Compressed activity log database storage

chrome/browser/extensions/activity_log/database_interned_string.h

+// Copyright 2013 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef CHROME_BROWSER_EXTENSIONS_ACTIVITY_LOG_DATABASE_INTERNED_STRING_H_
+#define CHROME_BROWSER_EXTENSIONS_ACTIVITY_LOG_DATABASE_INTERNED_STRING_H_
+
+#include <map>
+#include <string>
+
+#include "base/basictypes.h"
+
+namespace sql {
+class Connection;
+}  // namespace sql
+
+namespace extensions {
+
+// A class for maintaining a persistent mapping between strings and integers.
+// This is used to help compress the contents of the activity log database on
+// disk by replacing repeated strings by smaller integers.
+//
+// The mapping from integers to strings is maintained in a database table, but
+// the mapping is also cached in memory.
+//
+// The database table used to store the strings is configurable, but its layout
+// is fixed: it always consists of just two columns, "id" and "value".
+//
+// All calls to DatabaseStringTable must occur on the database thread.
+class DatabaseStringTable {
+ public:
+  explicit DatabaseStringTable(const std::string& table);
+
+  ~DatabaseStringTable();
+
+  // Initialize the database table.  This will create the table if it does not
+  // exist.  Returns true on success; false on error.
+  bool Initialize(sql::Connection* connection);
+
+  // Interns a string in the database table and sets *id to the corresponding
+  // integer.  If the string already exists, the existing number is returned;
+  // otherwise, new database row is inserted with the new string.  Returns true
+  // on success and false on database error.
+  bool StringToInt(sql::Connection* connection,
+                   const std::string& value,
+                   int64* id);
+
+  // Looks up an integer value and converts it to a string (which is stored in
+  // *value).  Returns true on success.  A false return does not necessarily
+  // indicate a database error; it might simply be that the value cannot be
+  // found.
+  bool IntToString(sql::Connection* connection, int64 id, std::string* value);
+
+  // Clear the in-memory cache; this should be called if the underlying
+  // database table has been manipulated and the cache may be stale.
+  void ClearCache();
+
+ private:
+  // In-memory caches of recently accessed values.
+  std::map<int64, std::string> id_to_value_;
+  std::map<std::string, int64> value_to_id_;
+
+  // The name of the database table where the mapping is stored.
+  std::string table_;
+
+  DISALLOW_COPY_AND_ASSIGN(DatabaseStringTable);
+};
+
+}  // namespace extensions
+
+#endif  // CHROME_BROWSER_EXTENSIONS_ACTIVITY_LOG_DATABASE_INTERNED_STRING_H_