expMovingAvg: trick for first N points

src/datasource-zabbix/timeseries.js

@@ -269,15 +269,43 @@ function simpleMovingAverage(datapoints, n) {
   return sma;
 }
 
-function expMovingAverage(datapoints, a) {
-  // Calculate a from window size
-  if (a > 1) {
-    a = 2 / (a + 1);
-  }
+function expMovingAverage(datapoints, n) {
   let ema = [datapoints[0]];
   let ema_prev = datapoints[0][POINT_VALUE];
   let ema_cur;
-  for (let i = 1; i < datapoints.length; i++) {
+  let a;
+
+  if (n > 1) {
+    // Calculate a from window size
+    a = 2 / (n + 1);
+
+    // Initial window, use simple moving average
+    let w_avg = null;
+    let w_count = 0;
+    for (let 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 (let 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;