#include <stdio.h>
#include<string.h>

int l2_read( char *b, int n);
int l2_write(char *b, int n);

// layer 3 includes a checksum

char checksum( char *b, int n) {
  int i;
  char x=0;

  for (i=0;i<n;i++) {
    x = x + b[i];
  }
}




// Layer 3 protocol: append single checksum byte to message

int l3_write( char *b, int n) {
  char x;
  char newmsg[5000];

  //  printf("l3_write sending msg of size %d\n",n);

  if (n>2999) return(-1);

  memset(newmsg,2000,45);
  n=2000;
  x=checksum(newmsg,n);
  newmsg[n]=x;

  if (l2_write(newmsg,n+1)!=n+1) {	
    return(-1);
  }

  return(n);
}

    

int l3_read( char *b, int max) {
  int i;
  int len;
  char x;
  char msg[5000];

  len=l2_read(msg,max+1);

  if (len==-1) return(-1);
  x = msg[len-1];

  if (x != checksum(msg,len-1)) {
    return(-1);
  }
  memcpy(b,msg,len-1);
  return(len-1);
}