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SquidLib platform-independent logic and utility code. Please refer to https://github.com/SquidPony/SquidLib .
There is a newer version: 3.0.6
/*
 * MIT License
 *
 * Copyright (c) 2017 Justin Kunimune
 *
 * Permission is hereby granted, free of charge, to any person obtaining a copy
 * of this software and associated documentation files (the "Software"), to deal
 * in the Software without restriction, including without limitation the rights
 * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
 * copies of the Software, and to permit persons to whom the Software is
 * furnished to do so, subject to the following conditions:
 *
 * The above copyright notice and this permission notice shall be included in all
 * copies or substantial portions of the Software.
 *
 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
 * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
 * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
 * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
 * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
 * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
 * SOFTWARE.
 */

package squidpony.squidgrid.mapping;

/**
 * Added to SquidLib by Tommy Ettinger on 7/4/2018, using MIT-licensed work by Justin Kunimune from
 * his Map-Projections repo.
 * @author jkunimune
 * @author Tommy Ettinger
 */
public class ProjectionTools {
    /**
     * Performs a definite integral using Simpson's rule and a constant step size; hard-coded to integrate a
     * hyperellipse function.
     * @param a The start of the integration region
     * @param b The end of the integration region (must be greater than a)
     * @param h The step size (must be positive)
     * @param kappa the kappa value of the hyperellipse
     * @return some magic stuff needed for Tobler Hyperelliptical maps
     */
    public static double simpsonIntegrateHyperellipse(double a, double b, double h, double kappa) {
        double sum = 0, ik = 1/kappa;
        for (double x = a; x < b; x += h) {
            if (x+h > b) h = b-x;
            sum += h/6*(Math.pow(1 - Math.pow(Math.abs(x), kappa), ik) 
                    + 4*Math.pow(1 - Math.pow(Math.abs(x + h * 0.5), kappa), ik) 
                    + Math.pow(1 - Math.pow(Math.abs(x + h), kappa), ik));
        }
        return sum;
    }

    /**
     * Solves a simple ODE using Simpson's rule and a constant step size; hard-coded to solve a hyperelliptical map
     * projection task.
     * @param T The maximum time value at which to sample (must be positive)
     * @param y the double array to fill with samples; must not be null and must have length 1 or greater
     * @param h The internal step size (must be positive)
     * @param alpha part of the hyperelliptical projection's parameters 
     * @param kappa part of the hyperelliptical projection's parameters
     * @param epsilon calculated beforehand using {@link #simpsonIntegrateHyperellipse(double, double, double, double)}
     * @return y, after modifications
     */
    public static double[] simpsonODESolveHyperellipse(final double T, final double[] y, final double h, final double alpha, final double kappa, final double epsilon)
    {
        final int n = y.length - 1;
        double t = 0;
        double sum = 0;
        for (int i = 0; i <= n; i++) {
            while (t < i * T / n) {
                final double tph = Math.min(t + h, i * T / n);
                sum += (tph - t) / 6 * (Math.abs((alpha + (1-alpha)*Math.pow(1 - Math.pow(Math.abs(t), kappa), 1.0/kappa)) / (alpha + (1-alpha)*epsilon))
                        + 4 * Math.abs((alpha + (1-alpha)*Math.pow(1 - Math.pow(Math.abs((t + tph) * 0.5), kappa), 1.0/kappa)) / (alpha + (1-alpha)*epsilon))
                        + Math.abs((alpha + (1-alpha)*Math.pow(1 - Math.pow(Math.abs(tph), kappa), 1.0/kappa)) / (alpha + (1-alpha)*epsilon)));
                t = tph;
            }
            y[i] = sum;
        }
        return y;
    }

    /**
     * Part of computing a hyperellipse; takes only a y parameter corresponding to the y on a map and a kappa parameter
     * used by Tobler's hyperelliptical projection to determine shape.
     * @param y y on a map, usually -1.0 to 1.0
     * @param kappa one of the Tobler parameters
     * @return I'm guessing the actual y used after hyperelliptical distortion; not sure
     */
    public static double hyperellipse(double y, double kappa) {
        return Math.pow(1 - Math.pow(Math.abs(y),kappa), 1/kappa);
    }

}