Write UUID transformation more portably

The previous version did not handle endianness correctly. MSVC
refused to compile the code that assumed we knew what we were doing,
which turned out to be a good thing.

src/gromacs/hardware/tests/device_management.cpp

@@ -44,6 +44,8 @@
 
 #include "config.h"
 
+#include <cstdint>
+
 #include <algorithm>
 #include <memory>
 #include <string>
@@ -106,36 +108,65 @@ TEST(DevicesManagerTest, Serialization)
     }
 }
 
+template<std::size_t N>
+uint32_t uint32FromBytes(const std::array<std::byte, N>& data, const std::size_t byteOffset)
+{
+    if (byteOffset + sizeof(uint32_t) > N)
+    {
+        throw std::invalid_argument("byteOffset would read out of bounds");
+    }
+
+    // Integer to copy the bytes to
+    uint32_t result;
+
+    // Copy the bytes, assuming little-endian layout. The compiler
+    // generally elides this away.
+    std::memcpy(&result, &(data[byteOffset]), sizeof(result));
+    if (GMX_INTEGER_BIG_ENDIAN)
+    {
+        // Change the endianness to match the hardware
+        const auto* ptr = reinterpret_cast<const uint8_t*>(&result);
+        return (ptr[0] << 24) | (ptr[1] << 16) | (ptr[2] << 8) | ptr[3];
+    }
+    else
+    {
+        return result;
+    }
+}
+
 std::string uuidToString(const std::array<std::byte, 16>& uuid)
 {
     // Write a string in the frequently used 8-4-4-4-12 format,
     // xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx, where every x represents 4 bits
     std::string result;
     result.reserve(37);
-    const auto* uuidIt = uuid.cbegin();
-    for (int i = 0; i < 4; ++i, ++uuidIt)
+
+    // Compilers tend to think uuid.size() is not a constant expression!
+    constexpr std::size_t uuidWidthInBytes = 16;
+    std::size_t byteOffset = 0;
+    for (int i = 0; i < 4; ++i, byteOffset += sizeof(uint32_t))
     {
-        result += gmx::formatString("%2.2x", static_cast<unsigned int>(*uuidIt));
+        result += gmx::formatString("%2.2x", uint32FromBytes<uuidWidthInBytes>(uuid, byteOffset));
     }
     result.append(1, '-');
-    for (int i = 0; i < 2; ++i, ++uuidIt)
+    for (int i = 0; i < 2; ++i, byteOffset += sizeof(uint32_t))
     {
-        result += gmx::formatString("%2.2x", static_cast<unsigned int>(*uuidIt));
+        result += gmx::formatString("%2.2x", uint32FromBytes<uuidWidthInBytes>(uuid, byteOffset));
     }
     result.append(1, '-');
-    for (int i = 0; i < 2; ++i, ++uuidIt)
+    for (int i = 0; i < 2; ++i, byteOffset += sizeof(uint32_t))
     {
-        result += gmx::formatString("%2.2x", static_cast<unsigned int>(*uuidIt));
+        result += gmx::formatString("%2.2x", uint32FromBytes<uuidWidthInBytes>(uuid, byteOffset));
     }
     result.append(1, '-');
-    for (int i = 0; i < 2; ++i, ++uuidIt)
+    for (int i = 0; i < 2; ++i, byteOffset += sizeof(uint32_t))
     {
-        result += gmx::formatString("%2.2x", static_cast<unsigned int>(*uuidIt));
+        result += gmx::formatString("%2.2x", uint32FromBytes<uuidWidthInBytes>(uuid, byteOffset));
     }
     result.append(1, '-');
-    for (int i = 0; i < 6; ++i, ++uuidIt)
+    for (int i = 0; i < 6; ++i, byteOffset += sizeof(uint32_t))
     {
-        result += gmx::formatString("%2.2x", static_cast<unsigned int>(*uuidIt));
+        result += gmx::formatString("%2.2x", uint32FromBytes<uuidWidthInBytes>(uuid, byteOffset));
     }
     return result;
 }