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Round ceil, floor, round, roundeven, trunc, rint, lrint on the integer significand #1486

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Merged
mborland merged 1 commit into from
Oct 3, 2026
Merged

Round ceil, floor, round, roundeven, trunc, rint, lrint on the integer significand #1486

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64 changes: 2 additions & 62 deletions include/boost/decimal/detail/cmath/ceil.hpp
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Original file line number Diff line number Diff line change
Expand Up @@ -6,14 +6,10 @@
#define BOOST_DECIMAL_DETAIL_CMATH_CEIL_HPP

#include <boost/decimal/fwd.hpp>
#include <boost/decimal/detail/config.hpp>
#include <boost/decimal/detail/type_traits.hpp>
#include <boost/decimal/detail/power_tables.hpp>
#include <boost/decimal/detail/apply_sign.hpp>
#include <boost/decimal/detail/concepts.hpp>
#include <boost/decimal/detail/cmath/impl/round_integral.hpp>
#include <boost/decimal/detail/config.hpp>
#include <boost/decimal/detail/cmath/fpclassify.hpp>
#include <boost/decimal/detail/cmath/frexp10.hpp>

#ifndef BOOST_DECIMAL_BUILD_MODULE
#include <type_traits>
Expand All @@ -27,63 +23,7 @@ BOOST_DECIMAL_EXPORT template <typename T>
constexpr auto ceil BOOST_DECIMAL_PREVENT_MACRO_SUBSTITUTION (const T val) noexcept
BOOST_DECIMAL_REQUIRES(detail::is_decimal_floating_point_v, T)
{
using DivType = typename T::significand_type;
constexpr int inverse_eps {std::is_same<DivType, std::uint32_t>::value ? 7 :
std::is_same<DivType, std::uint64_t>::value ? 16 : 34};

constexpr T zero {0, 0};
constexpr T one {1, 0};
constexpr T max_comp_value {1u, inverse_eps};
const auto fp {fpclassify(val)};

switch (fp)
{
case FP_ZERO:
case FP_NAN:
case FP_INFINITE:
return val;
default:
static_cast<void>(val);
}

if (val > zero && val >= max_comp_value)
{
return val;
}

int exp_ptr {};
auto new_sig {frexp10(val, &exp_ptr)};
const auto abs_exp {detail::make_positive_unsigned(exp_ptr)};
const bool is_neg {val < zero};

const auto sig_dig {detail::precision_v<T>};
auto decimal_digits {static_cast<unsigned>(sig_dig)};
const auto zero_digits {detail::remove_trailing_zeros(new_sig).number_of_removed_zeros};
const auto non_zero_exp {exp_ptr + static_cast<int>(zero_digits)};

if (non_zero_exp >= 0)
{
// If the value is an integer, nothing should occur
return val;
}

if (sig_dig > abs_exp)
{
decimal_digits = abs_exp;
}
else if (exp_ptr < 1 && abs_exp >= sig_dig)
{
return is_neg ? zero : one;
}

new_sig /= detail::pow10<DivType>(decimal_digits);
if (!is_neg)
{
++new_sig;
}
new_sig *= 10U;

return T{new_sig, exp_ptr + static_cast<int>(decimal_digits) - 1, is_neg};
return detail::round_integral_impl(val, rounding_mode::fe_dec_upward);
}

} // namespace decimal
Expand Down
56 changes: 2 additions & 54 deletions include/boost/decimal/detail/cmath/floor.hpp
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Original file line number Diff line number Diff line change
Expand Up @@ -8,13 +8,8 @@
#include <boost/decimal/fwd.hpp>
#include <boost/decimal/detail/config.hpp>
#include <boost/decimal/detail/type_traits.hpp>
#include <boost/decimal/detail/power_tables.hpp>
#include <boost/decimal/detail/apply_sign.hpp>
#include <boost/decimal/detail/cmath/fpclassify.hpp>
#include <boost/decimal/detail/cmath/frexp10.hpp>
#include "../int128.hpp"
#include <boost/decimal/detail/concepts.hpp>
#include <boost/decimal/detail/construction_sign.hpp>
#include <boost/decimal/detail/cmath/impl/round_integral.hpp>

