-- cgit commit-filter that turns git object names and configurable text
-- references in commit messages into links. Use it with the commit-filter or
-- repo.commit-filter setting and the lua: prefix.
--
--     commit-filter=lua:/path/to/link-commits.lua
--
-- cgit hands the filter the message already HTML-escaped, so this only wraps
-- matches in anchors. No external dependencies. Runs on Lua 5.1 through 5.4
-- and LuaJIT.
--
-- Two kinds of thing are linked, object names (runs of hex that look like git
-- hashes) and any number of text-reference rules you define, each a pattern
-- and a URL. Both are configured among the values below. All matches are resolved
-- in a single left-to-right pass, so nothing is ever linked twice.


-- Object names (git hashes). Handled specially, because the length rule cannot
-- be written as a plain Lua pattern.
--
-- Recognition is by shape, since a commit-filter cannot ask the repository
-- whether a hash is real. Any hex run within the length bounds is linked,
-- whatever mix of digits and letters it has, so abbreviated and all-digit
-- hashes are both caught. The cost is that a long hex-looking number can now
-- and then link to an object that does not exist, which cgit renders as a
-- harmless "bad object name" page. Shape matching is inherently approximate,
-- the length bounds are the only filter.
local objects = {
	-- Set false to stop linking bare hashes.
	enabled = true,
	-- A hex run within these lengths is linked. Git abbreviations run about 7
	-- to 12 characters, full names are 40 (sha1) or 64 (sha256).
	min_length = 7,
	max_length = 64,
	-- Link target, %s is replaced with the matched hash. "./?id=%s" is relative
	-- to the current page and works for the common virtual-root layout.
	url = "./?id=%s",
}

-- Text-reference rules. Each rule is a Lua pattern with ONE capture and a URL
-- where %s is replaced by that capture, percent-encoded. The whole match is
-- shown, the capture is what goes in the URL. Rules are tried in order and the
-- leftmost match on the line wins, so put more specific patterns first. Leave
-- the list empty to link only object names.
--
-- Lua patterns are not regular expressions. There is no alternation and no
-- {n,m} repetition. %d is a digit, %a a letter, %w a letter or digit, %x a hex
-- digit, and a literal magic character is escaped with %, so a literal '-' is
-- '%-'. Reference: https://www.lua.org/manual/5.1/manual.html#5.4.1
--
-- Patterns run against the escaped message, so '&', '<' and '>' reach them as
-- '&amp;', '&lt;' and '&gt;'. Match those entity spellings rather than the
-- bare character, and keep a pattern from ending part way through one, since
-- the matched run is what gets wrapped in the anchor.
local rules = {
	{ pattern = "#(%d+)", url = "https://bugs.example.com/?bug=%s" },
	-- { pattern = "CVE%-(%d%d%d%d%-%d+)", url = "https://www.cve.org/CVERecord?id=CVE-%s" },
	-- { pattern = "!(%d+)",               url = "https://gitlab.example.com/group/repo/-/merge_requests/%s" },
	-- { pattern = "RFC%s?(%d+)",          url = "https://www.rfc-editor.org/rfc/rfc%s" },
}


local chunks = {}

-- Percent-encode everything but the URL-unreserved characters, so a captured
-- value cannot break out of the href attribute or the URL.
local function url_encode(s)
	return (string.gsub(s, "[^%w._~-]", function(c)
		return string.format("%%%02X", string.byte(c))
	end))
end

-- Build one anchor. url_template has %s where the encoded capture goes, display
-- is the text shown. A function replacement is used so a '%' in the encoded
-- value is not treated as a gsub reference.
local function make_link(url_template, capture, display)
	local encoded = url_encode(capture)
	local href = string.gsub(url_template, "%%s", function() return encoded end)
	return '<a href="' .. href .. '">' .. display .. '</a>'
end

-- Collect every candidate match as {s, e, pri, link}. A lower pri wins a tie on
-- the same start position.
local function collect(text)
	local cands = {}
	for pri, rule in ipairs(rules) do
		-- A malformed pattern is an operator error, skip that rule rather than
		-- failing the whole page.
		pcall(function()
			local init = 1
			while init <= #text do
				local s, e, cap = string.find(text, rule.pattern, init)
				if not s then break end
				if cap == nil then
					cap = string.sub(text, s, e)
				end
				cands[#cands + 1] = {
					s = s, e = e, pri = pri,
					link = make_link(rule.url, cap, string.sub(text, s, e)),
				}
				init = (e >= s) and e + 1 or s + 1
			end
		end)
	end
	if objects.enabled then
		local objpri = #rules + 1
		local init = 1
		while init <= #text do
			local s, e, run = string.find(text, "%f[%w](%x+)%f[%W]", init)
			if not s then break end
			if #run >= objects.min_length and #run <= objects.max_length then
				cands[#cands + 1] = {
					s = s, e = e, pri = objpri,
					link = make_link(objects.url, run, run),
				}
			end
			init = e + 1
		end
	end
	return cands
end

function filter_open(...)
	chunks = {}
end

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

function filter_close()
	local text = table.concat(chunks)
	local cands = collect(text)
	table.sort(cands, function(a, b)
		if a.s ~= b.s then
			return a.s < b.s
		end
		return a.pri < b.pri
	end)
	local out = {}
	local i = 1
	for _, c in ipairs(cands) do
		-- Skip a candidate that overlaps one already emitted.
		if c.s >= i then
			out[#out + 1] = string.sub(text, i, c.s - 1)
			out[#out + 1] = c.link
			i = c.e + 1
		end
	end
	out[#out + 1] = string.sub(text, i)
	html(table.concat(out))
	return 0
end