--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/artifacts/src/main/java/org/dive4elements/river/artifacts/model/fixings/fitting/ChiSquare.java	Mon Dec 02 17:56:15 2019 +0100
@@ -0,0 +1,52 @@
+/** Copyright (C) 2017 by Bundesanstalt für Gewässerkunde
+ * Software engineering by
+ *  Björnsen Beratende Ingenieure GmbH
+ *  Dr. Schumacher Ingenieurbüro für Wasser und Umwelt
+ *
+ * This file is Free Software under the GNU AGPL (>=v3)
+ * and comes with ABSOLUTELY NO WARRANTY! Check out the
+ * documentation coming with Dive4Elements River for details.
+ */
+package org.dive4elements.river.artifacts.model.fixings.fitting;
+
+import java.util.Arrays;
+
+import org.apache.commons.math3.exception.MathIllegalArgumentException;
+import org.apache.commons.math3.stat.descriptive.AbstractUnivariateStatistic;
+import org.apache.commons.math3.stat.descriptive.UnivariateStatistic;
+
+/**
+ * @author Gernot Belger
+ */
+final class ChiSquare extends AbstractUnivariateStatistic {
+
+    private final double[] observation;
+
+    public ChiSquare(final double[] observation) {
+        this.observation = observation;
+    }
+
+    @Override
+    public UnivariateStatistic copy() {
+        /* stateless, hence we can return this */
+        return this;
+    }
+
+    @Override
+    public double evaluate(final double[] values, final int begin, final int length) throws MathIllegalArgumentException {
+        test(values, begin, length, true);
+
+        final double[] residualsWeights = new double[length];
+        Arrays.fill(residualsWeights, 0, length - 1, 1.0);
+
+        double cost = 0;
+
+        for (int i = begin; i < begin + length; i++) {
+            final double residual = values[i + begin] - this.observation[i + begin];
+            // final double wresiduals = residual * FastMath.sqrt(residualsWeights[i]);
+            cost += residualsWeights[i] * residual * residual;
+        }
+
+        return cost;
+    }
+}
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