Up the Newton iterations in cbrt

Author: ckormanyos

include/boost/decimal/detail/cmath/cbrt.hpp

@@ -131,14 +131,14 @@ constexpr auto cbrt_impl(const T x) noexcept
                   (five + gx * (seventy + gx * 56))
                 / (numbers::cbrt2_v<T> * (fourteen + gx * (seventy + gx * 20)));
 
-            // Perform 2, 3 or 4 Newton-Raphson iterations depending on precision.
+            // Perform 3, 4 or 5 Newton-Raphson iterations depending on precision.
             // Note from above, we start with slightly more than 2 decimal digits
             // of accuracy.
 
             constexpr int iter_loops
             {
-                  std::numeric_limits<T>::digits10 < 10 ? 2
-                : std::numeric_limits<T>::digits10 < 20 ? 3 : 4
+                  std::numeric_limits<T>::digits10 < 10 ? 3
+                : std::numeric_limits<T>::digits10 < 20 ? 4 : 5
             };
 
             for (int idx = 0; idx < iter_loops; ++idx)

include/boost/decimal/detail/cmath/sqrt.hpp

@@ -123,7 +123,7 @@ constexpr auto sqrt_impl(const T x) noexcept
                   (one + gx * ((one + gx) * 20))
                 / (numbers::sqrt2_v<T> * ((gx * 4) * (five + gx) + five));
 
-            // Perform 2, 3 or 4 Newton-Raphson iterations depending on precision.
+            // Perform 3, 4 or 5 Newton-Raphson iterations depending on precision.
             // Note from above, we start with slightly more than 2 decimal digits
             // of accuracy.
 

test/github_issue_1110.cpp

@@ -43,8 +43,7 @@ auto test() -> void
 
         strm << std::setprecision(std::numeric_limits<boost::decimal::decimal128_t>::digits10)<< cbr;
 
-        // TODO: This tolerance isn't the best
-        BOOST_TEST_CSTR_EQ(strm.str().c_str(), "1.000000000000000000000000000000641");
+        BOOST_TEST_CSTR_EQ(strm.str().c_str(), "1.000000000000000000000000000000003");
     }
 
     const boost::decimal::decimal128_t lgt { log10(sum) };