void PAlgebraModDerived<type>::decodePlaintext(
vector<RX>& alphas, const RX& ptxt, const MappingData<type>& mappingData) const
{
long nSlots = zMStar.getNSlots();
if (isDryRun()) {
alphas.assign(nSlots, RX::zero());
return;
}
// First decompose p into CRT components
vector<RX> CRTcomps(nSlots); // allocate space for CRT component
CRT_decompose(CRTcomps, ptxt); // CRTcomps[i] = p mod facors[i]
if (mappingData.degG==1) {
alphas = CRTcomps;
return;
}
alphas.resize(nSlots);
REBak bak; bak.save(); mappingData.contextForG.restore();
for (long i=0; i<nSlots; i++) {
REX te;
conv(te, CRTcomps[i]); // lift i'th CRT componnet to mod G(X)
te %= mappingData.rmaps[i]; // reduce CRTcomps[i](Y) mod Qi(Y), over (Z_2[X]/G(X))
// the free term (no Y component) should be our answer (as a poly(X))
alphas[i] = rep(ConstTerm(te));
}
}
template<> void PAlgebraModTmpl<zz_pX,vec_zz_pX,zz_pXModulus>::decodePlaintext(
vector<zz_pX>& alphas, const zz_pX& ptxt, const zz_pX &G,
const vector<zz_pX>& maps) const
{
unsigned nSlots = zmStar.NSlots();
if (nSlots==0 || maps.size()!= nSlots ||deg(G)<1|| zmStar.OrdTwo()%deg(G)!=0){
alphas.resize(0); return;
}
// First decompose p into CRT componsnt
vector<zz_pX> CRTcomps(nSlots); // allocate space for CRT component
CRT_decompoe(CRTcomps, ptxt); // CRTcomps[i] = p mod facors[i]
// Next convert each entry in CRTcomps[] back to base-G representation
// (this is roughly the inverse of the embedInSlots operation)
// maps contains all the base-G ==> base-Fi maps
// SHAI: The code below is supposed to be Nigel's code, borrowed partly
// from the constructor Algebra::Algebra(...) and partly from the
// function AElement::to_type(2). I don't really unerstand that code,
// so hopefully nothing was lost in translation
alphas.resize(nSlots);
zz_pE::init(G); // the extension field R[X]/G(X)
for (unsigned i=0; i<nSlots; i++) {
// We need to lift Fi from R[Y] to (R[X]/G(X))[Y]
zz_pEX Qi; conv(Qi,(zz_pX&)factors[i]);
vec_zz_pEX FRts=SFCanZass(Qi); // factor Fi over Z_2[X]/G(X)
// need to choose the right factor, the one that gives us back X
int j;
for (j=0; j<FRts.length(); j++) {
// lift maps[i] to (R[X]/G(X))[Y] and reduce mod j'th factor of Fi
zz_pEX GRti = to_zz_pEX((zz_pX&)maps[i]);
GRti %= FRts[j];
if (IsX(rep(coeff(GRti,0)))) { // is GRti == X?
Qi = FRts[j]; // If so, we found the right factor
break;
} // If this does not happen then move to the next factor of Fi
}
zz_pEX te; conv(te,CRTcomps[i]); // lift i'th CRT componnet to mod G(X)
te %= Qi; // reduce CRTcomps[i](Y) mod Qi(Y), over (Z_2[X]/G(X))
// the free term (no Y component) should be our answer (as a poly(X))
alphas[i] = rep(coeff(te,0));
}
}
