add variance to notification usage stats

import android.os.SystemClock;

import android.service.notification.StatusBarNotification;

import android.util.Log;

import java.io.PrintWriter;

import java.util.HashMap;

     */

    public static class Aggregate {

        long numSamples;

        long sum;

        long avg;

        double avg;

        double sum2;

        double var;

        public void addSample(long sample) {

            // Welford's "Method for Calculating Corrected Sums of Squares"

            // http://www.jstor.org/stable/1266577?seq=2

            numSamples++;

            sum += sample;

            avg = sum / numSamples;

            final double n = numSamples;

            final double delta = sample - avg;

            avg += (1.0 / n) * delta;

            sum2 += ((n - 1) / n) * delta * delta;

            final double divisor = numSamples == 1 ? 1.0 : n - 1.0;

            var = sum2 / divisor;

        }

        @Override

        public String toString() {

            return "Aggregate{" +

                    "numSamples=" + numSamples +

                    ", sum=" + sum +

                    ", avg=" + avg +

                    ", var=" + var +

                    '}';

        }

    }