fixes the bug with CKKS bootstrapping for N=2^17 (#1015)

Co-authored-by: Yuriy Polyakov <ypolyakod@dualitytech.com>

Author: yspolyakov

src/pke/lib/scheme/ckksrns/ckksrns-fhe.cpp

@@ -181,9 +181,7 @@ void FHECKKSRNS::EvalBootstrapSetup(const CryptoContextImpl<DCRTPoly>& cc, std::
         // Extract the modulus prior to bootstrapping
         double qDouble = GetBigModulus(cryptoParams);
 
-        int exponent{0};
-        std::frexp(qDouble, &exponent);
-        uint128_t factor = static_cast<uint128_t>(1) << static_cast<uint32_t>(exponent - 1);
+        uint128_t factor = ((uint128_t)1 << (static_cast<uint32_t>(std::round(std::log2(qDouble)))));
         double pre       = (compositeDegree > 1) ? 1.0 : qDouble / factor;
         double k         = (cryptoParams->GetSecretKeyDist() == SPARSE_TERNARY) ? K_SPARSE : 1.0;
         double scaleEnc  = pre / k;
@@ -311,9 +309,7 @@ void FHECKKSRNS::EvalBootstrapPrecompute(const CryptoContextImpl<DCRTPoly>& cc,
     // Extract the modulus prior to bootstrapping
     double qDouble = GetBigModulus(cryptoParams);
 
-    int exponent{0};
-    std::frexp(qDouble, &exponent);
-    uint128_t factor = static_cast<uint128_t>(1) << static_cast<uint32_t>(exponent - 1);
+    uint128_t factor = ((uint128_t)1 << (static_cast<uint32_t>(std::round(std::log2(qDouble)))));
     double pre       = qDouble / factor;
     double k         = (cryptoParams->GetSecretKeyDist() == SPARSE_TERNARY) ? K_SPARSE : 1.0;
     double scaleEnc  = (compositeDegree > 1) ? 1.0 / k : pre / k;