Add 64-bit test set of addition and subtraction using downward round

Author: mborland

test/Jamfile

@@ -128,6 +128,7 @@ run test_decimal64_fast_stream.cpp ;
 run test_decimal64_stream.cpp ;
 #run test_decimal128_basis.cpp ;
 run test_decimal_quantum.cpp ;
+run test_downward_rounding.cpp ;
 run test_dpd_conversions.cpp ;
 run test_edges_and_behave.cpp ;
 run test_edit_members.cpp ;

test/test_downward_rounding.cpp

@@ -0,0 +1,57 @@
+// Copyright 2025 Matt Borland
+// Distributed under the Boost Software License, Version 1.0.
+// https://www.boost.org/LICENSE_1_0.txt
+
+// Use compile-time rounding mode change so the test run on all platforms
+#define BOOST_DECIMAL_FE_DEC_DOWNWARD
+
+#include <boost/decimal.hpp>
+#include <boost/core/lightweight_test.hpp>
+#include <functional>
+#include <iostream>
+#include <iomanip>
+
+template <typename T, typename Func>
+void test(const char* lhs_str, const char* rhs_str, const char* result_str, Func f)
+{
+    const T lhs {lhs_str};
+    const T rhs {rhs_str};
+    const T result {result_str};
+
+    const T func_result {f(lhs, rhs)};
+
+    BOOST_TEST_EQ(func_result, result);
+}
+
+template <typename T>
+void test_add(const char* lhs, const char* rhs, const char* result)
+{
+    std::cerr << std::setprecision(std::numeric_limits<T>::max_digits10);
+    return test<T>(lhs, rhs, result, std::plus<>());
+}
+
+template <typename T>
+void test_sub(const char* lhs, const char* rhs, const char* result)
+{
+    std::cerr << std::setprecision(std::numeric_limits<T>::max_digits10);
+    return test<T>(lhs, rhs, result, std::minus<>());
+}
+
+int main()
+{
+    using namespace boost::decimal;
+
+    test_add<decimal64_t>("1e+2", "-1e-383", "99.99999999999999");
+    test_add<decimal64_t>("1e+1", "-1e-383", "9.999999999999999");
+
+    test_add<decimal_fast64_t>("1e+2", "-1e-383", "99.99999999999999");
+    test_add<decimal_fast64_t>("1e+1", "-1e-383", "9.999999999999999");
+
+    test_sub<decimal64_t>("1e+2", "1e-383", "99.99999999999999");
+    test_sub<decimal64_t>("1e+1", "1e-383", "9.999999999999999");
+
+    test_sub<decimal_fast64_t>("1e+2", "1e-383", "99.99999999999999");
+    test_sub<decimal_fast64_t>("1e+1", "1e-383", "9.999999999999999");
+
+    return boost::report_errors();
+}