I do not have to take 1st string's legth in consideration...  I am doing kmp failure function tabulation...  for suffix match of string b with prefixes of a and thus ...  only b's string length is taken care of ....  plz suggest some test case where I might be wrong..
what m doing is this
--------------------------
// n is the string length of b
 f.resize(n+2);

  f[0]=-1;
  int x;
  for(int i=1;i<=n;i++)
    {
      x=f[i-1];
      while(x>=0&&a[x+1]!=b[i-1])
        x=f[x];

      if(a[x+1]==b[i-1])
        f[i]=x+1;
      else
        f[i]=-1;
    }
--------------------------------------------