         <<< MOVIES::DISK$SYSDATA:[NOTES$LIBRARY]DOLLAR_INFO.NOTE;1 >>>
                      -< Dollar File System Information >-
================================================================================
Note 86.7                Additional Product Requirements                  7 of 8
LEDER1::PETTENGILL "mulp"                           116 lines  30-APR-1994 03:31
                            -< ctm_ssi_checksum.c >-
--------------------------------------------------------------------------------
/*
*****************************************************************************
**                                                                           *
**  DIGITAL EQUIPMENT CORPORATION, MAYNARD, MASSACHUSETTS.                   *
**  COPYRIGHT ©   1992                                                       *
**  ALL RIGHTS RESERVED.                                                     *
**                                                                           *
**  THIS SOFTWARE IS FURNISHED UNDER A LICENSE AND MAY BE USED AND COPIED    *
**  ONLY IN  ACCORDANCE WITH  THE  TERMS  OF  SUCH  LICENSE  AND WITH THE    *
**  INCLUSION OF THE ABOVE COPYRIGHT NOTICE. THIS SOFTWARE OR  ANY  OTHER    *
**  COPIES THEREOF MAY NOT BE PROVIDED OR OTHERWISE MADE AVAILABLE TO ANY    *
**  OTHER PERSON.  NO TITLE TO AND OWNERSHIP OF  THE  SOFTWARE IS  HEREBY    *
**  TRANSFERRED.                                                             *
**                                                                           *
**  THE INFORMATION IN THIS SOFTWARE IS  SUBJECT TO CHANGE WITHOUT NOTICE    *
**  AND  SHOULD  NOT  BE  CONSTRUED AS  A COMMITMENT BY DIGITAL EQUIPMENT    *
**  CORPORATION.                                                             *
**                                                                           *
**  DIGITAL ASSUMES NO RESPONSIBILITY FOR THE USE  OR  RELIABILITY OF ITS    *
**  SOFTWARE ON EQUIPMENT WHICH IS NOT SUPPLIED BY DIGITAL.                  *
**                                                                           *
**                                                                           *
******************************************************************************
*/

/* 
** Module: CTM_SSI_CHECKSUM 
** 
** Description: 
**
**      This Module contains a routine that computes a buffer's checksum
** 	and stores the results in the user defined aggregate. It also
** 	contains a routine that checks the validity of a transmitted
**      checksum aggregate.
**
** Author: Stefano Santoro
**
*/

#define CTM_SSI_CHECKSUM
#include "ctm_ssi_checksum"

/*
** Routine: CmpChkSm (Compute CheckSum)
**
** Parameters:
**
**      Bff:		Buffer
**      BffSz:          Buffer Size
**
** Description:
**
**	CmpChkSm computes the checksum of a buffer of BffSz lenght,
**	and then formats the user defined Checksum aggregate
**
** Returns:
**
**      The integer form of the computed checksum aggregate
**
** Author: Stefano Santoro
**
*/
unsigned int CmpChkSm( char *Bff, int BffSz) {

  T_ChkSmAgr ChkSm;
  unsigned int j, j1, c0, c1, j2;

  j1 = c0 = c1 = 0;

  while( j1 < BffSz) {
    j2 = (BffSz < (j1 + 4102) ? BffSz : (j1 + 4102));
    for (j = j1; j < j2; j++) {
      c0 += Bff[j];
      c1 += c0;
    };
    c0 %= CHECKSUM_MODULO;
    c1 %= CHECKSUM_MODULO;
    j1 = j2;
  }
  
  ChkSm.ByChr[0] = (unsigned char)c0;
  ChkSm.ByChr[1] = (unsigned char)c1;
  ChkSm.ByWrd[1] = ~ChkSm.ByWrd[0];

  return(ChkSm.ByInt);
}

/*
** Routine: VldChkSm (Validate CheckSum)
**
** Parameters:
**
**      PrmChkSm:	Checksum to be validated
**
** Description:
**
**	VldChkSm validates the checksum aggregate by checking wether
**      ones side one's complement is equal to the other side.
**
** Returns:
**
**      0: checksum aggregate is not valid
**      1: checksum aggregate is valid
**
** Author: Stefano Santoro
**
*/
int VldChkSm( unsigned int PrmChkSm) {

  T_ChkSmAgr ChkSm;

  ChkSm.ByInt = PrmChkSm;
  ChkSm.ByWrd[1] = ~ChkSm.ByWrd[1];

  return (ChkSm.ByWrd[0] == ChkSm.ByWrd[1]);
}