expMovingAvg: trick for first N points

dist/datasource-zabbix/timeseries.js

@@ -252,15 +252,43 @@ System.register(['lodash', './utils'], function (_export, _context) {
     return sma;
   }
 
-  function expMovingAverage(datapoints, a) {
-    // Calculate a from window size
-    if (a > 1) {
-      a = 2 / (a + 1);
-    }
+  function expMovingAverage(datapoints, n) {
     var ema = [datapoints[0]];
     var ema_prev = datapoints[0][POINT_VALUE];
     var ema_cur = void 0;
-    for (var i = 1; i < datapoints.length; i++) {
+    var a = void 0;
+
+    if (n > 1) {
+      // Calculate a from window size
+      a = 2 / (n + 1);
+
+      // Initial window, use simple moving average
+      var w_avg = null;
+      var w_count = 0;
+      for (var j = n; j > 0; j--) {
+        if (datapoints[n - j][POINT_VALUE] !== null) {
+          w_avg += datapoints[n - j][POINT_VALUE];
+          w_count++;
+        }
+      }
+      if (w_count > 0) {
+        w_avg = w_avg / w_count;
+        // Actually, we should set timestamp from datapoints[n-1] and start calculation of EMA from n.
+        // But in order to start EMA from first point (not from Nth) we should expand time range and request N additional
+        // points outside left side of range. We can't do that, so this trick is used for pretty view of first N points.
+        // We calculate AVG for first N points, but then start from 2nd point, not from Nth. In general, it means we
+        // assume that previous N values (0-N, 0-(N-1), ..., 0-1) have the same average value as a first N values.
+        ema = [[w_avg, datapoints[0][POINT_TIMESTAMP]]];
+        ema_prev = w_avg;
+        n = 1;
+      }
+    } else {
+      // Use predefined a and start from 1st point (use it as initial EMA value)
+      a = n;
+      n = 1;
+    }
+
+    for (var i = n; i < datapoints.length; i++) {
       if (datapoints[i][POINT_VALUE] !== null) {
         ema_cur = a * datapoints[i][POINT_VALUE] + (1 - a) * ema_prev;
         ema_prev = ema_cur;