CoCoA » CoCoALib

Here is a complete example:

  RingElem CutLF(RingElem& f)
  {
    const SparsePolyRing& P = owner(f);
    if (IsZero(f)) return f;
    RingElem ans(P);
    do
    {
      P->myMoveLMToBack(raw(ans), raw(f));
    }
    while (!IsZero(f) && (CmpWDeg(LPP(f), LPP(ans)) == 0));
    return ans;
  }

  void program()
  {
    GlobalManager CoCoAFoundations;

    ring P = NewPolyRing(RingQQ(), symbols("x,y,z"));
    RingElem f = ReadExpr(P,"x+y+z+1");
    RingElem fpow = power(f,199);
    RingElem fpow2 = fpow;
    const long n = NumTerms(fpow);
    long count = 0;
    // LOOP 1:
    double t0 = CpuTime();
    while (!IsZero(fpow))
    {
      RingElem lffpow = CutLF(fpow);
      count += NumTerms(lffpow);
    }
    cout << "loop1 time: " << CpuTime() - t0 << endl;
    cout << count -  n << endl;

    // LOOP 2:
    double t1 = CpuTime();
    while (!IsZero(fpow2))
      fpow2 -= LF(fpow2);
    cout << "loop2 time: " << CpuTime() - t1 << endl;
  }

I think this issue is relatively unimportant, hence the "low" priority.
I have put it on redmine just so that we do not forget it.

There was also a question about the name: I have called it CutLF. Another possibility could be MoveLF similar to MoveLM? This would require 2 args, one being the destination (and what should happen if the destination is not zero or in the wrong ring?)

After some reflection and after chatting to Anna we have decided that CutLF should also give an error (like LF) when the arg is zero. I will check in shortly.