--- /dev/null
+/*
+ * Copyright (c) 2003-2005 The BISON Project
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU Lesser General Public License version 2 as
+ * published by the Free Software Foundation.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+ * GNU Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public License
+ * along with this program; if not, write to the Free Software
+ * Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
+ *
+ */
+
+package peersim.util;
+
+/**
+ * This class provides extended statistical informations about the inspected
+ * distribution. In particular, it provides functions to compute the skewness
+ * (the 3rd degree moment) and the kurtosis (4th degree moment).
+ *
+ * @author Gian Paolo Jesi
+ */
+public class MomentStats extends IncrementalStats {
+
+ private double cubicsum, quadsum; // incremental sums
+
+ /** Calls {@link #reset} */
+ public MomentStats() {
+ reset();
+ }
+
+ public void reset() {
+ super.reset();
+ cubicsum = quadsum = 0.0;
+ }
+
+ public void add(double item, int k) {
+ for(int i=0; i<k; ++i)
+ {
+ super.add(item,1);
+ cubicsum += item * item * item;
+ quadsum += item * cubicsum;
+ }
+ }
+
+ /** Outputs on a single line the superclass statistics postfixed by the
+ * current skewness and kurtosis.
+ */
+ public String toString() {
+ return super.toString()+" "+getSkewness()+" "+getKurtosis();
+ }
+
+ /** Computes the skewness on the node values distribution and
+ * returns the asymmetry coefficient. It gives an indication about the
+ * distribution symmetry compared to the average.
+ *
+ *@return The skewness value as a double.
+ */
+ public double getSkewness() {
+ int n = this.getN();
+ double m3 = (((double)n) / (n-1)) * (cubicsum/n - Math.pow(getAverage(), 3) );
+ return ( m3 / Math.pow(getStD(), 3 ) );
+ }
+
+ /** Computes the kurtosis on the node values distribution and
+ * returns the flatness coefficient. It gives an indication about the
+ * distribution sharpness or flatness.
+ *
+ * @return The kurtosis momentus value as a double.
+ */
+ public double getKurtosis(){
+ int n = this.getN();
+ double m4 = (((double)n) / (n-1)) * (quadsum/n - Math.pow(getAverage(), 4) );
+ return ( m4 / Math.pow(getStD(), 4) )-3;
+ }
+
+}
