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+// Copyright (c) 2011 The LevelDB Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file. See the AUTHORS file for names of contributors.
+//
+// Thread safety
+// -------------
+//
+// Writes require external synchronization, most likely a mutex.
+// Reads require a guarantee that the SkipList will not be destroyed
+// while the read is in progress. Apart from that, reads progress
+// without any internal locking or synchronization.
+//
+// Invariants:
+//
+// (1) Allocated nodes are never deleted until the SkipList is
+// destroyed. This is trivially guaranteed by the code since we
+// never delete any skip list nodes.
+//
+// (2) The contents of a Node except for the next/prev pointers are
+// immutable after the Node has been linked into the SkipList.
+// Only Insert() modifies the list, and it is careful to initialize
+// a node and use release-stores to publish the nodes in one or
+// more lists.
+//
+// ... prev vs. next pointer ordering ...
+
+#include <assert.h>
+#include <stdlib.h>
+#include "port/port.h"
+#include "util/arena.h"
+#include "util/random.h"
+
+namespace leveldb {
+
+class Arena;
+
+template<typename Key, class Comparator>
+class SkipList {
+ private:
+ struct Node;
+
+ 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;
+
+ // Iteration over the contents of a skip list
+ class Iterator {
+ public:
+ // Initialize an iterator over the specified list.
+ // The returned iterator is not valid.
+ explicit Iterator(const SkipList* list);
+
+ // Returns true iff the iterator is positioned at a valid node.
+ bool Valid() const;
+
+ // Returns the key at the current position.
+ // REQUIRES: Valid()
+ const Key& key() const;
+
+ // Advances to the next position.
+ // REQUIRES: Valid()
+ void Next();
+
+ // Advances to the previous position.
+ // REQUIRES: Valid()
+ void Prev();
+
+ // Advance to the first entry with a key >= target
+ void Seek(const Key& target);
+
+ // Position at the first entry in list.
+ // Final state of iterator is Valid() iff list is not empty.
+ void SeekToFirst();
+
+ // Position at the last entry in list.
+ // Final state of iterator is Valid() iff list is not empty.
+ void SeekToLast();
+
+ private:
+ const SkipList* list_;
+ Node* node_;
+ // Intentionally copyable
+ };
+
+ 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_; // Height of the entire list
+
+ inline int GetMaxHeight() const {
+ return static_cast<int>(
+ reinterpret_cast<intptr_t>(max_height_.NoBarrier_Load()));
+ }
+
+ // Read/written only by Insert().
+ Random rnd_;
+
+ Node* NewNode(const Key& key, int height);
+ int RandomHeight();
+ bool Equal(const Key& a, const Key& b) const { return (compare_(a, b) == 0); }
+
+ // Return true if key is greater than the data stored in "n"
+ bool KeyIsAfterNode(const Key& key, Node* n) const;
+
+ // Return the earliest node that comes at or after key.
+ // Return NULL if there is no such node.
+ //
+ // If prev is non-NULL, fills prev[level] with pointer to previous
+ // node at "level" for every level in [0..max_height_-1].
+ Node* FindGreaterOrEqual(const Key& key, Node** prev) const;
+
+ // Return the latest node with a key < key.
+ // Return head_ if there is no such node.
+ Node* FindLessThan(const Key& key) const;
+
+ // Return the last node in the list.
+ // Return head_ if list is empty.
+ Node* FindLast() const;
+
+ // No copying allowed
+ SkipList(const SkipList&);
+ void operator=(const SkipList&);
+};
+
+// Implementation details follow
+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_[n].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_[n].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_[n].NoBarrier_Load());
+ }
+ void NoBarrier_SetNext(int n, Node* x) {
+ assert(n >= 0);
+ next_[n].NoBarrier_Store(x);
+ }
+
+ private:
+ // Array of length equal to the node height. next_[0] is lowest level link.
+ port::AtomicPointer next_[1];
+};
+
+template<typename Key, class Comparator>
+typename SkipList<Key,Comparator>::Node*
+SkipList<Key,Comparator>::NewNode(const Key& key, int height) {
+ char* mem = arena_->AllocateAligned(
+ sizeof(Node) + sizeof(port::AtomicPointer) * (height - 1));
+ return new (mem) Node(key);
+}
+
+template<typename Key, class Comparator>
+inline SkipList<Key,Comparator>::Iterator::Iterator(const SkipList* list) {
+ list_ = list;
+ node_ = NULL;
+}
+
+template<typename Key, class Comparator>
+inline bool SkipList<Key,Comparator>::Iterator::Valid() const {
+ return node_ != NULL;
+}
+
+template<typename Key, class Comparator>
+inline const Key& SkipList<Key,Comparator>::Iterator::key() const {
+ assert(Valid());
+ return node_->key;
+}
+
+template<typename Key, class Comparator>
+inline void SkipList<Key,Comparator>::Iterator::Next() {
+ assert(Valid());
+ node_ = node_->Next(0);
+}
+
+template<typename Key, class Comparator>
+inline void SkipList<Key,Comparator>::Iterator::Prev() {
+ // Instead of using explicit "prev" links, we just search for the
+ // last node that falls before key.
