CS360 Final -- December 11, 1999. Question 3

You've just been hired to take the place of ``Hank the hacker,'' who was fired because was too cavalier of a programmer and human being for your conservative-minded company. As your first job, you've been handed a piece of Hank's code. Here it is:

jmp_buf Env;
int CS;
Dllist Tmp;
Dllist D;

static rec_call()
{
  if (Tmp == D) longjmp(Env, 1);
  Tmp = Tmp->flink;
  CS++;
  rec_call();
}

int dlls(Dllist d)
{
  if (setjmp(Env) == 1) return CS;
  CS = 0;
  D = d;
  Tmp = d->flink;
  rec_call();
}

Part 1: It is obvious that Hank deserved to be fired. Rewrite this code with no global variables, no recursion, and no use of setjmp/longjmp.

Part 2: When Hank was confronted with this code, his explanation was the following: ``I inherited this code from this functional programming loser. All I did was make it more efficient.'' Here is the code that Hank inherited.

static int rec_call(Dllist d, Dllist tmp, int cs)
{
  if (tmp == d) return cs; else return rec_call(d, tmp->flink, cs+1);
}

int dlls(Dllist d)
{
  return rec_call(d, d->flink, 0);
}

Hank is right. He did make the code more efficient in two distinct ways. One involves the global variables, and one involves setjmp/longjmp. Explain the two ways in which Hank's code is more efficient than his predecessor's code.

Part 3: Your code, Hank's code, and his predecessor's code all have the same computational (big-O) complexity with respect to the size of the list d. If d has n elements, what is the computational complexity of all of your codes?

Part 4: Explain why your code is even more efficient than Hank's.