#ifndef BOOST_DECIMAL_BUILD_MODULE
#include <type_traits>
Expand All @@ -28,54 +23,7 @@ BOOST_DECIMAL_EXPORT template <typename T>
constexpr auto floor BOOST_DECIMAL_PREVENT_MACRO_SUBSTITUTION (const T val) noexcept
BOOST_DECIMAL_REQUIRES(detail::is_decimal_floating_point_v, T)
{
using DivType = typename T::significand_type;

constexpr T zero {0, 0};
constexpr T neg_one {1U, 0, construction_sign::negative};
const auto fp {fpclassify(val)};

switch (fp)
{
case FP_ZERO:
case FP_NAN:
case FP_INFINITE:
return val;
default:
static_cast<void>(val);
}

int exp_ptr {};
auto new_sig {frexp10(val, &exp_ptr)};
const auto abs_exp {detail::make_positive_unsigned(exp_ptr)};
const bool is_neg {val < zero};

const auto sig_dig {detail::precision_v<T>};
auto decimal_digits {static_cast<unsigned>(sig_dig)};
const auto zero_digits {detail::remove_trailing_zeros(new_sig).number_of_removed_zeros};
const auto non_zero_exp {exp_ptr + static_cast<int>(zero_digits)};

if (non_zero_exp >= 0)
{
// If the value is an integer, nothing should occur
return val;
}

if (sig_dig > abs_exp)
{
decimal_digits = abs_exp;
}
else if (exp_ptr < 1 && abs_exp >= sig_dig)
{
return is_neg ? neg_one : zero;
}

new_sig /= detail::pow10<DivType>(decimal_digits);
if (is_neg)
{
++new_sig;
}

return T{new_sig, exp_ptr + static_cast<int>(decimal_digits), is_neg};
return detail::round_integral_impl(val, rounding_mode::fe_dec_downward);
}

} // namespace decimal
Expand Down
100 changes: 100 additions & 0 deletions include/boost/decimal/detail/cmath/impl/round_integral.hpp
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Original file line number Diff line number Diff line change
@@ -0,0 +1,100 @@
// Copyright 2026 Shen-Ta Hsieh
// Distributed under the Boost Software License, Version 1.0.
// https://www.boost.org/LICENSE_1_0.txt

#ifndef BOOST_DECIMAL_DETAIL_CMATH_IMPL_ROUND_INTEGRAL_HPP
#define BOOST_DECIMAL_DETAIL_CMATH_IMPL_ROUND_INTEGRAL_HPP

#include <boost/decimal/detail/config.hpp>
#include <boost/decimal/detail/type_traits.hpp>
#include <boost/decimal/detail/attributes.hpp>
#include <boost/decimal/detail/power_tables.hpp>
#include <boost/decimal/detail/cmath/fpclassify.hpp>
#include <boost/decimal/detail/cmath/decompose.hpp>
#include <boost/decimal/detail/add_impl.hpp>
#include <boost/decimal/cfenv.hpp>

namespace boost {
namespace decimal {
namespace detail {

// Rounds c.sig * 10^c.exp, with c.exp < 0, to an integer in the mode round:
// one division by 10^-c.exp
template <typename T, typename Components>
BOOST_DECIMAL_FORCE_INLINE constexpr auto round_integral_sig(const Components& c, const rounding_mode round) noexcept -> typename T::significand_type
{
using sig_type = typename T::significand_type;

const auto shift {-static_cast<int>(c.exp)};
sig_type q {0U};
bool inc {};

if (shift > precision_v<T>)
{
// 0 < |x| < 0.1
inc = round == (c.sign ? rounding_mode::fe_dec_downward : rounding_mode::fe_dec_upward);
}
else
{
const auto p {pow10(static_cast<sig_type>(shift))};
q = static_cast<sig_type>(c.sig / p);
const auto r {static_cast<sig_type>(c.sig - q * p)};
if (r == 0U)
{
return q;
}

switch (round)
{
case rounding_mode::fe_dec_upward:
inc = !c.sign;
break;
case rounding_mode::fe_dec_downward:
inc = c.sign;
break;
case rounding_mode::fe_dec_to_nearest_from_zero:
inc = r >= p - r;
break;
case rounding_mode::fe_dec_to_nearest:
inc = r > p - r || (r == p - r && (static_cast<std::uint32_t>(q) & 1U) == 1U);
break;
case rounding_mode::fe_dec_toward_zero:
default:
break;
}
}

if (inc)
{
++q;
}

return q;
}

// Rounds x to an integer in the mode round. The result has exponent 0, and a zero result
// keeps the sign of x.
template <typename T>
constexpr auto round_integral_impl(const T x, const rounding_mode round) noexcept -> T
{
#ifndef BOOST_DECIMAL_FAST_MATH
if (!isfinite(x))
{
return x;
}
#endif

const auto c {decompose(x)};
if (c.exp >= 0 || c.sig == 0U)
{
return x;
}

return pack_in_range<T>(round_integral_sig<T>(c, round), 0, c.sign);
}

} // namespace detail
} // namespace decimal
} // namespace boost

#endif // BOOST_DECIMAL_DETAIL_CMATH_IMPL_ROUND_INTEGRAL_HPP
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