chrome/browser/extensions/activity_log/database_string_table.h

   1 // Copyright 2013 The Chromium 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.
   4
   5 #ifndef CHROME_BROWSER_EXTENSIONS_ACTIVITY_LOG_DATABASE_STRING_TABLE_H_
   6 #define CHROME_BROWSER_EXTENSIONS_ACTIVITY_LOG_DATABASE_STRING_TABLE_H_
   7
   8 #include <map>
   9 #include <string>
  10
  11 #include "base/basictypes.h"
  12 #include "base/gtest_prod_util.h"
  13
  14 namespace sql {
  15 class Connection;
  16 }  // namespace sql
  17
  18 namespace extensions {
  19
  20 // A class for maintaining a persistent mapping between strings and integers.
  21 // This is used to help compress the contents of the activity log database on
  22 // disk by replacing repeated strings by smaller integers.
  23 //
  24 // The mapping from integers to strings is maintained in a database table, but
  25 // the mapping is also cached in memory.
  26 //
  27 // The database table used to store the strings is configurable, but its layout
  28 // is fixed: it always consists of just two columns, "id" and "value".
  29 //
  30 // All calls to DatabaseStringTable must occur on the database thread.
  31 class DatabaseStringTable {
  32  public:
  33   explicit DatabaseStringTable(const std::string& table);
  34
  35   ~DatabaseStringTable();
  36
  37   // Initialize the database table.  This will create the table if it does not
  38   // exist.  Returns true on success; false on error.
  39   bool Initialize(sql::Connection* connection);
  40
  41   // Interns a string in the database table and sets *id to the corresponding
  42   // integer.  If the string already exists, the existing number is returned;
  43   // otherwise, new database row is inserted with the new string.  Returns true
  44   // on success and false on database error.
  45   bool StringToInt(sql::Connection* connection,
  46                    const std::string& value,
  47                    int64* id);
  48
  49   // Looks up an integer value and converts it to a string (which is stored in
  50   // *value).  Returns true on success.  A false return does not necessarily
  51   // indicate a database error; it might simply be that the value cannot be
  52   // found.
  53   bool IntToString(sql::Connection* connection, int64 id, std::string* value);
  54
  55   // Clears the in-memory cache; this should be called if the underlying
  56   // database table has been manipulated and the cache may be stale.
  57   void ClearCache();
  58
  59  private:
  60   // Reduces the size of the cache if too many entries are held in it.
  61   void PruneCache();
  62
  63   // In-memory caches of recently accessed values.
  64   std::map<int64, std::string> id_to_value_;
  65   std::map<std::string, int64> value_to_id_;
  66
  67   // The name of the database table where the mapping is stored.
  68   std::string table_;
  69
  70   FRIEND_TEST_ALL_PREFIXES(DatabaseStringTableTest, Prune);
  71
  72   DISALLOW_COPY_AND_ASSIGN(DatabaseStringTable);
  73 };
  74
  75 }  // namespace extensions
  76
  77 #endif  // CHROME_BROWSER_EXTENSIONS_ACTIVITY_LOG_DATABASE_STRING_TABLE_H_