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 */
17package org.apache.commons.math.analysis.interpolation;
18
19import org.apache.commons.math.exception.DimensionMismatchException;
20import org.apache.commons.math.exception.util.LocalizedFormats;
21import org.apache.commons.math.exception.NumberIsTooSmallException;
22import org.apache.commons.math.analysis.polynomials.PolynomialFunction;
23import org.apache.commons.math.analysis.polynomials.PolynomialSplineFunction;
24import 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 */
31public 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     */
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