From: http://pastebin.com/NHDdrbYV

LuceneNetExtensions.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace System.Linq
{
    public static class LuceneNetExtensions
    {
        /// <summary>
        /// <para>
        /// Replacement for list.OrderBy(?).Take(n)
        /// </para>
        /// <para>
        /// 1- Low Memory usage (since this method requires only ~"2*n" elements be in memory)
        /// </para>
        /// 2- Speed up by an order of magnitude especially when the input is large.
        /// </summary>
        public static IEnumerable<T> TakeOrdered<T, TKey>(this IEnumerable<T> list, int n, Func<T, TKey> keySelector, bool ascending = true) where TKey : IComparable<TKey>
        {

            var pq = new MyPriorityQueue<T>(n,
                    ascending
                        ?
                        (Func<T, T, bool>)((a, b) => keySelector(a).CompareTo(keySelector(b)) >= 0)
                        :
                        (Func<T, T, bool>)((a, b) => keySelector(a).CompareTo(keySelector(b)) < 0)
                );

            Stack<T> stack = new Stack<T>();

            int count = 0;
            foreach (T item in list)
            {
                pq.InsertWithOverflow(item);
                count++;
            }

            int min = Math.Min(count, n);

            for (int i = 0; i < min; i++)
                stack.Push(pq.Pop());

            for (int i = 0; i < min; i++)
                yield return stack.Pop();

        }

        public static IEnumerable<T> TakeOrdered<T>(this IEnumerable<T> list, int n, bool ascending = true) where T : IComparable<T>
        {
            return TakeOrdered(list, n, x => x,ascending);
        }


        class MyPriorityQueue<T> : Lucene.Net.Util.PriorityQueue<T>
        {
            Func<T, T, bool> _LessThan;

            internal MyPriorityQueue(int n, Func<T, T, bool> lessThan)
            {
                _LessThan = lessThan;
                Initialize(n);
            }

            public override bool LessThan(T a, T b)
            {
                return _LessThan(a, b);
            }
        }
    }
}

#region Lucene.Net
/// --------------------------------------------------------------------------------------
/// PriorityQueue from Lucene.Net
/// http://incubator.apache.org/lucene.net/
/// 
/// Remove this part if you already have a reference to Lucene.Net
/// --------------------------------------------------------------------------------------

/* 
 * 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.
 */

namespace Lucene.Net.Util
{

    /// <summary>A PriorityQueue maintains a partial ordering of its elements such that the
    /// least element can always be found in constant time.  Put()'s and pop()'s
    /// require log(size) time.
    /// 
    /// <p/><b>NOTE</b>: This class pre-allocates a full array of
    /// length <code>maxSize+1</code>, in {@link #initialize}.
    /// 
    /// </summary>
    public abstract class PriorityQueue<T>
    {
        private int size;
        private int maxSize;
        protected internal T[] heap;

        /// <summary>Determines the ordering of objects in this priority queue.  Subclasses
        /// must define this one method. 
        /// </summary>
        public abstract bool LessThan(T a, T b);

        /// <summary> This method can be overridden by extending classes to return a sentinel
        /// object which will be used by {@link #Initialize(int)} 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.<br/>
        /// 
        /// Those sentinel values should always compare worse than any non-sentinel
        /// value (i.e., {@link #LessThan(Object, Object)} should always favor the
        /// non-sentinel values).<br/>
        /// 
        /// By default, this method returns false, 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.<br/>
        /// 
        /// If this method is extended to return a non-null value, then the following
        /// usage pattern is recommended:
        /// 
        /// <pre>
        /// // extends getSentinelObject() to return a non-null value.
        /// PriorityQueue pq = new MyQueue(numHits);
        /// // save the 'top' element, which is guaranteed to not be null.
        /// MyObject pqTop = (MyObject) 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();
        /// </pre>
        /// 
        /// <b>NOTE:</b> if this method returns a non-null value, it will be called by
        /// {@link #Initialize(int)} {@link #Size()} times, relying on a new object to
        /// be returned and will not check if it's null again. Therefore you should
        /// ensure any call to this method creates a new instance and behaves
        /// consistently, e.g., it cannot return null if it previously returned
        /// non-null.
        /// 
        /// </summary>
        /// <returns> the sentinel object to use to pre-populate the queue, or null if
        /// sentinel objects are not supported.
        /// </returns>
        protected internal virtual T GetSentinelObject()
        {
            return default(T);
        }

