Fix GUID generation (#3159)

Author: josesimoes

src/CLR/CorLib/corlib_native_System_Guid.cpp

@@ -3,32 +3,31 @@
 // Portions Copyright (c) Microsoft Corporation.  All rights reserved.
 // See LICENSE file in the project root for full license information.
 //
-#include "CorLib.h"
-
-//
-//  Generate a new GUID
-//
-//  Based on the version 4 GUID (random)
-//  http://guid.one/guid/make
 
+#include "CorLib.h"
 
-HRESULT Library_corlib_native_System_Guid::GenerateNewGuid___STATIC__SZARRAY_U1( CLR_RT_StackFrame& stack )
+HRESULT Library_corlib_native_System_Guid::GenerateNewGuid___STATIC__SZARRAY_U1(CLR_RT_StackFrame &stack)
 {
     NANOCLR_HEADER();
 
-    CLR_RT_Random       rand;
-    CLR_UINT8*          buf;
-    CLR_RT_HeapBlock&   top = stack.PushValueAndClear();
- 
+    CLR_RT_Random rand;
+    CLR_UINT8 *buf;
+    CLR_RT_HeapBlock &top = stack.PushValueAndClear();
+
     // Create a array of 16 bytes on top of stack to return
-    NANOCLR_CHECK_HRESULT(CLR_RT_HeapBlock_Array::CreateInstance( top, 16, g_CLR_RT_WellKnownTypes.m_UInt8 ));
+    NANOCLR_CHECK_HRESULT(CLR_RT_HeapBlock_Array::CreateInstance(top, 16, g_CLR_RT_WellKnownTypes.m_UInt8));
     buf = top.DereferenceArray()->GetFirstElement();
 
     rand.Initialize();
-    rand.NextBytes(buf, 16);           // fill with random numbers
 
-    buf[7] =  (buf[7] & 0x0f) | 0x40;  // Set verion
-    buf[9] =  (buf[7] & 0x3f) | 0x80;  // Set variant
-    
+    // Generate 16 random bytes for the GUID
+    rand.NextBytes(buf, 16);
+
+    // Set the version (version 4, bits 12-15 of time_hi_and_version)
+    buf[7] = (buf[7] & 0x0F) | 0x40;
+
+    // Set the variant (the two most significant bits of clock_seq_hi_and_reserved must be 10)
+    buf[8] = (buf[8] & 0x3F) | 0x80;
+
     NANOCLR_NOCLEANUP();
 }