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383 lines
11 KiB
383 lines
11 KiB
// Copyright (c) 2011 The LevelDB Authors. All rights reserved.
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// Use of this source code is governed by a BSD-style license that can be
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// found in the LICENSE file. See the AUTHORS file for names of contributors.
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#ifndef STORAGE_LEVELDB_DB_SKIPLIST_H_
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#define STORAGE_LEVELDB_DB_SKIPLIST_H_
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// Thread safety
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// -------------
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//
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// Writes require external synchronization, most likely a mutex.
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// Reads require a guarantee that the SkipList will not be destroyed
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// while the read is in progress. Apart from that, reads progress
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// without any internal locking or synchronization.
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//
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// Invariants:
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//
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// (1) Allocated nodes are never deleted until the SkipList is
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// destroyed. This is trivially guaranteed by the code since we
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// never delete any skip list nodes.
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//
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// (2) The contents of a Node except for the next/prev pointers are
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// immutable after the Node has been linked into the SkipList.
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// Only Insert() modifies the list, and it is careful to initialize
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// a node and use release-stores to publish the nodes in one or
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// more lists.
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//
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// ... prev vs. next pointer ordering ...
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#include <atomic>
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#include <cassert>
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#include <cstdlib>
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#include "util/arena.h"
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#include "util/random.h"
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namespace leveldb {
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class Arena;
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template<typename Key, class Comparator>
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class SkipList {
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private:
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struct Node;
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public:
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// Create a new SkipList object that will use "cmp" for comparing keys,
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// and will allocate memory using "*arena". Objects allocated in the arena
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// must remain allocated for the lifetime of the skiplist object.
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explicit SkipList(Comparator cmp, Arena* arena);
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// Insert key into the list.
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// REQUIRES: nothing that compares equal to key is currently in the list.
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void Insert(const Key& key);
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// Returns true iff an entry that compares equal to key is in the list.
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bool Contains(const Key& key) const;
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// Iteration over the contents of a skip list
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class Iterator {
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public:
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// Initialize an iterator over the specified list.
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// The returned iterator is not valid.
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explicit Iterator(const SkipList* list);
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// Returns true iff the iterator is positioned at a valid node.
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bool Valid() const;
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// Returns the key at the current position.
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// REQUIRES: Valid()
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const Key& key() const;
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// Advances to the next position.
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// REQUIRES: Valid()
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void Next();
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// Advances to the previous position.
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// REQUIRES: Valid()
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void Prev();
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// Advance to the first entry with a key >= target
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void Seek(const Key& target);
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// Position at the first entry in list.
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// Final state of iterator is Valid() iff list is not empty.
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void SeekToFirst();
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// Position at the last entry in list.
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// Final state of iterator is Valid() iff list is not empty.
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void SeekToLast();
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private:
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const SkipList* list_;
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Node* node_;
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// Intentionally copyable
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};
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private:
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enum { kMaxHeight = 12 };
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// Immutable after construction
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Comparator const compare_;
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Arena* const arena_; // Arena used for allocations of nodes
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Node* const head_;
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// Modified only by Insert(). Read racily by readers, but stale
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// values are ok.
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std::atomic<int> max_height_; // Height of the entire list
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inline int GetMaxHeight() const {
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return max_height_.load(std::memory_order_relaxed);
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}
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// Read/written only by Insert().
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Random rnd_;
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Node* NewNode(const Key& key, int height);
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int RandomHeight();
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bool Equal(const Key& a, const Key& b) const { return (compare_(a, b) == 0); }
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// Return true if key is greater than the data stored in "n"
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bool KeyIsAfterNode(const Key& key, Node* n) const;
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// Return the earliest node that comes at or after key.
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// Return nullptr if there is no such node.
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//
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// If prev is non-null, fills prev[level] with pointer to previous
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// node at "level" for every level in [0..max_height_-1].
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Node* FindGreaterOrEqual(const Key& key, Node** prev) const;
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// Return the latest node with a key < key.
