ident: implement member identifier

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

ident/member.go

@@ -0,0 +1,84 @@
+package ident
+
+import (
+	"encoding/base64"
+	"encoding/binary"
+	"unsafe"
+)
+
+// M represents a unique member identifier.
+type M struct {
+	// A per-system value incremented by some unspecified amount every time the
+	// metadata of a member first appears to the backend.
+	Serial uint64
+	// An instant in time, some time after the corresponding member metadata
+	// first appeared to the backend, represented in nanoseconds since
+	// 1970-01-01.
+	Time uint64
+	// Randomly generated value. The implementation must guarantee that the same
+	// value cannot be emitted for a Time value.
+	ID uint64
+
+	// Underlying system.
+	System S
+}
+
+const (
+	// SizeMember is the size of the binary representation of [M].
+	SizeMember = SizeSystem * 2
+	// EncodedSizeMember is the size of the string representation of [M].
+	EncodedSizeMember = SizeMember / 3 * 4
+)
+
+// Encode appends the canonical string representation of mid to dst and returns
+// the extended buffer.
+func (mid *M) Encode(dst []byte) []byte {
+	var buf [SizeMember - SizeSystem]byte
+
+	p := buf[:]
+	binary.LittleEndian.PutUint64(p, mid.Serial)
+	p = p[8:]
+	binary.LittleEndian.PutUint64(p, mid.Time)
+	p = p[8:]
+	binary.LittleEndian.PutUint64(p, mid.ID)
+	dst = base64.URLEncoding.AppendEncode(dst, buf[:])
+
+	return mid.System.Encode(dst)
+}
+
+// String returns the canonical string representation of mid.
+func (mid *M) String() string {
+	s := mid.Encode(nil)
+	return unsafe.String(unsafe.SliceData(s), len(s))
+}
+
+// MarshalText returns the result of Encode.
+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}
+	}
+
+	var buf [SizeMember - SizeSystem]byte
+	if n, err := base64.URLEncoding.Decode(
+		buf[:],
+		data[:EncodedSizeMember-EncodedSizeSystem],
+	); err != nil {
+		return err
+	} else if n != SizeMember-SizeSystem {
+		return ErrNewline
+	}
+
+	p := buf[:]
+	mid.Serial = binary.LittleEndian.Uint64(p)
+	p = p[8:]
+	mid.Time = binary.LittleEndian.Uint64(p)
+	p = p[8:]
+	mid.ID = binary.LittleEndian.Uint64(p)
+
+	return mid.System.UnmarshalText(data[EncodedSizeMember-EncodedSizeSystem:])
+}