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Fix nextafter and nexttowards #850

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Merged
mborland merged 3 commits into from
Feb 1, 2025
Merged

Fix nextafter and nexttowards #850

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ce09f20
Add test set for issue 798
mborland Jan 31, 2025
b5ecfcd
Improve nextafter
mborland Jan 31, 2025
f8a3b3f
Fix test to match new behavior
mborland Jan 31, 2025
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55 changes: 49 additions & 6 deletions include/boost/decimal/detail/cmath/next.hpp
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Original file line number Diff line number Diff line change
Expand Up @@ -12,21 +12,66 @@
#include <boost/decimal/detail/type_traits.hpp>
#include <boost/decimal/detail/concepts.hpp>
#include <boost/decimal/detail/config.hpp>
#include <boost/decimal/detail/attributes.hpp>
#include <boost/decimal/detail/cmath/modf.hpp>
#include <boost/decimal/detail/cmath/abs.hpp>
#include <boost/decimal/detail/cmath/round.hpp>
#include <boost/decimal/detail/cmath/ilogb.hpp>
#include <boost/decimal/detail/cmath/fpclassify.hpp>
#include <boost/decimal/detail/cmath/frexp10.hpp>

#ifndef BOOST_DECIMAL_BUILD_MODULE
#include <type_traits>
#include <limits>
#include <cstdint>
#endif

namespace boost {
namespace decimal {

namespace detail {

template <BOOST_DECIMAL_DECIMAL_FLOATING_TYPE T1>
constexpr auto nextafter_impl(T1 val, bool direction) noexcept -> T1
{
constexpr T1 zero {0};

// Val < direction = +
// Val > direction = -
const auto abs_val {abs(val)};

if (val == zero)
{
const auto min_val {direction ? std::numeric_limits<T1>::denorm_min() :
-std::numeric_limits<T1>::denorm_min()};
return min_val;
}
if (abs_val > zero && abs_val < std::numeric_limits<T1>::epsilon())
{
const auto min_val {direction ? val + std::numeric_limits<T1>::min() :
val - std::numeric_limits<T1>::min()};
return min_val;
}

const auto val_eps {direction ? val + std::numeric_limits<T1>::epsilon() :
val - std::numeric_limits<T1>::epsilon()};

// If adding epsilon does nothing then we need to manipulate the representation
if (val == val_eps)
{
int exp {} ;
auto significand {frexp10(val, &exp)};

direction ? significand++ : significand--;

return T1{significand, exp};
}
else
{
return val_eps;
}
}

} // namespace detail

BOOST_DECIMAL_EXPORT
template <BOOST_DECIMAL_DECIMAL_FLOATING_TYPE T1,
BOOST_DECIMAL_DECIMAL_FLOATING_TYPE T2>
Expand All @@ -48,12 +93,10 @@ constexpr auto nextafter(T1 val, T2 direction) noexcept
return direction;
}
#endif
else if (val < direction)
else
{
return val + std::numeric_limits<T1>::epsilon();
return detail::nextafter_impl(val, val < direction);
}

return val - std::numeric_limits<T1>::epsilon();
}

BOOST_DECIMAL_EXPORT template <typename T>
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1 change: 1 addition & 0 deletions test/Jamfile
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Expand Up @@ -49,6 +49,7 @@ run crash_report_1.cpp ;
run github_issue_426.cpp ;
run github_issue_448.cpp ;
run-fail github_issue_519.cpp ;
run github_issue_798.cpp ;
run github_issue_799.cpp ;
run github_issue_802.cpp ;
run github_issue_805.cpp ;
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93 changes: 93 additions & 0 deletions test/github_issue_798.cpp
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Original file line number Diff line number Diff line change
@@ -0,0 +1,93 @@
// (C) Copyright Matt Borland 2025.
// Use, modification and distribution are subject to the
// Boost Software License, Version 1.0. (See accompanying file
// LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)

#include <boost/decimal.hpp>
#include <boost/core/lightweight_test.hpp>
#include <boost/decimal/detail/cmath/next.hpp>
#include <iomanip>
#include <limits>

template <typename T>
void test_zero()
{
constexpr T zero {0};
constexpr T one {1};

const auto next_after_zero {boost::decimal::nextafter(zero, one)};

BOOST_TEST_GT(next_after_zero, zero);
BOOST_TEST_LT(next_after_zero, zero + 2*std::numeric_limits<T>::min());
}

template <typename T>
void test_eps()
{
constexpr T eps {std::numeric_limits<T>::epsilon()};
constexpr T one {1};

const auto next_after_eps {boost::decimal::nextafter(eps, one)};

BOOST_TEST_GT(next_after_eps, eps);
BOOST_TEST_LT(next_after_eps, eps + 2*std::numeric_limits<T>::epsilon());
}

template <typename T>
void test_one()
{
constexpr T one {1};
constexpr T two {2};

const auto next_after_one {boost::decimal::nextafter(one, two)};