        /// <summary>Subclass constructors must call this. </summary>
        protected internal void Initialize(int maxSize)
        {
            size = 0;
            int heapSize;
            if (0 == maxSize)
                // We allocate 1 extra to avoid if statement in top()
                heapSize = 2;
            else
            {
                if (maxSize == Int32.MaxValue)
                {
                    // 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:
                    heapSize = Int32.MaxValue;
                }
                else
                {
                    // NOTE: we add +1 because all access to heap is
                    // 1-based not 0-based.  heap[0] is unused.
                    heapSize = maxSize + 1;
                }
            }
            heap = new T[heapSize];
            this.maxSize = maxSize;

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

        /// <summary> Adds an Object to a PriorityQueue in log(size) time. If one tries to add
        /// more objects than maxSize from initialize a RuntimeException
        /// (ArrayIndexOutOfBound) is thrown.
        /// 
        /// </summary>
        /// <deprecated> use {@link #Add(Object)} which returns the new top object,
        /// saving an additional call to {@link #Top()}.
        /// </deprecated>
        [Obsolete("use Add(Object) which returns the new top object, saving an additional call to Top().")]
        public void Put(T element)
        {
            size++;
            heap[size] = element;
            UpHeap();
        }

        /// <summary> Adds an Object to a PriorityQueue in log(size) time. If one tries to add
        /// more objects than maxSize from initialize an
        /// {@link ArrayIndexOutOfBoundsException} 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 element to the PriorityQueue in log(size) time if either the
        /// PriorityQueue is not full, or not lessThan(element, top()).
        /// 
        /// </summary>
        /// <param name="element">
        /// </param>
        /// <returns> true if element is added, false otherwise.
        /// </returns>
        /// <deprecated> use {@link #InsertWithOverflow(Object)} instead, which
        /// encourages objects reuse.
        /// </deprecated>
        [Obsolete("use InsertWithOverflow(Object) instead, which encourages objects reuse.")]
        public virtual bool Insert(T element)
        {
            return !element.Equals(InsertWithOverflow(element));
        }

        /// <summary> insertWithOverflow() is the same as insert() except its
        /// return value: 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 null
        /// if the queue wasn't yet full with maxSize elements.
        /// </summary>
        public virtual T InsertWithOverflow(T element)
        {
            if (size < maxSize)
            {
                Put(element);
                return default(T);
            }
            else if (size > 0 && !LessThan(element, heap[1]))
            {
                T ret = heap[1];
                heap[1] = element;
                AdjustTop();
                return ret;
            }
            else
            {
                return element;
            }
        }

        /// <summary>Returns the least element of the PriorityQueue in constant time. </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.
            return heap[1];
        }

        /// <summary>Removes and returns the least element of the PriorityQueue 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(T); // permit GC of objects
                size--;
                DownHeap(); // adjust heap
                return result;
            }
            else
                return default(T);
        }

        /// <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
        /// 
        /// <pre>
        /// pq.top().change();
        /// pq.adjustTop();
        /// </pre>
        /// 
        /// instead of
        /// 
        /// <pre>
        /// o = pq.pop();
        /// o.change();
        /// pq.push(o);
        /// </pre>
        /// 
        /// </summary>
        /// <deprecated> use {@link #UpdateTop()} which returns the new top element and
        /// saves an additional call to {@link #Top()}.
        /// </deprecated>
        [Obsolete("use UpdateTop() which returns the new top element and saves an additional call to Top()")]
        public void AdjustTop()
        {
            DownHeap();
        }

        /// <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
        /// 
        /// <pre>
        /// pq.top().change();
        /// pq.updateTop();
        /// </pre>
        /// 
        /// instead of
        /// 
        /// <pre>
        /// o = pq.pop();
        /// o.change();
        /// pq.push(o);
        /// </pre>
        /// 
        /// </summary>
        /// <returns> the new 'top' element.
        /// </returns>
        public T UpdateTop()
        {
            DownHeap();
            return heap[1];
        }

        /// <summary>Returns the number of elements currently stored in the PriorityQueue. </summary>
        public int Size()
        {
            return size;
        }

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

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

        public int URShift(int number, int bits)
        {
            return (int)(((uint)number) >> bits);
        }
    }
}
#endregion