'   Code courtesy of UtterAccess Wiki
'   http://www.utteraccess.com/wiki/index.php/Category:FunctionLibrary
'   Original submission by Dennis Kuhn (aka doctor9)
'   on 8/13/10
'
'   This code is not to be altered or distributed,
'   except as part of an application.
'   You are free to use it in any application,
'   provided this copyright notice is left unchanged.

Public Function Base10ToBaseLetter(lngNumber As Long) As String

'   In concept, it works like this:
'   If you've got a number like 156, you'd parse it out as:
'   (1) hundreds, (5) tens, and (6) ones.
'   This function basically does the same thing, only working down the powers of 26.
'   For example, 69 in base 10 is converted to Base Letter like this:
'   2 x 26^1 plus 17 x 26^0.  Using letters to represent the 2 & 17, we get "BQ".
   
   Dim intCounter As Integer, strTemp As String
   
   strTemp = ""

'   Skip down thru powers of 26 until we find a value less than the argument value
   For intCounter = 6 To 0 Step -1
'       Once we've started extracting value from the argument, force further
'       extractions from that point on, even zero values.
       If lngNumber > 26 ^ intCounter Or Len(strTemp) > 0 Then
'           Add the base-26 value to the string...
           strTemp = strTemp & Chr$(64 + Int(lngNumber / (26 ^ intCounter)))
'           Subtract the equivalent demical value from the number
           lngNumber = _
               lngNumber - ((26 ^ intCounter) * (Int(lngNumber / (26 ^ intCounter))))
       End If
   Next intCounter
   
'   A little cleanup next.  Base Letter doesn't have a "zero" equivalent, so when
'   a capital "A" (the equivalent of 1) is reduced by 1, it becomes an "@" symbol.
'   We need to convert "@" to "Z", and then reduce the next significant "digit" by one
'   to accomodate this.
   
'   Cascade carried-over values
   While InStr(strTemp, "@") > 0
'       Starting on the left, make the value just left of the "@" one step lower,
'       then convert the "@" to a "Z"
       strTemp = Trim(Left(strTemp, InStr(strTemp, "@") - 2) & _
           Chr$(Asc(Mid(strTemp, InStr(strTemp, "@") - 1, 1)) - 1) & "Z" & _
           Mid(strTemp, InStr(strTemp, "@") + 1, 7))
'       Remove leading "@" character if necessary
       While Left(strTemp, 1) = "@"
           strTemp = Trim(Mid(strTemp, 2, 7))
       Wend
   Wend
   
'   Return the final string value
   Base10ToBaseLetter = strTemp
   
End Function

Public Function BaseLetterToBase10(strInput As String) As Long

'   Move through the string, converting each letter to a number value by multiplying
'   the letter position (A=1, B=2, etc.) by the power of 26 indicated by it's position
'   in the string.

   Dim intChar As Integer
   
   If Len(strInput & "") > 0 Then
       BaseLetterToBase10 = 0
       For intChar = 1 To Len(strInput)
           BaseLetterToBase10 = BaseLetterToBase10 + _
               (Asc(Mid(strInput, intChar, 1)) - 64) * 26 ^ (Len(strInput) - intChar)
       Next intChar
   End If

End Function

? Base10ToBaseLetter(704)
AAB

? BaseLetterToBase10("AAB")
704

Function NextLetters(strInput As String) As String

   Dim lngTemp As Long
   
   lngTemp = BaseLetterToBase10(strInput)
   lngTemp = lngTemp + 1
   NextLetters = Base10ToBaseLetter(lngTemp)

End Function