logb impl

Author: mborland

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