metric: new pkg refactored from log for #90

pkg/metric/metric.go

@@ -0,0 +1,63 @@
+package metric
+
+import (
+	"container/list"
+	"expvar"
+	"strings"
+	"time"
+)
+
+// TickerFunc is the function signature accepted by AddTickerFunc, will be called once per minute.
+type TickerFunc func()
+
+var tickerFuncChan = make(chan TickerFunc)
+
+func init() {
+	go metricsTicker()
+}
+
+// AddTickerFunc adds a new function callback to the list of metrics TickerFuncs that get
+// called each minute.
+func AddTickerFunc(f TickerFunc) {
+	tickerFuncChan <- f
+}
+
+// Push adds the metric to the end of the list and returns a comma separated string of the
+// previous 61 entries.  We return 61 instead of 60 (an hour) because the chart on the client
+// tracks deltas between these values - there is nothing to compare the first value against.
+func Push(history *list.List, ev expvar.Var) string {
+	history.PushBack(ev.String())
+	if history.Len() > 61 {
+		history.Remove(history.Front())
+	}
+	return joinStringList(history)
+}
+
+// metricsTicker calls the current list of TickerFuncs once per minute.
+func metricsTicker() {
+	funcs := make([]TickerFunc, 0)
+	ticker := time.NewTicker(time.Minute)
+
+	for {
+		select {
+		case <-ticker.C:
+			for _, f := range funcs {
+				f()
+			}
+		case f := <-tickerFuncChan:
+			funcs = append(funcs, f)
+		}
+	}
+}
+
+// joinStringList joins a List containing strings by commas.
+func joinStringList(listOfStrings *list.List) string {
+	if listOfStrings.Len() == 0 {
+		return ""
+	}
+	s := make([]string, 0, listOfStrings.Len())
+	for e := listOfStrings.Front(); e != nil; e = e.Next() {
+		s = append(s, e.Value.(string))
+	}
+	return strings.Join(s, ",")
+}