ident: decode twitch vod name

There are a few unknown fields, represented as is for now.

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

ident.go

@@ -0,0 +1,142 @@
+package streamdata
+
+import (
+	"bytes"
+	"encoding/hex"
+	"errors"
+	"strconv"
+	"unsafe"
+)
+
+const (
+	// identFields is the expected number of fields in the [Ident] wire format.
+	identFields = 7
+	// identSeparator is the byte separating [Ident] fields.
+	identSeparator = '-'
+
+	// identFF0Len is the expected size of the first free-form [Ident] field.
+	identFF0Len = 4
+	// identFF123Len is the expected size of the second, third, fourth free-form
+	// [Ident] fields.
+	identFF123Len = 2
+	// identFF4Len is the expected size of the fifth free-form [Ident] field.
+	identFF4Len = 6
+
+	// IdentFFLen is the total size of all free-form fields.
+	IdentFFLen = identFF0Len + identFF123Len*3 + identFF4Len
+)
+
+// identFF are segment lengths of [Ident.Data].
+var identFF = []int{
+	identFF0Len,
+	identFF123Len,
+	identFF123Len,
+	identFF123Len,
+	identFF4Len,
+}
+
+// Ident represents the unique identifier of a VOD, returned by Twitch.
+type Ident struct {
+	// An incrementing internal identifier, decimal encoding.
+	Serial uint64
+	// Internal identifier specific to the channel, decimal encoding.
+	Channel uint64
+	// Constant-size remaining data, unknown semantics, hexadecimal encoding.
+	Data [IdentFFLen]byte
+}
+
+// Encode encodes a canonical representation of ident.
+func (ident *Ident) Encode() []byte {
+	data := make(
+		[]byte, 0,
+		20 /* decimal numbers with one extra byte of headroom */ +
+			identFields-1+
+			IdentFFLen*2,
+	)
+
+	data = strconv.AppendUint(data, ident.Serial, 10)
+	data = append(data, identSeparator)
+	data = strconv.AppendUint(data, ident.Channel, 10)
+
+	ff := ident.Data[:]
+	for _, el := range identFF {
+		data = append(data, identSeparator)
+		data = hex.AppendEncode(data, ff[:el])
+		ff = ff[el:]
+	}
+	return data
+}
+
+// MarshalText encodes a canonical representation of ident.
+func (ident *Ident) MarshalText() (data []byte, _ error) {
+	return ident.Encode(), nil
+}
+
+// String returns the result of MarshalText as a string.
+func (ident *Ident) String() string {
+	data := ident.Encode()
+	return unsafe.String(unsafe.SliceData(data), len(data))
+}
+
+// atoi is like [strconv.Atoi], but with the resulting error unwrapped for
+// cleaner error messages. The caller must ensure data is never modified.
+func atoi(data []byte) (v uint64, err error) {
+	v, err = strconv.ParseUint(
+		unsafe.String(unsafe.SliceData(data), len(data)),
+		10, 64,
+	)
+	if err != nil {
+		var numError *strconv.NumError
+		if errors.As(err, &numError) && numError != nil {
+			err = numError.Unwrap()
+		}
+	}
+	return
+}
+
+// IdentFieldError describes an [Ident] representation with not enough fields.
+type IdentFieldError int
+
+func (e IdentFieldError) Error() string {
+	return "got " + strconv.Itoa(int(e)) +
+		" field(s) instead of " + strconv.Itoa(identFields)
+}
+
+// IdentFFError describes an [Ident] representation with an irregular size
+// free-form field.
+type IdentFFError struct {
+	Got, Want int
+}
+
+func (e *IdentFFError) Error() string {
+	return "got " + strconv.Itoa(e.Got) + " bytes for a " +
+		strconv.Itoa(e.Want) + "-byte long free-form field"
+}
+
+// UnmarshalText strictly decodes an unsuffixed VOD name returned by Twitch.
+func (ident *Ident) UnmarshalText(data []byte) (err error) {
+	fields := bytes.SplitN(data, []byte{identSeparator}, identFields)
+	if l := IdentFieldError(len(fields)); l != identFields {
+		return l
+	}
+
+	if ident.Serial, err = atoi(fields[0]); err != nil {
+		return
+	}
+	if ident.Channel, err = atoi(fields[1]); err != nil {
+		return
+	}
+
+	var n int
+	buf := ident.Data[:]
+	for i, el := range identFF {
+		field := fields[2+i]
+		if n, err = hex.Decode(buf, field); err != nil {
+			return err
+		} else if n != el {
+			return &IdentFFError{n, el}
+		}
+		buf = buf[el:]
+	}
+	return nil
+}