A Tour of NTL: Tips for Getting the Best Performance out of NTL

Make sure you run the configuration wizard when you install NTL. This is the default behaviour in the makefile in the Unix distribution, so don't change this; in the Windows distribution, there is unfortunately no easy way to run the wizard.

In time-critical code, avoid creating unnecessary temporary objects. For example, instead of

ZZ InnerProduct(const ZZ *a, const ZZ *b, long n)
{
   long i;
   ZZ res;
   for (i = 0; i < n; i++)
      res += a[i] * b[i];
   return res;
}

write this as

ZZ InnerProduct(const ZZ *a, const ZZ *b, long n)
{
   long i;
   ZZ res, t;
   for (i = 0; i < n; i++) {
      mul(t, a[i], b[i]);
      add(res, res, t);
   }
   return res;
}

The first version of InnerProduct creates and destroys a temporary object, holding the value a[i]*b[i], in every loop iteration. The second does not.

If you use the class ZZ_p, try to avoid switching the modulus too often, as this can be a rather expensive operation. If you must switch the modulus often, use the class ZZ_pContext to save the information associated with the modulus (see ZZ_p.txt). The same holds for analogous classes, such as zz_p and GF2E.