Merge pull request #901 from sys27/bugfix/fact-formatting

#895 - Incorrect formatting of factorial.

Author: sys27

xFunc.Maths/Analyzers/Formatters/CommonFormatter.cs

@@ -103,7 +103,12 @@ public virtual string Analyze(Exp exp)
 
     /// <inheritdoc />
     public virtual string Analyze(Fact exp)
-        => ToString(exp, "{0}!");
+    {
+        if (exp.Argument is BinaryExpression)
+            return ToString(exp, "({0})!");
+
+        return ToString(exp, "{0}!");
+    }
 
     /// <inheritdoc />
     public virtual string Analyze(Floor exp)

xFunc.Tests/Analyzers/Formatters/CommonFormatterTest.cs

@@ -124,6 +124,14 @@ public void FactToStringTest()
         Assert.That(exp.ToString(), Is.EqualTo("5!"));
     }
 
+    [Test]
+    public void FactWithBinaryToStringTest()
+    {
+        var exp = new Fact(new Add(Number.One, Number.Two));
+
+        Assert.That(exp.ToString(), Is.EqualTo("(1 + 2)!"));
+    }
+
     [Test]
     public void FloorToStringTest()
     {