-- 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 its place.
--
-- SUPPORTED LUA
--
-- Lua 5.1 through 5.4 and LuaJIT. The Scintillua version matters too. Recent
-- Scintillua (6.x) needs Lua 5.3 or newer to load its lexers, older Scintillua
-- releases still load under 5.1 and 5.2. Pick a Scintillua release that matches
-- the Lua cgit is built against.
--
-- 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.
--
-- 1. lpeg, the parsing module, for the Lua 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
--        # Fedora
--        sudo dnf install lua-lpeg
--        # Alpine
--        sudo apk add lua5.1-lpeg
--        # or with LuaRocks, matched to your Lua version
--        sudo 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
--     the scintillua_dirs list in the CONFIGURATION block below
--
-- so either set the variable in the web server environment, or place (or
-- symlink) the scintillua directory next to your cgitrc.
--
-- SECURITY
--
-- Every probed directory is placed on package.path and its Lua is executed in
-- cgit's process. Make sure none of them is writable by other users, or someone
-- who can write there gains code execution as the web server. On macOS in
-- particular, /opt/homebrew/share is group-writable by default.
--
-- LIMITATIONS
--
-- cgit sends the filter output through a C string sink that stops at the first
-- NUL byte, so a blob containing a NUL is truncated there. This affects binary
-- files that slip past cgit's text detection, not ordinary source.
--
-- OUTPUT
--
-- Tokens are wrapped in <span> elements carrying the hl- classes that
-- assets/cgit.css styles. Every input byte up to the first NUL is preserved, so
-- the line number gutter stays aligned.

--
-- ===== CONFIGURATION =====
--

-- Files larger than this many bytes are served escaped but unhighlighted, so a
-- huge blob does not cost a lexing pass. Kept well below cgit's max-blob-size.
local max_bytes = 512 * 1024

-- Environment variable that, when set, points straight at the Scintillua
-- lexers directory and is used alone.
local scintillua_env = "CGIT_SCINTILLUA_PATH"

-- Directories probed for the lexers when that variable is not set. The
-- directory of $CGIT_CONFIG, when set, is tried ahead of these. Keep every one
-- of these unwritable by others, see the SECURITY note above.
local scintillua_dirs = {
	"/usr/local/share/scintillua/lexers",
	"/usr/share/scintillua/lexers",
	"/opt/homebrew/share/scintillua/lexers",
}

-- Scintillua tag name (its first dotted component) to a cgit css class. Only
-- the six classes below exist in assets/cgit.css. Add a class there and a row
-- here to style more token kinds. Tokens with no row render as plain text,
-- which is what most themes want for operators and identifiers.
local css = {
	comment      = "hl-comment",
	string       = "hl-string",
	regex        = "hl-string",
	number       = "hl-number",
	constant     = "hl-number",
	keyword      = "hl-keyword",
	preprocessor = "hl-keyword",
	tag          = "hl-keyword",
	label        = "hl-keyword",
	annotation   = "hl-keyword",
	type         = "hl-type",
	class        = "hl-type",
	attribute    = "hl-type",
	["function"] = "hl-func",
}

-- Extension to lexer-name fixes for the fallback path, used only when this
-- Scintillua has no detect(). Most extensions already equal their lexer name,
-- these are the frequent exceptions. A wrong guess just falls back to plain
-- text, so there is no harm in listing best-effort entries.
local ext_lexer = {
	py = "python", js = "javascript", ts = "typescript",
	rb = "ruby", pl = "perl", pm = "perl", sh = "bash",
	md = "markdown", htm = "html", yml = "yaml",
	rs = "rust", c = "ansi_c", h = "ansi_c",
}

--
-- =========================
--

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

local escape_map = { ["&"] = "&amp;", ["<"] = "&lt;", [">"] = "&gt;" }

-- Escape the three HTML metacharacters in a single pass.
local function escape(s)
	return (string.gsub(s, "[&<>]", escape_map))
end

local function scintillua_path()
	local env = os.getenv(scintillua_env)
	if env then
		return env
	end
	local candidates = {}
	local config = os.getenv("CGIT_CONFIG")
	if config then
		local dir = string.match(config, "^(.*)/[^/]+$")
		if dir then
			candidates[#candidates + 1] = dir .. "/scintillua/lexers"
		end
	end
	for _, d in ipairs(scintillua_dirs) do
		candidates[#candidates + 1] = d
	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

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

local function load_lexer_name(name)
	if name == nil then
		return nil
	end
	local ok, lex = pcall(lexer_mod.load, name)
	if ok and lex then
		return lex
	end
	return nil
end

-- Resolve a lexer for the file, preferring Scintillua's own filename detection
-- when this version provides it, then an extension map, then the raw extension.
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 lex = load_lexer_name(lang)
			if lex then
				return lex
			end
		end
	end
	local ext = string.match(name, "%.([^.]+)$")
	if not ext then
		return nil
	end
	ext = string.lower(ext)
	return load_lexer_name(ext_lexer[ext]) or load_lexer_name(ext)
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
	-- Fallback, escaped plain text emitted in slices so a large blob does not
	-- cost a full-size second copy all at once.
	local n = #text
	if n == 0 then
		html("")
		return 0
	end
	local pos = 1
	while pos <= n do
		html(escape(string.sub(text, pos, pos + 65535)))
		pos = pos + 65536
	end
	return 0
end