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/ident_test.go

@@ -117,15 +117,17 @@ func TestFM(t *testing.T) {
 
 		{"valid", ident.F[ident.M, *ident.M]{
 			I: &ident.M{
-				Serial: 0xfdfdfdfdfdfdfdfd,
+				PartM: ident.PartM{
+					Serial: 0xfdfdfdfdfdfdfdfd,
 
-				Time: uint64(time.Date(
+					Time: uint64(time.Date(
-					0xfd, 7, 15,
+						0xfd, 7, 15,
-					23, 59, 59, 0xcab,
+						23, 59, 59, 0xcab,
-					time.UTC,
+						time.UTC,
-				).UnixNano()),
+					).UnixNano()),
 
-				ID: 0x2e736e64,
+					ID: 0x2e736e64,
+				},
 
 				System: ident.S{
 					Site: ident.TrivialSite,

ident/member.go

@@ -6,8 +6,8 @@ import (
 	"unsafe"
 )
 
-// M represents a unique member identifier.
+// PartM represents the first half of [M].
-type M struct {
+type PartM struct {
 	// A per-system value incremented by some unspecified amount every time the
 	// metadata of a member first appears to the backend.
 	Serial uint64
@@ -18,7 +18,11 @@ type M struct {
 	// Randomly generated value. The implementation must guarantee that the same
 	// value cannot be emitted for a Time value.
 	ID uint64
+}
 
+// M represents a unique member identifier.
+type M struct {
+	PartM
 	// Underlying system.
 	System S
 }
@@ -35,19 +39,35 @@ const (
 // EncodedSize returns the number of bytes appended by Encode.
 func (*M) EncodedSize() int { return EncodedSizeMember }
 
-// Encode appends the canonical string representation of mid to dst and returns
+// Encode appends the canonical string representation of pm to dst and returns
 // the extended buffer.
-func (mid *M) Encode(dst []byte) []byte {
+func (pm *PartM) Encode(dst []byte) []byte {
 	var buf [SizeMember - SizeSystem]byte
 
 	p := buf[:]
-	binary.LittleEndian.PutUint64(p, mid.Serial)
+	binary.LittleEndian.PutUint64(p, pm.Serial)
 	p = p[8:]
-	binary.LittleEndian.PutUint64(p, mid.Time)
+	binary.LittleEndian.PutUint64(p, pm.Time)
 	p = p[8:]
-	binary.LittleEndian.PutUint64(p, mid.ID)
+	binary.LittleEndian.PutUint64(p, pm.ID)
-	dst = base64.URLEncoding.AppendEncode(dst, buf[:])
+	return base64.URLEncoding.AppendEncode(dst, buf[:])
+}
 
+// String returns the canonical string representation of pm.
+func (pm *PartM) String() string {
+	s := pm.Encode(nil)
+	return unsafe.String(unsafe.SliceData(s), len(s))
+}
+
+// MarshalText returns the result of Encode.
+func (pm *PartM) MarshalText() (data []byte, err error) {
+	return pm.Encode(nil), nil
+}
+
+// Encode appends the canonical string representation of mid to dst and returns
+// the extended buffer.
+func (mid *M) Encode(dst []byte) []byte {
+	dst = mid.PartM.Encode(dst)
 	return mid.System.Encode(dst)
 }
 
@@ -62,16 +82,16 @@ func (mid *M) MarshalText() (data []byte, err error) {
 	return mid.Encode(nil), nil
 }
 
-// UnmarshalText strictly decodes data into mid.
+// UnmarshalText strictly decodes data into pm.
-func (mid *M) UnmarshalText(data []byte) error {
+func (pm *PartM) UnmarshalText(data []byte) error {
-	if len(data) != EncodedSizeMember {
+	if len(data) != EncodedSizeMember-EncodedSizeSystem {
-		return &UnexpectedSizeError{data, EncodedSizeMember}
+		return &UnexpectedSizeError{data, EncodedSizeMember - EncodedSizeSystem}
 	}
 
 	var buf [SizeMember - SizeSystem]byte
 	if n, err := base64.URLEncoding.Decode(
 		buf[:],
-		data[:EncodedSizeMember-EncodedSizeSystem],
+		data,
 	); err != nil {
 		return err
 	} else if n != SizeMember-SizeSystem {
@@ -79,11 +99,23 @@ func (mid *M) UnmarshalText(data []byte) error {
 	}
 
 	p := buf[:]
-	mid.Serial = binary.LittleEndian.Uint64(p)
+	pm.Serial = binary.LittleEndian.Uint64(p)
 	p = p[8:]
-	mid.Time = binary.LittleEndian.Uint64(p)
+	pm.Time = binary.LittleEndian.Uint64(p)
 	p = p[8:]
-	mid.ID = binary.LittleEndian.Uint64(p)
+	pm.ID = binary.LittleEndian.Uint64(p)
+	return nil
+}
 
+// UnmarshalText strictly decodes data into mid.
+func (mid *M) UnmarshalText(data []byte) error {
+	if len(data) != EncodedSizeMember {
+		return &UnexpectedSizeError{data, EncodedSizeMember}
+	}
+	if err := mid.PartM.UnmarshalText(
+		data[:EncodedSizeMember-EncodedSizeSystem],
+	); err != nil {
+		return err
+	}
 	return mid.System.UnmarshalText(data[EncodedSizeMember-EncodedSizeSystem:])
 }

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(
+				Time: uint64(time.Date(
-				0xfd, 7, 15,
+					0xfd, 7, 15,
-				23, 59, 59, 0xcab,
+					23, 59, 59, 0xcab,
-				time.UTC,
+					time.UTC,
-			).UnixNano()),
+				).UnixNano()),
 
-			ID: 0x2e736e64,
+				ID: 0x2e736e64,
+			},
 
 			System: ident.S{
 				Site: ident.TrivialSite,