// 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.

#ifndef STORAGE_LEVELDB_DB_SKIPLIST_H_

#define STORAGE_LEVELDB_DB_SKIPLIST_H_

// 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 <atomic>

#include <cassert>

#include <cstdlib>

#include "util/arena.h"

#include "util/random.h"

namespace leveldb {

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);

  SkipList(const SkipList&) = delete;

  SkipList& 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;

  // 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 };

  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); }

  // 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 nullptr 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;

  // 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.

  std::atomic<int> max_height_;  // Height of the entire list

  // Read/written only by Insert().

  Random rnd_;

};

// 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* const node_memory = arena_->AllocateAligned(

      sizeof(Node) + sizeof(std::atomic<Node*>) * (height - 1));

  return new (node_memory) Node(key);

}

template <typename Key, class Comparator>

inline SkipList<Key, Comparator>::Iterator::Iterator(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, nullptr);

}

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() {

  // Increase height with probability 1 in kBranching

  static const unsigned int kBranching = 4;

  int height = 1;

  while (height < kMaxHeight && rnd_.OneIn(kBranching)) {

    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 != nullptr) && (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 != nullptr) 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 == nullptr || 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 == nullptr) {

      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_(1),

      rnd_(0xdeadbeef) {

  for (int i = 0; i < kMaxHeight; i++) {

    head_->SetNext(i, nullptr);

  }

}

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 == nullptr || !Equal(key, x->key));

  int height = RandomHeight();

  if (height > GetMaxHeight()) {

    for (int i = GetMaxHeight(); i < height; i++) {

      prev[i] = head_;

    }

    // 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);

  }

  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, nullptr);

  if (x != nullptr && Equal(key, x->key)) {

    return true;

  } else {

    return false;

  }

}

}  // namespace leveldb

#endif  // STORAGE_LEVELDB_DB_SKIPLIST_H_