Excel UDF Next Highest Prime Number For Excel

Function NextHighestPrimeNumber(StartingNumber As Double) As Variant
Dim CeilingTest As Double
Dim i As Double
'PGCodeRider
'Calculates answer to 10 trillion in 4 seconds on my laptop, which is pretty good.

If StartingNumber < 11 Then

            If StartingNumber > 6 Then
                NextHighestPrimeNumber = 11

            ElseIf StartingNumber > 4 Then
                NextHighestPrimeNumber = 7

            ElseIf StartingNumber > 2 Then
                NextHighestPrimeNumber = 5

            ElseIf StartingNumber > 0 Then
                NextHighestPrimeNumber = 3

            ElseIf StartingNumber = 0 Then
                NextHighestPrimeNumber = 1

            Else
                NextHighestPrimeNumber = "Pick A Positive Integer"

            End If

        Exit Function

Else

'Create Array (Skips 3 and 5 because those are hard coded to check for to save time of calculating unnecassary square roots
    ReDim prime_array(6) As Double
        prime_array(0) = 7
        prime_array(1) = 11
        prime_array(2) = 13
        prime_array(3) = 17
        prime_array(4) = 19
        prime_array(5) = 23
        prime_array(6) = 29

    'zz failed code: If StartingNumber Mod 2 = 0 Then if over Long Limit (2.1 billion)

    If StartingNumber - (Int(StartingNumber / 2) * 2) = 0 Then
        StartingNumber = StartingNumber - 1
    End If
End If


NewNumber:
        StartingNumber = StartingNumber + 2

        If StartingNumber - (Int(StartingNumber / 3) * 3) = 0 Then
            GoTo NewNumber

        ElseIf StartingNumber - (Int(StartingNumber / 5) * 5) = 0 Then
            GoTo NewNumber

        Else
            CeilingTest = Int(VBA.Sqr(StartingNumber))
        End If


'Array loop
    For i = 0 To UBound(prime_array)
        If prime_array(i) > CeilingTest Then
            NextHighestPrimeNumber = StartingNumber
            Exit Function

        ElseIf StartingNumber - (Int(StartingNumber / prime_array(i)) * prime_array(i)) = 0 Then
            GoTo NewNumber

        End If
    Next i


ExpandArray:
    ReDim Preserve prime_array(UBound(prime_array) + 1)
    prime_array(UBound(prime_array)) = AddArrayFactorMember(prime_array())


    If StartingNumber - (Int(StartingNumber / prime_array(UBound(prime_array))) * prime_array(UBound(prime_array))) = 0 Then
        GoTo NewNumber

    'test if bigger than ceiling
    ElseIf prime_array(UBound(prime_array)) > CeilingTest Then
        NextHighestPrimeNumber = StartingNumber
        Exit Function

    Else
        GoTo ExpandArray
    End If

End Function

Private Function AddArrayFactorMember(Old_Array() As Double) As Double
Dim i As Double
Dim StartingNumber As Double: StartingNumber = Old_Array(UBound(Old_Array) - 1)


NewNumber:
    StartingNumber = StartingNumber + 2

    If StartingNumber - (Int(StartingNumber / 3) * 3) = 0 Then
        GoTo NewNumber

    ElseIf StartingNumber - (Int(StartingNumber / 5) * 5) = 0 Then
        GoTo NewNumber

    Else
        CeilingTest = Int(VBA.Sqr(StartingNumber))

    End If


    For i = 0 To UBound(Old_Array)
        If Old_Array(i) > CeilingTest Then
            AddArrayFactorMember = StartingNumber
            Exit Function

        ElseIf StartingNumber - (Int(StartingNumber / Old_Array(i)) * Old_Array(i)) = 0 Then
            GoTo NewNumber

        End If

    Next i

End Function