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// Return head_ if there is no such node.
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Node* FindLessThan(const Key& key) const;
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// Return the last node in the list.
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// Return head_ if list is empty.
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Node* FindLast() const;
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// No copying allowed
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SkipList(const SkipList&);
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void operator=(const SkipList&);
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};
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// Implementation details follow
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template<typename Key, class Comparator>
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struct SkipList<Key, Comparator>::Node {
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explicit Node(const Key& k) : key(k) { }
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Key const key;
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// Accessors/mutators for links. Wrapped in methods so we can
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// add the appropriate barriers as necessary.
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Node* Next(int n) {
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assert(n >= 0);
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// Use an 'acquire load' so that we observe a fully initialized
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// version of the returned Node.
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return next_[n].load(std::memory_order_acquire);
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}
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void SetNext(int n, Node* x) {
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assert(n >= 0);
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// Use a 'release store' so that anybody who reads through this
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// pointer observes a fully initialized version of the inserted node.
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next_[n].store(x, std::memory_order_release);
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}
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// No-barrier variants that can be safely used in a few locations.
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Node* NoBarrier_Next(int n) {
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assert(n >= 0);
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return next_[n].load(std::memory_order_relaxed);
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}
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void NoBarrier_SetNext(int n, Node* x) {
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assert(n >= 0);
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next_[n].store(x, std::memory_order_relaxed);
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}
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private:
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// Array of length equal to the node height. next_[0] is lowest level link.
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std::atomic<Node*> next_[1];
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};
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template<typename Key, class Comparator>
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typename SkipList<Key, Comparator>::Node*
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SkipList<Key, Comparator>::NewNode(const Key& key, int height) {
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char* const node_memory = arena_->AllocateAligned(
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sizeof(Node) + sizeof(std::atomic<Node*>) * (height - 1));
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return new (node_memory) Node(key);
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}
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template<typename Key, class Comparator>
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inline SkipList<Key, Comparator>::Iterator::Iterator(const SkipList* list) {
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list_ = list;
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node_ = nullptr;
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}
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template<typename Key, class Comparator>
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inline bool SkipList<Key, Comparator>::Iterator::Valid() const {
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return node_ != nullptr;
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}
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template<typename Key, class Comparator>
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inline const Key& SkipList<Key, Comparator>::Iterator::key() const {
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assert(Valid());
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return node_->key;
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}
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template<typename Key, class Comparator>
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inline void SkipList<Key, Comparator>::Iterator::Next() {
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assert(Valid());
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node_ = node_->Next(0);
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}
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template<typename Key, class Comparator>
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inline void SkipList<Key, Comparator>::Iterator::Prev() {
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// Instead of using explicit "prev" links, we just search for the
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// last node that falls before key.
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assert(Valid());
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node_ = list_->FindLessThan(node_->key);
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if (node_ == list_->head_) {
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node_ = nullptr;
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}
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}
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template<typename Key, class Comparator>
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inline void SkipList<Key, Comparator>::Iterator::Seek(const Key& target) {
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node_ = list_->FindGreaterOrEqual(target, nullptr);
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}
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template<typename Key, class Comparator>
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inline void SkipList<Key, Comparator>::Iterator::SeekToFirst() {
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node_ = list_->head_->Next(0);
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}
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template<typename Key, class Comparator>
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inline void SkipList<Key, Comparator>::Iterator::SeekToLast() {
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node_ = list_->FindLast();
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if (node_ == list_->head_) {
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node_ = nullptr;
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}
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}
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template<typename Key, class Comparator>
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int SkipList<Key, Comparator>::RandomHeight() {
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// Increase height with probability 1 in kBranching
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static const unsigned int kBranching = 4;
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int height = 1;
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while (height < kMaxHeight && ((rnd_.Next() % kBranching) == 0)) {
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height++;
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}
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assert(height > 0);
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assert(height <= kMaxHeight);
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return height;
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}
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template<typename Key, class Comparator>
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bool SkipList<Key, Comparator>::KeyIsAfterNode(const Key& key, Node* n) const {
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// null n is considered infinite
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return (n != nullptr) && (compare_(n->key, key) < 0);
