Add IBufferWriter.WriteGuidBigEndian

Author: iamcarbon

Src/Fido2/Extensions/IBufferWriterExtensions.cs

@@ -8,19 +8,43 @@ internal static class IBufferWriterExtensions
 {
     public static void WriteUInt16BigEndian(this IBufferWriter<byte> writer, ushort value)
     {
-        Span<byte> buffer = stackalloc byte[2];
+        var buffer = writer.GetSpan(2);
 
         BinaryPrimitives.WriteUInt16BigEndian(buffer, value);
 
-        writer.Write(buffer);
+        writer.Advance(2);
     }
 
     public static void WriteUInt32BigEndian(this IBufferWriter<byte> writer, uint value)
     {
-        Span<byte> buffer = stackalloc byte[4];
+        var buffer = writer.GetSpan(4);
 
         BinaryPrimitives.WriteUInt32BigEndian(buffer, value);
 
-        writer.Write(buffer);
+        writer.Advance(4);
+    }
+
+    public static void WriteGuidBigEndian(this IBufferWriter<byte> writer, Guid value)
+    {
+        var buffer = writer.GetSpan(16);
+
+        _ = value.TryWriteBytes(buffer);
+
+        if (BitConverter.IsLittleEndian)
+        {
+            SwapBytes(buffer, 0, 3);
+            SwapBytes(buffer, 1, 2);
+            SwapBytes(buffer, 4, 5);
+            SwapBytes(buffer, 6, 7);
+        }
+
+        writer.Advance(16);
+    }
+
+    private static void SwapBytes(Span<byte> bytes, int index1, int index2)
+    {
+        var temp = bytes[index1];
+        bytes[index1] = bytes[index2];
+        bytes[index2] = temp;
     }
 }