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+// Copyright (c) 2011-present, Facebook, Inc. All rights reserved.
+// This source code is licensed under both the GPLv2 (found in the
+// COPYING file in the root directory) and Apache 2.0 License
+// (found in the LICENSE.Apache file in the root directory).
+//
+// 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 ...
+//
+
+#pragma once
+#include <assert.h>
+#include <stdlib.h>
+#include <atomic>
+#include "memory/allocator.h"
+#include "port/port.h"
+#include "util/random.h"
+
+namespace ROCKSDB_NAMESPACE {
+
+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 "*allocator". Objects allocated in the
+ // allocator must remain allocated for the lifetime of the skiplist object.
+ explicit SkipList(Comparator cmp, Allocator* allocator,
+ int32_t max_height = 12, int32_t branching_factor = 4);
+ // No copying allowed
+ SkipList(const SkipList&) = delete;
+ void operator=(const SkipList&) = delete;
+
+ // 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;
+
+ // Return estimated number of entries smaller than `key`.
+ uint64_t EstimateCount(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);
+
+ // Change the underlying skiplist used for this iterator
+ // This enables us not changing the iterator without deallocating
+ // an old one and then allocating a new one
+ void SetList(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);
+
+ // Retreat to the last entry with a key <= target
+ void SeekForPrev(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:
+ const uint16_t kMaxHeight_;
+ const uint16_t kBranching_;
+ const uint32_t kScaledInverseBranching_;
+
+ // Immutable after construction
+ Comparator const compare_;
+ Allocator* const allocator_; // Allocator used for allocations of nodes
+
+ Node* const head_;
+
+ // Modified only by Insert(). Read racily by readers, but stale
+ // values are ok.
+ std::atomic<int> max_height_; // Height of the entire list
+
+ // Used for optimizing sequential insert patterns. Tricky. prev_[i] for
+ // i up to max_height_ is the predecessor of prev_[0] and prev_height_
+ // is the height of prev_[0]. prev_[0] can only be equal to head before
+ // insertion, in which case max_height_ and prev_height_ are 1.
+ Node** prev_;
+ int32_t prev_height_;
+
+ inline int GetMaxHeight() const {
+ return max_height_.load(std::memory_order_relaxed);
+ }
+
+ Node* NewNode(const Key& key, int height);
+ int RandomHeight();
+ bool Equal(const Key& a, const Key& b) const { return (compare_(a, b) == 0); }
+ bool LessThan(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;
+
+ // Returns the earliest node with a key >= key.
+ // Return nullptr if there is no such node.
+ Node* FindGreaterOrEqual(const Key& key) const;
+
+ // Return the latest node with a key < key.
+ // Return head_ if there is no such node.
+ // Fills prev[level] with pointer to previous node at "level" for every
+ // level in [0..max_height_-1], if prev is non-null.
+ Node* FindLessThan(const Key& key, Node** prev = nullptr) const;
+
+ // Return the last node in the list.
+ // Return head_ if list is empty.
+ Node* FindLast() const;
+};
+
+// 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 (next_[n].load(std::memory_order_acquire));
+ }
+ 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].store(x, std::memory_order_release);
+ }
+
+ // No-barrier variants that can be safely used in a few locations.
+ Node* NoBarrier_Next(int n) {
+ assert(n >= 0);
+ return next_[n].load(std::memory_order_relaxed);
+ }
+ void NoBarrier_SetNext(int n, Node* x) {
+ assert(n >= 0);
+ next_[n].store(x, std::memory_order_relaxed);
+ }
+
+ private:
+ // Array of length equal to the node height. next_[0] is lowest level link.
