-- cgit email-filter that puts a Gravatar icon next to an author name. Enable
-- it with the email-filter or repo.email-filter setting and the lua: prefix,
-- so it runs in cgit's embedded interpreter with no per-request process. The
-- companion email-libravatar.lua is the same filter pointed at Libravatar
-- instead.
--
--     email-filter=lua:/path/to/email-gravatar.lua
--
-- Runs on Lua 5.1, 5.2, 5.3, 5.4 and LuaJIT. Lua 5.5 is not supported, because
-- luaossl has no 5.5 build.
--
-- The one dependency is luaossl, the OpenSSL binding that provides
-- openssl.digest, from <https://github.com/wahern/luaossl>. The reliable
-- cross-platform install is LuaRocks, matched to the Lua version cgit is built
-- against, and it needs the OpenSSL development headers present. Without it
-- the filter draws no icons and leaves the names as they are.
--
--     # Debian and Ubuntu
--     sudo apt install luarocks libssl-dev
--     sudo luarocks --lua-version 5.1 install luaossl
--
--     # Fedora
--     sudo dnf install luarocks openssl-devel
--     sudo luarocks --lua-version 5.1 install luaossl
--
--     # macOS with Homebrew
--     brew install luarocks openssl
--     luarocks install luaossl OPENSSL_DIR="$(brew --prefix openssl)"
--
-- Every page view sends the visitor's IP address and a hash of each
-- committer's email to a third-party service, so leave this filter off if
-- that is not acceptable for your instance. Addresses are hashed with
-- SHA-256, the form Gravatar documents.

local has_digest, digest = pcall(require, "openssl.digest")


-- The size is in pixels and serves both as the image asked of the service
-- and as the width and height attributes. The default image is what
-- Gravatar draws for an address it has never seen. The endpoint is https so
-- the icon is not blocked as mixed content on an https page.
local avatar_size = 13
local default_image = "retro"
local base_url = "https://gravatar.com/avatar/"


-- One name reaches this filter as an open, writes and a close, so what the
-- open works out is kept here for the close.
local buffer = ""
local avatar_hash = nil

local function hash_hex(input)
	local raw = digest.new("sha256"):final(input)
	local hex = ""
	for i = 1, #raw do
		hex = hex .. string.format("%.2x", string.byte(raw, i))
	end
	return hex
end

-- Lua patterns backtrack, so "^%s*(.-)%s*$" would cost the square of a long
-- run of spaces inside the address. Finding each end directly stays linear.
local function trim(text)
	local first = text:find("%S")
	if first == nil then
		return ""
	end
	local last = #text
	while text:find("^%s", last) do
		last = last - 1
	end
	return text:sub(first, last)
end

-- cgit can hand over the address still wrapped in angle brackets, and the
-- service hashes the trimmed lowercase form, so an address that skipped this
-- would hash to something the service has never heard of. A missing or empty
-- address becomes nil, which is how the caller knows to draw no icon.
local function normalize_email(email)
	if email == nil then
		return nil
	end
	-- The first "<" and the last ">" are found directly, because "<(.*)>"
	-- backtracks to the end of the address from every "<" in it.
	local open = email:find("<", 1, true)
	local close = email:find(">[^>]*$")
	if open and close and close > open then
		email = email:sub(open + 1, close - 1)
	end
	email = trim(email):lower()
	if email == "" then
		return nil
	end
	return email
end

function filter_open(email, page)
	buffer = ""
	local address = normalize_email(email)
	if address == nil or not has_digest then
		avatar_hash = nil
	else
		avatar_hash = hash_hex(address)
	end
end

function filter_write(text)
	buffer = buffer .. text
end

-- A log page can carry one icon per commit, so the attributes let the
-- browser defer the fetches for icons still off screen and keep decoding
-- off the critical path. The icon only decorates the name beside it, so its
-- alt text is empty and a screen reader goes straight to the name.
function filter_close()
	if avatar_hash == nil then
		html(buffer)
	else
		html("<img src='" .. base_url .. avatar_hash ..
			"?s=" .. avatar_size ..
			"&amp;d=" .. default_image ..
			"' alt='' width='" .. avatar_size ..
			"' height='" .. avatar_size ..
			"' loading='lazy' decoding='async'> " .. buffer)
	end
	return 0
end