LevelDB源碼分析：skiplist

LevelDB中的skiplist實現(xiàn)方式基本上和中的實現(xiàn)方式類似。它向外暴露接口非常簡單，如下：

public:
??//?Create?a?new?SkipList?object?that?will?use?"cmp"?for?comparing?keys,
??//?and?will?allocate?memory?using?"*arena".??Objects?allocated?in?the?arena
??//?must?remain?allocated?for?the?lifetime?of?the?skiplist?object.
??explicit?SkipList(Comparator?cmp,?Arena*?arena);

??//?Insert?key?into?the?list.
??//?REQUIRES:?nothing?that?compares?equal?to?key?is?currently?in?the?list.
??void?Insert(const?Key&?key);

??//?Returns?true?iff?an?entry?that?compares?equal?to?key?is?in?the?list.
??bool?Contains(const?Key&?key)?const

private成員變量：

private:
??enum?{?kMaxHeight?=?12?};

??//?Immutable?after?construction
??Comparator?const?compare_;
??Arena*?const?arena_;????//?Arena?used?for?allocations?of?nodes

??Node*?const?head_;

??//?Modified?only?by?Insert().??Read?racily?by?readers,?but?stale
??//?values?are?ok.
??port::AtomicPointer?max_height_;???

??//?Read/written?only?by?Insert().
??Random?rnd_;

一.構(gòu)造函數(shù)

templateSkipList::SkipList(Comparator?cmp,?Arena*?arena)
????:?compare_(cmp),
??????arena_(arena),
??????head_(NewNode(0?/*?any?key?will?do?*/,?kMaxHeight)),
??????max_height_(reinterpret_cast(1)),
??????rnd_(0xdeadbeef)?{
??for?(int?i?=?0;?i?<?kMaxHeight;?i++)?{
????head_->SetNext(i,?NULL);
??}
}

重點注意下head_和rnd_的初始化，NewNode方法如下。

templatetypename?SkipList::Node*
SkipList::NewNode(const?Key&?key,?int?height)?{
??char*?mem?=?arena_->AllocateAligned(
??????sizeof(Node)?+?sizeof(port::AtomicPointer)?*?(height?-?1));
??return?new?(mem)?Node(key);
}

這里為什么是“height-1”詳見：LevelDb源碼分析之五：skiplist（1）。

new (mem) Node(key)使用了placement new技巧，詳見：C++中使用placement new

rnd_是一個Random類型的變量，使用0xdeadbeef進(jìn)行初始化，Random詳見LevelDB源碼分析之七：Random

二.插入函數(shù)

templatevoid?SkipList::Insert(const?Key&?key)?{
??//?TODO(opt):?We?can?use?a?barrier-free?variant?of?FindGreaterOrEqual()
??//?here?since?Insert()?is?externally?synchronized.
??//?prev記錄的是查詢路徑，下面需要使用prev來修改新生成結(jié)點的指針??
??//?prev相當(dāng)于LevelDb源碼分析之五：skiplist（1）中的update
??//?整個流程與LevelDb源碼分析之五：skiplist（1）相似，這里不再詳細(xì)解釋
??Node*?prev[kMaxHeight];
??//?返回大于等于key的結(jié)點或者NULL，原因詳見FindGreaterOrEqual的分析
??Node*?x?=?FindGreaterOrEqual(key,?prev);

??//?Our?data?structure?does?not?allow?duplicate?insertion
??//?不允許插入重復(fù)的值??
??assert(x?==?NULL?||?!Equal(key,?x->key));
??//?產(chǎn)生一個隨機(jī)層數(shù)height
??int?height?=?RandomHeight();
??//?如果height大于原最大層數(shù)，則更新prev，并更新最大層數(shù)
??if?(height?>?GetMaxHeight())?{
????for?(int?i?=?GetMaxHeight();?i?<?height;?i++)?{
??????prev[i]?=?head_;
????}
????//fprintf(stderr,?"Change?height?from?%d?to?%dn",?max_height_,?height);

????//?It?is?ok?to?mutate?max_height_?without?any?synchronization
????//?with?concurrent?readers.??A?concurrent?reader?that?observes
????//?the?new?value?of?max_height_?will?see?either?the?old?value?of
????//?new?level?pointers?from?head_?(NULL),?or?a?new?value?set?in
????//?the?loop?below.??In?the?former?case?the?reader?will
????//?immediately?drop?to?the?next?level?since?NULL?sorts?after?all
????//?keys.??In?the?latter?case?the?reader?will?use?the?new?node.
????max_height_.NoBarrier_Store(reinterpret_cast(height));
??}
??//?創(chuàng)建一個待插入的結(jié)點x，從低到高一層層插入
??x?=?NewNode(key,?height);
??//?逐層更新結(jié)點的指針，和普通鏈表插入一樣?
??for?(int?i?=?0;?i?<?height;?i++)?{
????//?NoBarrier_SetNext()?suffices?since?we?will?add?a?barrier?when
????//?we?publish?a?pointer?to?"x"?in?prev[i].
????x->NoBarrier_SetNext(i,?prev[i]->NoBarrier_Next(i));
????prev[i]->SetNext(i,?x);
??}
}

插入函數(shù)里調(diào)用了私有函數(shù)FindGreaterOrEqual。FindGreaterOrEqual中完成查詢操作，就是向下（level控制）和向右（x控制）移動過程，并不斷將經(jīng)過路徑保存到參數(shù)prev中。

templatetypename?SkipList::Node*?SkipList::FindGreaterOrEqual(const?Key&?key,?Node**?prev)
????const?{
??Node*?x?=?head_;
??int?level?=?GetMaxHeight()?-?1;
??//?從最高層往下，每層都查找插入位置，當(dāng)遍歷到該層的尾部（x->next[level]==NULL）??
??//?也沒有找到比key大的結(jié)點時，將該層的最后一個結(jié)點的指針放到prev[level]中。??
??//?如果在某層中找到了比key大或等于key的結(jié)點時，將該結(jié)點之前的那個比key小的結(jié)點的指針??
??//?放到prev[level]中。??
??while?(true)?{
????Node*?next?=?x->Next(level);
????if?(KeyIsAfterNode(key,?next))?{
??????//?Keep?searching?in?this?list
??????x?=?next;
????}?else?{
??????if?(prev?!=?NULL)?prev[level]?=?x;
	??//?當(dāng)查到第一層時，有兩種情況：
	??//?1.第一層中有滿足要求的結(jié)點，此時next剛好是不小于key的那個結(jié)點
	??//?2.第一層中沒有滿足要求的結(jié)點，此時實際上到了尾部，next=NULL
??????if?(level?==?0)?{
????????return?next;
??????}?else?{
????????//?Switch?to?next?list
????????level--;
??????}
????}
??}
}

三.查找函數(shù)

查找操作基本上就是調(diào)用函數(shù)上面的函數(shù)FindGreaterOrEqual實現(xiàn)。

templatebool?SkipList::Contains(const?Key&?key)?const?{
??Node*?x?=?FindGreaterOrEqual(key,?NULL);
??if?(x?!=?NULL?&&?Equal(key,?x->key))?{
????return?true;
??}?else?{
????return?false;
??}
}

需要注意的是，LevelDB中沒有提供顯式的刪除節(jié)點操作，但實際上是可以刪除的，因為當(dāng)我們插入數(shù)據(jù)時，key的形式為key:value，當(dāng)刪除數(shù)據(jù)時，則插入key:deleted類似刪除的標(biāo)記，等到Compaction再刪除。