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}
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template<typename Key, class Comparator>
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typename SkipList<Key, Comparator>::Node*
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SkipList<Key, Comparator>::FindGreaterOrEqual(const Key& key,
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Node** prev) const {
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Node* x = head_;
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int level = GetMaxHeight() - 1;
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while (true) {
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Node* next = x->Next(level);
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if (KeyIsAfterNode(key, next)) {
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// Keep searching in this list
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x = next;
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} else {
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if (prev != nullptr) prev[level] = x;
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if (level == 0) {
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return next;
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} else {
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// Switch to next list
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level--;
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}
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}
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}
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}
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template<typename Key, class Comparator>
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typename SkipList<Key, Comparator>::Node*
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SkipList<Key, Comparator>::FindLessThan(const Key& key) const {
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Node* x = head_;
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int level = GetMaxHeight() - 1;
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while (true) {
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assert(x == head_ || compare_(x->key, key) < 0);
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Node* next = x->Next(level);
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if (next == nullptr || compare_(next->key, key) >= 0) {
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if (level == 0) {
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return x;
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} else {
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// Switch to next list
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level--;
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}
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} else {
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x = next;
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}
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}
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}
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template<typename Key, class Comparator>
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typename SkipList<Key, Comparator>::Node* SkipList<Key, Comparator>::FindLast()
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const {
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Node* x = head_;
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int level = GetMaxHeight() - 1;
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while (true) {
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Node* next = x->Next(level);
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if (next == nullptr) {
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if (level == 0) {
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return x;
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} else {
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// Switch to next list
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level--;
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}
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} else {
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x = next;
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}
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}
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}
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template<typename Key, class Comparator>
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SkipList<Key, Comparator>::SkipList(Comparator cmp, Arena* arena)
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: compare_(cmp),
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arena_(arena),
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head_(NewNode(0 /* any key will do */, kMaxHeight)),
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max_height_(1),
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rnd_(0xdeadbeef) {
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for (int i = 0; i < kMaxHeight; i++) {
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head_->SetNext(i, nullptr);
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}
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}
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template<typename Key, class Comparator>
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void SkipList<Key, Comparator>::Insert(const Key& key) {
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// TODO(opt): We can use a barrier-free variant of FindGreaterOrEqual()
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// here since Insert() is externally synchronized.
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Node* prev[kMaxHeight];
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Node* x = FindGreaterOrEqual(key, prev);
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// Our data structure does not allow duplicate insertion
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assert(x == nullptr || !Equal(key, x->key));
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int height = RandomHeight();
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if (height > GetMaxHeight()) {
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for (int i = GetMaxHeight(); i < height; i++) {
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prev[i] = head_;
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}
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// It is ok to mutate max_height_ without any synchronization
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// with concurrent readers. A concurrent reader that observes
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// the new value of max_height_ will see either the old value of
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// new level pointers from head_ (nullptr), or a new value set in
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// the loop below. In the former case the reader will
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// immediately drop to the next level since nullptr sorts after all
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// keys. In the latter case the reader will use the new node.
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max_height_.store(height, std::memory_order_relaxed);
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}
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x = NewNode(key, height);
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for (int i = 0; i < height; i++) {
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// NoBarrier_SetNext() suffices since we will add a barrier when
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// we publish a pointer to "x" in prev[i].
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x->NoBarrier_SetNext(i, prev[i]->NoBarrier_Next(i));
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prev[i]->SetNext(i, x);
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}
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}
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template<typename Key, class Comparator>
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bool SkipList<Key, Comparator>::Contains(const Key& key) const {
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Node* x = FindGreaterOrEqual(key, nullptr);
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if (x != nullptr && Equal(key, x->key)) {
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return true;
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} else {
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return false;
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}
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}
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} // namespace leveldb
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#endif // STORAGE_LEVELDB_DB_SKIPLIST_H_
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