??????在系統(tǒng)中，我們經(jīng)常會(huì)遇到這樣的需求：將大量（比如幾十萬(wàn)、甚至上百萬(wàn)）的對(duì)象進(jìn)行排序，然后只需要取出最Top的前N名作為排行榜的數(shù)據(jù)，這即是一個(gè)TopN算法。常見(jiàn)的解決方案有三種：

（1）直接使用List的Sort方法進(jìn)行處理。

（2）使用排序二叉樹(shù)進(jìn)行排序，然后取出前N名。

（3）使用最大堆排序，然后取出前N名。

??????第一種方案的性能是最差的，后兩種方案性能會(huì)好一些，但是還是不能滿足我們的需求。最主要的原因在于使用二叉樹(shù)和最大堆排序時(shí)，都是對(duì)所有的對(duì)象進(jìn)行排序，而不是將代價(jià)花費(fèi)在我們需要的少數(shù)的TopN上。為此，我自己實(shí)現(xiàn)了TopNOrderedContainer來(lái)解決這個(gè)問(wèn)題。

??????思路是這樣的，使用一個(gè)長(zhǎng)度為N的數(shù)組，來(lái)存放最Top的N個(gè)對(duì)象，越Top的對(duì)象其在數(shù)組中的Index就越小。這樣，每次加入一個(gè)對(duì)象時(shí)，就與Index最大的那個(gè)對(duì)象比較，如果比其更Top，則交換兩個(gè)對(duì)象的位置。如果被交換的對(duì)象是數(shù)組中的最后一個(gè)對(duì)象（Index最大），則該對(duì)象會(huì)被拋棄。如此，可以保證容器中始終保持的都是最Top的N個(gè)對(duì)象。

??????接下來(lái)我們看具體的實(shí)現(xiàn)。

??????如果一個(gè)對(duì)象要參與TopN排行榜，則其必須實(shí)現(xiàn)IOrdered接口，表明其可以被Top排序。

??? ///?<summary>
????///?IOrdered?參與排行榜排序的對(duì)象必須實(shí)現(xiàn)的接口。
????///?</summary>
????///?<typeparam?name="TOrderedObj">參與排行榜排序的對(duì)象的類型</typeparam>
????public?interface?IOrdered<TOrderedObj>
????{
????????bool?IsTopThan(TOrderedObj?other);
????}

??????之所以使用泛型參數(shù)TOrderedObj，是為了避免派生類在實(shí)現(xiàn)IsTopThan方法時(shí)，需要將參數(shù)other進(jìn)行向下轉(zhuǎn)換。

??????接下來(lái)是TopNOrderedContainer實(shí)現(xiàn)的源碼：

??? ///?<summary>
????///?TopNOrderedContainer?用于始終保持排行榜前N名的Object。該實(shí)現(xiàn)是線程安全的。
????///?zhuweisky?2009.05.23
????///?</summary>
????///?<typeparam?name="TID">被排名的對(duì)象的標(biāo)志類型</typeparam>
????///?<typeparam?name="TObj">被排名的對(duì)象類型</typeparam>
????public?class?TopNOrderedContainer<TObj>?where?TObj?:?IOrdered<TObj>
????{
????????private?TObj[]?orderedArray?=?null;
????????private?int?validObjCount?=?0;
????????private?SmartRWLocker?smartRWLocker?=?new?SmartRWLocker();

????????#region?TopNumber
????????private?int?topNumber?=?10;
????????public?int?TopNumber
????????{
????????????get?{?return?topNumber;?}
????????????set?{?topNumber?=?value;?}
????????}?
????????#endregion

????????#region?Ctor
????????public?TopNOrderedContainer()?{?}
????????public?TopNOrderedContainer(int?_topNumber)
????????{
????????????this.topNumber?=?_topNumber;
????????}
????????#endregion

????????#region?Initialize
????????public?void?Initialize()
????????{
????????????if?(this.topNumber?<?1)
????????????{
????????????????throw?new?Exception("The?value?of?TopNumber?must?greater?than?0?");
????????????}

????????????this.orderedArray?=?new?TObj[this.topNumber];
????????}?
????????#endregion

????????#region?Add?List
????????public?void?Add(IList<TObj>?list)
????????{
????????????if?(list?==?null)
????????????{
????????????????return;
????????????}

????????????using?(this.smartRWLocker.Lock(AccessMode.Write))
????????????{
????????????????foreach?(TObj?obj?in?list)
????????????????{
????????????????????this.DoAdd(obj);
????????????????}
????????????}
????????}?
????????#endregion

????????#region?Add
????????public?void?Add(TObj?obj)
????????{
????????????using?(this.smartRWLocker.Lock(AccessMode.Write))
????????????{
????????????????this.DoAdd(obj);
????????????}
????????}?
????????#endregion????????

