From 2618ca43244147a1aaf942c1d68936eb9635884d Mon Sep 17 00:00:00 2001
From: jatin <jatinchowdhury18@gmail.com>
Date: Sun, 7 Jan 2024 14:32:40 -0800
Subject: [PATCH] Working on C++17 compatibility

---
 include/math_approx/src/basic_math.hpp        | 24 +++++++++++++++++++
 include/math_approx/src/log_approx.hpp        |  8 +++----
 .../math_approx/src/polylogarithm_approx.hpp  |  2 ++
 include/math_approx/src/pow_approx.hpp        |  4 ++--
 4 files changed, 32 insertions(+), 6 deletions(-)

diff --git a/include/math_approx/src/basic_math.hpp b/include/math_approx/src/basic_math.hpp
index b0348da..8a7a501 100644
--- a/include/math_approx/src/basic_math.hpp
+++ b/include/math_approx/src/basic_math.hpp
@@ -71,4 +71,28 @@ xsimd::batch<T> select (xsimd::batch_bool<T> q, xsimd::batch<T> t, xsimd::batch<
     return xsimd::select (q, t, f);
 }
 #endif
+
+#if ! __cpp_lib_bit_cast
+// bit_cast requirement.
+template <typename From, typename To>
+using is_bitwise_castable = std::integral_constant<bool,
+                                                   (sizeof (From) == sizeof (To)) && std::is_trivially_copyable<From>::value && std::is_trivially_copyable<To>::value>;
+
+// compiler support is needed for bitwise copy with constexpr.
+template <typename To, typename From>
+inline typename std::enable_if<is_bitwise_castable<From, To>::value, To>::type bit_cast (const From& from) noexcept
+{
+    union U
+    {
+        U() {};
+        char storage[sizeof (To)] {};
+        typename std::remove_const<To>::type dest;
+    } u; // instead of To dest; because To doesn't require DefaultConstructible.
+    std::memcpy (&u.dest, &from, sizeof from);
+    return u.dest;
+}
+#else
+using std::bit_cast;
+#endif
+
 } // namespace math_approx
diff --git a/include/math_approx/src/log_approx.hpp b/include/math_approx/src/log_approx.hpp
index e778690..9ba64d6 100644
--- a/include/math_approx/src/log_approx.hpp
+++ b/include/math_approx/src/log_approx.hpp
@@ -105,11 +105,11 @@ namespace log_detail
 template <typename Base, int order, bool C1_continuous, typename Log2ProviderType = log_detail::Log2Provider>
 constexpr float log (float x)
 {
-    const auto vi = std::bit_cast<int32_t> (x);
+    const auto vi = bit_cast<int32_t> (x);
     const auto ex = vi & 0x7f800000;
     const auto e = (ex >> 23) - 127;
     const auto vfi = (vi - ex) | 0x3f800000;
-    const auto vf = std::bit_cast<float> (vfi);
+    const auto vf = bit_cast<float> (vfi);
 
     constexpr auto log2_base_r = 1.0f / Base::log2_base;
     return log2_base_r * ((float) e + Log2ProviderType::template log2_approx<float, order, C1_continuous> (vf));
@@ -119,11 +119,11 @@ constexpr float log (float x)
 template <typename Base, int order, bool C1_continuous, typename Log2ProviderType = log_detail::Log2Provider>
 constexpr double log (double x)
 {
-    const auto vi = std::bit_cast<int64_t> (x);
+    const auto vi = bit_cast<int64_t> (x);
     const auto ex = vi & 0x7ff0000000000000;
     const auto e = (ex >> 52) - 1023;
     const auto vfi = (vi - ex) | 0x3ff0000000000000;
-    const auto vf = std::bit_cast<double> (vfi);
+    const auto vf = bit_cast<double> (vfi);
 
     constexpr auto log2_base_r = 1.0 / Base::log2_base;
     return log2_base_r * ((double) e + Log2ProviderType::template log2_approx<double, order, C1_continuous> (vf));
diff --git a/include/math_approx/src/polylogarithm_approx.hpp b/include/math_approx/src/polylogarithm_approx.hpp
index c28b894..2339436 100644
--- a/include/math_approx/src/polylogarithm_approx.hpp
+++ b/include/math_approx/src/polylogarithm_approx.hpp
@@ -156,6 +156,7 @@ constexpr T li2 (T x)
     return r + select (sign, li2_reduce, -li2_reduce);
 }
 
+#if defined(XSIMD_HPP)
 /**
  * Approximation of the "dilogarithm" function for all inputs.
  *
@@ -218,4 +219,5 @@ xsimd::batch<T> li2 (const xsimd::batch<T>& x)
     const auto li2_reduce = li2_0_half<order> (y);
     return r + select (sign, li2_reduce, -li2_reduce);
 }
+#endif
 } // namespace math_approx
diff --git a/include/math_approx/src/pow_approx.hpp b/include/math_approx/src/pow_approx.hpp
index a48efb6..68c218d 100644
--- a/include/math_approx/src/pow_approx.hpp
+++ b/include/math_approx/src/pow_approx.hpp
@@ -148,7 +148,7 @@ constexpr float pow (float x)
     const auto f = x - (float) l;
     const auto vi = (l + 127) << 23;
 
-    return std::bit_cast<float> (vi) * pow_detail::pow2_approx<float, order, C1_continuous> (f);
+    return bit_cast<float> (vi) * pow_detail::pow2_approx<float, order, C1_continuous> (f);
 }
 
 /** approximation for pow(Base, x) (64-bit) */
@@ -162,7 +162,7 @@ constexpr double pow (double x)
     const auto d = x - (double) l;
     const auto vi = (l + 1023) << 52;
 
-    return std::bit_cast<double> (vi) * pow_detail::pow2_approx<double, order, C1_continuous> (d);
+    return bit_cast<double> (vi) * pow_detail::pow2_approx<double, order, C1_continuous> (d);
 }
 
 #if defined(XSIMD_HPP)