from collections.abc import Sequence

_to_numpy = {}
_from_numpy = {}

def converts_to_numpy(msgtype, plural=False):
    def decorator(f):
        _to_numpy[msgtype, plural] = f
        return f
    return decorator

def converts_from_numpy(msgtype, plural=False):
    def decorator(f):
        _from_numpy[msgtype, plural] = f
        return f
    return decorator

def numpify(msg, *args, **kwargs):
    if msg is None:
        return

    conv = _to_numpy.get((msg.__class__, False))
    if not conv and isinstance(msg, Sequence):
        if not msg:
            raise ValueError("Cannot determine the type of an empty Collection")
        conv = _to_numpy.get((msg[0].__class__, True))


    if not conv:
        raise ValueError(
            "Unable to convert message {} - only supports {}".format(
            msg.__class__.__name__,
            ', '.join(cls.__name__ + ("[]" if pl else '')
                          for cls, pl in _to_numpy.keys())
        ))

    return conv(msg, *args, **kwargs)

def msgify(msg_type, numpy_obj, *args, **kwargs):
    conv = _from_numpy.get((msg_type, kwargs.pop('plural', False)))
    if not conv:
        raise ValueError("Unable to build message {} - only supports {}".format(
            msg_type.__name__,
            ', '.join(cls.__name__ + ("[]" if pl else '')
                          for cls, pl in _to_numpy.keys())
        ))
    return conv(numpy_obj, *args, **kwargs)
