
Function I25Encode(StringNumber)

   Dim asPattern(), sSTART, sSTOP

   Redim asPattern(10)

   sSTART = "NNNN"              ' start and stop patterns can be found in fig. 3
   sSTOP  = "WNN"

   asPattern(0) = "NNWWN"       ' these patterns can be found in fig. 1
   asPattern(1) = "WNNNW"
   asPattern(2) = "NWNNW"
   asPattern(3) = "WWNNN"
   asPattern(4) = "NNWNW"
   asPattern(5) = "WNWNN"
   asPattern(6) = "NWWNN"
   asPattern(7) = "NNNWW"
   asPattern(8) = "WNNWN"
   asPattern(9) = "NWNWN"

   ' validate argument

   If (Len(StringNumber) Mod 2) <> 0 Then
 
      ' the number of characters in the argument must be odd

      I25Encode = ""
      Exit Function

   End If

      
   If Not IsNumeric(StringNumber) Then
    
      ' argument must be numeric

      I25Encode = ""
      Exit Function

   Else

      If (InStr(StringNumber, ".") <> 0) Or (InStr(StringNumber, ",") <> 0) Then

         ' argument is numeric but contains invalid characters to us
 
         I25Encode = ""
         Exit Function

      End If

   End If

   ' encode and interleave argument

   Dim sEncodedSTR, sUnit

   Dim iCharRead,   sDigit1, sDigit2, i

   sEncodedSTR = ""

   For iCharRead = 1 To Len(StringNumber) Step 2

       sDigit1 = asPattern( Asc( Mid( StringNumber, iCharRead, 1 ) ) - 48 )

       sDigit2 = asPattern( Asc( Mid( StringNumber, iCharRead + 1, 1 ) ) - 48 )

       sUnit = ""

       For i = 1 To 5

           sUnit = sUnit & Mid( sDigit1, i, 1 ) & Mid( sDigit2, i, 1 )

       Next

       sEncodedSTR = sEncodedSTR & sUnit

   Next

   I25Encode = sSTART & sEncodedSTR & sSTOP

End Function