Combination, permutation problem...

A,A,A
A,A,B
A,A,C
A,B,A
A,B,B
A,B,C
A,C,A
A,C,B
A,C,C
B,A,A
B,A,B
B,A,C
B,B,A
B,B,B
B,B,C
B,C,A
B,C,B
B,C,C
C,A,A
C,A,B
C,A,C
C,B,A
C,B,B
C,B,C
C,C,A
C,C,B
C,C,C

                                              

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Public Class Form1
 
    Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
        Dim Rev As New Revision("<>N", "<<<")
        Dim curRev As String
 
        curRev = Rev.CurrentRevision
        While curRev.Length = 3
            Debug.Print(curRev)
            curRev = Rev.NextRevision
        End While
    End Sub
 
End Class
 
Public Class Revision
 
    Private chars As String
    Private values() As Char
    Private curRevision As System.Text.StringBuilder
 
    Public Sub New()
        Me.DefaultRevision()
    End Sub
 
    Public Sub New(ByVal validChars As String)
        If validChars.Length > 0 Then
            chars = validChars.ToUpper()
            values = chars.ToCharArray()
            curRevision = New System.Text.StringBuilder(values(0))
        Else
            Me.DefaultRevision()
        End If
    End Sub
 
    Public Sub New(ByVal validChars As String, ByVal startingRevision As String)
        Me.New(validChars)
        curRevision = New System.Text.StringBuilder(startingRevision.ToUpper())
        Dim i As Integer
        For i = 0 To curRevision.Length - 1
            If Array.IndexOf(values, curRevision.Chars(i)) = -1 Then
                curRevision = New System.Text.StringBuilder(values(0))
                MessageBox.Show("Revision has been reset." & vbCrLf & "Current Revision = " & Me.CurrentRevision, "Starting Revision contains an Invalid Character", MessageBoxButtons.OK, MessageBoxIcon.Exclamation)
                Exit For
            End If
        Next
    End Sub
 
    Private Sub DefaultRevision()
        chars = "ABCDEFGHIJKLMNOPQRSTUVWXYZ"
        values = chars.ToCharArray
        curRevision = New System.Text.StringBuilder(values(0))
    End Sub
 
    Public ReadOnly Property ValidChars() As String
        Get
            Return chars
        End Get
    End Property
 
    Public ReadOnly Property CurrentRevision() As String
        Get
            Return curRevision.ToString()
        End Get
    End Property
 
    Public Function NextRevision(Optional ByVal numRevisions As Integer = 1) As String
        Dim forward As Boolean = (numRevisions > 0)
        numRevisions = Math.Abs(numRevisions)
        Dim i As Integer
        For i = 1 To numRevisions
            If forward Then
                Me.Increment()
            Else
                Me.Decrement()
            End If
        Next
        Return Me.CurrentRevision
    End Function
 
    Private Sub Increment()
        Dim curChar As Char = curRevision.Chars(curRevision.Length - 1)
        Dim index As Integer = Array.IndexOf(values, curChar)
        If index < (chars.Length - 1) Then
            index = index + 1
            curRevision.Chars(curRevision.Length - 1) = values(index)
        Else
            curRevision.Chars(curRevision.Length - 1) = values(0)
            Dim i As Integer
            Dim startPosition As Integer = curRevision.Length - 2
            For i = startPosition To 0 Step -1
                curChar = curRevision.Chars(i)
                index = Array.IndexOf(values, curChar)
                If index < (values.Length - 1) Then
                    index = index + 1
                    curRevision.Chars(i) = values(index)
                    Exit Sub
                Else
                    curRevision.Chars(i) = values(0)
                End If
            Next
            curRevision.Insert(0, values(0))
        End If
    End Sub
 
    Private Sub Decrement()
        Dim curChar As Char = curRevision.Chars(curRevision.Length - 1)
        Dim index As Integer = Array.IndexOf(values, curChar)
        If index > 0 Then
            index = index - 1
            curRevision.Chars(curRevision.Length - 1) = values(index)
        Else
            curRevision.Chars(curRevision.Length - 1) = values(values.Length - 1)
            Dim i As Integer
            Dim startPosition As Integer = curRevision.Length - 2
            For i = startPosition To 0 Step -1
                curChar = curRevision.Chars(i)
                index = Array.IndexOf(values, curChar)
                If index > 0 Then
                    index = index - 1
                    curRevision.Chars(i) = values(index)
                    Exit Sub
                Else
                    curRevision.Chars(i) = values(values.Length - 1)
                End If
            Next
            curRevision.Remove(0, 1)
            If curRevision.Length = 0 Then
                curRevision.Insert(0, values(0))
            End If
        End If
    End Sub
 
End Class

                                              

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