+ std::atomic<Node*> next_[1];
+};
+
+template<typename Key, class Comparator>
+typename SkipList<Key, Comparator>::Node*
+SkipList<Key, Comparator>::NewNode(const Key& key, int height) {
+ char* mem = allocator_->AllocateAligned(
+ sizeof(Node) + sizeof(std::atomic<Node*>) * (height - 1));
+ return new (mem) Node(key);
+}
+
+template<typename Key, class Comparator>
+inline SkipList<Key, Comparator>::Iterator::Iterator(const SkipList* list) {
+ SetList(list);
+}
+
+template<typename Key, class Comparator>
+inline void SkipList<Key, Comparator>::Iterator::SetList(const SkipList* list) {
+ list_ = list;
+ node_ = nullptr;
+}
+
+template<typename Key, class Comparator>
+inline bool SkipList<Key, Comparator>::Iterator::Valid() const {
+ return node_ != nullptr;
+}
+
+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_ = nullptr;
+ }
+}
+
+template<typename Key, class Comparator>
+inline void SkipList<Key, Comparator>::Iterator::Seek(const Key& target) {
+ node_ = list_->FindGreaterOrEqual(target);
+}
+
+template <typename Key, class Comparator>
+inline void SkipList<Key, Comparator>::Iterator::SeekForPrev(
+ const Key& target) {
+ Seek(target);
+ if (!Valid()) {
+ SeekToLast();
+ }
+ while (Valid() && list_->LessThan(target, key())) {
+ Prev();
+ }
+}
+
+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_ = nullptr;
+ }
+}
+
+template<typename Key, class Comparator>
+int SkipList<Key, Comparator>::RandomHeight() {
+ auto rnd = Random::GetTLSInstance();
+
+ // Increase height with probability 1 in kBranching
+ int height = 1;
+ while (height < kMaxHeight_ && rnd->Next() < kScaledInverseBranching_) {
+ 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 {
+ // nullptr n is considered infinite
+ return (n != nullptr) && (compare_(n->key, key) < 0);
+}
+
+template<typename Key, class Comparator>
+typename SkipList<Key, Comparator>::Node* SkipList<Key, Comparator>::
+ FindGreaterOrEqual(const Key& key) const {
+ // Note: It looks like we could reduce duplication by implementing
+ // this function as FindLessThan(key)->Next(0), but we wouldn't be able
+ // to exit early on equality and the result wouldn't even be correct.
+ // A concurrent insert might occur after FindLessThan(key) but before
+ // we get a chance to call Next(0).
+ Node* x = head_;
+ int level = GetMaxHeight() - 1;
+ Node* last_bigger = nullptr;
+ while (true) {
+ assert(x != nullptr);
+ Node* next = x->Next(level);
+ // Make sure the lists are sorted
+ assert(x == head_ || next == nullptr || KeyIsAfterNode(next->key, x));
+ // Make sure we haven't overshot during our search
+ assert(x == head_ || KeyIsAfterNode(key, x));
+ int cmp = (next == nullptr || next == last_bigger)
+ ? 1 : compare_(next->key, key);
+ if (cmp == 0 || (cmp > 0 && level == 0)) {
+ return next;
+ } else if (cmp < 0) {
+ // Keep searching in this list
+ x = next;
+ } else {
+ // Switch to next list, reuse compare_() result
+ last_bigger = next;
+ level--;
+ }
+ }
+}
+
+template<typename Key, class Comparator>
+typename SkipList<Key, Comparator>::Node*
+SkipList<Key, Comparator>::FindLessThan(const Key& key, Node** prev) const {
+ Node* x = head_;
+ int level = GetMaxHeight() - 1;
+ // KeyIsAfter(key, last_not_after) is definitely false
+ Node* last_not_after = nullptr;
+ while (true) {
+ assert(x != nullptr);
+ Node* next = x->Next(level);
+ assert(x == head_ || next == nullptr || KeyIsAfterNode(next->key, x));
+ assert(x == head_ || KeyIsAfterNode(key, x));
+ if (next != last_not_after && KeyIsAfterNode(key, next)) {
+ // Keep searching in this list
+ x = next;
+ } else {
+ if (prev != nullptr) {
+ prev[level] = x;
+ }
+ if (level == 0) {
