from stog.modules.encoder_base import _EncoderBase


class Seq2VecEncoder(_EncoderBase):
    """
    A ``Seq2VecEncoder`` is a ``Module`` that takes as input a sequence of vectors and returns a
    single vector.  Input shape: ``(batch_size, sequence_length, input_dim)``; output shape:
    ``(batch_size, output_dim)``.

    We add two methods to the basic ``Module`` API: :func:`get_input_dim()` and :func:`get_output_dim()`.
    You might need this if you want to construct a ``Linear`` layer using the output of this encoder,
    or to raise sensible errors for mis-matching input dimensions.
    """
    def get_input_dim(self) -> int:
        """
        Returns the dimension of the vector input for each element in the sequence input
        to a ``Seq2VecEncoder``. This is `not` the shape of the input tensor, but the
        last element of that shape.
        """
        raise NotImplementedError

    def get_output_dim(self) -> int:
        """
        Returns the dimension of the final vector output by this ``Seq2VecEncoder``.  This is `not`
        the shape of the returned tensor, but the last element of that shape.
        """
        raise NotImplementedError