+ assert(Valid());
+ node_ = list_->FindLessThan(node_->key);
+ if (node_ == list_->head_) {
+ node_ = NULL;
+ }
+}
+
+template<typename Key, class Comparator>
+inline void SkipList<Key,Comparator>::Iterator::Seek(const Key& target) {
+ node_ = list_->FindGreaterOrEqual(target, NULL);
+}
+
+template<typename Key, class Comparator>
+inline void SkipList<Key,Comparator>::Iterator::SeekToFirst() {
+ node_ = list_->head_->Next(0);
+}
+
+template<typename Key, class Comparator>
+inline void SkipList<Key,Comparator>::Iterator::SeekToLast() {
+ node_ = list_->FindLast();
+ if (node_ == list_->head_) {
+ node_ = NULL;
+ }
+}
+
+template<typename Key, class Comparator>
+int SkipList<Key,Comparator>::RandomHeight() {
+ // Increase height with probability 1 in kBranching
+ static const unsigned int kBranching = 4;
+ int height = 1;
+ while (height < kMaxHeight && ((rnd_.Next() % kBranching) == 0)) {
+ height++;
+ }
+ assert(height > 0);
+ assert(height <= kMaxHeight);
+ return height;
+}
+
+template<typename Key, class Comparator>
+bool SkipList<Key,Comparator>::KeyIsAfterNode(const Key& key, Node* n) const {
+ // NULL n is considered infinite
+ return (n != NULL) && (compare_(n->key, key) < 0);
+}
+
+template<typename Key, class Comparator>
+typename SkipList<Key,Comparator>::Node* SkipList<Key,Comparator>::FindGreaterOrEqual(const Key& key, Node** prev)
+ const {
+ Node* x = head_;
+ int level = GetMaxHeight() - 1;
+ 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;
+ if (level == 0) {
+ return next;
+ } else {
+ // Switch to next list
+ level--;
+ }
+ }
+ }
+}
+
+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;
+ }
+ }
+}
+
+template<typename Key, class Comparator>
+typename SkipList<Key,Comparator>::Node* SkipList<Key,Comparator>::FindLast()
+ const {
+ Node* x = head_;
+ int level = GetMaxHeight() - 1;
+ while (true) {
+ Node* next = x->Next(level);
+ if (next == NULL) {
+ if (level == 0) {
+ return x;
+ } else {
+ // Switch to next list
+ level--;
+ }
+ } else {
+ x = next;
+ }
+ }
+}
+
+template<typename Key, class Comparator>
+SkipList<Key,Comparator>::SkipList(Comparator cmp, Arena* arena)
+ : compare_(cmp),
+ arena_(arena),
+ head_(NewNode(0 /* any key will do */, kMaxHeight)),
+ max_height_(reinterpret_cast<void*>(1)),
+ rnd_(0xdeadbeef) {
+ for (int i = 0; i < kMaxHeight; i++) {
+ head_->SetNext(i, NULL);
+ }
+}
+
+template<typename Key, class Comparator>
+void SkipList<Key,Comparator>::Insert(const Key& key) {
+ // TODO(opt): We can use a barrier-free variant of FindGreaterOrEqual()
+ // here since Insert() is externally synchronized.
+ Node* prev[kMaxHeight];
+ Node* x = FindGreaterOrEqual(key, prev);
+
+ // Our data structure does not allow duplicate insertion
+ assert(x == NULL || !Equal(key, x->key));
+
+ int height = RandomHeight();
+ if (height > GetMaxHeight()) {
+ for (int i = GetMaxHeight(); i < height; i++) {
+ prev[i] = head_;
+ }
+ //fprintf(stderr, "Change height from %d to %d\n", 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<void*>(height));
+ }
+
+ x = NewNode(key, height);
+ 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);
+ }
+}
+
+template<typename Key, class Comparator>
+bool SkipList<Key,Comparator>::Contains(const Key& key) const {
+ Node* x = FindGreaterOrEqual(key, NULL);
+ if (x != NULL && Equal(key, x->key)) {
+ return true;
+ } else {
+ return false;
+ }
+}
+
+} // namespace leveldb