+ return x;
+ } else {
+ // Switch to next list, reuse KeyIUsAfterNode() result
+ last_not_after = next;
+ level--;
+ }
+ }
+ }
+}
+
+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 == nullptr) {
+ if (level == 0) {
+ return x;
+ } else {
+ // Switch to next list
+ level--;
+ }
+ } else {
+ x = next;
+ }
+ }
+}
+
+template <typename Key, class Comparator>
+uint64_t SkipList<Key, Comparator>::EstimateCount(const Key& key) const {
+ uint64_t count = 0;
+
+ Node* x = head_;
+ int level = GetMaxHeight() - 1;
+ while (true) {
+ assert(x == head_ || compare_(x->key, key) < 0);
+ Node* next = x->Next(level);
+ if (next == nullptr || compare_(next->key, key) >= 0) {
+ if (level == 0) {
+ return count;
+ } else {
+ // Switch to next list
+ count *= kBranching_;
+ level--;
+ }
+ } else {
+ x = next;
+ count++;
+ }
+ }
+}
+
+template <typename Key, class Comparator>
+SkipList<Key, Comparator>::SkipList(const Comparator cmp, Allocator* allocator,
+ int32_t max_height,
+ int32_t branching_factor)
+ : kMaxHeight_(static_cast<uint16_t>(max_height)),
+ kBranching_(static_cast<uint16_t>(branching_factor)),
+ kScaledInverseBranching_((Random::kMaxNext + 1) / kBranching_),
+ compare_(cmp),
+ allocator_(allocator),
+ head_(NewNode(0 /* any key will do */, max_height)),
+ max_height_(1),
+ prev_height_(1) {
+ assert(max_height > 0 && kMaxHeight_ == static_cast<uint32_t>(max_height));
+ assert(branching_factor > 0 &&
+ kBranching_ == static_cast<uint32_t>(branching_factor));
+ assert(kScaledInverseBranching_ > 0);
+ // Allocate the prev_ Node* array, directly from the passed-in allocator.
+ // prev_ does not need to be freed, as its life cycle is tied up with
+ // the allocator as a whole.
+ prev_ = reinterpret_cast<Node**>(
+ allocator_->AllocateAligned(sizeof(Node*) * kMaxHeight_));
+ for (int i = 0; i < kMaxHeight_; i++) {
+ head_->SetNext(i, nullptr);
+ prev_[i] = head_;
+ }
+}
+
+template<typename Key, class Comparator>
+void SkipList<Key, Comparator>::Insert(const Key& key) {
+ // fast path for sequential insertion
+ if (!KeyIsAfterNode(key, prev_[0]->NoBarrier_Next(0)) &&
+ (prev_[0] == head_ || KeyIsAfterNode(key, prev_[0]))) {
+ assert(prev_[0] != head_ || (prev_height_ == 1 && GetMaxHeight() == 1));
+
+ // Outside of this method prev_[1..max_height_] is the predecessor
+ // of prev_[0], and prev_height_ refers to prev_[0]. Inside Insert
+ // prev_[0..max_height - 1] is the predecessor of key. Switch from
+ // the external state to the internal
+ for (int i = 1; i < prev_height_; i++) {
+ prev_[i] = prev_[0];
+ }
+ } else {
+ // TODO(opt): we could use a NoBarrier predecessor search as an
+ // optimization for architectures where memory_order_acquire needs
+ // a synchronization instruction. Doesn't matter on x86
+ FindLessThan(key, prev_);
+ }
+
+ // Our data structure does not allow duplicate insertion
+ assert(prev_[0]->Next(0) == nullptr || !Equal(key, prev_[0]->Next(0)->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_ (nullptr), or a new value set in
+ // the loop below. In the former case the reader will
+ // immediately drop to the next level since nullptr sorts after all
+ // keys. In the latter case the reader will use the new node.
+ max_height_.store(height, std::memory_order_relaxed);
+ }
+
+ Node* 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);
+ }
+ prev_[0] = x;
+ prev_height_ = height;
+}
+
+template<typename Key, class Comparator>
+bool SkipList<Key, Comparator>::Contains(const Key& key) const {
+ Node* x = FindGreaterOrEqual(key);
+ if (x != nullptr && Equal(key, x->key)) {
+ return true;
+ } else {
+ return false;
+ }
+}
+
+} // namespace ROCKSDB_NAMESPACE