[shared] StorageExpressionModel: memoize the fact that the expression is

circular to avoid lagging when approximating the model multiple times.

apps/shared/storage_expression_model.cpp

@@ -14,7 +14,8 @@ namespace Shared {
 StorageExpressionModel::StorageExpressionModel(Storage::Record record) :
   Storage::Record(record),
   m_expression(),
-  m_layout()
+  m_layout(),
+  m_circular(-1)
 {
 }
 
@@ -27,6 +28,13 @@ void StorageExpressionModel::text(char * buffer, size_t bufferSize) const {
   }
 }
 
+bool StorageExpressionModel::isCircularlyDefined(Poincare::Context * context) const {
+  if (m_circular == -1) {
+    m_circular = Expression::ExpressionWithoutSymbols(expression(context), *context).isUninitialized();
+  }
+  return m_circular;
+}
+
 Expression StorageExpressionModel::expression(Poincare::Context * context) const {
   if (m_expression.isUninitialized()) {
     assert(!isNull());
@@ -63,6 +71,7 @@ bool StorageExpressionModel::isEmpty() {
 void StorageExpressionModel::tidy() {
   m_layout = Layout();
   m_expression = Expression();
+  m_circular = 0;
 }
 
 Ion::Storage::Record::ErrorStatus StorageExpressionModel::setContent(const char * c) {
@@ -102,6 +111,7 @@ Ion::Storage::Record::ErrorStatus StorageExpressionModel::setExpressionContent(E
    * the m_layout and m_expression. */
   m_layout = Layout();
   m_expression = Expression();
+  m_circular = 0;
   return error;
 }
 

apps/shared/storage_expression_model.h

@@ -32,11 +32,13 @@ public:
 protected:
   virtual size_t metaDataSize() const = 0;
   Ion::Storage::Record record() const;
+  bool isCircularlyDefined(Poincare::Context * context) const;
   mutable Poincare::Expression m_expression;
   mutable Poincare::Layout m_layout;
 private:
   void * expressionAddressForValue(Ion::Storage::Record::Data recordData) const;
   size_t expressionSizeForValue(Ion::Storage::Record::Data recordData) const;
+  mutable int m_circular;
 };
 
 }

apps/shared/storage_function.cpp

@@ -69,6 +69,9 @@ int StorageFunction::nameWithArgument(char * buffer, size_t bufferSize, char arg
 
 template<typename T>
 T StorageFunction::templatedApproximateAtAbscissa(T x, Poincare::Context * context) const {
+  if (isCircularlyDefined(context)) {
+    return NAN;
+  }
   const char unknownX[2] = {Poincare::Symbol::UnknownX, 0};
   return expression(context).approximateWithValueForSymbol(unknownX, x, *context, Preferences::sharedPreferences()->angleUnit());
 }