Merge pull request #1128 from cppalliance/1112

Obey current rounding mode with rint/lrint/nearbyint

Author: mborland

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

@@ -28,20 +28,15 @@ namespace decimal {
 
 namespace detail {
 
-template <BOOST_DECIMAL_INTEGRAL T1, BOOST_DECIMAL_INTEGRAL T2>
-constexpr auto rint_impl(T1& sig, T2 exp, const bool)
+template <BOOST_DECIMAL_DECIMAL_FLOATING_TYPE TargetType, BOOST_DECIMAL_INTEGRAL T1, BOOST_DECIMAL_INTEGRAL T2>
+constexpr auto rint_impl(T1& sig, T2 exp, const bool is_neg)
 {
     const T2 abs_exp { (exp < T2(0)) ? -exp : exp };
 
-    sig /= detail::pow10(static_cast<T1>(abs_exp - 1));
+    const auto res {detail::impl::divmod(sig, detail::pow10(static_cast<T1>(abs_exp - 1)))};
+    sig = res.quotient;
 
-    const auto trailing_num {static_cast<std::uint32_t>(sig % 10U)};
-    sig /= 10U;
-
-    if (trailing_num >= 5U)
-    {
-        ++sig;
-    }
+    detail::fenv_round<TargetType>(sig, is_neg, res.remainder != 0U);
 }
 
 // MSVC 14.1 warns of unary minus being applied to unsigned type from numeric_limits::min
@@ -97,7 +92,7 @@ constexpr auto lrint_impl(const T num) noexcept -> Int
         return 0;
     }
 
-    detail::rint_impl(sig, expptr, is_neg);
+    detail::rint_impl<T>(sig, expptr, is_neg);
 
     auto res {static_cast<Int>(sig)};
     if (is_neg)
@@ -147,7 +142,7 @@ constexpr auto rint(const T num) noexcept
         return is_neg ? -zero : zero;
     }
 
-    detail::rint_impl(sig, expptr, is_neg);
+    detail::rint_impl<T>(sig, expptr, is_neg);
 
     return {sig, 0, is_neg};
 }

test/Jamfile

@@ -48,6 +48,7 @@ run-fail benchmark_uint256.cpp ;
 
 run compare_dec128_and_fast.cpp ;
 compile-fail concepts_test.cpp ;
+
 run crash_report_1.cpp ;
 run github_issue_426.cpp ;
 run github_issue_448.cpp ;
@@ -69,6 +70,8 @@ run github_issue_1057.cpp ;
 compile-fail github_issue_1087.cpp ;
 run github_issue_1091.cpp ;
 run github_issue_1094.cpp ;
+run github_issue_1112.cpp ;
+
 run link_1.cpp link_2.cpp link_3.cpp ;
 run quick.cpp ;
 run random_decimal32_comp.cpp ;

test/github_issue_1112.cpp

@@ -0,0 +1,63 @@
+// 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/1112
+
+#include <boost/decimal.hpp>
+#include <boost/core/lightweight_test.hpp>
+#include <random>
+
+static std::mt19937_64 rng(42);
+std::uniform_int_distribution<> dist(-1, -1);
+
+using namespace boost::decimal;
+
+template <typename T>
+void reproducer()
+{
+    const auto dec_res {static_cast<int>(nearbyint(T{2325, -1}))};
+    const auto dbl_res {static_cast<int>(std::nearbyint(232.5))};
+    BOOST_TEST_EQ(dec_res, dbl_res);
+}
+
+template <typename T>
+void test_rounding_up()
+{
+    const auto mode {boost::decimal::fesetround(rounding_mode::fe_dec_upward)};
+    if (mode != rounding_mode::fe_dec_default)
+    {
+        const T value {2325, dist(rng)}; // The result will be wrong if computed at compile time
+        const auto dec_res {static_cast<int>(nearbyint(value))};
+        BOOST_TEST_EQ(dec_res, 233);
+    }
+}
+
+template <typename T>
+void test_rounding_down()
+{
+    const auto mode {boost::decimal::fesetround(rounding_mode::fe_dec_downward)};
+    if (mode != rounding_mode::fe_dec_default)
+    {
+        const T value {2325, dist(rng)}; // The result will be wrong if computed at compile time
+        const auto dec_res {static_cast<int>(nearbyint(value))};
+        BOOST_TEST_EQ(dec_res, 232);
+    }
+}
+
+int main()
+{
+    reproducer<decimal32_t>();
+    reproducer<decimal64_t>();
+    reproducer<decimal128_t>();
+
+    test_rounding_up<decimal32_t>();
+    test_rounding_up<decimal64_t>();
+    test_rounding_up<decimal128_t>();
+
+    test_rounding_down<decimal32_t>();
+    test_rounding_down<decimal64_t>();
+    test_rounding_down<decimal128_t>();
+
+    return boost::report_errors();
+}