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With inspiration from other libraries
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/*
 * 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.partitioning;

import org.apache.commons.math3.geometry.Space;

/** Utility class checking if inside nodes can be found
 * on the plus and minus sides of an hyperplane.
 * @param  Type of the space.
 * @since 3.4
 */
class InsideFinder {

    /** Region on which to operate. */
    private final Region region;

    /** Indicator of inside leaf nodes found on the plus side. */
    private boolean plusFound;

    /** Indicator of inside leaf nodes found on the plus side. */
    private boolean minusFound;

    /** Simple constructor.
     * @param region region on which to operate
     */
    InsideFinder(final Region region) {
        this.region = region;
        plusFound  = false;
        minusFound = false;
    }

    /** Search recursively for inside leaf nodes on each side of the given hyperplane.

     * The algorithm used here is directly derived from the one
     * described in section III (Binary Partitioning of a BSP
     * Tree) of the Bruce Naylor, John Amanatides and William
     * Thibault paper Merging
     * BSP Trees Yields Polyhedral Set Operations Proc. Siggraph
     * '90, Computer Graphics 24(4), August 1990, pp 115-124, published
     * by the Association for Computing Machinery (ACM)..

     * @param node current BSP tree node
     * @param sub sub-hyperplane
     */
    public void recurseSides(final BSPTree node, final SubHyperplane sub) {

        if (node.getCut() == null) {
            if ((Boolean) node.getAttribute()) {
                // this is an inside cell expanding across the hyperplane
                plusFound  = true;
                minusFound = true;
            }
            return;
        }

        final Hyperplane hyperplane = node.getCut().getHyperplane();
        final SubHyperplane.SplitSubHyperplane split = sub.split(hyperplane);
        switch (split.getSide()) {
        case PLUS :
            // the sub-hyperplane is entirely in the plus sub-tree
            if (node.getCut().split(sub.getHyperplane()).getSide() == Side.PLUS) {
                if (!region.isEmpty(node.getMinus())) {
                    plusFound  = true;
                }
            } else {
                if (!region.isEmpty(node.getMinus())) {
                    minusFound = true;
                }
            }
            if (!(plusFound && minusFound)) {
                recurseSides(node.getPlus(), sub);
            }
            break;
        case MINUS :
            // the sub-hyperplane is entirely in the minus sub-tree
            if (node.getCut().split(sub.getHyperplane()).getSide() == Side.PLUS) {
                if (!region.isEmpty(node.getPlus())) {
                    plusFound  = true;
                }
            } else {
                if (!region.isEmpty(node.getPlus())) {
                    minusFound = true;
                }
            }
            if (!(plusFound && minusFound)) {
                recurseSides(node.getMinus(), sub);
            }
            break;
        case BOTH :
            // the sub-hyperplane extends in both sub-trees

            // explore first the plus sub-tree
            recurseSides(node.getPlus(), split.getPlus());

            // if needed, explore the minus sub-tree
            if (!(plusFound && minusFound)) {
                recurseSides(node.getMinus(), split.getMinus());
            }
            break;
        default :
            // the sub-hyperplane and the cut sub-hyperplane share the same hyperplane
            if (node.getCut().getHyperplane().sameOrientationAs(sub.getHyperplane())) {
                if ((node.getPlus().getCut() != null) || ((Boolean) node.getPlus().getAttribute())) {
                    plusFound  = true;
                }
                if ((node.getMinus().getCut() != null) || ((Boolean) node.getMinus().getAttribute())) {
                    minusFound = true;
                }
            } else {
                if ((node.getPlus().getCut() != null) || ((Boolean) node.getPlus().getAttribute())) {
                    minusFound = true;
                }
                if ((node.getMinus().getCut() != null) || ((Boolean) node.getMinus().getAttribute())) {
                    plusFound  = true;
                }
            }
        }

    }

    /** Check if inside leaf nodes have been found on the plus side.
     * @return true if inside leaf nodes have been found on the plus side
     */
    public boolean plusFound() {
        return plusFound;
    }

    /** Check if inside leaf nodes have been found on the minus side.
     * @return true if inside leaf nodes have been found on the minus side
     */
    public boolean minusFound() {
        return minusFound;
    }

}