--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/util/statistics.lua	Tue Jan 20 12:31:32 2015 +0000
@@ -0,0 +1,131 @@
+local t_sort = table.sort
+local m_floor = math.floor;
+local time = require "socket".gettime;
+
+local function nop_function() end
+
+local function percentile(arr, length, pc)
+	local n = pc/100 * (length + 1);
+	local k, d = m_floor(n), n%1;
+	if k == 0 then
+		return arr[1];
+	elseif k >= length then
+		return arr[length];
+	end
+	return arr[k] + d*(arr[k+1] - arr[k]);
+end
+
+local function new_registry(config)
+	config = config or {};
+	local duration_sample_interval = config.duration_sample_interval or 5;
+	local duration_max_samples = config.duration_max_stored_samples or 5000;
+
+	local registry = {};
+	local methods;
+	methods = {
+		amount = function (name, initial)
+			local v = initial or 0;
+			registry[name] = function () return "amount", v; end
+			return function (new_v) v = new_v; end
+		end;
+		counter = function (name, initial)
+			local v = initial or 0;
+			registry[name] = function () return "amount", v; end
+			return function (delta)
+				v = v + delta;
+			end;
+		end;
+		rate = function (name)
+			local since, n = time(), 0;
+			registry[name] = function ()
+				local t = time();
+				local stats = {
+					rate = n/(t-since);
+					count = n;
+				};
+				since, n = t, 0;
+				return "rate", stats.rate, stats;
+			end;
+			return function ()
+				n = n + 1;
+			end;
+		end;
+		duration = function (name)
+			local events, last_event = {}, 0;
+			local n_actual_events = 0;
+			local since = time();
+
+			registry[name] = function ()
+				local n_stored_events = #events;
+				t_sort(events);
+				local sum = 0;
+				for i = 1, n_stored_events do
+					sum = sum + events[i];
+				end
+
+				local new_time = time();
+
+				local stats = {
+					samples = events;
+					sample_count = n_stored_events;
+					count = n_actual_events,
+					rate = n_actual_events/(new_time-since);
+					average = n_stored_events > 0 and sum/n_stored_events or 0,
+					min = events[1],
+					max = events[n_stored_events],
+				};
+
+				events, last_event = {}, 0;
+				n_actual_events = 0;
+				since = new_time;
+
+				return "duration", stats.average, stats;
+			end;
+
+			return function ()
+				n_actual_events = n_actual_events + 1;
+				if n_actual_events%duration_sample_interval > 0 then
+					return nop_function;
+				end
+
+				local start_time = time();
+				return function ()
+					local end_time = time();
+					local duration = end_time - start_time;
+					last_event = (last_event%duration_max_samples) + 1;
+					events[last_event] = duration;
+				end
+			end;
+		end;
+
+		get_stats = function ()
+			return registry;
+		end;
+	};
+	return methods;
+end
+
+return {
+	new = new_registry;
+	get_histogram = function (duration, n_buckets)
+		n_buckets = n_buckets or 100;
+		local events, n_events = duration.samples, duration.sample_count;
+		if not (events and n_events) then
+			return nil, "not a valid duration stat";
+		end
+		local histogram = {};
+
+		for i = 1, 100, 100/n_buckets do
+			histogram[i] = percentile(events, n_events, i);
+		end
+		return histogram;
+	end;
+
+	get_percentile = function (duration, pc)
+		local events, n_events = duration.samples, duration.sample_count;
+		if not (events and n_events) then
+			return nil, "not a valid duration stat";
+		end
+		return percentile(events, n_events, pc);
+	end;
+}