Added error handling for EvalPoly and EvalChebyshev/Degree (#561)

* Added fixes and error handling to EvalChebyshevCoefficients(), Degree() and EvalPoly()

* Changed a comment and replaced ssize_t with int32_t as ssize_t is not a part of standard C/C++

---------

Co-authored-by: Dmitriy Suponitskiy <dsuponitskiy@dualitytech.com>

src/core/lib/math/chebyshev.cpp

@@ -35,6 +35,7 @@
  */
 
 #include "math/chebyshev.h"
+#include "utils/exception.h"
 
 #include <cmath>
 #include <cstdint>
@@ -44,6 +45,9 @@
 namespace lbcrypto {
 
 std::vector<double> EvalChebyshevCoefficients(std::function<double(double)> func, double a, double b, uint32_t degree) {
+    if (!degree) {
+        OPENFHE_THROW(config_error, "The degree of approximation can not be zero");
+    }
     // the number of coefficients to be generated should be degree+1 as zero is also included
     size_t coeffTotal{degree+1};
     double bMinusA = 0.5 * (b - a);

src/pke/include/scheme/ckksrns/ckksrns-utils.h

@@ -53,9 +53,9 @@ struct longDiv {
 };
 
 /**
- * Gets the degree of a polynomial specified by its coefficients.
- *
- * @param &coefficients vector of coefficients of a polynomial.
+ * @brief Gets the degree of a polynomial specified by its coefficients, which is the index of
+ * the last non-zero element in the coefficients. If all the coefficients are zero, it returns 0.
+ * @param coefficients vector of coefficients of a polynomial (can not be empty)
  * @return the integer degree of the polynomial.
  */
 uint32_t Degree(const std::vector<double>& coefficients);

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

@@ -124,7 +124,16 @@ Ciphertext<DCRTPoly> AdvancedSHECKKSRNS::EvalPoly(ConstCiphertext<DCRTPoly> x,
 
 Ciphertext<DCRTPoly> AdvancedSHECKKSRNS::EvalPolyLinear(ConstCiphertext<DCRTPoly> x,
                                                         const std::vector<double>& coefficients) const {
-    uint32_t k = coefficients.size() - 1;
+    const size_t coefficientsSize = coefficients.size();
+    if (coefficientsSize == 0) {
+        OPENFHE_THROW(math_error, "The coefficients vector can not be empty");
+    }
+
+    uint32_t k = coefficientsSize - 1;
+    if (k == 0) {
+        OPENFHE_THROW(math_error, "The coefficients vector should have, at least, 2 elements");
+    }
+
     if (coefficients[k] == 0)
         OPENFHE_THROW(math_error, "EvalPolyLinear: The highest-order coefficient cannot be set to 0.");
 

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

@@ -116,19 +116,20 @@ inline bool IsNotEqualOne(double val) {
     return false;
 }
 
-/*Return the degree of the polynomial described by coefficients,
-which is the index of the last non-zero element in the coefficients - 1.
-Don't throw an error if all the coefficients are zero, but return 0. */
 uint32_t Degree(const std::vector<double>& coefficients) {
-    uint32_t deg = 1;
-    for (int i = coefficients.size() - 1; i > 0; i--) {
-        if (coefficients[i] == 0) {
-            deg += 1;
-        }
-        else
+    const size_t coefficientsSize = coefficients.size();
+    if (!coefficientsSize) {
+        OPENFHE_THROW(math_error, "The coefficients vector can not be empty");
+    }
+
+    int32_t indx = coefficientsSize;
+    while (--indx >= 0) {
+        if (coefficients[indx])
             break;
     }
-    return coefficients.size() - deg;
+
+    // indx becomes negative (-1) only when all coefficients are zeroes. in this case we return 0
+    return static_cast<uint32_t>((indx < 0) ? 0 : indx);
 }
 
 /* f and g are vectors of coefficients of the two polynomials. We assume their dominant