levelDB跳表实现
2016-03-02 23:22
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跳表的原理就是利用随机性建立索引,加速搜索,并且简化代码实现难度。具体的跳表原理不再赘述,主要是看了levelDB有一些实现细节的东西,凸显自己写的实现不足之处。
去除冗余的key
template<typename Key, class Comparator> struct SkipList<Key,Comparator>::Node { explicit Node(const Key& k) : key(k) { } Key const key; // Accessors/mutators for links. Wrapped in methods so we can // add the appropriate barriers as necessary. Node* Next(int n) { assert(n >= 0); // Use an 'acquire load' so that we observe a fully initialized // version of the returned Node. return reinterpret_cast<Node*>(next_ .Acquire_Load()); } void SetNext(int n, Node* x) { assert(n >= 0); // Use a 'release store' so that anybody who reads through this // pointer observes a fully initialized version of the inserted node. next_ .Release_Store(x); } // No-barrier variants that can be safely used in a few locations. Node* NoBarrier_Next(int n) { assert(n >= 0); return reinterpret_cast<Node*>(next_ .NoBarrier_Load()); } void NoBarrier_SetNext(int n, Node* x) { assert(n >= 0); next_ .NoBarrier_Store(x); } private: // Array of length equal to the node height. next_[0] is lowest level link. port::AtomicPointer next_[1]; };
这里使用一个Node节点表示所有相同key,不同高度的节点集合,仅保留了key和不同高度的向右指针,并且使用NewNode来动态分配随即高度的向右指针集合,而next_就指向这指针集合。这也是c/c++ tricky的地方。
#include <stdio.h> struct Node { char str[1]; }; int main() { char* mem = new char[4]; for (int i = 0; i < 4; i++) { mem[i] = i + '0'; } Node* node = (Node*)mem; char* const pstr = node->str; for (int i = 0; i < 4; i++) { printf("%c", pstr[i]); } return 0; }
就像上面这个简单的sample,成员str可以作为指针指向从数组下标0开始的元素,并且不受申明时的限制,不局限于大小1,索引至分配的最大的内存地址。
简易随机数生成
uint32_t Next() { static const uint32_t M = 2147483647L; // 2^31-1 static const uint64_t A = 16807; // bits 14, 8, 7, 5, 2, 1, 0 // We are computing // seed_ = (seed_ * A) % M, where M = 2^31-1 // // seed_ must not be zero or M, or else all subsequent computed values // will be zero or M respectively. For all other values, seed_ will end // up cycling through every number in [1,M-1] uint64_t product = seed_ * A; // Compute (product % M) using the fact that ((x << 31) % M) == x. seed_ = static_cast<uint32_t>((product >> 31) + (product & M)); // The first reduction may overflow by 1 bit, so we may need to // repeat. mod == M is not possible; using > allows the faster // sign-bit-based test. if (seed_ > M) { seed_ -= M; } return seed_; }
可以看到,他使用A和M对种子进行运算,达到一定数据范围内不会重复的数集,而里面对于(product % M),使用(product >> 31) + (product & M)进行运算优化,考虑右移和与操作的代价远小于取余操作。
简洁清晰的私有帮助方法,帮助寻找小于指定key的节点
template<typename Key, class Comparator> typename SkipList<Key,Comparator>::Node* SkipList<Key,Comparator>::FindLessThan(const Key& key) const { Node* x = head_; int level = GetMaxHeight() - 1; while (true) { assert(x == head_ || compare_(x->key, key) < 0); Node* next = x->Next(level); if (next == NULL || compare_(next->key, key) >= 0) { if (level == 0) { return x; } else { // Switch to next list level--; } } else { x = next; } } }
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