PriorityQueue的Java实现

借助heap数据结构实现。

以小顶heap为例（说明值越小优先级越高，如距离），代码如下：

//  PriorityQueue.java
//  Java Spatial Index Library
//  Copyright (C) 2008 aled@users.sourceforge.net
//
//  This library is free software; you can redistribute it and/or
//  modify it under the terms of the GNU Lesser General Public
//  License as published by the Free Software Foundation; either
//  version 2.1 of the License, or (at your option) any later version.
//
//  This library is distributed in the hope that it will be useful,
//  but WITHOUT ANY WARRANTY; without even the implied warranty of
//  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
//  Lesser General Public License for more details.
//
//  You should have received a copy of the GNU Lesser General Public
//  License along with this library; if not, write to the Free Software
//  Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307 USA

package buaa.act.ucar.moi;

import java.io.Serializable;

import gnu.trove.list.array.TFloatArrayList;
import gnu.trove.list.array.TIntArrayList;

/**
 * <p>
 * Priority Queue that stores values as ints and priorities as floats. Uses a
 * Heap to sort the priorities; the values are sorted "in step" with the
 * priorities.
 * </p>
 * <p>
 * A Heap is simply an array that is kept semi sorted; in particular if the
 * elements of the array are arranged in a tree structure; ie
 * </p>
 * 
 * <pre>
 *                                   00
 *                                 /     
 *                             01          02
 *                            /          /  
 *                          03    04    05    06
 *                          /    /    /    /
 *                        07 08 09 10 11 12 13 14
 * </pre>
 * <p>
 * then each parent is kept sorted with respect to it's immediate children. E.g.
 * 00 < 01, 00 < 02, 02 < 05, 02 < 06
 * </p>
 * <p>
 * This means that the array appears to be sorted, as long as we only ever look
 * at element 0.
 * </p>
 * <p>
 * Inserting new elements is much faster than if the entire array was kept
 * sorted; a new element is appended to the array, and then recursively swapped
 * with each parent to maintain the "parent is sorted w.r.t it's children"
 * property.
 * </p>
 * <p>
 * To return the "next" value it is necessary to remove the root element. The
 * last element in the array is placed in the root of the tree, and is
 * recursively swapped with one of it's children until the "parent is sorted
 * w.r.t it's children" property is restored.
 * </p>
 * <p>
 * Random access is slow (eg for deleting a particular value), and is not
 * implemented here - if this functionality is required, then a heap probably
 * isn't the right data structure.
 * </p>
 */
public class PriorityQueue implements Serializable {
    private static final long serialVersionUID = -5653506138757217673L;
    public static final boolean SORT_ORDER_ASCENDING = true;
    public static final boolean SORT_ORDER_DESCENDING = false;

    private TIntArrayList values = null;
    private TFloatArrayList priorities = null;
    private boolean sortOrder = SORT_ORDER_ASCENDING;

    private static boolean INTERNAL_CONSISTENCY_CHECKING = false;

    public PriorityQueue(boolean sortOrder) {
        this(sortOrder, 10);
    }

    public PriorityQueue(boolean sortOrder, int initialCapacity) {
        this.sortOrder = sortOrder;
        values = new TIntArrayList(initialCapacity);
        priorities = new TFloatArrayList(initialCapacity);
    }

    /**
     * @param p1
     * @param p2
     * @return true if p1 has an earlier sort order than p2.
     */
    private boolean sortsEarlierThan(float p1, float p2) {
        if (sortOrder == SORT_ORDER_ASCENDING) {
            return p1 < p2;
        }
        return p2 < p1;
    }

    // to insert a value, append it to the arrays, then
    // reheapify by promoting it to the correct place.
    public void insert(int value, float priority) {
        values.add(value);
        priorities.add(priority);

        promote(values.size() - 1, value, priority);
    }

    private void promote(int index, int value, float priority) {
        // Consider the index to be a "hole"; i.e. don't swap priorities/values
        // when moving up the tree, simply copy the parent into the hole and
        // then consider the parent to be the hole.
        // Finally, copy the value/priority into the hole.
        while (index > 0) {
            int parentIndex = (index - 1) / 2;
            float parentPriority = priorities.get(parentIndex);

            if (sortsEarlierThan(parentPriority, priority)) {
                break;
            }

            // copy the parent entry into the current index.
            values.set(index, values.get(parentIndex));
            priorities.set(index, parentPriority);
            index = parentIndex;
        }

        values.set(index, value);
        priorities.set(index, priority);

        if (INTERNAL_CONSISTENCY_CHECKING) {
            check();
        }
    }

    public int size() {
        return values.size();
    }

    public void clear() {
        values.clear();
        priorities.clear();
    }

    public void reset() {
        values.reset();
        priorities.reset();
    }

    public int getValue() {
        return values.get(0);
    }

    public float getPriority() {
        return priorities.get(0);
    }

    private void demote(int index, int value, float priority) {
        int childIndex = (index * 2) + 1; // left child

        while (childIndex < values.size()) {
            float childPriority = priorities.get(childIndex);

            if (childIndex + 1 < values.size()) {
                float rightPriority = priorities.get(childIndex + 1);
                if (sortsEarlierThan(rightPriority, childPriority)) {
                    childPriority = rightPriority;
                    childIndex++; // right child
                }
            }

            if (sortsEarlierThan(childPriority, priority)) {
                priorities.set(index, childPriority);
                values.set(index, values.get(childIndex));
                index = childIndex;
                childIndex = (index * 2) + 1;
            } else {
                break;
            }
        }

        values.set(index, value);
        priorities.set(index, priority);
    }

    // get the value with the lowest priority
    // creates a "hole" at the root of the tree.
    // The algorithm swaps the hole with the appropriate child, until
    // the last entry will fit correctly into the hole (ie is lower
    // priority than its children)
    public int pop() {
        int ret = values.get(0);

        // record the value/priority of the last entry
        int lastIndex = values.size() - 1;
        int tempValue = values.get(lastIndex);
        float tempPriority = priorities.get(lastIndex);

        values.removeAt(lastIndex);
        priorities.removeAt(lastIndex);

        if (lastIndex > 0) {
            demote(0, tempValue, tempPriority);
        }

        if (INTERNAL_CONSISTENCY_CHECKING) {
            check();
        }

        return ret;
    }

    public void setSortOrder(boolean sortOrder) {
        if (this.sortOrder != sortOrder) {
            this.sortOrder = sortOrder;
            // reheapify the arrays
            for (int i = (values.size() / 2) - 1; i >= 0; i--) {
                demote(i, values.get(i), priorities.get(i));
            }
        }
        if (INTERNAL_CONSISTENCY_CHECKING) {
            check();
        }
    }

    private void check() {
        // for each entry, check that the child entries have a lower or equal
        // priority
        int lastIndex = values.size() - 1;

        for (int i = 0; i < values.size() / 2; i++) {
            float currentPriority = priorities.get(i);

            int leftIndex = (i * 2) + 1;
            if (leftIndex <= lastIndex) {
                float leftPriority = priorities.get(leftIndex);
                if (sortsEarlierThan(leftPriority, currentPriority)) {
                    System.err.println("Internal error in PriorityQueue");
                }
            }

            int rightIndex = (i * 2) + 2;
            if (rightIndex <= lastIndex) {
                float rightPriority = priorities.get(rightIndex);
                if (sortsEarlierThan(rightPriority, currentPriority)) {
                    System.err.println("Internal error in PriorityQueue");
                }
            }
        }
    }
}