Merge pull request #346 from cppalliance/logb

Author: mborland

include/boost/decimal/cmath.hpp

@@ -53,6 +53,7 @@
 #include <boost/decimal/detail/cmath/tgamma.hpp>
 #include <boost/decimal/detail/cmath/trunc.hpp>
 #include <boost/decimal/detail/cmath/nan.hpp>
+#include <boost/decimal/detail/cmath/logb.hpp>
 #include <boost/decimal/numbers.hpp>
 
 // Macros from 3.6.2

include/boost/decimal/decimal128.hpp

@@ -214,6 +214,12 @@ class decimal128 final
 
     friend constexpr auto d128_mod_impl(decimal128 lhs, decimal128 rhs, const decimal128& q, decimal128& r) noexcept -> void;
 
+    template <typename T>
+    friend constexpr auto ilogb(T d) noexcept -> std::enable_if_t<detail::is_decimal_floating_point_v<T>, int>;
+
+    template <typename T>
+    friend constexpr auto logb(T num) noexcept -> std::enable_if_t<detail::is_decimal_floating_point_v<T>, T>;
+
 public:
     // 3.2.4.1 construct/copy/destroy
     constexpr decimal128() noexcept = default;

include/boost/decimal/decimal32.hpp

@@ -165,6 +165,9 @@ class decimal32 final // NOLINT(cppcoreguidelines-special-member-functions,hicpp
     template <typename T>
     friend constexpr auto ilogb(T d) noexcept -> std::enable_if_t<detail::is_decimal_floating_point_v<T>, int>;
 
+    template <typename T>
+    friend constexpr auto logb(T num) noexcept -> std::enable_if_t<detail::is_decimal_floating_point_v<T>, T>;
+
     // Debug bit pattern
     friend constexpr auto from_bits(std::uint32_t bits) noexcept -> decimal32;
     friend BOOST_DECIMAL_CXX20_CONSTEXPR auto to_bits(decimal32 rhs) noexcept -> std::uint32_t;

include/boost/decimal/decimal64.hpp

@@ -208,6 +208,9 @@ class decimal64 final
     template <typename T>
     friend constexpr auto ilogb(T d) noexcept -> std::enable_if_t<detail::is_decimal_floating_point_v<T>, int>;
 
+    template <typename T>
+    friend constexpr auto logb(T num) noexcept -> std::enable_if_t<detail::is_decimal_floating_point_v<T>, T>;
+
 public:
     // 3.2.3.1 construct/copy/destroy
     constexpr decimal64() noexcept = default;

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

@@ -11,9 +11,8 @@
 #include <boost/decimal/fwd.hpp> // NOLINT(llvm-include-order)
 #include <boost/decimal/detail/type_traits.hpp>
 
-namespace boost { namespace decimal {
-
-// TODO(mborland): Allow conversion between decimal types via a promotion system
+namespace boost {
+namespace decimal {
 
 template <typename T>
 constexpr auto ilogb(T d) noexcept -> std::enable_if_t<detail::is_decimal_floating_point_v<T>, int>

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

@@ -0,0 +1,44 @@
+// Copyright 2023 Matt Borland
+// Distributed under the Boost Software License, Version 1.0.
+// https://www.boost.org/LICENSE_1_0.txt
+
+#ifndef BOOST_DECIMAL_DETAIL_CMATH_LOGB_HPP
+#define BOOST_DECIMAL_DETAIL_CMATH_LOGB_HPP
+
+#include <cmath>
+#include <type_traits>
+
+#include <boost/decimal/fwd.hpp> // NOLINT(llvm-include-order)
+#include <boost/decimal/detail/type_traits.hpp>
+
+namespace boost {
+namespace decimal {
+
+template <typename T>
+constexpr auto logb(T num) noexcept -> std::enable_if_t<detail::is_decimal_floating_point_v<T>, T>
+{
+    const auto fpc {fpclassify(num)};
+
+    if (fpc == FP_ZERO)
+    {
+        return -std::numeric_limits<T>::infinity();
+    }
+    else if (fpc == FP_INFINITE)
+    {
+        return std::numeric_limits<T>::infinity();
+    }
+    else if (fpc == FP_NAN)
+    {
+        return num;
+    }
+
+    const auto offset = detail::num_digits(num.full_significand()) - 1;
+    const auto expval = static_cast<int>(static_cast<int>(num.unbiased_exponent()) + offset);
+
+    return static_cast<T>(expval);
+}
+
+}
+}
+
+#endif //BOOST_DECIMAL_DETAIL_CMATH_LOGB_HPP

test/test_cmath.cpp

@@ -610,12 +610,41 @@ void test_ilogb()
         BOOST_TEST_EQ(ilogb(Dec(1, 0)), 398);
         BOOST_TEST_EQ(ilogb(Dec(10, 0)), 399);
     }
+    else
+    {
+        BOOST_TEST_EQ(ilogb(Dec(1, 0)), 6176);
+        BOOST_TEST_EQ(ilogb(Dec(10, 0)), 6177);
+    }
 
     BOOST_TEST_EQ(ilogb(Dec(0)), FP_ILOGB0);
     BOOST_TEST_EQ(ilogb(std::numeric_limits<Dec>::infinity()), INT_MAX);
     BOOST_TEST_EQ(ilogb(std::numeric_limits<Dec>::quiet_NaN()), FP_ILOGBNAN);
 }
 
+template <typename Dec>
+void test_logb()
+{
+    BOOST_DECIMAL_IF_CONSTEXPR (std::is_same<Dec, decimal32>::value)
+    {
+        BOOST_TEST_EQ(ilogb(Dec(1, 0)), Dec(101));
+        BOOST_TEST_EQ(ilogb(Dec(10, 0)), Dec(102));
+    }
+    else BOOST_DECIMAL_IF_CONSTEXPR (std::is_same<Dec, decimal64>::value)
+    {
+        BOOST_TEST_EQ(ilogb(Dec(1, 0)), Dec(398));
+        BOOST_TEST_EQ(ilogb(Dec(10, 0)), Dec(399));
+    }
+    else
+    {
+        BOOST_TEST_EQ(ilogb(Dec(1, 0)), 6176);
+        BOOST_TEST_EQ(ilogb(Dec(10, 0)), 6177);
+    }
+
+    BOOST_TEST_EQ(logb(Dec(0)), -std::numeric_limits<Dec>::infinity());
+    BOOST_TEST_EQ(logb(std::numeric_limits<Dec>::infinity()), std::numeric_limits<Dec>::infinity());
+    BOOST_TEST(isnan(logb(std::numeric_limits<Dec>::quiet_NaN())));
+}
+
 template <typename Dec>
 void test_sqrt()
 {
@@ -1422,6 +1451,11 @@ int main()
 
     test_ilogb<decimal32>();
     test_ilogb<decimal64>();
+    test_ilogb<decimal128>();
+
+    test_logb<decimal32>();
+    test_logb<decimal64>();
+    test_logb<decimal128>();
 
     test_sqrt<decimal32>();
     test_sqrt<decimal64>();