-- cgit commit-filter that turns git object names and configurable text
-- references in a commit message into links, named by the commit-filter or
-- repo.commit-filter setting in cgitrc. cgit hands over the message already
-- HTML-escaped, so all this does is wrap matches in anchors. Every match is
-- resolved in one left-to-right pass, so nothing is ever linked twice. The
-- two tables below are the whole configuration, and the filter runs on Lua
-- 5.1 through 5.4 and LuaJIT with nothing outside the standard library.
--
--     commit-filter=lua:/path/to/link-commits.lua


-- Object names are handled apart from the rules below because the length
-- bound on them 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, so any hex run within the bounds is linked whatever mix of digits and
-- letters it has and abbreviated and all-digit names are both caught. The
-- cost is that a long hex-looking number now and then links to an object that
-- does not exist, which cgit renders as a harmless "bad object name" page.
local objects = {
	-- Set false to stop linking bare hashes.
	enabled = true,
	-- Git abbreviations run about 7 to 12 characters, and a full name is 40
	-- characters for sha1 or 64 for sha256.
	min_length = 7,
	max_length = 64,
	-- Link target, where %s is replaced with the matched hash. The relative
	-- form is resolved against the current page and works for the common
	-- virtual-root layout.
	url = "./?id=%s",
}

-- Text-reference rules, each one a Lua pattern with a single capture and a
-- URL where %s is replaced by that capture, percent-encoded. The whole match
-- is what gets shown and the capture is only what goes into the URL. Rules are
-- tried in order and the leftmost match on the line wins, so put the more
-- specific patterns first, and an empty list leaves only object names linked.
--
-- 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
-- dash is '%-'. The whole set is in the reference manual at
-- 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 out of the URL itself.
local function url_encode(s)
	return (string.gsub(s, "[^%w._~-]", function(c)
		return string.format("%%%02X", string.byte(c))
	end))
end

-- Build one anchor from a URL template holding %s and the text to show. The
-- replacement is a function so that a '%' in the encoded value is not taken
-- for 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 in the message. Priority records which rule
-- found it, and the lower priority wins a tie on the same start position.
local function collect(text)
	local candidates = {}
	for priority, rule in ipairs(rules) do
		-- A malformed pattern is an operator error, so skip that
		-- rule rather than fail the whole page.
		pcall(function()
			local init = 1
			while init <= #text do
				local start, stop, capture =
					string.find(text, rule.pattern, init)
				if not start then break end
				if capture == nil then
					capture = string.sub(text, start, stop)
				end
				candidates[#candidates + 1] = {
					start = start, stop = stop, priority = priority,
					link = make_link(rule.url, capture,
						string.sub(text, start, stop)),
				}
				-- An empty match still has to advance the
				-- scan, or it never reaches the end.
				init = (stop >= start) and stop + 1 or start + 1
			end
		end)
	end
	if objects.enabled then
		local priority = #rules + 1
		local init = 1
		while init <= #text do
			local start, stop, run =
				string.find(text, "%f[%w](%x+)%f[%W]", init)
			if not start then break end
			if #run >= objects.min_length and #run <= objects.max_length then
				candidates[#candidates + 1] = {
					start = start, stop = stop, priority = priority,
					link = make_link(objects.url, run, run),
				}
			end
			init = stop + 1
		end
	end
	return candidates
end

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

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

function filter_close()
	local text = table.concat(chunks)
	local candidates = collect(text)
	table.sort(candidates, function(a, b)
		if a.start ~= b.start then
			return a.start < b.start
		end
		return a.priority < b.priority
	end)
	local out = {}
	local pos = 1
	for _, candidate in ipairs(candidates) do
		-- A candidate reaching back into one already emitted is
		-- dropped, so no run of text is ever wrapped twice.
		if candidate.start >= pos then
			out[#out + 1] = string.sub(text, pos, candidate.start - 1)
			out[#out + 1] = candidate.link
			pos = candidate.stop + 1
		end
	end
	out[#out + 1] = string.sub(text, pos)
	html(table.concat(out))
	return 0
end