/**********************************************************
			Checksum.h
provide checksum functions using 1's complement for both IP 
header checksum and ICMP checksum

***********************************************************/

#include "sr_checksum.h"

/*
*  Author: Denny Chen Dai
*
*  Compute checksum for outgoing packet, store checksum field 
*  
*  base: the beginning of the data chunck
*  checksum: the field for storing 16bit checksum 
*  packet_size: number of bytes (8bit)
*/
void sr_checksum_send(uint16_t *base, uint16_t *checksum, int packet_size){

		/*puts("****sr_checksum_send****");*/
		uint16_t *pw = base, *pb = base;
		uint32_t sum = 0;  
		int i;
		pw = base;

		/*first zero out checksum field*/
		*checksum = 0x00; 	

		/*compute sum*/
		for(i=0; i< packet_size / sizeof(uint16_t); i++){
			sum = sum + *pw; 
			pw++;
		}

		/*even size handling*/
		if(packet_size % 2 != 0){
			puts("odd numbr length...");
			uint8_t *p =  (uint8_t *)pw;
			p++;
			*p = 0x00;
			sum = sum + *pw;  
		}

		/*printf(" Check sum:  %08X \n", sum);*/
		pb = (uint16_t*)&sum; 
		uint16_t high =  *pb;
		pb++;
		uint16_t low = *pb;
		uint16_t check = low + high; 		
		*checksum = ~check; 
		/*	printf("Check sum : %04X \n", *checksum);*/
}


/*
*	Author: Denny Chen Dai
* 	Perform error checksum on receiving packet
* 	
*	base: the base pointer of the packet field
* 	packet_size: number of bytes
*   Return: if no error, return 1, else return 0
*/
int sr_checksum_receive(uint16_t *base, int packet_size){

		/*puts("****sr_checksum_receive****");*/
		uint16_t *pw = base, *pb = base;
		uint32_t sum = 0;  
		int i;
		pw = base;
		
		/*compute sum*/
		for(i=0; i< packet_size / sizeof(uint16_t); i++){
			sum = sum + *pw; 
			pw++;
		}		

		/*handle even size*/
		if(packet_size % 2 != 0){
			puts("odd numbr length...");
			uint8_t *p =  (uint8_t *)pw;
			p++;
			*p = 0x00;
			sum = sum + *pw;  
		}


		/*printf(" Check sum:  %08X \n", sum);*/		
		pb = (uint16_t*)&sum; 
		uint16_t high =  *pb;
		pb++;
		uint16_t low = *pb;
		uint16_t check_sum = low + high; 
		/*printf("Check sum : %04X \n", check_sum);*/

		if(check_sum == 0xFFFF)
			return 1;
		else return 0;
}