă",g="ğ",G="Ğ"},
		["H"] = {o="ő",O="Ő",u="ű",U="Ű"},
		["."] = {z="ż",Z="Ż"},
		["="] = {a="ā",A="Ā",e="ē",E="Ē",i="ī",I="Ī",o="ō",O="Ō",u="ū",U="Ū"},
	}

	-- \'{a} / \'a etc.
	s = s:gsub("\\([\"'`%^~%.=vucH])%s*%{?%s*([A-Za-z])%s*%}?", function(acc, letter)
		local m = accents[acc]
		return (m and m[letter]) or letter
	end)

	-- common ligatures/specials
	s = s:gsub("\\ae", "æ"):gsub("\\AE", "Æ")
	s = s:gsub("\\oe", "œ"):gsub("\\OE", "Œ")
	s = s:gsub("\\aa", "å"):gsub("\\AA", "Å")
	s = s:gsub("\\o", "ø"):gsub("\\O", "Ø")
	s = s:gsub("\\ss", "ß")

	-- remove brace-protection used for capitalization
	s = s:gsub("[{}]", "")

	return s
end

local function read_balanced_braces(src, i)
	-- src[i] must be "{"
	local n = #src
	local depth = 0
	local start_inner = nil
	local j = i
	while j <= n do
		local ch = src:sub(j, j)
		local prev = (j > 1) and src:sub(j-1, j-1) or ""

		if ch == "{" and prev ~= "\\" then
			depth = depth + 1
			if depth == 1 then
				start_inner = j + 1
			end
		elseif ch == "}" and prev ~= "\\" then
			depth = depth - 1
			if depth == 0 then
				local inner = src:sub(start_inner or (i+1), j - 1)
				return inner, j + 1
			end
		end
		j = j + 1
	end
	return nil, i -- unbalanced
end

local function read_quoted(src, i)
	-- src[i] must be '"'
	local n = #src
	local j = i + 1
	local out = {}
	while j <= n do
		local ch = src:sub(j, j)
		local prev = (j > 1) and src:sub(j-1, j-1) or ""
		if ch == '"' and prev ~= "\\" then
			return table.concat(out), j + 1
		end
		table.insert(out, ch)
		j = j + 1
	end
	return table.concat(out), i
end

local function skip_ws_and_commas(src, i)
	local n = #src
	while i <= n do
		local ch = src:sub(i, i)
		if ch == " " or ch == "\n" or ch == "\r" or ch == "\t" or ch == "," then
			i = i + 1
		else
			break
		end
	end
	return i
end

local function read_word(src, i)
	local n = #src
	local j = i
	while j <= n do
		local ch = src:sub(j, j)
		if ch:match("[%w_%-%:%.]") then
			j = j + 1
		else
			break
		end
	end
	return src:sub(i, j - 1), j
end

local function read_value(src, i, closing_char)
	i = skip_ws_and_commas(src, i)
	local ch = src:sub(i, i)
	if ch == "{" then
		local inner, ni = read_balanced_braces(src, i)
		return inner or "", ni
	elseif ch == '"' then
		local inner, ni = read_quoted(src, i)
		return inner or "", ni
	else
		-- bareword until comma or closing_char
		local n = #src
		local j = i
		while j <= n do
			local c = src:sub(j, j)
			if c == "," or c == closing_char then
				break
			end
			j = j + 1
		end
		return src:sub(i, j - 1), j
	end
end

local function parse_bibtex(src)
	src = trim(src or "")
	if src == "" then
		return nil, "empty input"
	end

	local at = src:find("@", 1, true)
	if not at then
		return nil, "no @ found"
	end
	src = src:sub(at)

	-- entry type
	local entry_type = src:match("^@%s*([%a]+)")
	if not entry_type then
		return nil, "cannot read entry type"
	end
	entry_type = u.lower(entry_type)

	-- opening delimiter
	local open_pos = src:find("{", 1, true) or src:find("(", 1, true)
	if not open_pos then
		return nil, "no opening { or ("
	end
	local open_ch = src:sub(open_pos, open_pos)
	local close_ch = (open_ch == "{") and "}" or ")"

	local i = open_pos + 1
	i = skip_ws_and_commas(src, i)

