/****************************************************************
 *								*
 *	Copyright 2005, 2009 Fidelity Information Services, Inc	*
 *								*
 *	This source code contains the intellectual property	*
 *	of its copyright holder(s), and is made available	*
 *	under a license.  If you do not know the terms of	*
 *	the license, please stop and do not read further.	*
 *								*
 ****************************************************************/
#include "mdef.h"
#include "gdsroot.h"
#include "gdsbt.h"
#include "gdsfhead.h"
#include "jnl_get_checksum.h"

/*
 * Input :
 *	buff   	 : Pointer to the input buffer whose checksum needs to be computed.
 *	bufflen  : Buffer size in bytes
 *
 * Returns:
 *	Computed checksum.
 *
 * Algorithm:
 * 	For every 512 (DISK_BLOCK_SIZE) byte block of the input buffer, the first 16 and last 16 bytes are considered for checksum.
 *	The way this is implemented is as follows.
 *	For the 0th 512-byte block, the first 16 bytes are considered.
 *	For the 1st 512-byte block, the last 16 bytes of the 0th block and the first 16 bytes of the 1st block are considered.
 *		This is taken as a contiguous sequence of 32-bytes (i.e. CHKSUM_SEGLEN4 uint4s)
 *	For the 2nd 512-byte block, the last 16 bytes of the 1st block and the first 16 bytes of the 2nd block are considered.
 *		This is taken as a contiguous sequence of 32-bytes (i.e. CHKSUM_SEGLEN4 uint4s)
 *	And so on until the input buffer length is exceeded.
 * 	If bufflen is not divisible by 4, it is ROUNDED DOWN to the nearest 4-byte (uint4) multiple.
 */
uint4 jnl_get_checksum(uint4 *buff, sgmnt_addrs *csa, int bufflen)
{
	uint4			*top, *blk_base, *blk_top, blen, checksum;
#	ifdef GTM_CRYPT
	DEBUG_ONLY(
		sm_uc_ptr_t	orig_buff = NULL;
	)

	if (NULL != csa && (csa->hdr->is_encrypted))
	{
		DBG_ENSURE_PTR_IS_VALID_GLOBUFF(csa, csa->hdr, (sm_uc_ptr_t)buff);
		DEBUG_ONLY(orig_buff = (unsigned char *)buff;)
		buff = (uint4 *)GDS_ANY_ENCRYPTGLOBUF(buff, csa);
		DBG_ENSURE_PTR_IS_VALID_ENCTWINGLOBUFF(csa, csa->hdr, (sm_uc_ptr_t)buff);
	}
#	endif
	checksum = INIT_CHECKSUM_SEED;
	for (blen = bufflen / SIZEOF(*buff), top = buff + blen, blk_top = buff + CHKSUM_SEGLEN4 / 2; buff < top ;)
	{
		if (blk_top > top)
			blk_top = top;
		for ( ; buff < blk_top; buff++)
			checksum = ADJUST_CHECKSUM(checksum, *buff);
		blk_top = (uint4 *)((sm_uc_ptr_t)buff + DISK_BLOCK_SIZE);
		buff = blk_top - CHKSUM_SEGLEN4;
	}
	/* It is theoretically possible the computed checksum calculates to 0. Since we use a 0 to imply checksum
	 * was never computed, we need to return a non-zero value so in this case return INIT_CHECKSUM_SEED
	 */
	if (!checksum)
		checksum = INIT_CHECKSUM_SEED;
	return checksum;
}