/*---------------*/
/* checkShouldBe */
/*               *-------------------------------------------------*/
/* Computes a checksum for a packet.  This can be used to set the  */
/* checksum and also to verify it.  It does not use exactly the    */
/* same method as the Internet checksum described in Kurose&Ross.  */
/*                                                                 */
/* Author:   J. Blustein                                           */
/* Version:  28 July 2003                                          */
/*                                                                 */
/* To Set A Checksum (for an outgoing packet)                      */
/*  `packet' must have the correct data (for sequence number, ACK, */
/*  message data to be passed to the other host) stored in it.     */
/*  This function will return the value that should be placed      */
/*  in the checksum field.                                         */
/*                                                                 */
/* To Verify A Checksum (for an incoming packet)                   */
/*  `packet' should not be changed since it arrived from the other */
/*  host.  This function will return what the proper checksum      */
/*  should be.  To see if it the checksum is okay or not compare   */
/*  the value returned by this function with the checksum stored   */
/*  in the checksum field:                                         */
/*    if (checkShouldBe(packet) == pkt_check(packet)) {            */
/*       // the packet is probably okay                            */
/*    } else {                                                     */
/*       // the packet is not okay                                 */
/*    }                                                            */
/*                                                                 */
/* Note:                                                           */
/*  this function does not compute the Internet checksum.  That    */
/*  method uses bit twiddling and is too complex for a simple      */
/*  simulation like this assignment.  If however you are           */
/*  interested in how to twiddle bits in C then check out          */
/*  Dr. Dobb's Journal  for August 1995 and the code at <URL:      */
/*  http://www.csd.uwo.ca/~jamie/BitVectors/README.html>.          */
/*-----------------------------------------------------------------*/
int
checkShouldBe(packet_t packet) {
  char * data;
  int    checksum;

  data = pkt_data(packet);
  checksum = 0;
  while(NULL != data && '\0' != *data) {
    checksum += (int)*data;
    data++;
  }
  checksum += pkt_ack(packet) + pkt_seq(packet);
  checksum = ~checksum;

  return checksum;
} /* checkShouldBe() */

/* EOF for checksum.c */