PriorityQueue.cs

using J2N.Numerics;
using Lucene.Net.Support;
using System;
using System.Collections.Generic;
using System.Diagnostics.CodeAnalysis;
using System.Runtime.CompilerServices;

namespace Lucene.Net.Util
{
    /*
     * 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.
     */

    /// <summary>
    /// A <see cref="PriorityQueue{T}"/> maintains a partial ordering of its elements such that the
    /// element with least priority can always be found in constant time. Put()'s and Pop()'s
    /// require log(size) time.
    ///
    /// <para/><b>NOTE</b>: this class will pre-allocate a full array of
    /// length <c>maxSize+1</c> if instantiated via the
    /// <see cref="PriorityQueue(int, bool)"/> constructor with
    /// <c>prepopulate</c> set to <c>true</c>. That maximum
    /// size can grow as we insert elements over the time.
    /// <para/>
    /// @lucene.internal
    /// </summary>
#if FEATURE_SERIALIZABLE
    [Serializable]
#endif
    public abstract class PriorityQueue<T>
    {
        private int size = 0;
        private readonly int maxSize;
        private readonly T[] heap;

        protected PriorityQueue(int maxSize) // LUCENENET specific - made protected instead of public
            : this(maxSize, true)
        {
        }

        protected PriorityQueue(int maxSize, bool prepopulate) // LUCENENET specific - made protected instead of public
        {
            int heapSize;
            if (0 == maxSize)
            {
                // We allocate 1 extra to avoid if statement in top()
                heapSize = 2;
            }
            else
            {
                if (maxSize > ArrayUtil.MAX_ARRAY_LENGTH)
                {
                    // Don't wrap heapSize to -1, in this case, which
                    // causes a confusing NegativeArraySizeException.
                    // Note that very likely this will simply then hit
                    // an OOME, but at least that's more indicative to
                    // caller that this values is too big.  We don't +1
                    // in this case, but it's very unlikely in practice
                    // one will actually insert this many objects into
                    // the PQ:
                    // Throw exception to prevent confusing OOME:
                    throw new ArgumentOutOfRangeException(nameof(maxSize), "maxSize must be <= " + ArrayUtil.MAX_ARRAY_LENGTH + "; got: " + maxSize); // LUCENENET specific - changed from IllegalArgumentException to ArgumentOutOfRangeException (.NET convention)
                }
                else
                {
                    // NOTE: we add +1 because all access to heap is
                    // 1-based not 0-based.  heap[0] is unused.
                    heapSize = maxSize + 1;
                }
            }
            // T is unbounded type, so this unchecked cast works always:
            T[] h = new T[heapSize];
            this.heap = h;
            this.maxSize = maxSize;

            if (prepopulate)
            {
                // If sentinel objects are supported, populate the queue with them
                T sentinel = GetSentinelObject();
                if (!EqualityComparer<T>.Default.Equals(sentinel, default))
                {
                    heap[1] = sentinel;
                    for (int i = 2; i < heap.Length; i++)
                    {
                        heap[i] = GetSentinelObject();
                    }
                    size = maxSize;
                }
            }
        }

        /// <summary>
        /// Determines the ordering of objects in this priority queue.  Subclasses
        /// must define this one method. </summary>
        /// <returns> <c>true</c> if parameter <paramref name="a"/> is less than parameter <paramref name="b"/>. </returns>
        protected internal abstract bool LessThan(T a, T b); // LUCENENET: Internal for testing

        /// <summary>
        /// This method can be overridden by extending classes to return a sentinel
        /// object which will be used by the <see cref="PriorityQueue(int, bool)"/>
        /// constructor to fill the queue, so that the code which uses that queue can always
        /// assume it's full and only change the top without attempting to insert any new
        /// object.
        /// <para/>
        /// Those sentinel values should always compare worse than any non-sentinel
        /// value (i.e., <see cref="LessThan(T, T)"/> should always favor the
        /// non-sentinel values).
        /// <para/>
        /// By default, this method returns <c>false</c>, which means the queue will not be
        /// filled with sentinel values. Otherwise, the value returned will be used to
        /// pre-populate the queue. Adds sentinel values to the queue.
        /// <para/>
        /// If this method is extended to return a non-null value, then the following
        /// usage pattern is recommended:
        ///
        /// <code>
        /// // extends GetSentinelObject() to return a non-null value.
        /// PriorityQueue&lt;MyObject&gt; pq = new MyQueue&lt;MyObject&gt;(numHits);
        /// // save the 'top' element, which is guaranteed to not be null.
        /// MyObject pqTop = pq.Top;
        /// &lt;...&gt;
        /// // now in order to add a new element, which is 'better' than top (after
        /// // you've verified it is better), it is as simple as:
        /// pqTop.Change().
        /// pqTop = pq.UpdateTop();
        /// </code>
        /// <para/>
        /// <b>NOTE:</b> if this method returns a non-<c>null</c> value, it will be called by
        /// the <see cref="PriorityQueue(int, bool)"/> constructor
        /// <see cref="Count"/> times, relying on a new object to be returned and will not
        /// check if it's <c>null</c> again. Therefore you should ensure any call to this
        /// method creates a new instance and behaves consistently, e.g., it cannot
        /// return <c>null</c> if it previously returned non-<c>null</c>.
        /// </summary>
        /// <returns> The sentinel object to use to pre-populate the queue, or <c>null</c> if
        ///         sentinel objects are not supported. </returns>
        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        protected virtual T GetSentinelObject()
        {
            return default;
        }

