onec_sum.c

来自「minix操作系统最新版本（3.1.1）的源代码」· C语言 代码 · 共 62 行

/*	oneC_sum() - One complement's checksum		Author: Kees J. Bot
 *								8 May 1995
 * See RFC 1071, "Computing the Internet checksum"
 */

#include <sys/types.h>
#include <net/gen/oneCsum.h>

u16_t oneC_sum(U16_t prev, void *data, size_t size)
{
	u8_t *dptr;
	size_t n;
	u16_t word;
	u32_t sum;
	int swap= 0;

	sum= prev;
	dptr= data;
	n= size;

	swap= ((size_t) dptr & 1);
	if (swap) {
		sum= ((sum & 0xFF) << 8) | ((sum & 0xFF00) >> 8);
		if (n > 0) {
			((u8_t *) &word)[0]= 0;
			((u8_t *) &word)[1]= dptr[0];
			sum+= (u32_t) word;
			dptr+= 1;
			n-= 1;
		}
	}

	while (n >= 8) {
		sum+= (u32_t) ((u16_t *) dptr)[0]
		    + (u32_t) ((u16_t *) dptr)[1]
		    + (u32_t) ((u16_t *) dptr)[2]
		    + (u32_t) ((u16_t *) dptr)[3];
		dptr+= 8;
		n-= 8;
	}

	while (n >= 2) {
		sum+= (u32_t) ((u16_t *) dptr)[0];
		dptr+= 2;
		n-= 2;
	}

	if (n > 0) {
		((u8_t *) &word)[0]= dptr[0];
		((u8_t *) &word)[1]= 0;
		sum+= (u32_t) word;
	}

	sum= (sum & 0xFFFF) + (sum >> 16);
	if (sum > 0xFFFF) sum++;

	if (swap) {
		sum= ((sum & 0xFF) << 8) | ((sum & 0xFF00) >> 8);
	}
	return sum;
}