Optimised Expression::hash().

By storing the context names rather than querying them, we avoid having to enter python in Expression::hash(). In a test case generating a scene with 100 CustomAttributes nodes each with a python expression connected, this gives a 23% reduction in initial traversal time, and a 65% reduction in traversal time once the scene is in the ValuePlug cache.

Fixes #1034.

include/Gaffer/Expression.h

@@ -79,7 +79,7 @@ class Expression : public ComputeNode
 				virtual void inPlugs( std::vector<std::string> &plugPaths ) = 0;
 				/// Must fill names with the names of all context values the expression
 				/// wishes to read.
-				virtual void contextNames( std::vector<std::string> &names ) = 0;
+				virtual void contextNames( std::vector<IECore::InternedString> &names ) = 0;
 				/// Must execute the expression in the specified context, using the values
 				/// provided by proxyInputs and setting the result in proxyOutput.
 				virtual void execute( const Context *context, const std::vector<const ValuePlug *> &proxyInputs, ValuePlug *proxyOutput ) = 0;
@@ -112,6 +112,7 @@ class Expression : public ComputeNode
 		void updatePlugs( const std::string &outPlugPath, std::vector<std::string> &inPlugPaths );
 
 		EnginePtr m_engine;
+		std::vector<IECore::InternedString> m_contextNames;
 
 };
 

src/Gaffer/Expression.cpp

@@ -138,9 +138,7 @@ void Expression::hash( const ValuePlug *output, const Context *context, IECore::
 		}
 		if( m_engine )
 		{
-			std::vector<std::string> contextNames;
-			m_engine->contextNames( contextNames );
-			for( std::vector<std::string>::const_iterator it = contextNames.begin(); it != contextNames.end(); it++ )
+			for( std::vector<IECore::InternedString>::const_iterator it = m_contextNames.begin(); it != m_contextNames.end(); it++ )
 			{
 				const IECore::Data *d = context->get<IECore::Data>( *it, 0 );
 				if( d )
@@ -209,6 +207,8 @@ void Expression::plugSet( Plug *plug )
 				{
 					m_engine->inPlugs( inPlugPaths );
 					outPlugPath = m_engine->outPlug();
+					m_contextNames.clear();
+					m_engine->contextNames( m_contextNames );
 				}
 
 				updatePlugs( outPlugPath, inPlugPaths );

src/GafferBindings/ExpressionBinding.cpp

@@ -103,7 +103,7 @@ class EngineWrapper : public Expression::Engine, public IECorePython::Wrapper<Ex
 			}
 		}
 
-		virtual void contextNames( std::vector<std::string> &names )
+		virtual void contextNames( std::vector<IECore::InternedString> &names )
 		{
 			IECorePython::ScopedGILLock gilLock;
 			try