Generate a receipt number in this range - vb.net

Question

How would I go about generating a unique receipt number in the following range:
GA00000-GZ99999? I am not allowed to use the 'I' and 'O' letters so GI00000-GI99999 & GO00000-GO99999 would be excluded.
Ideally, I'd like to create this in T-SQL but can also do it in VB.Net. This number will be stored in SQL and I can access it prior to generating the next one. They do not have to be sequential.
Thanks.

Answer 1

Create an array of all characters that are permitted as the first digit.
For any numeric receipt id n the receipt code will be G + array[n/1000000] + n % 1000000.

Answer 2

keep a suffix pointer. when your index wraps over 99999, increment it. Check for your special cases and exhaustion (e.g. Z).

Answer 3

Here's a VB.NET version to get the next sequential number, given the previous one. I think I covered all the cases.
Private Const FirstReceiptNumber As String = "GA00000"
Public Function GenerateReceiptNumber(ByVal lastNumber As String) As String
If lastNumber.Length <> 7 Then
Throw New ArgumentException("Incorrect length", "lastNumber")
End If
If lastNumber.StartsWith("G") = False Then
Throw New ArgumentException("Incorrect start character", "lastNumber")
End If
Dim letterPortion As Char = lastNumber.Chars(1)
If letterPortion < "A"c Or letterPortion > "Z"c Then
Throw New ArgumentException("Non-letter second character", "lastNumber")
End If
If letterPortion = "I"c Or letterPortion = "O"c Then
Throw New ArgumentException("Invalid second character", "lastNumber")
End If
Dim numericPortionString As String = lastNumber.Substring(2)
Dim numericPortion As Integer
If Integer.TryParse(numericPortionString, numericPortion) = False Then
Throw New ArgumentException("Invalid numeric portion", "lastNumber")
End If
If numericPortion = 99999 Then
If letterPortion = "Z"c Then
Throw New ArgumentException("No more receipt numbers possible", "lastNumber")
End If
numericPortion = 0
letterPortion = letterPortion + Chr(1)
If letterPortion = "I"c Or letterPortion = "O"c Then
letterPortion = letterPortion + Chr(1)
End If
Else
numericPortion = numericPortion + 1
End If
Return String.Format("G{0}{1:00000}", letterPortion, numericPortion)
End Function

Answer 4

I decided to do it as the following:
CREATE FUNCTION [dbo].[fn_generateReceiptNumber]()
RETURNS NCHAR(7)
AS
BEGIN
-- Declare the return variable here
DECLARE #result NCHAR(7);
DECLARE #prefix NCHAR(1);
DECLARE #suffix INT;
DECLARE #currentMax NCHAR(7);
SELECT #currentMax = MAX(ISNULL(fp.CustomReceiptNo, 'GA00001')) FROM dbo.FinPayment fp;
SELECT #prefix = SUBSTRING(#currentMax,2,1);
SELECT #suffix = CAST(SUBSTRING(#currentMax,3,7) AS INT);
IF((#suffix + 1) > 99999)
BEGIN
SELECT #suffix = 0;
END
ELSE
BEGIN
SELECT #suffix = #suffix + 1;
END
IF(#suffix = 0)
BEGIN
IF(#prefix = 'Z')
BEGIN
RETURN -1;
END
ELSE
BEGIN
IF(NCHAR(UNICODE(#prefix)+1) IN ('I', 'O'))
BEGIN
SELECT #prefix = NCHAR(UNICODE(#prefix)+2);
END
ELSE
BEGIN
SELECT #prefix = NCHAR(UNICODE(#prefix)+1);
END
END
END
-- Return the result of the function
SELECT #result = NCHAR(71) + #prefix + CAST(RIGHT('00000' + RTRIM(#suffix), 5) AS NCHAR(5));
RETURN #result;
END
GO
Thank you everyone for the input.
Steve.