Module CollateDecollate

    Sub Main()
        Dim s1 As String = "abc"
        Dim s2 As String = "def"

        Dim collatedResult As String = Collate(s1, s2)
        Console.WriteLine($"Collated: {collatedResult}") ' Output: adbecf

        Dim decollatedResultEven As String = Decollate(collatedResult, 0)
        Console.WriteLine($"Decollated (Even): {decollatedResultEven}") ' Output: abc

        Dim decollatedResultOdd As String = Decollate(collatedResult, 1)
        Console.WriteLine($"Decollated (Odd): {decollatedResultOdd}") ' Output: def
    End Sub

    Function Collate(ByVal s1 As String, ByVal s2 As String) As String
        Dim minLength As Integer = Math.Min(s1.Length, s2.Length)
        Dim result As New Text.StringBuilder()

        For i As Integer = 0 To minLength - 1
            result.Append(s1(i))
            result.Append(s2(i))
        Next

        If s1.Length > minLength Then
            result.Append(s1.Substring(minLength))
        End If

        If s2.Length > minLength Then
            result.Append(s2.Substring(minLength))
        End If

        Return result.ToString()
    End Function

    Function Decollate(ByVal s As String, ByVal i As Integer) As String
        Dim result As New Text.StringBuilder()

        For j As Integer = i To s.Length - 1 Step 2
            result.Append(s(j))
        Next

        Return result.ToString()
    End Function
End Module