// emitDepth from the provider io.Reader.
// If emitDepth is < 0, values at every depth will be emitted.
func NewDecoder(r io.Reader, emitDepth int) *Decoder {
	d := &Decoder{
		scanner:   newScanner(r),
		emitDepth: emitDepth,
		scratch:   &scratch{data: make([]byte, 1024)},
		metaCh:    make(chan *MetaValue, 128),
	}
	if emitDepth < 0 {
		d.emitDepth = 0
		d.emitRecursive = true
	}
	return d