move gotd fork into repo. (#111)

- update to latest telegram layer
- remove some references to fields in tg.Entities that don't exist in
the schema
- originally added here:
https://github.com/beeper/td/commit/820929062a2ba0104397bc01235ab58a9cff780e
  - referenced here
-
https://github.com/mautrix/telegramgo/commit/124f0967ed195b5a380c9bd02e170ada9710dde3
-
https://github.com/mautrix/telegramgo/commit/4205047aab2e0639217148b5d125bfaab668bd8e

pkg/gotd/telegram/message/entity/write.go

@@ -0,0 +1,108 @@
+package entity
+
+import (
+	"io"
+	"unicode/utf8"
+)
+
+// ComputeLength returns length of s encoded as UTF-16 string.
+//
+// While Telegram API docs state that they expect the number of UTF-8
+// code points, in fact they are talking about UTF-16 code units.
+func ComputeLength(s string) int {
+	// From utf16 package.
+	n := 0
+	for _, v := range s {
+		n += utf16RuneLen(v)
+	}
+	return n
+}
+
+// ComputeLengthBytes returns length of s encoded as UTF-16 string.
+//
+// While Telegram API docs state that they expect the number of UTF-8
+// code points, in fact they are talking about UTF-16 code units.
+func ComputeLengthBytes(s []byte) (n int) {
+	// From utf16 package.
+	var i int
+	for i < len(s) {
+		v, size := utf8.DecodeRune(s[i:])
+		i += size
+		n += utf16RuneLen(v)
+	}
+	return n
+}
+
+func utf16RuneLen(v rune) int {
+	const (
+		surrSelf = 0x10000
+		maxRune  = '\U0010FFFF' // Maximum valid Unicode code point.
+	)
+
+	if surrSelf <= v && v <= maxRune {
+		return 2
+	}
+	return 1
+}
+
+func (b *Builder) appendMessage(s string, formats ...Formatter) *Builder {
+	if s == "" {
+		return b
+	}
+
+	offset := b.utf16length
+	length := ComputeLength(s)
+
+	b.appendEntities(offset, length, utf8entity{
+		offset: b.message.Len(),
+		length: len(s),
+	}, formats...)
+	_, _ = b.WriteString(s)
+	return b
+}
+
+func (b *Builder) appendEntities(offset, length int, u utf8entity, formats ...Formatter) *Builder {
+	b.lastFormatIndex = len(b.entities)
+	for i := range formats {
+		b.entities = append(b.entities, formats[i](offset, length))
+		b.lengths = append(b.lengths, u)
+	}
+	return b
+}
+
+var _ = []interface {
+	io.Writer
+	io.StringWriter
+	io.ByteWriter
+	WriteRune(rune) (int, error)
+}{
+	(*Builder)(nil),
+}
+
+// Write implements io.Writer.
+func (b *Builder) Write(s []byte) (int, error) {
+	n, err := b.message.Write(s)
+	b.utf16length += ComputeLengthBytes(s)
+	return n, err
+}
+
+// WriteString implements io.StringWriter.
+func (b *Builder) WriteString(s string) (int, error) {
+	n, err := b.message.WriteString(s)
+	b.utf16length += ComputeLength(s)
+	return n, err
+}
+
+// WriteByte implements io.ByteWriter.
+func (b *Builder) WriteByte(s byte) error {
+	err := b.message.WriteByte(s)
+	b.utf16length++
+	return err
+}
+
+// WriteRune implements rune writer.
+func (b *Builder) WriteRune(s rune) (int, error) {
+	n, err := b.message.WriteRune(s)
+	b.utf16length += utf16RuneLen(s)
+	return n, err
+}