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diff --git a/src/de/lmu/ifi/dbs/elki/distance/distancefunction/probabilistic/ChiSquaredDistanceFunction.java b/src/de/lmu/ifi/dbs/elki/distance/distancefunction/probabilistic/ChiSquaredDistanceFunction.java
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+package de.lmu.ifi.dbs.elki.distance.distancefunction.probabilistic;
+
+/*
+ This file is part of ELKI:
+ Environment for Developing KDD-Applications Supported by Index-Structures
+
+ Copyright (C) 2011
+ Ludwig-Maximilians-Universität München
+ Lehr- und Forschungseinheit für Datenbanksysteme
+ ELKI Development Team
+
+ This program is free software: you can redistribute it and/or modify
+ it under the terms of the GNU Affero General Public License as published by
+ the Free Software Foundation, either version 3 of the License, or
+ (at your option) any later version.
+
+ 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 Affero General Public License for more details.
+
+ You should have received a copy of the GNU Affero General Public License
+ along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+import de.lmu.ifi.dbs.elki.data.NumberVector;
+import de.lmu.ifi.dbs.elki.data.spatial.SpatialComparable;
+import de.lmu.ifi.dbs.elki.distance.distancefunction.AbstractSpatialDoubleDistanceFunction;
+import de.lmu.ifi.dbs.elki.utilities.Alias;
+import de.lmu.ifi.dbs.elki.utilities.documentation.Reference;
+import de.lmu.ifi.dbs.elki.utilities.optionhandling.AbstractParameterizer;
+
+/**
+ * Chi-Squared distance function, symmetric version.
+ * 
+ * Reference:
+ * <p>
+ * J. Puzicha, J.M. Buhmann, Y. Rubner, C. Tomasi<br />
+ * Empirical evaluation of dissimilarity measures for color and texture<br />
+ * Proc. 7th IEEE International Conference on Computer Vision
+ * </p>
+ * 
+ * @author Erich Schubert
+ */
+@Alias("chisq")
+@Reference(authors = "J. Puzicha, J.M. Buhmann, Y. Rubner, C. Tomasi", title = "Empirical evaluation of dissimilarity measures for color and texture", booktitle = "Proc. 7th IEEE International Conference on Computer Vision", url = "http://dx.doi.org/10.1109/ICCV.1999.790412")
+public class ChiSquaredDistanceFunction extends AbstractSpatialDoubleDistanceFunction {
+  /**
+   * Static instance. Use this!
+   */
+  public static final ChiSquaredDistanceFunction STATIC = new ChiSquaredDistanceFunction();
+
+  /**
+   * Constructor for the Chi-Squared distance function.
+   * 
+   * @deprecated Use static instance!
+   */
+  @Deprecated
+  public ChiSquaredDistanceFunction() {
+    super();
+  }
+
+  @Override
+  public double doubleDistance(NumberVector<?> v1, NumberVector<?> v2) {
+    final int dim = dimensionality(v1, v2);
+    double agg = 0.;
+    for (int d = 0; d < dim; d++) {
+      final double xd = v1.doubleValue(d), yd = v2.doubleValue(d);
+      final double di = xd - yd;
+      final double si = xd + yd;
+      if (!(si > 0. || si < 0.) || !(di > 0. || di < 0.)) {
+        continue;
+      }
+      agg += di * di / si;
+    }
+    return 2. * agg;
+  }
+
+  @Override
+  public double doubleMinDist(SpatialComparable mbr1, SpatialComparable mbr2) {
+    final int dim = dimensionality(mbr1, mbr2);
+    double agg = 0.;
+    for (int d = 0; d < dim; d++) {
+      final double min1 = mbr1.getMin(d), max1 = mbr1.getMax(d);
+      final double min2 = mbr2.getMin(d), max2 = mbr2.getMax(d);
+      final double diff; // Minimum difference
+      if (max1 < min2) {
+        diff = min2 - max1;
+      } else if (max2 < min1) {
+        diff = max2 - min1;
+      } else {
+        continue; // 0.
+      }
+      final double si = max1 + max2; // Maximum sum
+      if (!(si > 0. || si < 0.) || !(diff > 0. || diff < 0.)) {
+        continue;
+      }
+      agg += diff * diff / si;
+    }
+    return 2. * agg;
+  }
+
+  @Override
+  public String toString() {
+    return "ChiSquaredDistance";
+  }
+
+  @Override
+  public boolean equals(Object obj) {
+    if (obj == null) {
+      return false;
+    }
+    if (obj == this) {
+      return true;
+    }
+    if (this.getClass().equals(obj.getClass())) {
+      return true;
+    }
+    return super.equals(obj);
+  }
+
+  /**
+   * Parameterization class, using the static instance.
+   * 
+   * @author Erich Schubert
+   * 
+   * @apiviz.exclude
+   */
+  public static class Parameterizer extends AbstractParameterizer {
+    @Override
+    protected ChiSquaredDistanceFunction makeInstance() {
+      return STATIC;
+    }
+  }
+}