'----------------------------------------------------------------------------------------
'    Name:   CRC_DEMO_001.TIG
'    Type:   TIGER-BASIC(tm) Source Code
'
' Purpose:   Short demo of new CRC function for subsequent use for easy calculation of
'	     CRCs in large objects as sequential data streams of indefinite length,
'	     files from SmartMedia FLASH cards, or any other kind of large data sources.
'
'  (C) - Copyright Wilke Technology, P.O.Box 1727, D-52018 Aachen, Germany
'----------------------------------------------------------------------------------------
'
' Thank you for using BASIC Tigers in your products. If you have questions, ideas
' or special needs, please contact your next distributor or the Tiger support team
' and visit our web site:
'
'	Wilke Technology GmbH
'	The Tiger Support Team
'	P.O.Box 1727,       D-52018 Aachen, Germany
'	Krefelder Str. 147, D-52070 Aachen, Germany
'
'	email: support@wilke-technology.com  (english)
'	email: support@wilke.de              (german)
'	Phone: +49 (241) 918 900    Mo to Fr, 7:00 to 16:00 (GMT)
'	Fax:   +49 (241) 918 9068
'
' New information, new drivers and free downloads see:
'
'	www.wilke-technology.com	     (english)
'	www.wilke.de                         (german)
'
' Sincerely,
'
'	Your Tiger Support Team
'
'----------------------------------------------------------------------------------------
'
' Calculate CRC:	CRC  (A$, POS, LEN, START, STEP, CRC)
'
'
' Comparison:	In contrast to "CALC_CRC" there is NO function value !
'		This new function lends itself to calculate the CRC in large objects
'		as files (e.g. from SmartMedia FLASH cards) over many megabytes.
'
'
'	CRC  (A$, POS, LEN, START, STEP, CRC)
'			    =====        ===	<--  "===="  Input + Output !!!
'
'
' The function calculates a CRC in a SOURCE string. The parameters "POS" and "LEN"
' determine, from which position and how many bytes should be taken.
'
'   START = Starting value of FACTOR	-->  also starting value:  -->  gives NEXT START value for
'								        the following function call
'    STEP = Stepping width of FACTOR
'
'     LEN = Length in bytes: 1...64K-1 (for now: limited)
'
'     CRC = Entry value for CRC		-->  also starting value:  Resulting CRC
'	    Start CRC is summed up
'
'
' Note:	Value range of FACTOR = 0...FFFF (unsigned)
'	FACTOR jumps at the end of the range back to the next, small value
'
'
' Build 32-bit CRC:	1. Byte * (FAKTOR)           --> sum up
' (byte-by-byte)	2. Byte * (FACTOR+1*STEP)    --> sum up
'			3. Byte * (FACTOR+2*STEP)    --> sum up
'			4. Byte * (FACTOR+3*STEP)    --> sum up
'			    .			.
'			    .			.
'		      100. Byte * (FACTOR+199*STEP)  --> sum up
'			    .			.
'			    .			.
'			   Sum = 32 bit = 8 HEX digits
'				   (on overrun: don't care !!)
'
'
' Note:	To get a useful protection by the CRC:
'	Starting value = odd
'	Stepping width = even
'
'	To possibly get the maximum number of factors = 32k
'	choose the stepping width wisely.
'	E.g. Step = 6 (32K factors) is good, Step = 8 (8K factors)
'	is not so good.
'
'	Example:	Start = 7,  Step = 6
'
'	7,13,19,25, ... 65533,3,9,15, .... 65535,5,11,17, ....
'
'
'	Also good:  STEP = 2  (logical)  ---> gives 32K factors
'----------------------------------------------------------------------------------------

TASK MAIN

  STRING A$ (1K)
  LONG POS,  XLEN,  START,  XSTEP,  XCRC

  A$	= FILL$ ("the quick ", 1K)
  POS	= 0
  XLEN	= 2
  START	= 7
  XSTEP	= 4
  XCRC	= 0

'-----------------------------------------------------------------------------------------
' Repeated use of the CRC function is easily possible
'-----------------------------------------------------------------------------------------
' CRC  (A$, POS,  LEN, START,  STEP, CRC)
'                      =====         ===	<--  "===="  Input + Output !!!
  CRC  (A$, POS, XLEN, START, XSTEP, XCRC)
  CRC  (A$, POS, XLEN, START, XSTEP, XCRC)
  CRC  (A$, POS, XLEN, START, XSTEP, XCRC)
  CRC  (A$, POS, XLEN, START, XSTEP, XCRC)
  CRC  (A$, POS, XLEN, START, XSTEP, XCRC)

END