1 files changed, 66 insertions, 0 deletions
diff --git a/leveldb/db/snapshot.h b/leveldb/db/snapshot.h
new file mode 100644
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--- /dev/null
+++ b/leveldb/db/snapshot.h
@@ -0,0 +1,66 @@
+// 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_SNAPSHOT_H_
+#define STORAGE_LEVELDB_DB_SNAPSHOT_H_
+
+#include "leveldb/db.h"
+
+namespace leveldb {
+
+class SnapshotList;
+
+// Snapshots are kept in a doubly-linked list in the DB.
+// Each Snapshot corresponds to a particular sequence number.
+class Snapshot {
+ public:
+  SequenceNumber number_;  // const after creation
+
+ private:
+  friend class SnapshotList;
+
+  // Snapshot is kept in a doubly-linked circular list
+  Snapshot* prev_;
+  Snapshot* next_;
+
+  SnapshotList* list_;                 // just for sanity checks
+};
+
+class SnapshotList {
+ public:
+  SnapshotList() {
+    list_.prev_ = &list_;
+    list_.next_ = &list_;
+  }
+
+  bool empty() const { return list_.next_ == &list_; }
+  Snapshot* oldest() const { assert(!empty()); return list_.next_; }
+  Snapshot* newest() const { assert(!empty()); return list_.prev_; }
+
+  const Snapshot* New(SequenceNumber seq) {
+    Snapshot* s = new Snapshot;
+    s->number_ = seq;
+    s->list_ = this;
+    s->next_ = &list_;
+    s->prev_ = list_.prev_;
+    s->prev_->next_ = s;
+    s->next_->prev_ = s;
+    return s;
+  }
+
+  void Delete(const Snapshot* s) {
+    assert(s->list_ == this);
+    s->prev_->next_ = s->next_;
+    s->next_->prev_ = s->prev_;
+    delete s;
+  }
+
+ private:
+  // Dummy head of doubly-linked list of snapshots
+  Snapshot list_;
+};
+
+}
+
+#endif  // STORAGE_LEVELDB_DB_SNAPSHOT_H_