add a case to FormulaChecker

src/NoStringEvaluating.Tests/UnitTests/Data/CheckFormula.cs

@@ -46,5 +46,10 @@ public static IEnumerable<FormulaModel> Get()
         yield return CreateTestModelToCheck(";;;", false);
         yield return CreateTestModelToCheck(",", false);
         yield return CreateTestModelToCheck(",,", false);
+        yield return CreateTestModelToCheck("?", false);
+        yield return CreateTestModelToCheck("#", false);
+        yield return CreateTestModelToCheck("?!!", false);
+        yield return CreateTestModelToCheck(string.Empty, false);
+        yield return CreateTestModelToCheck("   ", false);
     }
 }

src/NoStringEvaluating/Models/FormulaChecker/FormulaCheckerMistakeType.cs

@@ -31,17 +31,17 @@ public enum FormulaCheckerMistakeType
     OperatorBetweenCurrentAndNextNode = 4,
 
     /// <summary>
-    /// Between prevNode and nextNode must be an operator, not node"
+    /// Between prevNode and nextNode must be an operator, not node
     /// </summary>
     OperatorBetweenPrevAndNextNode = 5,
 
     /// <summary>
-    /// Before node must be a number or a closed bracket, not prevNodeName"
+    /// Before node must be a number or a closed bracket, not prevNodeName
     /// </summary>
     NumberBeforeNode = 6,
 
     /// <summary>
-    /// After node must be a number or an opened bracket, not nextNodeName"
+    /// After node must be a number or an opened bracket, not nextNodeName
     /// </summary>
     NumberAfterNode = 7,
 
@@ -54,4 +54,9 @@ public enum FormulaCheckerMistakeType
     /// Two or more function chars in a sequence
     /// </summary>
     DoubledFunctionCharNodes = 9,
+
+    /// <summary>
+    /// Empty or wrong formula
+    /// </summary>
+    EmptyOrWrongFormula = 10,
 }

src/NoStringEvaluating/NoStringEvaluating.csproj

@@ -8,12 +8,12 @@
 		<Description>Fast low memory consuming mathematical evaluation without endless string parsing! Parses string formula once and uses its object sequence in each evaluation.</Description>
 		<PackageLicenseExpression>MIT</PackageLicenseExpression>
 		<PackageIcon>logo.png</PackageIcon>
-		<PackageReleaseNotes>.NET 8</PackageReleaseNotes>
+		<PackageReleaseNotes>Add a new case to FormulaChecker (empty formula or meaningless chars)</PackageReleaseNotes>
 		<PackageTags>Math, Mathematics, Mathematical-Expression, Expressions, Parser, Formula, Evaluator, Calculator, Solve, Calculation, Logic, Condition, Custom, Function, Math-Parser, Expression-Evaluator, Formula-Parser, Object-Pooling, NoString, RPN</PackageTags>
 		<PackageProjectUrl>https://github.com/KovtunV/NoStringEvaluating</PackageProjectUrl>
 		<RepositoryUrl>https://github.com/KovtunV/NoStringEvaluating</RepositoryUrl>
 		<RepositoryType>git</RepositoryType>
-		<Version>2.6.0</Version>
+		<Version>2.6.1</Version>
 		<PackageReadmeFile></PackageReadmeFile>
 	</PropertyGroup>
 

src/NoStringEvaluating/Services/Checking/FormulaChecker.cs

@@ -29,6 +29,7 @@ public CheckFormulaResult CheckSyntax(ReadOnlySpan<char> formula)
         var mistakes = new List<FormulaCheckerModel>();
         var nodes = _formulaParser.ParseWithoutRpn(formula);
 
+        CheckEmpty(mistakes, nodes);
         CheckBracketsCount(mistakes, nodes, 0, nodes.Count);
         CheckEmptyBrackets(mistakes, nodes, 0, nodes.Count);
         CheckNotOperatorableNodes(mistakes, nodes, 0, nodes.Count);
@@ -71,6 +72,15 @@ private static int GetNextIndex(List<BaseFormulaNode> nodes, int start, int end)
         return end;
     }
 
+    private static void CheckEmpty(List<FormulaCheckerModel> mistakes, List<BaseFormulaNode> nodes)
+    {
+        if (nodes.Count is 0)
+        {
+            var mistakeItem = CreateMistakeModel(FormulaCheckerMistakeType.EmptyOrWrongFormula, "Formula doesn't contain any meaningful characters");
+            mistakes.Add(mistakeItem);
+        }
+    }
+
     #region Function
 
     private bool TryCheckFunction(List<FormulaCheckerModel> mistakes, List<BaseFormulaNode> nodes, ref int index)