guid: Improve version and variant handling

Signed-off-by: Kevin Parsons <kevpar@microsoft.com>

pkg/guid/guid.go

@@ -52,10 +52,11 @@ func NewV4() (GUID, error) {
 		return GUID{}, err
 	}
 
-	b[6] = (b[6] & 0x0f) | 0x40 // Version 4 (randomly generated)
-	b[8] = (b[8] & 0x3f) | 0x80 // RFC4122 variant
+	g := FromArray(b)
+	g.setVersion(4) // Version 4 means randomly generated.
+	g.setVariant(VariantRFC4122)
 
-	return FromArray(b), nil
+	return g, nil
 }
 
 func fromArray(b [16]byte, order binary.ByteOrder) GUID {
@@ -150,6 +151,25 @@ func FromString(s string) (GUID, error) {
 	return g, nil
 }
 
+func (g *GUID) setVariant(v Variant) {
+	d := g.Data4[0]
+	switch v {
+	case VariantNCS:
+		d = (d & 0x7f)
+	case VariantRFC4122:
+		d = (d & 0x3f) | 0x80
+	case VariantMicrosoft:
+		d = (d & 0x1f) | 0xc0
+	case VariantFuture:
+		d = (d & 0x0f) | 0xe0
+	case VariantUnknown:
+		fallthrough
+	default:
+		panic(fmt.Sprintf("invalid variant: %d", v))
+	}
+	g.Data4[0] = d
+}
+
 // Variant returns the GUID variant, as defined in RFC 4122.
 func (g GUID) Variant() Variant {
 	b := g.Data4[0]
@@ -165,6 +185,10 @@ func (g GUID) Variant() Variant {
 	return VariantUnknown
 }
 
+func (g *GUID) setVersion(v Version) {
+	g.Data3 = (g.Data3 & 0x0fff) | (uint16(v) << 12)
+}
+
 // Version returns the GUID version, as defined in RFC 4122.
 func (g GUID) Version() Version {
 	return Version((g.Data3 & 0xF000) >> 12)