Tan bug fixed (#570)

So basically, before that `tan` checked if the angle is divided
by `pi / 2`. If yes, it should be zero.

But it misses a condition: it also should not be integer. Basically,
whenever you get `pi / 2` as a modulo of dividing the angle by
`pi`, then `tan` turns into NaN. Whereas it just checked dividing
by `pi / 2` and did not care about case when `angle` is `pi` or
any integer by `pi`.

Issue #568

Author: WhiteBlackGoose

Sources/AngouriMath/AngouriMath/Functions/Evaluation/Evaluation.Continuous/Evaluation.Continuous.Trigonometry.Classes.cs

@@ -25,8 +25,13 @@ internal static bool IsCosDefinitelyZero(Entity angle)
                 // is it of the form n / m * pi?
                 && MathS.UnsafeAndInternal.DivideByEntityStrict(angle, MathS.pi)?.Evaled is Rational angleRat
 
-                // if n / m is integer, it is definitely 0
-                && angleRat % TrigonometricAngleExpansion.OneHalf == 0;
+                // if n / m is integer + 1/2
+                
+                // if it's divided by 1/2
+                && angleRat % TrigonometricAngleExpansion.OneHalf == 0
+                
+                // but is not integer
+                && angleRat is not Integer;
     }
 
     partial record Entity

Sources/Tests/UnitTests/PatternsTest/TrigTest.cs

@@ -255,6 +255,12 @@ public void SymbolicCosineFormTest(string angle)
                         .ShouldBeNotNull()
                         .ShouldApproximatelyEqual(MathS.Cos(angle))
                 );
+
+        [Fact]
+        public void TestTan1() => "tan(pi)".ToEntity().InnerSimplified.ShouldBe(0);
+        
+        [Fact]
+        public void TestTan2() => "tan(pi / 1)".ToEntity().InnerSimplified.ShouldBe(0);
     }
 }