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#       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                 :            :   SnapshotImpl(SequenceNumber sequence_number)
#      20                 :       2193 :       : sequence_number_(sequence_number) {}
#      21                 :            : 
#      22                 :          0 :   SequenceNumber sequence_number() const { return sequence_number_; }
#      23                 :            : 
#      24                 :            :  private:
#      25                 :            :   friend class SnapshotList;
#      26                 :            : 
#      27                 :            :   // SnapshotImpl is kept in a doubly-linked circular list. The SnapshotList
#      28                 :            :   // implementation operates on the next/previous fields direcly.
#      29                 :            :   SnapshotImpl* prev_;
#      30                 :            :   SnapshotImpl* next_;
#      31                 :            : 
#      32                 :            :   const SequenceNumber sequence_number_;
#      33                 :            : 
#      34                 :            : #if !defined(NDEBUG)
#      35                 :            :   SnapshotList* list_ = nullptr;
#      36                 :            : #endif  // !defined(NDEBUG)
#      37                 :            : };
#      38                 :            : 
#      39                 :            : class SnapshotList {
#      40                 :            :  public:
#      41                 :       2193 :   SnapshotList() : head_(0) {
#      42                 :       2193 :     head_.prev_ = &head_;
#      43                 :       2193 :     head_.next_ = &head_;
#      44                 :       2193 :   }
#      45                 :            : 
#      46                 :        108 :   bool empty() const { return head_.next_ == &head_; }
#      47                 :          0 :   SnapshotImpl* oldest() const {
#      48                 :          0 :     assert(!empty());
#      49                 :          0 :     return head_.next_;
#      50                 :          0 :   }
#      51                 :          0 :   SnapshotImpl* newest() const {
#      52                 :          0 :     assert(!empty());
#      53                 :          0 :     return head_.prev_;
#      54                 :          0 :   }
#      55                 :            : 
#      56                 :            :   // Creates a SnapshotImpl and appends it to the end of the list.
#      57                 :          0 :   SnapshotImpl* New(SequenceNumber sequence_number) {
#      58                 :          0 :     assert(empty() || newest()->sequence_number_ <= sequence_number);
#      59                 :            : 
#      60                 :          0 :     SnapshotImpl* snapshot = new SnapshotImpl(sequence_number);
#      61                 :            : 
#      62                 :          0 : #if !defined(NDEBUG)
#      63                 :          0 :     snapshot->list_ = this;
#      64                 :          0 : #endif  // !defined(NDEBUG)
#      65                 :          0 :     snapshot->next_ = &head_;
#      66                 :          0 :     snapshot->prev_ = head_.prev_;
#      67                 :          0 :     snapshot->prev_->next_ = snapshot;
#      68                 :          0 :     snapshot->next_->prev_ = snapshot;
#      69                 :          0 :     return snapshot;
#      70                 :          0 :   }
#      71                 :            : 
#      72                 :            :   // Removes a SnapshotImpl from this list.
#      73                 :            :   //
#      74                 :            :   // The snapshot must have been created by calling New() on this list.
#      75                 :            :   //
#      76                 :            :   // The snapshot pointer should not be const, because its memory is
#      77                 :            :   // deallocated. However, that would force us to change DB::ReleaseSnapshot(),
#      78                 :            :   // which is in the API, and currently takes a const Snapshot.
#      79                 :          0 :   void Delete(const SnapshotImpl* snapshot) {
#      80                 :          0 : #if !defined(NDEBUG)
#      81                 :          0 :     assert(snapshot->list_ == this);
#      82                 :          0 : #endif  // !defined(NDEBUG)
#      83                 :          0 :     snapshot->prev_->next_ = snapshot->next_;
#      84                 :          0 :     snapshot->next_->prev_ = snapshot->prev_;
#      85                 :          0 :     delete snapshot;
#      86                 :          0 :   }
#      87                 :            : 
#      88                 :            :  private:
#      89                 :            :   // Dummy head of doubly-linked list of snapshots
#      90                 :            :   SnapshotImpl head_;
#      91                 :            : };
#      92                 :            : 
#      93                 :            : }  // namespace leveldb
#      94                 :            : 
#      95                 :            : #endif  // STORAGE_LEVELDB_DB_SNAPSHOT_H_