*
* Creates next string key from the passed one using specified set of characters
*
* Parameters
*	1 - Current Key
*	2 - Defines the list of allowed characters
*		BASEnn - See DO CASE in the body of the program
*		CUSTOM - the list of character as parameter 3
*	3 - List of characters
*
* Returns 	Next key
*
*   Note 1  Routine ignores (doesn't change) positions with the characters not in the specified list
*   Note 2  When max possible value is reached, the next return value will be the min possible value
*
 
FUNCTION NextKey
LPARAMETERS tcOldVal, tcOpt, tcCharList
LOCAL lcNewVal, i, lcDigits, lcLetters, lnCharListLen, lcOldChar, lcNewChar, lcCharList, lnPos, lcOpt
LOCAL lnNextPos
 
lcOpt = IIF(EMPTY(tcOpt), "BASE10", UPPER(tcOpt))
* Get the list of appropriate characters
lcCharList = NextKeyFillCharList(lcOpt, tcCharList)
lnCharListLen = LEN(lcCharList)
lcNewVal = tcOldVal
 
* Scan string from the right to the left
FOR i = LEN(lcNewVal) TO 1 STEP -1
 
	lcOldChar = SUBSTR(tcOldVal, i, 1)
	* Is the current charater in the list?
	lnPos = AT(lcOldChar, lcCharList)
	IF lnPos = 0
		* Not in the list, keep it
		LOOP
	ENDIF
 
	* Get the next character position
	lnNextPos = (lnPos % lnCharListLen) + 1
	* Get the next character 
	lcNewChar = SUBSTR(lcCharList,lnNextPos,1)
	* Stuff it back in the string
	lcNewVal = STUFF(lcNewVal, i, 1, lcNewChar)
	* Check if we have to carry over to the next position
	IF lnNextPos > 1
		* We are done
		EXIT
	ENDIF
 
ENDFOR
 
RETURN lcNewVal
*------------------------------------------------------------
* Fill the list of characters based on character set requested
FUNCTION NextKeyFillCharList
LPARAMETERS tcCharSet, tcCharList
LOCAL lcCharList, lcDigits, lcLetters
* Fill string 'lcCharList' with appropriate characters
lcDigits = "0123456789"
lcLetters = "ABCDEFGHIJKLMNOPQRSTUVWXYZ"
 
DO CASE
CASE tcCharSet = "CUSTOM"  
	lcCharList = tcCharList
CASE tcCharSet == "BASE10" 
	* Just Digits
	lcCharList = lcDigits
CASE tcCharSet == "BASE16L" 
	* Hexadecimal in lower case
	lcCharList = lcDigits + "abcdef"
CASE tcCharSet == "BASE16" 
	* Hexadecimal in upper case
	lcCharList = lcDigits + "ABCDEF"
CASE tcCharSet == "BASE26L" 
	* Lower case letters
	lcCharList = LOWER(lcLetters)
CASE tcCharSet == "BASE26" 
	* Upper case letters
	lcCharList = lcLetters
CASE tcCharSet == "BASE36L" 
	* Digits + Lower case letters
	lcCharList = lcDigits + LOWER(lcLetters)
CASE tcCharSet == "BASE36" 
	* Digits + Upper case letters
	lcCharList = lcDigits + lcLetters
CASE tcCharSet == "BASE52" 
	* All letters
	lcCharList = lcLetters + LOWER(lcLetters)
CASE tcCharSet == "BASE62" 
	* Digits + All letters
	lcCharList = lcDigits + lcLetters + LOWER(lcLetters)
OTHERWISE
	* The same as BASE10
	lcCharList = lcDigits
ENDCASE
RETURN lcCharList 

* Rollover to min value (Base10)
? NextKey("999999")			&& 000000

* Next value (Base16)
? NextKey("999999", "Base16")  && 99999A

* Dash ('-') has not been changed
? NextKey("999-FFF", "Base16")  && 99A-000