PolInterp.java

/*
 * Class:        PolInterp
 * Description:  polynomial that interpolates through a set of points
 * Environment:  Java
 * Software:     SSJ 
 * Copyright (C) 2001  Pierre L'Ecuyer and Universite de Montreal
 * Organization: DIRO, Universite de Montreal
 * @author       
 * @since
 *
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 *
 */
package umontreal.ssj.functionfit;
 
import java.io.Serializable;
import umontreal.ssj.functions.Polynomial;
 
import umontreal.ssj.util.PrintfFormat;
import cern.colt.matrix.DoubleMatrix2D;
import cern.colt.matrix.impl.DenseDoubleMatrix2D;
import cern.colt.matrix.linalg.Algebra;

public class PolInterp extends Polynomial implements Serializable {
   private static final long serialVersionUID = -710451931485296501L;
   private static final Algebra alg = new Algebra();
   private double[] x;
   private double[] y;

   public PolInterp(double[] x, double[] y) {
      super(getCoefficients(x, y));
      this.x = x.clone();
      this.y = y.clone();
   }

   public static double[] getCoefficients(double[] x, double[] y) {
      if (x.length != y.length)
         throw new IllegalArgumentException("x and y must have the same length");
      if (x.length <= 1)
         throw new IllegalArgumentException("At least two points are needed");
      final DoubleMatrix2D u = new DenseDoubleMatrix2D(x.length, x.length);
      for (int i = 0; i < x.length; i++) {
         double v = 1;
         for (int j = 0; j < x.length; j++) {
            u.setQuick(i, j, v);
            v *= x[i];
         }
      }
      final DoubleMatrix2D yMat = new DenseDoubleMatrix2D(x.length, 1);
      yMat.viewColumn(0).assign(y);
      final DoubleMatrix2D bMat = alg.solve(u, yMat);
      return bMat.viewColumn(0).toArray();
   }

   public double[] getX() {
      return x.clone();
   }

   public double[] getY() {
      return y.clone();
   }

   public static String toString(double[] x, double[] y) {
      final StringBuilder sb = new StringBuilder("Points: ");
      for (int i = 0; i < x.length; i++) {
         if (i > 0)
            sb.append(", ");
         final String xstr = PrintfFormat.format(8, 3, 3, x[i]);
         final String ystr = PrintfFormat.format(8, 3, 3, y[i]);
         sb.append('(').append(xstr).append(", ").append(ystr).append(')');
      }
      return sb.toString();
   }

   @Override
   public String toString() {
      return toString(x, y);
   }
 
   public PolInterp clone() {
      final PolInterp p = (PolInterp) super.clone();
      p.x = x.clone();
      p.y = y.clone();
      return p;
   }
}