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.go

@@ -6,8 +6,8 @@ import (
 	"unsafe"
 )
 
-// M represents a unique member identifier.
-type M struct {
+// PartM represents the first half of [M].
+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)
-	dst = base64.URLEncoding.AppendEncode(dst, buf[:])
+	binary.LittleEndian.PutUint64(p, pm.ID)
+	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.
-func (mid *M) UnmarshalText(data []byte) error {
-	if len(data) != EncodedSizeMember {
-		return &UnexpectedSizeError{data, EncodedSizeMember}
+// UnmarshalText strictly decodes data into pm.
+func (pm *PartM) UnmarshalText(data []byte) error {
+	if len(data) != EncodedSizeMember-EncodedSizeSystem {
+		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:])
 }