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Java example source code file (ConvexHull2D.java)

This example Java source code file (ConvexHull2D.java) is included in the alvinalexander.com "Java Source Code Warehouse" project. The intent of this project is to help you "Learn Java by Example" TM.

Learn more about this Java project at its project page.

Java - Java tags/keywords

convexhull2d, insufficientdataexception, line, mathillegalargumentexception, regionfactory, segment, serializable, vector2d

The ConvexHull2D.java Java example source code

/*
 * Licensed to the Apache Software Foundation (ASF) under one or more
 * 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
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
package org.apache.commons.math3.geometry.euclidean.twod.hull;

import java.io.Serializable;

import org.apache.commons.math3.exception.InsufficientDataException;
import org.apache.commons.math3.exception.MathIllegalArgumentException;
import org.apache.commons.math3.exception.util.LocalizedFormats;
import org.apache.commons.math3.geometry.euclidean.twod.Euclidean2D;
import org.apache.commons.math3.geometry.euclidean.twod.Line;
import org.apache.commons.math3.geometry.euclidean.twod.Segment;
import org.apache.commons.math3.geometry.euclidean.twod.Vector2D;
import org.apache.commons.math3.geometry.hull.ConvexHull;
import org.apache.commons.math3.geometry.partitioning.Region;
import org.apache.commons.math3.geometry.partitioning.RegionFactory;
import org.apache.commons.math3.util.MathArrays;
import org.apache.commons.math3.util.Precision;

/**
 * This class represents a convex hull in an two-dimensional euclidean space.
 *
 * @since 3.3
 */
public class ConvexHull2D implements ConvexHull<Euclidean2D, Vector2D>, Serializable {

    /** Serializable UID. */
    private static final long serialVersionUID = 20140129L;

    /** Vertices of the hull. */
    private final Vector2D[] vertices;

    /** Tolerance threshold used during creation of the hull vertices. */
    private final double tolerance;

    /**
     * Line segments of the hull.
     * The array is not serialized and will be created from the vertices on first access.
     */
    private transient Segment[] lineSegments;

    /**
     * Simple constructor.
     * @param vertices the vertices of the convex hull, must be ordered
     * @param tolerance tolerance below which points are considered identical
     * @throws MathIllegalArgumentException if the vertices do not form a convex hull
     */
    public ConvexHull2D(final Vector2D[] vertices, final double tolerance)
        throws MathIllegalArgumentException {

        // assign tolerance as it will be used by the isConvex method
        this.tolerance = tolerance;

        if (!isConvex(vertices)) {
            throw new MathIllegalArgumentException(LocalizedFormats.NOT_CONVEX);
        }

        this.vertices = vertices.clone();
    }

    /**
     * Checks whether the given hull vertices form a convex hull.
     * @param hullVertices the hull vertices
     * @return {@code true} if the vertices form a convex hull, {@code false} otherwise
     */
    private boolean isConvex(final Vector2D[] hullVertices) {
        if (hullVertices.length < 3) {
            return true;
        }

        int sign = 0;
        for (int i = 0; i < hullVertices.length; i++) {
            final Vector2D p1 = hullVertices[i == 0 ? hullVertices.length - 1 : i - 1];
            final Vector2D p2 = hullVertices[i];
            final Vector2D p3 = hullVertices[i == hullVertices.length - 1 ? 0 : i + 1];

            final Vector2D d1 = p2.subtract(p1);
            final Vector2D d2 = p3.subtract(p2);

            final double crossProduct = MathArrays.linearCombination(d1.getX(), d2.getY(), -d1.getY(), d2.getX());
            final int cmp = Precision.compareTo(crossProduct, 0.0, tolerance);
            // in case of collinear points the cross product will be zero
            if (cmp != 0.0) {
                if (sign != 0.0 && cmp != sign) {
                    return false;
                }
                sign = cmp;
            }
        }

        return true;
    }

    /** {@inheritDoc} */
    public Vector2D[] getVertices() {
        return vertices.clone();
    }

    /**
     * Get the line segments of the convex hull, ordered.
     * @return the line segments of the convex hull
     */
    public Segment[] getLineSegments() {
        return retrieveLineSegments().clone();
    }

    /**
     * Retrieve the line segments from the cached array or create them if needed.
     *
     * @return the array of line segments
     */
    private Segment[] retrieveLineSegments() {
        if (lineSegments == null) {
            // construct the line segments - handle special cases of 1 or 2 points
            final int size = vertices.length;
            if (size <= 1) {
                this.lineSegments = new Segment[0];
            } else if (size == 2) {
                this.lineSegments = new Segment[1];
                final Vector2D p1 = vertices[0];
                final Vector2D p2 = vertices[1];
                this.lineSegments[0] = new Segment(p1, p2, new Line(p1, p2, tolerance));
            } else {
                this.lineSegments = new Segment[size];
                Vector2D firstPoint = null;
                Vector2D lastPoint = null;
                int index = 0;
                for (Vector2D point : vertices) {
                    if (lastPoint == null) {
                        firstPoint = point;
                        lastPoint = point;
                    } else {
                        this.lineSegments[index++] =
                                new Segment(lastPoint, point, new Line(lastPoint, point, tolerance));
                        lastPoint = point;
                    }
                }
                this.lineSegments[index] =
                        new Segment(lastPoint, firstPoint, new Line(lastPoint, firstPoint, tolerance));
            }
        }
        return lineSegments;
    }

    /** {@inheritDoc} */
    public Region<Euclidean2D> createRegion() throws InsufficientDataException {
        if (vertices.length < 3) {
            throw new InsufficientDataException();
        }
        final RegionFactory<Euclidean2D> factory = new RegionFactory();
        final Segment[] segments = retrieveLineSegments();
        final Line[] lineArray = new Line[segments.length];
        for (int i = 0; i < segments.length; i++) {
            lineArray[i] = segments[i].getLine();
        }
        return factory.buildConvex(lineArray);
    }
}

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