	-- citekey until first comma
	local comma = src:find(",", i, true)
	if not comma then
		return nil, "no comma after citekey"
	end
	local citekey = trim(src:sub(i, comma - 1))
	i = comma + 1

	local fields = {}
	while i <= #src do
		i = skip_ws_and_commas(src, i)
		local ch = src:sub(i, i)
		if ch == "" then break end
		if ch == close_ch then
			break
		end

		local name, ni = read_word(src, i)
		name = u.lower(trim(name or ""))
		i = ni

		i = skip_ws_and_commas(src, i)
		-- expect '=' (allow whitespace)
		if src:sub(i, i) == "=" then
			i = i + 1
		else
			-- skip forward to '=' if present
			local eq = src:find("=", i, true)
			if not eq then break end
			i = eq + 1
		end

		local rawv, nvi = read_value(src, i, close_ch)
		i = nvi

		local v = normalize_value(rawv)
		v = tex_to_unicode(v)

		if v ~= nil and v ~= "" then
			fields[name] = v
		end
	end

	return {
		entryType = entry_type,
		citekey = citekey,
		fields = fields
	}
end

local function month_to_name(m)
	if not m or m == "" then return nil end
	m = trim(m)
	local map = {
		["1"]="January",["2"]="February",["3"]="March",["4"]="April",["5"]="May",["6"]="June",
		["7"]="July",["8"]="August",["9"]="September",["10"]="October",["11"]="November",["12"]="December",
		jan="January",feb="February",mar="March",apr="April",may="May",jun="June",jul="July",aug="August",
		sep="September",sept="September",oct="October",nov="November",dec="December"
	}
	-- leading zeros in strings, numbers, etc.
	local n = tonumber(m)
	if n and n >= 1 and n <= 12 then
		return map[tostring(n)]
	end
	local low = u.lower(m)
	return map[low] or map[m] or m
end

local function split_authors(author_field)
	if not author_field or author_field == "" then return {} end
	local s = author_field

	-- split on " and " at brace depth 0
	local parts = {}
	local buf = {}
	local depth = 0
	local i = 1
	while i <= #s do
		local ch = s:sub(i,i)
		if ch == "{" then depth = depth + 1 end
		if ch == "}" and depth > 0 then depth = depth - 1 end

		if depth == 0 and string.lower(s:sub(i, i+4)) == " and " then
			table.insert(parts, trim(table.concat(buf)))
			buf = {}
			i = i + 5
		else
			table.insert(buf, ch)
			i = i + 1
		end
	end
	local last = trim(table.concat(buf))
	if last ~= "" then table.insert(parts, last) end

	local authors = {}
	for _, a in ipairs(parts) do
		a = trim(a)
		if a and a ~= "" then
			-- literal org author if whole thing brace-wrapped
			if a:match("^%b{}$") and not a:find(",") then
				table.insert(authors, { literal = strip_outer_braces(a) })
			elseif a:find(",") then
				local name_parts = mw.text.split(a, "%s*,%s*")
				if #name_parts == 3 then
					local last_name  = trim(name_parts[1] or "")
					local suffix     = normalize_suffix(name_parts[2])
					local first_name
					if (suffix) then
						first_name = trim(name_parts[3] or "")
					else
						first_name = trim(name_parts[2] or "")
						suffix     = normalize_suffix(name_parts[3])	
					end
					if suffix then
					    first_name  = first_name .. ' ' .. suffix
					end
					table.insert(authors, { last = last_name, first = first_name})
				else
					local first_chunk, last_chunk = a:match("^(.-),%s*(.+)$")
					local suffix = normalize_suffix(last_chunk)
					local first_name, last_name
					if (suffix) then
						-- if we can split "first_chunk" (note already in first last order)
						first_name, last_name = first_chunk:match("^(.-)%s+(.+)$")
						first_name = first_name .. ' ' .. suffix
					else
						first_name = last_chunk
						last_name  = first_chunk
					end
					table.insert(authors, { last = trim(last_name or ""), first = trim(first_name or "") })
				end
			else
				local tokens = mw.text.split(a, "%s+")
				local suffix = normalize_suffix(tokens[#tokens])
				if suffix then table.remove(tokens) end
				-- "First von Last" heuristic: fold trailing lowercase particles into last name
				if #tokens == 1 then
					table.insert(authors, { literal = tokens[1] })
				else
					local j = #tokens - 1
					while j >= 1 and tokens[j]:match("^[%l][%l%-%']*$") do
						j = j - 1
					end
					local first_name = table.concat(tokens, " ", 1, j)
					if (suffix) then
						first_name = first_name .. ' ' .. suffix
					end
					local last_name = table.concat(tokens, " ", j + 1, #tokens)
					table.insert(authors, { last = trim(last_name), first = trim(first_name) })
				end
			end
		end
	end
	return authors
end

