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.UnivariateFunction;
020    import org.apache.commons.math3.util.MathUtils;
021    import org.apache.commons.math3.util.MathArrays;
022    import org.apache.commons.math3.exception.NumberIsTooSmallException;
023    
024    /**
025     * Adapter for classes implementing the {@link UnivariateInterpolator}
026     * interface.
027     * The data to be interpolated is assumed to be periodic. Thus values that are
028     * outside of the range can be passed to the interpolation function: They will
029     * be wrapped into the initial range before being passed to the class that
030     * actually computes the interpolation.
031     *
032     * @version $Id: UnivariatePeriodicInterpolator.java 1379904 2012-09-01 23:54:52Z erans $
033     */
034    public class UnivariatePeriodicInterpolator
035        implements UnivariateInterpolator {
036        /** Default number of extension points of the samples array. */
037        public static final int DEFAULT_EXTEND = 5;
038        /** Interpolator. */
039        private final UnivariateInterpolator interpolator;
040        /** Period. */
041        private final double period;
042        /** Number of extension points. */
043        private final int extend;
044    
045        /**
046         * Builds an interpolator.
047         *
048         * @param interpolator Interpolator.
049         * @param period Period.
050         * @param extend Number of points to be appended at the beginning and
051         * end of the sample arrays in order to avoid interpolation failure at
052         * the (periodic) boundaries of the orginal interval. The value is the
053         * number of sample points which the original {@code interpolator} needs
054         * on each side of the interpolated point.
055         */
056        public UnivariatePeriodicInterpolator(UnivariateInterpolator interpolator,
057                                              double period,
058                                              int extend) {
059            this.interpolator = interpolator;
060            this.period = period;
061            this.extend = extend;
062        }
063    
064        /**
065         * Builds an interpolator.
066         * Uses {@link #DEFAULT_EXTEND} as the number of extension points on each side
067         * of the original abscissae range.
068         *
069         * @param interpolator Interpolator.
070         * @param period Period.
071         */
072        public UnivariatePeriodicInterpolator(UnivariateInterpolator interpolator,
073                                              double period) {
074            this(interpolator, period, DEFAULT_EXTEND);
075        }
076    
077        /**
078         * {@inheritDoc}
079         *
080         * @throws NumberIsTooSmallException if the number of extension points
081         * iss larger then the size of {@code xval}.
082         */
083        public UnivariateFunction interpolate(double[] xval,
084                                              double[] yval)
085            throws NumberIsTooSmallException {
086            if (xval.length < extend) {
087                throw new NumberIsTooSmallException(xval.length, extend, true);
088            }
089    
090            MathArrays.checkOrder(xval);
091            final double offset = xval[0];
092    
093            final int len = xval.length + extend * 2;
094            final double[] x = new double[len];
095            final double[] y = new double[len];
096            for (int i = 0; i < xval.length; i++) {
097                final int index = i + extend;
098                x[index] = MathUtils.reduce(xval[i], period, offset);
099                y[index] = yval[i];
100            }
101    
102            // Wrap to enable interpolation at the boundaries.
103            for (int i = 0; i < extend; i++) {
104                int index = xval.length - extend + i;
105                x[i] = MathUtils.reduce(xval[index], period, offset) - period;
106                y[i] = yval[index];
107    
108                index = len - extend + i;
109                x[index] = MathUtils.reduce(xval[i], period, offset) + period;
110                y[index] = yval[i];
111            }
112    
113            MathArrays.sortInPlace(x, y);
114    
115            final UnivariateFunction f = interpolator.interpolate(x, y);
116            return new UnivariateFunction() {
117                public double value(final double x) {
118                    return f.value(MathUtils.reduce(x, period, offset));
119                }
120            };
121        }
122    }