numeric/endian: Add constexpr endian conversion functions
These are as fast as the c library versions but they are also constexpr.
Change-Id: I95bb1562a5a15a7f9c57a3483afe364a0286da48
Signed-off-by: William A. Kennington III <wak@google.com>
diff --git a/include/meson.build b/include/meson.build
index 5785caf..1a1eb6f 100644
--- a/include/meson.build
+++ b/include/meson.build
@@ -9,6 +9,7 @@
'stdplus/hash.hpp',
'stdplus/hash/array.hpp',
'stdplus/hash/tuple.hpp',
+ 'stdplus/numeric/endian.hpp',
'stdplus/pinned.hpp',
'stdplus/raw.hpp',
'stdplus/signal.hpp',
diff --git a/include/stdplus/numeric/endian.hpp b/include/stdplus/numeric/endian.hpp
new file mode 100644
index 0000000..bd3199b
--- /dev/null
+++ b/include/stdplus/numeric/endian.hpp
@@ -0,0 +1,112 @@
+#pragma once
+#include <algorithm>
+#include <bit>
+#include <cstddef>
+#include <cstdint>
+#include <type_traits>
+
+namespace stdplus
+{
+
+namespace detail
+{
+
+template <typename T, size_t s = sizeof(T)>
+struct BswapFast
+{
+ static inline constexpr bool fast = false;
+};
+
+template <typename T>
+struct BswapFast<T, 2>
+{
+ static inline constexpr bool fast = true;
+ using type = uint16_t;
+
+ static constexpr uint16_t swapAligned(uint16_t n) noexcept
+ {
+ return __builtin_bswap16(n);
+ }
+};
+
+template <typename T>
+struct BswapFast<T, 4>
+{
+ static inline constexpr bool fast = true;
+ using type = uint32_t;
+
+ static constexpr uint32_t swapAligned(uint32_t n) noexcept
+ {
+ return __builtin_bswap32(n);
+ }
+};
+
+template <typename T>
+struct BswapFast<T, 8>
+{
+ static inline constexpr bool fast = true;
+ using type = uint64_t;
+
+ static constexpr uint64_t swapAligned(uint64_t n) noexcept
+ {
+ return __builtin_bswap64(n);
+ }
+};
+
+} // namespace detail
+
+template <typename T>
+constexpr T bswap(T n) noexcept
+{
+ static_assert(std::is_trivially_copyable_v<T>);
+ using F = detail::BswapFast<T>;
+ if constexpr (sizeof(T) == 1)
+ {}
+ else if constexpr (F::fast)
+ {
+ if constexpr (alignof(T) % alignof(typename F::type) == 0)
+ {
+ n = F::swapAligned(n);
+ }
+ else
+ {
+ typename F::type v;
+ auto i = reinterpret_cast<std::byte*>(&n);
+ auto o = reinterpret_cast<std::byte*>(&v);
+ std::copy(i, i + sizeof(T), o);
+ v = F::swapAligned(v);
+ std::copy(o, o + sizeof(T), i);
+ }
+ }
+ else
+ {
+ auto b = reinterpret_cast<std::byte*>(&n);
+ std::reverse(b, b + sizeof(n));
+ }
+ return n;
+}
+
+template <typename T>
+constexpr T hton(T n) noexcept
+{
+ if constexpr (std::endian::native == std::endian::big)
+ {
+ return n;
+ }
+ else if constexpr (std::endian::native == std::endian::little)
+ {
+ return bswap(n);
+ }
+ else
+ {
+ static_assert(std::is_same_v<T, void>);
+ }
+}
+
+template <typename T>
+constexpr T ntoh(T n) noexcept
+{
+ return hton(n);
+}
+
+} // namespace stdplus