fix: Add fallback byteswap/endian implementations for older abseil versions

src/s2/util/endian/endian.h

@@ -18,18 +18,83 @@
 
 #include <cstdint>
 
-#include "absl/numeric/bits.h"
 #include "absl/numeric/int128.h"
 #include "s2/util/gtl/unaligned.h"
 
+// Check for absl::byteswap availability (added in abseil LTS 2025.01.27)
+#if __has_include("absl/numeric/bits.h")
+#include "absl/numeric/bits.h"
+#endif
+
+// Provide fallback byteswap implementations for older abseil versions
 namespace s2endian {
+
+// Fallback byteswap for uint16_t
+inline uint16_t byteswap_u16(uint16_t x) {
+#if defined(__GNUC__) || defined(__clang__)
+  return __builtin_bswap16(x);
+#else
+  return static_cast<uint16_t>((x >> 8) | (x << 8));
+#endif
+}
+
+// Fallback byteswap for uint32_t
+inline uint32_t byteswap_u32(uint32_t x) {
+#if defined(__GNUC__) || defined(__clang__)
+  return __builtin_bswap32(x);
+#else
+  return ((x & 0xFF000000U) >> 24) |
+         ((x & 0x00FF0000U) >> 8) |
+         ((x & 0x0000FF00U) << 8) |
+         ((x & 0x000000FFU) << 24);
+#endif
+}
+
+// Fallback byteswap for uint64_t
+inline uint64_t byteswap_u64(uint64_t x) {
+#if defined(__GNUC__) || defined(__clang__)
+  return __builtin_bswap64(x);
+#else
+  return ((x & 0xFF00000000000000ULL) >> 56) |
+         ((x & 0x00FF000000000000ULL) >> 40) |
+         ((x & 0x0000FF0000000000ULL) >> 24) |
+         ((x & 0x000000FF00000000ULL) >> 8) |
+         ((x & 0x00000000FF000000ULL) << 8) |
+         ((x & 0x0000000000FF0000ULL) << 24) |
+         ((x & 0x000000000000FF00ULL) << 40) |
+         ((x & 0x00000000000000FFULL) << 56);
+#endif
+}
+
 // TODO(user): Trim unused Encoder functions and drop int128 API.
 inline absl::uint128 byteswap(absl::uint128 host_int) {
-  return absl::MakeUint128(absl::byteswap(absl::Uint128Low64(host_int)),
-                           absl::byteswap(absl::Uint128High64(host_int)));
+  return absl::MakeUint128(byteswap_u64(absl::Uint128Low64(host_int)),
+                           byteswap_u64(absl::Uint128High64(host_int)));
 }
+
 }  // namespace s2endian
 
+// Detect endianness at compile time
+namespace s2endian_detail {
+#if defined(__BYTE_ORDER__) && defined(__ORDER_LITTLE_ENDIAN__) && \
+    defined(__ORDER_BIG_ENDIAN__)
+#if __BYTE_ORDER__ == __ORDER_LITTLE_ENDIAN__
+constexpr bool kIsLittleEndian = true;
+#elif __BYTE_ORDER__ == __ORDER_BIG_ENDIAN__
+constexpr bool kIsLittleEndian = false;
+#else
+#error "Unknown endianness"
+#endif
+#elif defined(_WIN32) || defined(__x86_64__) || defined(__i386__) || \
+      defined(__aarch64__) || defined(__arm__)
+// Most common platforms are little-endian
+constexpr bool kIsLittleEndian = true;
+#else
+// Fallback: assume little-endian (safe for most platforms)
+constexpr bool kIsLittleEndian = true;
+#endif
+}  // namespace s2endian_detail
+
 // Utilities to convert numbers between the current hosts's native byte
 // order and little-endian byte order
 //
@@ -54,7 +119,7 @@ class LittleEndian {
   }
 
   static constexpr bool IsLittleEndian() {
-    return absl::endian::native == absl::endian::little;
+    return s2endian_detail::kIsLittleEndian;
   }
 
   // Functions to do unaligned loads and stores in little-endian order.
@@ -109,15 +174,16 @@ class LittleEndian {
   }
 
  private:
+  // Byteswap implementations for each type
+  static uint16_t do_byteswap(uint16_t x) { return s2endian::byteswap_u16(x); }
+  static uint32_t do_byteswap(uint32_t x) { return s2endian::byteswap_u32(x); }
+  static uint64_t do_byteswap(uint64_t x) { return s2endian::byteswap_u64(x); }
+  static absl::uint128 do_byteswap(absl::uint128 x) { return s2endian::byteswap(x); }
+
   template <typename T>
   static T byteswap_if_big_endian(T x) {
-    static_assert(absl::endian::native == absl::endian::big ||
-                  absl::endian::native == absl::endian::little,
-                  "Unsupported endianness.");
-    if constexpr (absl::endian::native == absl::endian::big) {
-      using absl::byteswap;
-      using s2endian::byteswap;
-      x = byteswap(x);
+    if constexpr (!s2endian_detail::kIsLittleEndian) {
+      return do_byteswap(x);
     }
     return x;
   }