        /// <summary>
        /// Adds an Object to a <see cref="PriorityQueue{T}"/> in log(size) time. If one tries to add
        /// more objects than <see cref="maxSize"/> from initialize and it is not possible to resize
        /// the heap, an <see cref="IndexOutOfRangeException"/> is thrown.
        /// </summary>
        /// <returns> The new 'top' element in the queue. </returns>
        public T Add(T element)
        {
            size++;
            heap[size] = element;
            UpHeap();
            return heap[1];
        }

        /// <summary>
        /// Adds an Object to a <see cref="PriorityQueue{T}"/> in log(size) time.
        /// If the given <paramref name="element"/> is smaller than then full
        /// heap's minimum, it won't be added.
        /// </summary>
        public virtual void Insert(T element) // LUCENENET specific - added as a more efficient way to insert value types without reuse
        {
            if (size < maxSize)
            {
                Add(element);
            }
            else if (size > 0 && !LessThan(element, heap[1]))
            {
                heap[1] = element;
                UpdateTop();
            }
        }

        /// <summary>
        /// Adds an Object to a <see cref="PriorityQueue{T}"/> in log(size) time.
        /// It returns the object (if any) that was
        /// dropped off the heap because it was full. This can be
        /// the given parameter (in case it is smaller than the
        /// full heap's minimum, and couldn't be added), or another
        /// object that was previously the smallest value in the
        /// heap and now has been replaced by a larger one, or <c>null</c>
        /// if the queue wasn't yet full with <see cref="maxSize"/> elements.
        /// </summary>
        public virtual T InsertWithOverflow(T element)
        {
            if (size < maxSize)
            {
                Add(element);
                return default;
            }
            else if (size > 0 && !LessThan(element, heap[1]))
            {
                T ret = heap[1];
                heap[1] = element;
                UpdateTop();
                return ret;
            }
            else
            {
                return element;
            }
        }

        /// <summary>
        /// Returns the least element of the <see cref="PriorityQueue{T}"/> in constant time.
        /// Returns <c>null</c> if the queue is empty. </summary>
        public T Top =>
            // We don't need to check size here: if maxSize is 0,
            // then heap is length 2 array with both entries null.
            // If size is 0 then heap[1] is already null.
            heap[1];

        /// <summary>
        /// Removes and returns the least element of the <see cref="PriorityQueue{T}"/> in log(size)
        /// time.
        /// </summary>
        public T Pop()
        {
            if (size > 0)
            {
                T result = heap[1]; // save first value
                heap[1] = heap[size]; // move last to first
                heap[size] = default; // permit GC of objects
                size--;
                DownHeap(); // adjust heap
                return result;
            }
            else
            {
                return default;
            }
        }

        /// <summary>
        /// Should be called when the Object at top changes values. Still log(n) worst
        /// case, but it's at least twice as fast to
        ///
        /// <code>
        /// pq.Top.Change();
        /// pq.UpdateTop();
        /// </code>
        ///
        /// instead of
        ///
        /// <code>
        /// o = pq.Pop();
        /// o.Change();
        /// pq.Push(o);
        /// </code>
        /// </summary>
        /// <returns> The new 'top' element. </returns>
        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        public T UpdateTop()
        {
            DownHeap();
            return heap[1];
        }

        /// <summary>
        /// Returns the number of elements currently stored in the <see cref="PriorityQueue{T}"/>.
        /// NOTE: This was size() in Lucene.
        /// </summary>
        public int Count => size;

        /// <summary>
        /// Removes all entries from the <see cref="PriorityQueue{T}"/>. </summary>
        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        public void Clear()
        {
            for (int i = 0; i <= size; i++)
            {
                heap[i] = default;
            }
            size = 0;
        }

        private void UpHeap()
        {
            int i = size;
            T node = heap[i]; // save bottom node
            int j = i.TripleShift(1);
            while (j > 0 && LessThan(node, heap[j]))
            {
                heap[i] = heap[j]; // shift parents down
                i = j;
                j = j.TripleShift(1);
            }
            heap[i] = node; // install saved node
        }

        private void DownHeap()
        {
            int i = 1;
            T node = heap[i]; // save top node
            int j = i << 1; // find smaller child
            int k = j + 1;
            if (k <= size && LessThan(heap[k], heap[j]))
            {
                j = k;
            }
            while (j <= size && LessThan(heap[j], node))
            {
                heap[i] = heap[j]; // shift up child
                i = j;
                j = i << 1;
                k = j + 1;
                if (k <= size && LessThan(heap[k], heap[j]))
                {
                    j = k;
                }
            }
            heap[i] = node; // install saved node
        }

        /// <summary>
        /// This method returns the internal heap array as T[].
        /// <para/>
        /// @lucene.internal
        /// </summary>
        [WritableArray]
        [SuppressMessage("Microsoft.Performance", "CA1819", Justification = "Lucene's design requires some writable array properties")]
        protected T[] HeapArray => heap;
    }
}