actube/src/cw/lw_checksum.c

/*
    This file is part of libcapwap.

    libcapwap is free software: you can redistribute it and/or modify
    it under the terms of the GNU General Public License as published by
    the Free Software Foundation, either version 3 of the License, or
    (at your option) any later version.

    libcapwap is distributed in the hope that it will be useful,
    but WITHOUT ANY WARRANTY; without even the implied warranty of
    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
    GNU General Public License for more details.

    You should have received a copy of the GNU General Public License
    along with Foobar.  If not, see <http://www.gnu.org/licenses/>.

*/

/**
 *@file
 *@brief Implements lw_checksum.
 *@addtogroup LW 
 *@{
 */ 


#include "lw.h"


/**
 * Calculate the 16-bit checksum for LWAPP Image Data message 
 * elements with opcode 3 (see RFC5412) 
 * This is also used by Cisco in CAPWAP
 * @param d pointer to data to calulate the checksum for
 * @param len length of data
 * @return the calculated checksum.
 */
uint16_t lw_checksum(uint8_t * d, int len)
{
	int32_t sum = 0;
	uint16_t *w = (uint16_t *) d;

	while (len > 1) {
		sum += ntohs(*w++);
		len -= 2;
	}

	if (len)
		sum += (*(uint8_t *) w) << 8;

	sum = (sum >> 16) + (sum & 0xffff);
	sum += (sum >> 16);

	return (~sum) & 0xffff;
}


/**
 *@}
 */