db/snapshot.h

   1 // Copyright (c) 2011 The LevelDB Authors. All rights reserved.
   2 // Use of this source code is governed by a BSD-style license that can be
   3 // found in the LICENSE file. See the AUTHORS file for names of contributors.
   4
   5 #ifndef STORAGE_LEVELDB_DB_SNAPSHOT_H_
   6 #define STORAGE_LEVELDB_DB_SNAPSHOT_H_
   7
   8 #include "db/dbformat.h"
   9 #include "leveldb/db.h"
  10
  11 namespace leveldb {
  12
  13 class SnapshotList;
  14
  15 // Snapshots are kept in a doubly-linked list in the DB.
  16 // Each SnapshotImpl corresponds to a particular sequence number.
  17 class SnapshotImpl : public Snapshot {
  18  public:
  19   SequenceNumber number_;  // const after creation
  20
  21  private:
  22   friend class SnapshotList;
  23
  24   // SnapshotImpl is kept in a doubly-linked circular list
  25   SnapshotImpl* prev_;
  26   SnapshotImpl* next_;
  27
  28   SnapshotList* list_;                 // just for sanity checks
  29 };
  30
  31 class SnapshotList {
  32  public:
  33   SnapshotList() {
  34     list_.prev_ = &list_;
  35     list_.next_ = &list_;
  36   }
  37
  38   bool empty() const { return list_.next_ == &list_; }
  39   SnapshotImpl* oldest() const { assert(!empty()); return list_.next_; }
  40   SnapshotImpl* newest() const { assert(!empty()); return list_.prev_; }
  41
  42   const SnapshotImpl* New(SequenceNumber seq) {
  43     SnapshotImpl* s = new SnapshotImpl;
  44     s->number_ = seq;
  45     s->list_ = this;
  46     s->next_ = &list_;
  47     s->prev_ = list_.prev_;
  48     s->prev_->next_ = s;
  49     s->next_->prev_ = s;
  50     return s;
  51   }
  52
  53   void Delete(const SnapshotImpl* s) {
  54     assert(s->list_ == this);
  55     s->prev_->next_ = s->next_;
  56     s->next_->prev_ = s->prev_;
  57     delete s;
  58   }
  59
  60  private:
  61   // Dummy head of doubly-linked list of snapshots
  62   SnapshotImpl list_;
  63 };
  64
  65 }  // namespace leveldb
  66
  67 #endif  // STORAGE_LEVELDB_DB_SNAPSHOT_H_