Merge pull request #812 from cppalliance/802

Fix calculation of resultant digits to keep us in the range of `uint128`

Author: mborland

include/boost/decimal/detail/mul_impl.hpp

@@ -125,7 +125,7 @@ constexpr auto d128_mul_impl(T1 lhs_sig, U1 lhs_exp, bool lhs_sign,
     const auto rhs_dig {detail::num_digits(rhs_sig)};
 
     // If we can avoid it don't do 256 bit multiplication because it is slow
-    if (lhs_dig * rhs_dig <= std::numeric_limits<uint128>::digits10)
+    if (lhs_dig + rhs_dig <= std::numeric_limits<uint128>::digits10)
     {
         auto res_sig {lhs_sig * rhs_sig};
         auto res_exp {lhs_exp + rhs_exp};

test/Jamfile

@@ -50,6 +50,7 @@ run github_issue_426.cpp ;
 run github_issue_448.cpp ;
 run-fail github_issue_519.cpp ;
 run github_issue_799.cpp ;
+run github_issue_802.cpp ;
 run link_1.cpp link_2.cpp link_3.cpp ;
 run quick.cpp ;
 run random_decimal32_comp.cpp ;

test/github_issue_802.cpp

@@ -0,0 +1,28 @@
+// Copyright 2025 Matt Borland
+// Distributed under the Boost Software License, Version 1.0.
+// https://www.boost.org/LICENSE_1_0.txt
+
+#include <boost/decimal.hpp>
+#include <boost/core/lightweight_test.hpp>
+#include <iostream>
+
+void fasting()
+{
+    using namespace boost::decimal;
+    using Dec = decimal128;
+    const Dec a{100'000,1};  // 6 dec digits significant
+    const Dec b{2'000'000,1}; // 7 dec digits significant
+    constexpr Dec ab{2, 13};
+
+    // Here we should be able to use the fast path instead of the slow path
+    // since 2e13 fits in a 128 bit uint
+    BOOST_TEST((a * b) == ab);
+}
+
+int main()
+{
+    fasting();
+
+    return boost::report_errors();
+}
+