// For an n-size array, compute the 2^n products
// products[j] = \prod_{i s.t. j_i=1} array[i]
// \times \prod_{i s.t. j_i=0}(a-array[i])
void computeAllProducts(/*Output*/CtPtrs& products,
/*Index*/const CtPtrs& array,
std::vector<zzX>* unpackSlotEncoding)
{
FHE_TIMER_START;
long nBits = array.size();
if (lsize(products)>0) {
long nBits2 = NTL::NumBits(lsize(products)-1); // ceil(log_2(size))
if (nBits>nBits2)
nBits=nBits2; // ignore extra bits in 'array'
}
if (nBits<1) return; // do nothing
assert(nBits <= 16); // Output cannot be bigger than 2^16
if (lsize(products)==0) // try to set the output size
products.resize(1L << nBits, &array);
for (long i=0; i<lsize(products); i++)
products[i]->clear();
// Check that we have enough levels, try to bootstrap otherwise
assert(array.ptr2nonNull() != nullptr);
long bpl = array.ptr2nonNull()->getContext().BPL();
if (findMinBitCapacity(array) < (NTL::NumBits(nBits)+1)*bpl) {
const Ctxt* ct = array.ptr2nonNull(); // find some non-null Ctxt
assert(unpackSlotEncoding!=nullptr);
assert(ct!=nullptr);
assert(ct->getPubKey().isBootstrappable());
packedRecrypt(array, *unpackSlotEncoding,
*(ct->getContext().ea), /*belowLevel=*/nBits +3);
}
if (findMinBitCapacity(array) < (NTL::NumBits(nBits)+1)*bpl)
throw std::logic_error("not enough levels for table lookup");
// Call the recursive function that copmutes the products
recursiveProducts(products, CtPtrs_slice(array,0,nBits));
}
// A counterpart of tableLookup. The input is an encrypted table T[]
// and an array of encrypted bits I[], holding the binary representation
// of an index i into T. This function increments by one the entry T[i].
void tableWriteIn(const CtPtrs& table, const CtPtrs& idx,
std::vector<zzX>* unpackSlotEncoding)
{
FHE_TIMER_START;
const Ctxt* ct = table.ptr2nonNull(); // find some non-null Ctxt
long size = lsize(table);
if (size==0) return;
std::vector<Ctxt> products(size, Ctxt(ZeroCtxtLike, *ct));
CtPtrs_vectorCt pWrap(products); // A wrapper
// Compute all products of ecnrypted bits =: b_i
computeAllProducts(pWrap, idx, unpackSlotEncoding);
// incrememnt each entry of T[i] by products[i]
NTL_EXEC_RANGE(lsize(table), first, last)
for(long i=first; i<last; i++)
*table[i] += products[i];
NTL_EXEC_RANGE_END
}