feat(byteswap): adding signed versions, feature flags

Also refactoring away from specialized approach and paving way for
(currently unimplemented) xor version

#77

src/estd/bit.h

@@ -1,6 +1,7 @@
 #pragma once
 
 #include "cstdint.h"
+#include "internal/feature/bit.h"
 
 namespace estd {
 
@@ -20,7 +21,8 @@ enum class endian
 
 }
 
-#if __cpp_lib_byteswap >= 202110L
+#if (FEATURE_ESTD_BYTESWAP_MODE == FEATURE_ESTD_BYTESWAP_AUTO && __cpp_lib_byteswap >= 202110L) || \
+    FEATURE_ESTD_BYTESWAP_MODE == FEATURE_ESTD_BYTESWAP_STD
 #include <bit>
 
 namespace estd {
@@ -32,38 +34,70 @@ using std::byteswap;
 #else
 namespace estd {
 
-template <class Unsigned>
-constexpr Unsigned byteswap(Unsigned n) noexcept;
+namespace detail {
+#if FEATURE_ESTD_BYTESWAP_MODE == FEATURE_ESTD_BYTESWAP_XOR
 
-#if __has_builtin(__builtin_bswap16)
-template <>
-constexpr uint16_t byteswap(uint16_t n) noexcept
+#error manual xor non gcc/ non std mode not yet supported
+
+#else // not xor (gcc)
+
+#if __has_builtin(__builtin_bswap16) || FEATURE_ESTD_BYTESWAP_MODE == FEATURE_ESTD_BYTESWAP_GCC
+
+constexpr uint16_t
+byteswap(uint16_t n) noexcept
+{
+    return __builtin_bswap16(n);
+}
+
+// ISO/IEC 9899:201x aka WG14 N1570 (https://www.open-std.org/jtc1/sc22/wg14/www/docs/n1570.pdf)
+// Section 6.2.6.2 indicates one signed bit is present, and section 3.4.1 immplies it's always
+// the high order bit.  This gives us confidence that we can swap bits around safely, trusting that
+// whoever "unswaps" them will end up with the expected high order signed bit.
+
+constexpr int16_t
+byteswap(int16_t n) noexcept
 {
     return __builtin_bswap16(n);
 }
+
 #endif
 
-#if __has_builtin(__builtin_bswap32)
-template <>
-constexpr uint32_t byteswap(uint32_t n) noexcept
+#if __has_builtin(__builtin_bswap32) || FEATURE_ESTD_BYTESWAP_MODE == FEATURE_ESTD_BYTESWAP_GCC
+
+constexpr uint32_t
+byteswap(uint32_t n) noexcept
 {
     return __builtin_bswap32(n);
 }
+
+constexpr int32_t
+byteswap(int32_t n) noexcept
+{
+    return __builtin_bswap32(n);
+}
+
 #endif
 
-#if __has_builtin(__builtin_bswap64)
+#if __has_builtin(__builtin_bswap64) || FEATURE_ESTD_BYTESWAP_MODE == FEATURE_ESTD_BYTESWAP_GCC
 
-template <>
-constexpr uint64_t byteswap(uint64_t n) noexcept
+constexpr uint64_t
+byteswap(uint64_t n) noexcept
 {
-#if __has_builtin(__builtin_bswap32)
     return __builtin_bswap64(n);
-#else
-    return 0;
-#endif
 }
+
 #endif
 
+#endif // not xor (gcc)
 }
 
+// Since spec indicates a templated function, we wrap up the specific ones
+// in detail.  This is better than specializing, since that can lead to linker ambiguities
+template <class T>
+constexpr T byteswap(T n) noexcept { return detail::byteswap(n); }
+
+}
+
+
+
 #endif

src/estd/internal/feature/bit.h

@@ -0,0 +1,10 @@
+#pragma once
+
+#define FEATURE_ESTD_BYTESWAP_AUTO  0       // Prefer STD, then GCC (if available), then XOR
+#define FEATURE_ESTD_BYTESWAP_STD   1       // Force STD mode, ignoring compiler indication of availability
+#define FEATURE_ESTD_BYTESWAP_GCC   2       // Force GCC mode, ignoring compiler indication of availability
+#define FEATURE_ESTD_BYTESWAP_XOR   3       // Force XOR (manual) mode, not implemented at this time
+
+#ifndef FEATURE_ESTD_BYTESWAP_MODE
+#define FEATURE_ESTD_BYTESWAP_MODE  FEATURE_ESTD_BYTESWAP_AUTO
+#endif

test/catch/bit-test.cpp

@@ -6,8 +6,17 @@ TEST_CASE("bit operations")
 {
     SECTION("byteswap")
     {
-        auto v = estd::byteswap<uint16_t>(0x1234);
+        SECTION("uint16_t")
+        {
+            auto v = estd::byteswap<uint16_t>(0x1234);
 
-        REQUIRE(v == 0x3412);
+            REQUIRE(v == 0x3412);
+        }
+        SECTION("int32_t")
+        {
+            auto v = estd::byteswap<int32_t>(0x1234);
+
+            REQUIRE(v == 0x34120000);
+        }
     }
 }