local time = require "util.time".now;

local new_metric_registry = require "util.openmetrics".new_metric_registry;

local render_histogram_le = require "util.openmetrics".render_histogram_le;

-- BEGIN of Metric implementations

-- Gauges

local gauge_metric_mt = {}

gauge_metric_mt.__index = gauge_metric_mt

local function new_gauge_metric()

	local metric = { value = 0 }

	setmetatable(metric, gauge_metric_mt)

	return metric

end

function gauge_metric_mt:set(value)

	self.value = value

end

function gauge_metric_mt:add(delta)

	self.value = self.value + delta

end

function gauge_metric_mt:reset()

	self.value = 0

end

function gauge_metric_mt:iter_samples()

	local done = false

	return function(_s)

		if done then

			return nil, true

		end

		done = true

		return "", nil, _s.value

	end, self

end

-- Counters

local counter_metric_mt = {}

counter_metric_mt.__index = counter_metric_mt

local function new_counter_metric()

	local metric = {

		_created = time(),

		value = 0,

	}

	setmetatable(metric, counter_metric_mt)

	return metric

end

function counter_metric_mt:set(value)

	self.value = value

end

function counter_metric_mt:add(value)

	self.value = (self.value or 0) + value

end

function counter_metric_mt:iter_samples()

	local step = 0

	return function(_s)

		step = step + 1

		if step == 1 then

			return "_created", nil, _s._created

		elseif step == 2 then

			return "_total", nil, _s.value

		else

			return nil, nil, true

		end

	end, self

end

function counter_metric_mt:reset()

	self.value = 0

end

-- Histograms

local histogram_metric_mt = {}

histogram_metric_mt.__index = histogram_metric_mt

local function new_histogram_metric(buckets)

	local metric = {

		_created = time(),

		_sum = 0,

		_count = 0,

	}

	-- the order of buckets matters unfortunately, so we cannot directly use

	-- the thresholds as table keys

	for i, threshold in ipairs(buckets) do

		metric[i] = {

			threshold = threshold,

			threshold_s = render_histogram_le(threshold),

			count = 0

		}

	end

	setmetatable(metric, histogram_metric_mt)

	return metric

end

function histogram_metric_mt:sample(value)

	-- According to the I-D, values must be part of all buckets

	for i, bucket in pairs(self) do

		if "number" == type(i) and value <= bucket.threshold then

			bucket.count = bucket.count + 1

		end

	end

	self._sum = self._sum + value

	self._count = self._count + 1

end

function histogram_metric_mt:iter_samples()

	local key = nil

	return function (_s)

		local data

		key, data = next(_s, key)

		if key == "_created" or key == "_sum" or key == "_count" then

			return key, nil, data

		elseif key ~= nil then

			return "_bucket", {["le"] = data.threshold_s}, data.count

		else

			return nil, nil, nil

		end

	end, self

end

function histogram_metric_mt:reset()

	self._created = time()

	self._count = 0

	self._sum = 0

	for i, bucket in pairs(self) do

		if "number" == type(i) then

			bucket.count = 0

		end

	end

end

-- Summary

local summary_metric_mt = {}

summary_metric_mt.__index = summary_metric_mt

local function new_summary_metric()

	-- quantiles are not supported yet

	local metric = {

		_created = time(),

		_sum = 0,

		_count = 0,

	}

	setmetatable(metric, summary_metric_mt)

	return metric

end

function summary_metric_mt:sample(value)

	self._sum = self._sum + value

	self._count = self._count + 1

end

function summary_metric_mt:iter_samples()

	local key = nil

	return function (_s)

		local data

		key, data = next(_s, key)

		return key, nil, data

	end, self

end

function summary_metric_mt:reset()

	self._created = time()

	self._count = 0

	self._sum = 0

end

local pull_backend = {

	gauge = new_gauge_metric,

	counter = new_counter_metric,

	histogram = new_histogram_metric,

	summary = new_summary_metric,

}

-- END of Metric implementations

local function new()

	return {

		metric_registry = new_metric_registry(pull_backend),

	}

end

return {

	new = new;

}