//-------------------------------------------------------------------
//	xsumdemo.cpp
//
//	This program illustrates the advantages of one's complement
//	addition for computing of checksums in network programming.
//
//	     compile using:  $ g++ xsumdemo.cpp -o xsumdemo
//	     execute using:  $ ./xsumdemo
//
//	programmer: ALLAN CRUSE
//	written on: 26 MAR 2008
//-------------------------------------------------------------------

#include <stdio.h>	// for printf(), perror() 
#include <fcntl.h>	// for open() 
#include <stdlib.h>	// for exit() 
#include <unistd.h>	// for read(), write(), close() 
#include <arpa/inet.h>	// for htons()

#define BUF_LEN	64

unsigned char	packet[ BUF_LEN ];

int main( int argc, char **argv )
{
	// initialize an array of bytes with random values 
	for (int i = 0; i < BUF_LEN; i++)
		packet[ i ] = (unsigned char)rand();

	// display the array in hexadecimal format
	printf( "\n  Sample packet-array of random data is shown below: \n" );
	for (int i = 0; i < BUF_LEN; i++) 
		{
		if ( ( i % 16 ) == 0 ) printf( "\n  %04X: ", i );
		printf( "%02X ", packet[ i ] );
		}
	printf( "\n\n" );

	// compute the normal 16-bit two's complement sum
	// using the CPU's native 'little endian' byte-order
	unsigned short	*wp = (unsigned short*)packet;
	unsigned int	sum = 0;
	for (int i = 0; i < BUF_LEN / 2; i++) sum += wp[i];
	sum &= 0xFFFF;
	printf( "\n  The 16-bit little-endian two's complement sum " );
	printf( "is %04X \n", sum );

	// recompute this sum using the network 'big endian' byte-order
	sum = 0;
	for (int i = 0; i < BUF_LEN / 2; i++) sum += htons( wp[ i ] );
	sum &= 0xFFFF;
	sum = htons( sum );
	printf( "\n     The 16-bit big-endian two's complement sum " );
	printf( "is %04X \n", sum );
	printf( "\n" );

	// now compute the 16-bit one's complement sum 
	// using the CPU's native 'little endian' byte-order
	sum = 0;
	for (int i = 0; i < BUF_LEN / 2; i++) sum += wp[i];
	sum += (sum >> 16);
	sum &= 0xFFFF;
	printf( "\n  The 16-bit little-endian one's complement sum " );
	printf( "is %04X \n", sum );

	// recompute this sum using the network 'big endian' byte-order
	sum = 0;
	for (int i = 0; i < BUF_LEN / 2; i++) sum += htons( wp[ i ] );
	sum += (sum >> 16);
	sum &= 0xFFFF;
	sum = htons( sum );
	printf( "\n     The 16-bit big-endian one's complement sum " );
	printf( "is %04X \n", sum );
	printf( "\n" );

	// display the concluding observation
	printf( "\n  These comparisons show us why the one's complement " );
	printf( "\n  checksum is preferred as being \'endian neutral\'. " );
	printf( "\n\n" );
}