-- Numbers from Module:Citation/CS1/Configuration - may need updating sometimes
local FREE_DOI_REGISTRANTS = {
	['1045'] =1, ['1074'] =1, ['1096'] =1, ['1100'] =1, ['1155'] =1, ['1186'] =1, ['1194'] =1, ['1371'] =1, ['1613'] =1, ['1629'] =1, ['1989'] =1, ['1999'] =1, ['2147'] =1,
	['2196'] =1, ['3285'] =1, ['3389'] =1, ['3390'] =1, ['3906'] =1, ['3748'] =1, ['3814'] =1, ['3847'] =1, ['3897'] =1, ['4061'] =1, ['4089'] =1, ['4103'] =1, ['4172'] =1,
	['4175'] =1, ['4230'] =1, ['4236'] =1, ['4239'] =1, ['4240'] =1, ['4249'] =1, ['4251'] =1, ['4252'] =1, ['4253'] =1, ['4254'] =1, ['4291'] =1, ['4292'] =1, ['4329'] =1,
	['4330'] =1, ['4331'] =1, ['5086'] =1, ['5194'] =1, ['5210'] =1, ['5306'] =1, ['5312'] =1, ['5313'] =1, ['5314'] =1, ['5315'] =1, ['5316'] =1, ['5317'] =1, ['5318'] =1,
	['5319'] =1, ['5320'] =1, ['5321'] =1, ['5334'] =1, ['5402'] =1, ['5409'] =1, ['5410'] =1, ['5411'] =1, ['5412'] =1, ['5492'] =1, ['5493'] =1, ['5494'] =1, ['5495'] =1,
	['5496'] =1, ['5497'] =1, ['5498'] =1, ['5499'] =1, ['5500'] =1, ['5501'] =1, ['5527'] =1, ['5528'] =1, ['5662'] =1, ['6064'] =1, ['6219'] =1, ['7167'] =1, ['7217'] =1,
	['7287'] =1, ['7482'] =1, ['7490'] =1, ['7554'] =1, ['7717'] =1, ['7759'] =1, ['7766'] =1, ['9778'] =1, ['11131']=1, ['11569']=1, ['11647']=1, ['11648']=1, ['12688']=1,
	['12703']=1, ['12715']=1, ['12942']=1, ['12998']=1, ['13105']=1, ['14256']=1, ['14293']=1, ['14303']=1, ['15215']=1, ['15347']=1, ['15412']=1, ['15560']=1, ['16995']=1,
    ['17645']=1, ['18637']=1, ['19080']=1, ['19173']=1, ['20944']=1, ['21037']=1, ['21105']=1, ['21468']=1, ['21767']=1, ['21425']=1, ['22261']=1, ['22323']=1, ['22459']=1,
    ['24105']=1, ['24196']=1, ['24966']=1, ['26775']=1, ['30845']=1, ['32545']=1, ['35711']=1, ['35712']=1, ['35713']=1, ['35995']=1, ['36648']=1, ['37126']=1, ['37532']=1,
    ['37871']=1, ['47128']=1, ['47622']=1, ['47959']=1, ['52437']=1, ['52975']=1, ['53288']=1, ['53347']=1, ['54081']=1, ['54947']=1, ['55667']=1, ['55914']=1, ['57009']=1,
    ['58647']=1, ['59081']=1,
}

local function doi_registrant(doi)
	-- expects canonical "10.<registrant>/<suffix>"
	return doi and doi:match("^10%.(%d+)/")
end

local function is_free_doi(doi)
	local r = doi_registrant(doi)
	return r and FREE_DOI_REGISTRANTS[r] or false
end

local function strip_doi(doi)
	if not doi or doi == "" then return nil end
	doi = trim(doi)
	doi = doi:gsub("^https?://(dx%.)?doi%.org/", "")
	doi = doi:gsub("^doi:%s*", "")
	return doi
end

local function wikitext_escape(s)
	if not s or s == "" then return s end
	-- keep it simple: avoid breaking template params
	s = s:gsub("|", "&#124;")
	return s
end

local cite_template_for_type = {
	article = "Cite journal",
	book = "Cite book",
	inbook = "Cite book",
	incollection = "Cite book",
	inproceedings = "Cite conference",
	conference = "Cite conference",
	techreport = "Cite report",
	report = "Cite report",
	phdthesis = "Cite thesis",
	mastersthesis = "Cite thesis",
	thesis = "Cite thesis",
	misc = "Cite web",
}

