Merge pull request #1275 from cppalliance/1260

Use the complete range of `int128` when performing 128-bit addition and subtraction

Author: mborland

include/boost/decimal/detail/add_impl.hpp

@@ -337,17 +337,17 @@ constexpr auto d128_add_impl(T lhs_sig, U lhs_exp, bool lhs_sign,
     auto& sign_smaller {abs_lhs_bigger ? rhs_sign : lhs_sign};
     auto& sign_bigger {abs_lhs_bigger ? lhs_sign : rhs_sign};
 
-    if (delta_exp <= 2)
+    if (delta_exp <= 3)
     {
         sig_bigger *= pow10(static_cast<boost::int128::uint128_t>(delta_exp));
         exp_bigger -= delta_exp;
         delta_exp = 0;
     }
     else
     {
-        sig_bigger *= 100U;
-        delta_exp -= 2;
-        exp_bigger -= 2;
+        sig_bigger *= 1000U;
+        delta_exp -= 3;
+        exp_bigger -= 3;
 
         if (delta_exp > 1)
         {

test/Jamfile

@@ -76,6 +76,7 @@ run github_issue_1107.cpp ;
 run github_issue_1110.cpp ;
 run github_issue_1112.cpp ;
 run github_issue_1174.cpp ;
+run github_issue_1260.cpp ;
 
 run link_1.cpp link_2.cpp link_3.cpp ;
 run quick.cpp ;

test/github_issue_1260.cpp

@@ -0,0 +1,35 @@
+// Copyright 2025 Matt Borland
+// Distributed under the Boost Software License, Version 1.0.
+// https://www.boost.org/LICENSE_1_0.txt
+//
+// See: https://github.com/cppalliance/decimal/issues/1260
+
+#include <boost/decimal.hpp>
+#include <boost/core/lightweight_test.hpp>
+
+template <typename T>
+void test()
+{
+    const T lhs {"1E34"};
+    const T rhs {"-0.51"};
+    const T res {"9999999999999999999999999999999999"};
+
+    const T add_val {lhs + rhs};
+    BOOST_TEST_EQ(add_val, res);
+
+    const T sub_val {lhs - boost::decimal::abs(rhs)};
+    BOOST_TEST_EQ(sub_val, res);
+}
+
+int main()
+{
+    // TODO(mborland): GCC 12+ in 32 bit compilations only with release mode fail
+    #ifndef __i386__
+
+    test<boost::decimal::decimal128_t>();
+    test<boost::decimal::decimal_fast128_t>();
+
+    #endif
+
+    return boost::report_errors();
+}