-- Server-side syntax highlighting for the tree and blob views, used with
-- the source-filter setting in cgitrc and the lua: prefix so it runs in
-- cgit's embedded interpreter with no per-request process.
--
--     source-filter=lua:/usr/lib/cgit/extensions/syntax-highlight.lua
--
-- Highlighting is deliberately not built into cgit itself. Without this
-- filter cgit serves plain escaped text, and any other program can take
-- this filter's place.
--
-- REQUIREMENTS
--
-- Two pieces, and BOTH must be installed. When either is missing the
-- filter serves plain escaped text by design, so uncolored code means
-- a missing dependency, not an error. Works with Lua 5.1 onward,
-- including LuaJIT.
--
-- 1. lpeg, the parsing module for the Lua that cgit is linked
--    against. Scintillua does NOT bundle it, it must come from the
--    system, and forgetting it is the usual reason nothing happens.
--
--        Debian and Ubuntu   sudo apt install lua-lpeg
--        macOS               luarocks --lua-version 5.1 install lpeg
--
-- 2. Scintillua, the lexer collection from the Textadept editor.
--    Roughly 120 languages as plain .lua files, nothing to compile.
--    Download a release and unpack it anywhere.
--
--        https://orbitalquark.github.io/scintillua/
--
-- The lexers are found by probing, in order
--
--     $CGIT_SCINTILLUA_PATH        (used alone when set, no fallback)
--     <dir of $CGIT_CONFIG>/scintillua/lexers
--     /usr/local/share/scintillua/lexers
--     /usr/share/scintillua/lexers
--     /opt/homebrew/share/scintillua/lexers
--
-- so either set the variable in the web server environment, or place
-- (or symlink) the scintillua directory next to your cgitrc. When
-- Scintillua or lpeg is missing the filter passes text through escaped
-- and unhighlighted, so it is safe to enable before the dependencies
-- are installed.
--
-- OUTPUT
--
-- Tokens are wrapped in <span> elements carrying the hl- classes that
-- assets/cgit.css styles. Every input byte is preserved, so the line
-- number gutter stays aligned.

-- Files larger than this are passed through without lexing.
local max_bytes = 512 * 1024

local function scintillua_path()
	local env = os.getenv("CGIT_SCINTILLUA_PATH")
	if env then
		return env
	end
	local candidates = {
		"/usr/local/share/scintillua/lexers",
		"/usr/share/scintillua/lexers",
		"/opt/homebrew/share/scintillua/lexers",
	}
	local config = os.getenv("CGIT_CONFIG")
	if config then
		local dir = string.match(config, "^(.*)/[^/]+$")
		if dir then
			table.insert(candidates, 1, dir .. "/scintillua/lexers")
		end
	end
	for _, dir in ipairs(candidates) do
		local f = io.open(dir .. "/lexer.lua", "r")
		if f then
			f:close()
			return dir
		end
	end
	return nil
end

-- Scintillua tag name (first dotted component) to cgit css class.
local css = {
	comment      = "hl-comment",
	string       = "hl-string",
	regex        = "hl-string",
	number       = "hl-number",
	keyword      = "hl-keyword",
	preprocessor = "hl-keyword",
	type         = "hl-type",
	class        = "hl-type",
	constant     = "hl-number",
	["function"] = "hl-func",
}

local lexer_mod = nil
local filename = ""
local chunks = {}

local function escape(s)
	s = string.gsub(s, "&", "&amp;")
	s = string.gsub(s, "<", "&lt;")
	s = string.gsub(s, ">", "&gt;")
	return s
end

local function load_scintillua()
	local dir = scintillua_path()
	if not dir then
		return nil
	end
	if not string.find(package.path, dir, 1, true) then
		package.path = dir .. "/?.lua;" .. package.path
	end
	local ok, mod = pcall(require, "lexer")
	-- A real Scintillua exposes load(); anything else on the path
	-- that happens to be called lexer is not usable.
	if ok and type(mod) == "table" and type(mod.load) == "function" then
		return mod
	end
	return nil
end

-- Resolve a lexer for the file, preferring Scintillua's own filename
-- detection when this version provides it.
local function lexer_for(name)
	if type(lexer_mod.detect) == "function" then
		local ok, lang = pcall(lexer_mod.detect, name)
		if ok and lang then
			local okl, lex = pcall(lexer_mod.load, lang)
			if okl then
				return lex
			end
		end
		return nil
	end
	local ext = string.match(name, "%.([^.]+)$")
	if not ext then
		return nil
	end
	local ok, lex = pcall(lexer_mod.load, string.lower(ext))
	if ok then
		return lex
	end
	return nil
end

local function highlight(text)
	local lex = lexer_for(filename)
	if not lex then
		return nil
	end
	local ok, tokens = pcall(lex.lex, lex, text)
	if not ok or type(tokens) ~= "table" then
		return nil
	end
	local out = {}
	local pos = 1
	for i = 1, #tokens, 2 do
		local tag = tokens[i]
		local fin = tokens[i + 1]
		local part = escape(string.sub(text, pos, fin - 1))
		local class = css[string.match(tag, "^[%w_]+")]
		if class and part ~= "" then
			part = "<span class='" .. class .. "'>" .. part .. "</span>"
		end
		out[#out + 1] = part
		pos = fin
	end
	-- Anything the lexer left unconsumed is kept, escaped.
	if pos <= #text then
		out[#out + 1] = escape(string.sub(text, pos))
	end
	return table.concat(out)
end

function filter_open(name)
	filename = name or ""
	chunks = {}
end

function filter_write(str)
	chunks[#chunks + 1] = str
end

function filter_close()
	local text = table.concat(chunks)
	chunks = {}
	if #text <= max_bytes then
		if lexer_mod == nil then
			lexer_mod = load_scintillua() or false
		end
		if lexer_mod then
			local ok, marked = pcall(highlight, text)
			if ok and marked then
				html(marked)
				return 0
			end
		end
	end
	html(escape(text))
	return 0
end