    001    /*
    002     * Licensed to the Apache Software Foundation (ASF) under one or more
    003     * contributor license agreements.  See the NOTICE file distributed with
    004     * this work for additional information regarding copyright ownership.
    005     * The ASF licenses this file to You under the Apache License, Version 2.0
    006     * (the "License"); you may not use this file except in compliance with
    007     * the License.  You may obtain a copy of the License at
    008     *
    009     *      http://www.apache.org/licenses/LICENSE-2.0
    010     *
    011     * Unless required by applicable law or agreed to in writing, software
    012     * distributed under the License is distributed on an "AS IS" BASIS,
    013     * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
    014     * See the License for the specific language governing permissions and
    015     * limitations under the License.
    016     */
    017    package org.apache.commons.math3.analysis.interpolation;
    018
    019    import org.apache.commons.math3.analysis.polynomials.PolynomialFunction;
    020    import org.apache.commons.math3.analysis.polynomials.PolynomialSplineFunction;
    021    import org.apache.commons.math3.util.MathArrays;
    022    import org.apache.commons.math3.exception.DimensionMismatchException;
    023    import org.apache.commons.math3.exception.NumberIsTooSmallException;
    024    import org.apache.commons.math3.exception.NonMonotonicSequenceException;
    025    import org.apache.commons.math3.exception.util.LocalizedFormats;
    026
    027    /**
    028     * Implements a linear function for interpolation of real univariate functions.
    029     *
    030     * @version $Id: LinearInterpolator.java 1379904 2012-09-01 23:54:52Z erans $
    031     */
    032    public class LinearInterpolator implements UnivariateInterpolator {
    033        /**
    034         * Computes a linear interpolating function for the data set.
    035         *
    036         * @param x the arguments for the interpolation points
    037         * @param y the values for the interpolation points
    038         * @return a function which interpolates the data set
    039         * @throws DimensionMismatchException if {@code x} and {@code y}
    040         * have different sizes.
    041         * @throws NonMonotonicSequenceException if {@code x} is not sorted in
    042         * strict increasing order.
    043         * @throws NumberIsTooSmallException if the size of {@code x} is smaller
    044         * than 2.
    045         */
    046        public PolynomialSplineFunction interpolate(double x[], double y[])
    047            throws DimensionMismatchException,
    048                   NumberIsTooSmallException,
    049                   NonMonotonicSequenceException {
    050            if (x.length != y.length) {
    051                throw new DimensionMismatchException(x.length, y.length);
    052            }
    053
    054            if (x.length < 2) {
    055                throw new NumberIsTooSmallException(LocalizedFormats.NUMBER_OF_POINTS,
    056                                                    x.length, 2, true);
    057            }
    058
    059            // Number of intervals.  The number of data points is n + 1.
    060            int n = x.length - 1;
    061
    062            MathArrays.checkOrder(x);
    063
    064            // Slope of the lines between the datapoints.
    065            final double m[] = new double[n];
    066            for (int i = 0; i < n; i++) {
    067                m[i] = (y[i + 1] - y[i]) / (x[i + 1] - x[i]);
    068            }
    069
    070            final PolynomialFunction polynomials[] = new PolynomialFunction[n];
    071            final double coefficients[] = new double[2];
    072            for (int i = 0; i < n; i++) {
    073                coefficients[0] = y[i];
    074                coefficients[1] = m[i];
    075                polynomials[i] = new PolynomialFunction(coefficients);
    076            }
    077
    078            return new PolynomialSplineFunction(x, polynomials);
    079        }
    080    }