simonz05

  // WriteTo implements io.WriterTo.
  // This may make multiple calls to the [Reader.Read] method of the underlying [Reader].
  // If the underlying reader supports the [Reader.WriteTo] method,
  // this calls the underlying [Reader.WriteTo] without buffering.
  func (b *Reader) WriteTo(w io.Writer) (n int64, err error) {
   b.lastByte = -1
   b.lastRuneSize = -1
  
   n, err = b.writeBuf(w)
   if err != nil {
    return
   }
  
   if r, ok := b.rd.(io.WriterTo); ok {
    m, err := r.WriteTo(w)
    n += m
    return n, err
   }
  
   if w, ok := w.(io.ReaderFrom); ok {
    m, err := w.ReadFrom(b.rd)
    n += m
    return n, err
   }
  
   if b.w-b.r < len(b.buf) {
    b.fill() // buffer not full
   }
  
   for b.r < b.w {
    // b.r < b.w => buffer is not empty
    m, err := b.writeBuf(w)
    n += m
    if err != nil {
     return n, err
    }
    b.fill() // buffer is empty
   }
  
   if b.err == io.EOF {
    b.err = nil
   }
  
   return n, b.readErr()
  }