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1 /*
2  * Licensed to the Apache Software Foundation (ASF) under one or more
3  * contributor license agreements.  See the NOTICE file distributed with
4  * this work for additional information regarding copyright ownership.
5  * The ASF licenses this file to You under the Apache License, Version 2.0
6  * (the "License"); you may not use this file except in compliance with
7  * the License.  You may obtain a copy of the License at
8  *
9  *      http://www.apache.org/licenses/LICENSE-2.0
10  *
11  * Unless required by applicable law or agreed to in writing, software
12  * distributed under the License is distributed on an "AS IS" BASIS,
13  * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
14  * See the License for the specific language governing permissions and
15  * limitations under the License.
16  */
17 package org.apache.commons.math.analysis.interpolation;
18 
19 import org.apache.commons.math.exception.DimensionMismatchException;
20 import org.apache.commons.math.exception.util.LocalizedFormats;
21 import org.apache.commons.math.exception.NumberIsTooSmallException;
22 import org.apache.commons.math.analysis.polynomials.PolynomialFunction;
23 import org.apache.commons.math.analysis.polynomials.PolynomialSplineFunction;
24 import org.apache.commons.math.util.MathUtils;
25 
26 /**
27  * Implements a linear function for interpolation of real univariate functions.
28  * @version $Revision$ $Date$
29  * @since 2.2
30  */
31 public class LinearInterpolator implements UnivariateRealInterpolator {
32     /**
33      * Computes a linear interpolating function for the data set.
34      * @param x the arguments for the interpolation points
35      * @param y the values for the interpolation points
36      * @return a function which interpolates the data set
37      * @throws DimensionMismatchException if {@code x} and {@code y}
38      * have different sizes.
39      * @throws org.apache.commons.math.exception.NonMonotonousSequenceException
40      * if {@code x} is not sorted in strict increasing order.
41      * @throws NumberIsTooSmallException if the size of {@code x} is smaller
42      * than 2.
43      */
interpolate(double x[], double y[])44     public PolynomialSplineFunction interpolate(double x[], double y[]) {
45         if (x.length != y.length) {
46             throw new DimensionMismatchException(x.length, y.length);
47         }
48 
49         if (x.length < 2) {
50             throw new NumberIsTooSmallException(LocalizedFormats.NUMBER_OF_POINTS,
51                                                 x.length, 2, true);
52         }
53 
54         // Number of intervals.  The number of data points is n + 1.
55         int n = x.length - 1;
56 
57         MathUtils.checkOrder(x);
58 
59         // Slope of the lines between the datapoints.
60         final double m[] = new double[n];
61         for (int i = 0; i < n; i++) {
62             m[i] = (y[i + 1] - y[i]) / (x[i + 1] - x[i]);
63         }
64 
65         PolynomialFunction polynomials[] = new PolynomialFunction[n];
66         final double coefficients[] = new double[2];
67         for (int i = 0; i < n; i++) {
68             coefficients[0] = y[i];
69             coefficients[1] = m[i];
70             polynomials[i] = new PolynomialFunction(coefficients);
71         }
72 
73         return new PolynomialSplineFunction(x, polynomials);
74     }
75 }
76