// PointDecodeFrom provides a generic implementation of Point.DecodeFrom,
// based on Point.Decode, or Point.Pick if r is a Cipher or cipher.Stream.
// The returned byte-count is valid only when decoding from a normal Reader,
// not when picking from a pseudorandom source.
func PointUnmarshalFrom(p abstract.Point, r io.Reader) (int, error) {
if strm, ok := r.(cipher.Stream); ok {
p.Pick(nil, strm)
return -1, nil // no byte-count when picking randomly
}
buf := make([]byte, p.MarshalSize())
n, err := io.ReadFull(r, buf)
if err != nil {
return n, err
}
return n, p.UnmarshalBinary(buf)
}