BOOST_TEST_GT(next_after_one, one);
BOOST_TEST_LT(next_after_one, one + 2*std::numeric_limits<T>::epsilon());
}

template <typename T>
void test_onek()
{
constexpr T onek {1024};
constexpr T twok {2048};

const auto next_after_onek {boost::decimal::nextafter(onek, twok)};

BOOST_TEST_GT(next_after_onek, onek);
BOOST_TEST_LT(next_after_onek, twok);
}

int main()
{
using namespace boost::decimal;

test_zero<decimal32>();
test_zero<decimal32_fast>();
test_zero<decimal64>();
test_zero<decimal64_fast>();
test_zero<decimal128>();
test_zero<decimal128_fast>();

test_eps<decimal32>();
test_eps<decimal32_fast>();
test_eps<decimal64>();
test_eps<decimal64_fast>();
test_eps<decimal128>();
test_eps<decimal128_fast>();

test_one<decimal32>();
test_one<decimal32_fast>();
test_one<decimal64>();
test_one<decimal64_fast>();
test_one<decimal128>();
test_one<decimal128_fast>();

test_onek<decimal32>();
test_onek<decimal32_fast>();
test_onek<decimal64>();
test_onek<decimal64_fast>();
test_onek<decimal128>();
test_onek<decimal128_fast>();

return boost::report_errors();
}
16 changes: 8 additions & 8 deletions test/test_cmath.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -1128,7 +1128,7 @@ void test_nextafter()
{
BOOST_TEST_EQ(ret_val, ret_dec); // LCOV_EXCL_LINE
}
else if (!BOOST_TEST(boost::math::float_distance(ret_val, ret_dec) < 10))
else if (!BOOST_TEST(boost::math::float_distance(ret_val, ret_dec) < 25))
{
// LCOV_EXCL_START
std::cerr << "Val 1: " << val1
Expand All @@ -1137,7 +1137,7 @@ void test_nextafter()
<< "\nDec 2: " << d2
<< "\nRet val: " << ret_val
<< "\nRet dec: " << ret_dec
<< "\nDist: " << boost::math::float_distance(ret_val, ret_dec);
<< "\nDist: " << boost::math::float_distance(ret_val, ret_dec) << std::endl;
// LCOV_EXCL_STOP
}
}
Expand All @@ -1147,8 +1147,8 @@ void test_nextafter()
BOOST_TEST(isnan(nextafter(Dec(1), std::numeric_limits<Dec>::quiet_NaN() * Dec(dist(rng)))));
BOOST_TEST(!isinf(nextafter(Dec(1), std::numeric_limits<Dec>::infinity() * Dec(dist(rng)))));
BOOST_TEST_EQ(nextafter(Dec(1), Dec(1)), Dec(1));
BOOST_TEST_EQ(nextafter(Dec(0), Dec(1)), std::numeric_limits<Dec>::epsilon());
BOOST_TEST_EQ(nextafter(Dec(0), Dec(-1)), -std::numeric_limits<Dec>::epsilon());
BOOST_TEST_EQ(nextafter(Dec(0), Dec(1)), std::numeric_limits<Dec>::denorm_min());
BOOST_TEST_EQ(nextafter(Dec(0), Dec(-1)), -std::numeric_limits<Dec>::denorm_min());
}

template <typename Dec>
Expand All @@ -1171,7 +1171,7 @@ void test_nexttoward()
{
BOOST_TEST_EQ(ret_val, ret_dec); // LCOV_EXCL_LINE
}
else if (!BOOST_TEST(boost::math::float_distance(ret_val, ret_dec) < 10))
else if (!BOOST_TEST(boost::math::float_distance(ret_val, ret_dec) < 25))
{
// LCOV_EXCL_START
std::cerr << "Val 1: " << val1
Expand All @@ -1180,16 +1180,16 @@ void test_nexttoward()
<< "\nDec 2: " << d2
<< "\nRet val: " << ret_val
<< "\nRet dec: " << ret_dec
<< "\nDist: " << boost::math::float_distance(ret_val, ret_dec);
<< "\nDist: " << boost::math::float_distance(ret_val, ret_dec) << std::endl;
// LCOV_EXCL_STOP
}
}

BOOST_TEST(isinf(nexttoward(std::numeric_limits<Dec>::infinity() * Dec(dist(rng)), 1)));
BOOST_TEST(isnan(nexttoward(std::numeric_limits<Dec>::quiet_NaN() * Dec(dist(rng)), 1)));
BOOST_TEST_EQ(nexttoward(Dec(1), 1), Dec(1));
BOOST_TEST_EQ(nexttoward(Dec(0), 1), std::numeric_limits<Dec>::epsilon());
BOOST_TEST_EQ(nexttoward(Dec(0), -1), -std::numeric_limits<Dec>::epsilon());
BOOST_TEST_EQ(nexttoward(Dec(0), 1), std::numeric_limits<Dec>::denorm_min());
BOOST_TEST_EQ(nexttoward(Dec(0), -1), -std::numeric_limits<Dec>::denorm_min());
}

template <typename T>
Expand Down