Method Scale

FHE/Rq_Element.cpp:207–273

}

void Rq_Element::Scale(const bigint& p)
{
  if (lev==0) { return; }

  if (a[0].is_zero() and a[1].is_zero())
  {
      lev = 0;
      return;
  }

  CODE_LOCATION_NO_SCOPE

  if (n_mults() == 0) {
	  //for some reason we scale but we have just one level
	  throw level_mismatch();
  }
  bigint p0=a[0].get_prime(),p1=a[1].get_prime(),p1i,lambda,n=p1*p;
  invMod(p1i,p1%p,p);

  // First multiply input by [p1]_p
  bigint te=p1%p;
  if (te>p/2) { te-=p; }
  modp tep;
  to_modp(tep,te,a[0].get_prD());
  mul(a[0],a[0],tep);
  to_modp(tep,te,a[1].get_prD());
  mul(a[1],a[1],tep);

  // Now compute delta
  Ring_Element b0(a[0].get_FFTD(),evaluation);
  Ring_Element b1(a[1].get_FFTD(),evaluation);
  // scope to ensure deconstruction of write iterators
  {
    auto poly_a1 = a[1];
    poly_a1.change_rep(polynomial);
    auto it = poly_a1.get_iterator();
    auto it0 = b0.get_write_iterator();
    auto it1 = b1.get_write_iterator();
    bigint half_n = n / 2;
    bigint delta;
    for (int i=0; i < a[1].get_FFTD().phi_m(); i++)
      {
        it.get(delta);
        lambda = delta;
        lambda *= p1i;
        lambda %= p;
        lambda *= p1;
        lambda -= delta;
        lambda %= n;
        if (lambda > half_n)
          lambda -= n;
        it0.get(lambda);
        it1.get(lambda);
      }
  }

  // Now add delta back onto a0
  Rq_Element bb(b0,b1);