MicrosphereProjectionInterpolatorTest

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 * contributor license agreements.  See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.
 * The ASF licenses this file to You 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
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 * distributed under the License is distributed on an "AS IS" BASIS,
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 * See the License for the specific language governing permissions and
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package org.apache.commons.math4.legacy.analysis.interpolation;

import org.apache.commons.math4.legacy.analysis.MultivariateFunction;
import org.apache.commons.math4.core.jdkmath.JdkMath;
import org.junit.Assert;
import org.junit.Test;

/**
 * Test case for the {@link MicrosphereProjectionInterpolator
 * "microsphere projection"} interpolator.
 */
public final class MicrosphereProjectionInterpolatorTest {
    /**
     * Test of interpolator for a plane.
     * <p>
     * y = 2 x<sub>1</sub> - 3 x<sub>2</sub> + 5
     */
    @Test
    public void testLinearFunction2D() {
        MultivariateFunction f = new MultivariateFunction() {
            @Override
            public double value(double[] x) {
                if (x.length != 2) {
                    throw new IllegalArgumentException();
                }
                return 2 * x[0] - 3 * x[1] + 5;
            }
        };

        final double darkFraction = 0.5;
        final double darkThreshold = 1e-2;
        final double background = Double.NaN;
        final double exponent = 1.1;
        final boolean shareSphere = true;
        final double noInterpolationTolerance = Math.ulp(1d);

        // N-dimensional interpolator.
        final MultivariateInterpolator interpolator
            = new MicrosphereProjectionInterpolator(2, 500,
                                                    darkFraction,
                                                    darkThreshold,
                                                    background,
                                                    exponent,
                                                    shareSphere,
                                                    noInterpolationTolerance);

        // 2D interpolator.
        final MultivariateInterpolator interpolator2D
            = new MicrosphereProjectionInterpolator(new InterpolatingMicrosphere2D(16,
                                                                                   darkFraction,
                                                                                   darkThreshold,
                                                                                   background),
                                                    exponent,
                                                    shareSphere,
                                                    noInterpolationTolerance);

        final double min = -1;
        final double max = 1;
        final double range = max - min;
        final int res = 5;
        final int n = res * res; // Number of sample points.
        final int dim = 2;
        double[][] x = new double[n][dim];
        double[] y = new double[n];
        int index = 0;
        for (int i = 0; i < res; i++) {
            final double x1Val = toCoordinate(min, range, res, i);
            for (int j = 0; j < res; j++) {
                final double x2Val = toCoordinate(min, range, res, j);
                x[index][0] = x1Val;
                x[index][1] = x2Val;
                y[index] = f.value(x[index]);
                ++index;
            }
        }

        final MultivariateFunction p = interpolator.interpolate(x, y);
        final MultivariateFunction p2D = interpolator2D.interpolate(x, y);

        double[] c = new double[dim];
        double expected;
        double result;
        double result2D;

        final int sampleIndex = 2;
        c[0] = x[sampleIndex][0];
        c[1] = x[sampleIndex][1];
        expected = f.value(c);
        result = p.value(c);
        result2D = p2D.value(c);
        Assert.assertEquals("on sample point (exact)", expected, result2D, JdkMath.ulp(1d));
        Assert.assertEquals("on sample point (ND vs 2D)", result2D, result, JdkMath.ulp(1d));

        // Interpolation.
        c[0] = 0.654321;
        c[1] = -0.345678;
        expected = f.value(c);
        result = p.value(c);
        result2D = p2D.value(c);
        Assert.assertEquals("interpolation (exact)", expected, result2D, 1e-1);
        Assert.assertEquals("interpolation (ND vs 2D)", result2D, result, 1e-1);

        // Extrapolation.
        c[0] = 0 - 1e-2;
        c[1] = 1 + 1e-2;
        expected = f.value(c);
        result = p.value(c);
        result2D = p2D.value(c);
        Assert.assertFalse(Double.isNaN(result));
        Assert.assertFalse(Double.isNaN(result2D));
        Assert.assertEquals("extrapolation (exact)", expected, result2D, 1e-1);
        Assert.assertEquals("extrapolation (ND vs 2D)", result2D, result, 1e-2);

        // Far away.
        c[0] = 20;
        c[1] = -30;
        result = p.value(c);
        Assert.assertTrue(result + " should be NaN", Double.isNaN(result));
        result2D = p2D.value(c);
        Assert.assertTrue(result2D + " should be NaN", Double.isNaN(result2D));
    }

    /**
     * @param min Minimum of the coordinate range.
     * @param range Extent of the coordinate interval.
     * @param res Number of pixels.
     * @param pixel Pixel index.
     */
    private static double toCoordinate(double min,
                                       double range,
                                       int res,
                                       int pixel) {
        return pixel * range / (res - 1) + min;
    }
}