--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/artifacts/src/main/java/org/dive4elements/river/artifacts/math/fitting/InvExp.java	Thu Apr 25 15:23:37 2013 +0200
@@ -0,0 +1,66 @@
+package org.dive4elements.river.artifacts.math.fitting;
+
+public class InvExp
+extends      Function
+{
+    public static final Derivative DERIVATIVE =
+        new Derivative("W'(Q) = 1/(log(a)*(Q-b))") {
+
+        @Override
+        public org.dive4elements.river.artifacts.math.Function
+            instantiate(double [] parameters)
+        {
+            final double a = parameters[1];
+            final double b = parameters[2];
+            final double loga = Math.log(a);
+
+            return new org.dive4elements.river.artifacts.math.Function() {
+                @Override
+                public double value(double Q) {
+                    return 1d/(loga*(Q-a));
+                }
+            };
+        }
+    };
+
+    public static final Function INSTANCE = new InvExp();
+
+    public InvExp() {
+        super(
+            "inv-exp",
+            "Q(W) = log((W-b)/m)/log(a)",
+            new String [] { "m", "a", "b" });
+    }
+
+    @Override
+    public double value(double W, double [] parameters) {
+        double m = parameters[0];
+        double a = parameters[1];
+        double b = parameters[2];
+        return Math.log((W-b)/m)/Math.log(a);
+    }
+
+    @Override
+    public double [] gradient(double Q, double [] parameters) {
+        double m = parameters[0];
+        double a = parameters[1];
+        double b = parameters[2];
+        double loga = Math.log(a);
+        return new double [] {
+            -1d/(loga*m),
+            -Math.log((Q-b)/m)/(a*loga*loga),
+            -1d/(loga*(Q-b))
+        };
+    }
+
+    @Override
+    public Derivative getDerivative() {
+        return DERIVATIVE;
+    }
+
+    @Override
+    public Function getInverse() {
+        return Exp.INSTANCE;
+    }
+}
+// vim:set ts=4 sw=4 si et sta sts=4 fenc=utf8 :