????????#region?GetTopN
????????public?TObj[]?GetTopN()
????????{
????????????using?(this.smartRWLocker.Lock(AccessMode.Read))
????????????{
????????????????return?(TObj[])this.orderedArray.Clone();
????????????}
????????}?
????????#endregion

????????#region?Private
????????#region?DoAdd
????????private?void?DoAdd(TObj?obj)
????????{
????????????if?(obj?==?null)
????????????{
????????????????return;
????????????}

????????????if?(this.validObjCount?<?this.topNumber)
????????????{
????????????????this.orderedArray[this.validObjCount]?=?obj;
????????????????this.Adjust(this.validObjCount);

????????????????++this.validObjCount;
????????????????return;
????????????}

????????????if?(this.orderedArray[this.topNumber?-?1].IsTopThan(obj))
????????????{
????????????????return;
????????????}

????????????this.orderedArray[this.topNumber?-?1]?=?obj;
????????????this.Adjust(this.topNumber?-?1);
????????}
????????#endregion

????????#region?Adjust
????????///?<summary>
????????///?Adjust?調(diào)整posIndex處的對(duì)象到合適的位置。
????????///?與相鄰前一個(gè)對(duì)象比較，如果當(dāng)前對(duì)象更加Top，則與前一個(gè)對(duì)象交換位置。
????????///?</summary>???????
????????private?void?Adjust(int?posIndex)
????????{
????????????TObj?obj?=?this.orderedArray[posIndex];
????????????for?(int?index?=?posIndex;?index?>?0;?index--)
????????????{
????????????????if?(obj.IsTopThan(this.orderedArray[index?-?1]))
????????????????{
????????????????????TObj?temp?=?this.orderedArray[index?-?1];
????????????????????this.orderedArray[index?-?1]?=?obj;
????????????????????this.orderedArray[index]?=?temp;
????????????????}
????????????????else
????????????????{
????????????????????break;
????????????????}
????????????}
????????}
????????#endregion
????????#endregion
????}

??????源碼面前毫無(wú)秘密。

??????但是有幾點(diǎn)我還是需要說(shuō)明一下：

（1）ESBasic.ObjectManagement.TopNOrderedContainer位于我的ESBasic.dll類庫(kù)中，其實(shí)現(xiàn)時(shí)用到的SmartRWLocker是一個(gè)讀寫鎖，也是ESBasic.dll類庫(kù)中的一員。你可以從這里下載ESBasic.dll直接試用。

（2）為何不將TopN排序直接實(shí)現(xiàn)為一個(gè)靜態(tài)方法，如：

??????public?static?TObj[]?GetTopN<TObj>(IList<TObj>?list)?where?TObj?:?IOrdered<TObj>

??????如果要是這樣實(shí)現(xiàn)，那我們就沒(méi)有辦法繼續(xù)動(dòng)態(tài)的Add新的TObj對(duì)象進(jìn)來(lái)，如果要達(dá)到這樣的目的，就只有構(gòu)造新的list，再次調(diào)用static GetTopN方法，如此會(huì)重復(fù)做一些工作。

??????最后，我們來(lái)測(cè)試一下TopNOrderedContainer與List.Sort方法的性能比較，測(cè)試的對(duì)象數(shù)目為500000個(gè)，取出Top20。測(cè)試代碼如下：??

????public?class?UserData?:?IOrdered<UserData>
????{
????????#region?UserID
????????private?string?userID;
????????public?string?UserID
????????{
????????????get?{?return?userID;?}
????????????set?{?userID?=?value;?}
????????}?
????????#endregion

????????#region?Score
????????private?int?score;
????????public?int?Score
????????{
????????????get?{?return?score;?}
????????????set?{?score?=?value;?}
????????}?
????????#endregion

????????public?UserData(string?_userID,?int?_score)
????????{
????????????this.userID?=?_userID;
????????????this.score?=?_score;
????????}

????????#region?IOrdered<string>?成員???????

????????public?bool?IsTopThan(UserData?other)
????????{
????????????return?this.Score?>?other.Score;
????????}

????????public?override?string?ToString()
????????{
????????????return?this.score.ToString();
????????}
????????#endregion
????}

?

??????? private?void?button4_Click(object?sender,?EventArgs?e)
????????{
????????????List<UserData>?list?=?new?List<UserData>();
????????????for?(int?i?=?0;?i?<?500000;?i++)
????????????{
????????????????list.Add(new?UserData("User"?+?i.ToString(),?i?*?i?*?i?-?3?*?i?*?i?+?4?*?i?+?8));
????????????}

????????????List<UserData>?list2?=?new?List<UserData>();
????????????for?(int?i?=?0;?i?<?500000;?i++)
????????????{
????????????????list2.Add(new?UserData("User"?+?i.ToString(),?i?*?i?*?i?-?3?*?i?*?i?+?4?*?i?+?8));
????????????}

????????????Stopwatch?stopwatch?=?new?Stopwatch();
????????????stopwatch.Start();
????????????list.Sort(this);
????????????stopwatch.Stop();
????????????long?ms1?=?stopwatch.ElapsedMilliseconds;

????????????stopwatch.Reset();
????????????stopwatch.Start();
????????????TopNOrderedContainer<UserData>?container?=?new?TopNOrderedContainer<UserData>(20);
????????????container.Initialize();
????????????container.Add(list2);
????????????UserData[]?res?=?container.GetTopN();
????????????stopwatch.Stop();
????????????long?ms2?=?stopwatch.ElapsedMilliseconds;
????????}???????

????????#region?IComparer<UserData>?成員
????????public?int?Compare(UserData?x,?UserData?y)
????????{
????????????return?(y.Score?-?x.Score);
????????}
????????#endregion

??????測(cè)試的結(jié)果顯示，使用List.Sort方法需要1287ms，而TopNOrderedContainer只花了78ms。

?

?

?

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