DbLib: Add single-row query cache

Since single-row queries are performed rapid-fire during certain actions like
stepping through the symbol browser, there is high value in caching them for
a small amount of time.  The default cache parameters will keep results for
10 seconds, which errs on the side of getting fresh data from the database
on most user interactions.

include/database/database_cache.h

@@ -0,0 +1,100 @@
+/*
+ * This program source code file is part of KiCad, a free EDA CAD application.
+ *
+ * Copyright (C) 2022 Jon Evans <[email protected]>
+ * Copyright (C) 2022 KiCad Developers, see AUTHORS.txt for contributors.
+ *
+ * This program is free software: you can redistribute it and/or modify it
+ * under the terms of the GNU General Public License as published by the
+ * Free Software Foundation, either version 3 of the License, or (at your
+ * option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful, but
+ * WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License along
+ * with this program.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+#ifndef KICAD_DATABASE_CACHE_H
+#define KICAD_DATABASE_CACHE_H
+
+#include <chrono>
+#include <list>
+#include <string>
+#include <unordered_map>
+
+#include <database/database_connection.h>
+
+
+class DATABASE_CACHE
+{
+public:
+    typedef std::pair<std::string, std::pair<time_t, DATABASE_CONNECTION::ROW>> CACHE_ENTRY;
+
+    DATABASE_CACHE( size_t aMaxSize, time_t aMaxAge ) :
+            m_maxSize( aMaxSize ),
+            m_maxAge( aMaxAge )
+    {}
+
+    void Put( const std::string& aQuery, const DATABASE_CONNECTION::ROW& aResult )
+    {
+        auto it = m_cache.find( aQuery );
+
+        time_t time = std::chrono::system_clock::to_time_t( std::chrono::system_clock::now() );
+
+        m_cacheMru.push_front( std::make_pair( aQuery,
+                                               std::make_pair( time, aResult ) ) );
+
+        if( it != m_cache.end() )
+        {
+            m_cacheMru.erase( it->second );
+            m_cache.erase( it );
+        }
+
+        m_cache[aQuery] = m_cacheMru.begin();
+
+        if( m_cache.size() > m_maxSize )
+        {
+            auto last = m_cacheMru.end();
+            last--;
+            m_cache.erase( last->first );
+            m_cacheMru.pop_back();
+        }
+    }
+
+    bool Get( const std::string& aQuery, DATABASE_CONNECTION::ROW& aResult )
+    {
+        auto it = m_cache.find( aQuery );
+
+        if( it == m_cache.end() )
+            return false;
+
+        time_t time = std::chrono::system_clock::to_time_t( std::chrono::system_clock::now() );
+
+        if( time - it->second->second.first > m_maxAge )
+        {
+            m_cacheMru.erase( it->second );
+            m_cache.erase( it );
+            return false;
+        }
+
+        m_cacheMru.splice( m_cacheMru.begin(), m_cacheMru, it->second );
+
+        aResult = it->second->second.second;
+        return true;
+    }
+
+    void SetMaxSize( size_t aMaxSize ) { m_maxSize = aMaxSize; }
+    void SetMaxAge( time_t aMaxAge ) { m_maxAge = aMaxAge; }
+
+private:
+    size_t m_maxSize;
+    time_t m_maxAge;
+    std::list<CACHE_ENTRY> m_cacheMru;
+    std::unordered_map<std::string, std::list<CACHE_ENTRY>::iterator> m_cache;
+};
+
+#endif //KICAD_DATABASE_CACHE_H