local function build_cite(data, forced_type)
	local f = data.fields
	local et = forced_type or data.entryType
	local tpl = cite_template_for_type[et] or "Cite journal"

	local params = {}

	-- authors
	local authors = split_authors(f.author)
	for i, a in ipairs(authors) do
		if a.literal then
			params["author" .. i] = a.literal
		else
			if a.last and a.last ~= "" then params["last" .. i] = a.last end
			if a.first and a.first ~= "" then params["first" .. i] = a.first end
		end
	end

	-- date/year/month
	local year = f.year
	local month = month_to_name(f.month)
	if year and month then
		params.date = month .. " " .. year
	elseif f.date then
		params.date = f.date
	elseif year then
		params.year = year
	end

	-- title-ish fields
	if f.title then params.title = f.title end
	if f.subtitle and not params.title then params.title = f.subtitle end

	-- container fields by type
	if tpl == "Cite journal" then
		if f.journal then params.journal = f.journal end
		if f.volume then params.volume = f.volume end
		if f.number then params.issue = f.number end
	elseif tpl == "Cite book" then
		if f.publisher then params.publisher = f.publisher end
		if f.address then params.location = f.address end
		if f.location then params.location = f.location end
		if f.edition then params.edition = f.edition end
		if f.series then params.series = f.series end
		if f.booktitle then params.title = f.booktitle end
	elseif tpl == "Cite conference" then
		if f.booktitle then params.booktitle = f.booktitle end
		if f.organization then params.publisher = f.organization end
	end

	-- common bibliographic bits
	if f.pages and f.pages ~= "{}" then
		local pg = normalize_pages(f.pages)
		if pg then params.pages = pg end
	end
	if f.url then params.url = f.url end
	if f.doi then
		local doi = strip_doi(f.doi)
		if doi and doi ~= "" then
			params.doi = doi
			if is_free_doi(doi) then
				params["doi-access"] = "free"
			end
		end
	end
	if f.isbn then params.isbn = f.isbn end
	if f.issn then params.issn = f.issn end
	if f.publisher and not params.publisher then params.publisher = f.publisher end

	-- stable-ish ordering for output
	local ordered = {}

	local function add(k)
		if params[k] and params[k] ~= "" then
			table.insert(ordered, {k = k, v = params[k]})
		end
	end

	-- author params (last/first or authorN)
	for i = 1, #authors do
		add("last" .. i); add("first" .. i); add("suffix" .. i); add("author" .. i)
	end

	add("date"); add("year")
	add("title")
	add("journal"); add("booktitle")
	add("publisher"); add("location")
	add("volume"); add("issue")
	add("pages")
	add("doi"); add("doi-access")
	add("isbn"); add("issn")
	add("url")

	local out = { "{{", tpl }
	for _, kv in ipairs(ordered) do
		table.insert(out, "|" .. kv.k .. "=" .. wikitext_escape(kv.v))
	end
	table.insert(out, "}}")
	return table.concat(out)
end

local function lc_or_nil(s)
	s = trim(s)
	if not s or s == "" then
		return nil
	end
	return mw.ustring.lower(s)  -- non-empty string
end

function p.main(frame)
	local args = frame.args
	local parent = frame:getParent() and frame:getParent().args or {}

	local bib = args[1]
	if (not bib or bib == "") and parent[1] and parent[1] ~= "" then
		bib = parent[1]
	end
	if (not bib or bib == "") then
		bib = args.BibTeX or args.BibTex or args.bibtex or parent.BibTeX or parent.BibTex or parent.bibtex
	end

    local forced = lc_or_nil(args.type) or lc_or_nil(parent.type)

	local debug = args.debug or parent.debug

	local data, err = parse_bibtex(bib or "")
	if not data then
		return '<span class="error">BibTeX parse error: ' .. wikitext_escape(err or "unknown") .. "</span>"
	end

	if debug and tostring(debug) ~= "" then
		return mw.text.nowiki(mw.text.jsonEncode(data))
	end
	
	local cite_wikitext = build_cite(data, forced)
	if lc_or_nil(args.display) or lc_or_nil(parent.display) then
		return "<nowiki>" .. cite_wikitext .. "</nowiki>"
	end

	return frame:preprocess(cite_wikitext)
end

return p