-- Log statistics to Prometheus
--
-- Copyright (C) 2014 Daurnimator
-- Copyright (C) 2018 Emmanuel Gil Peyrot <linkmauve@linkmauve.fr>
-- Copyright (C) 2021 Jonas Schäfer <jonas@zombofant.net>
--
-- This module is MIT/X11 licensed.

module:set_global();

local tostring = tostring;
local t_insert = table.insert;
local t_concat = table.concat;
local socket = require "socket";
local statsman = require "core.statsmanager";
local get_stats = statsman.get_stats;

local function escape(text)
	return text:gsub("\\", "\\\\"):gsub("\"", "\\\""):gsub("\n", "\\n");
end

local function escape_name(name)
	return name:gsub("[^A-Za-z0-9_]", "_"):gsub("^[^A-Za-z_]", "_%1");
end

local function get_timestamp()
	-- Using LuaSocket for that because os.time() only has second precision.
	return math.floor(socket.gettime() * 1000);
end

local function repr_help(metric, docstring)
	docstring = docstring:gsub("\\", "\\\\"):gsub("\n", "\\n");
	return "# HELP "..escape_name(metric).." "..docstring.."\n";
end

-- local allowed_types = { counter = true, gauge = true, histogram = true, summary = true, untyped = true };
-- local allowed_types = { "counter", "gauge", "histogram", "summary", "untyped" };
local function repr_type(metric, type_)
	-- if not allowed_types:contains(type_) then
	-- 	return;
	-- end
	return "# TYPE "..escape_name(metric).." "..type_.."\n";
end

local function repr_label(key, value)
	return key.."=\""..escape(value).."\"";
end

local function repr_labels(labels)
	local values = {}
	for key, value in pairs(labels) do
		t_insert(values, repr_label(escape_name(key), escape(value)));
	end
	if #values == 0 then
		return "";
	end
	return "{"..t_concat(values, ",").."}";
end

local function repr_sample(metric, labels, value, timestamp)
	return escape_name(metric)..repr_labels(labels).." "..value.." "..timestamp.."\n";
end

local allowed_extras = { min = true, max = true, average = true };
local function insert_extras(data, key, name, timestamp, extra)
	if not extra then
		return false;
	end
	local has_extra = false;
	for extra_name in pairs(allowed_extras) do
		if extra[extra_name] then
			local field = {
				value = extra[extra_name],
				labels = {
					["type"] = name,
					field = extra_name,
				},
				typ = "gauge";
				timestamp = timestamp,
			};
			t_insert(data[key], field);
			has_extra = true;
		end
	end
	return has_extra;
end

local function parse_stats()
	local timestamp = tostring(get_timestamp());
	local data = {};
	local stats, changed_only, extras = get_stats();
	for stat, value in pairs(stats) do
		-- module:log("debug", "changed_stats[%q] = %s", stat, tostring(value));
		local extra = extras[stat];
		local host, sect, name, typ = stat:match("^/([^/]+)/([^/]+)/(.+):(%a+)$");
		if host == nil then
			sect, name, typ = stat:match("^([^.]+)%.(.+):(%a+)$");
		elseif host == "*" then
			host = nil;
		end
		if sect:find("^mod_measure_.") then
			sect = sect:sub(13);
		elseif sect:find("^mod_statistics_.") then
			sect = sect:sub(16);
		end

		local key = escape_name("prosody_"..sect);
		local field = {
			value = value,
			labels = { ["type"] = name},
			-- TODO: Use the other types where it makes sense.
			typ = (typ == "rate" and "counter" or "gauge"),
			timestamp = timestamp,
		};
		if host then
			field.labels.host = host;
		end
		if data[key] == nil then
			data[key] = {};
		end
		if not insert_extras(data, key, name, timestamp, extra) then
			t_insert(data[key], field);
		end
	end
	return data;
end

local function get_metrics(event)
	local response = event.response;
	response.headers.content_type = "text/plain; version=0.0.4";
	if statsman.collect then
		statsman.collect()
	end

	local answer = {};
	for key, fields in pairs(parse_stats()) do
		t_insert(answer, repr_help(key, "TODO: add a description here."));
		t_insert(answer, repr_type(key, fields[1].typ));
		for _, field in pairs(fields) do
			t_insert(answer, repr_sample(key, field.labels, field.value, field.timestamp));
		end
	end
	return t_concat(answer, "");
end

function module.add_host(module)
	module:depends "http";
	module:provides("http", {
		default_path = "metrics";
		route = {
			GET = get_metrics;
		};
	});
end