ident: isolate non-system component of member

This enables more efficient representation in contexts inherently namespaced to the current system.

Signed-off-by: Yonah <contrib@gensokyo.uk>

ident/member_test.go

@@ -9,6 +9,48 @@ import (
 	"git.gensokyo.uk/cofront/cof-spec/ident"
 )
 
+func TestPartM(t *testing.T) {
+	t.Parallel()
+
+	rTestCases[ident.PartM, *ident.PartM]{
+		{"short", ident.PartM{}, nil, &ident.UnexpectedSizeError{
+			Data: nil,
+			Want: ident.EncodedSizeMember - ident.EncodedSizeSystem,
+		}},
+
+		{"malformed", ident.PartM{}, []byte{
+			0xfe, 0xe1, 0xde, 0xad,
+			0xfe, 0xe1, 0xde, 0xad,
+			0xfe, 0xe1, 0xde, 0xad,
+			0xfe, 0xe1, 0xde, 0xad,
+			0xfe, 0xe1, 0xde, 0xad,
+			0xfe, 0xe1, 0xde, 0xad,
+			0xfe, 0xe1, 0xde, 0xad,
+			0xfe, 0xe1, 0xde, 0xad,
+		}, base64.CorruptInputError(0)},
+
+		{"newline", ident.PartM{}, []byte(
+			"AAAA" + strings.Repeat("\n", ident.EncodedSizeMember-ident.EncodedSizeSystem-4),
+		), ident.ErrNewline},
+
+		{"valid", ident.PartM{
+			Serial: 0xfdfdfdfdfdfdfdfd,
+
+			Time: uint64(time.Date(
+				0xfd, 7, 15,
+				23, 59, 59, 0xcab,
+				time.UTC,
+			).UnixNano()),
+
+			ID: 0x2e736e64,
+		}, base64.URLEncoding.AppendEncode(nil, []byte{
+			/* serial: */ 0xfd, 0xfd, 0xfd, 0xfd, 0xfd, 0xfd, 0xfd, 0xfd,
+			/* time:   */ 0xab, 0x42, 0x42, 0xce, 0xf1, 0x92, 0x4a, 0x10,
+			/* id:     */ 0x64, 0x6e, 0x73, 0x2e, 0, 0, 0, 0,
+		}), nil},
+	}.run(t)
+}
+
 func TestM(t *testing.T) {
 	t.Parallel()
 
@@ -43,15 +85,17 @@ func TestM(t *testing.T) {
 		), ident.ErrNewline},
 
 		{"valid", ident.M{
-			Serial: 0xfdfdfdfdfdfdfdfd,
+			PartM: ident.PartM{
+				Serial: 0xfdfdfdfdfdfdfdfd,
 
-			Time: uint64(time.Date(
-				0xfd, 7, 15,
-				23, 59, 59, 0xcab,
-				time.UTC,
-			).UnixNano()),
+				Time: uint64(time.Date(
+					0xfd, 7, 15,
+					23, 59, 59, 0xcab,
+					time.UTC,
+				).UnixNano()),
 
-			ID: 0x2e736e64,
+				ID: 0x2e736e64,
+			},
 
 			System: ident.S{
 				Site: ident.TrivialSite,