Imports System
Imports System.Text
Public Module Rot13Program
''' <summary>
''' Applies the ROT13 substitution cipher to a string.
''' Applying ROT13 twice returns the original text.
''' Non-alphabetic characters are left unchanged.
''' </summary>
''' <param name="input">The text to encode or decode.</param>
''' <returns>The ROT13-transformed string.</returns>
Public Function Rot13(input As String) As String
If input Is Nothing Then
Throw New ArgumentNullException(NameOf(input))
End If
Dim result As New StringBuilder(input.Length)
For Each ch As Char In input
Select Case ch
Case "A"c To "Z"c
result.Append(Convert.ToChar(((Convert.ToInt32(ch) - Convert.ToInt32("A"c) + 13) Mod 26) + Convert.ToInt32("A"c)))
Case "a"c To "z"c
result.Append(Convert.ToChar(((Convert.ToInt32(ch) - Convert.ToInt32("a"c) + 13) Mod 26) + Convert.ToInt32("a"c)))
Case Else
' Keep numbers, spaces, punctuation, etc. unchanged.
result.Append(ch)
End Select
Next
Return result.ToString()
End Function
Public Sub Main()
Dim encoded As String = Rot13Program.Rot13("Hello, World! 123")
Console.WriteLine(encoded)
' Uryyb, Jbeyq!
Dim decoded As String = Rot13Program.Rot13(encoded)
Console.WriteLine(decoded)
' Hello, World!
End Sub
End Module
' run:
'
' Uryyb, Jbeyq! 123
' Hello, World! 123
'
