from ray.rllib.utils.framework import try_import_tf

tf1, tf, tfv = try_import_tf()


class RelativeMultiHeadAttention(tf.keras.layers.Layer if tf else object):
    """A RelativeMultiHeadAttention layer as described in [3].

    Uses segment level recurrence with state reuse.
    """

    def __init__(self,
                 out_dim,
                 num_heads,
                 head_dim,
                 rel_pos_encoder,
                 input_layernorm=False,
                 output_activation=None,
                 **kwargs):
        """Initializes a RelativeMultiHeadAttention keras Layer object.

        Args:
            out_dim (int):
            num_heads (int): The number of attention heads to use.
                Denoted `H` in [2].
            head_dim (int): The dimension of a single(!) attention head
                Denoted `D` in [2].
            rel_pos_encoder (:
            input_layernorm (bool): Whether to prepend a LayerNorm before
                everything else. Should be True for building a GTrXL.
            output_activation (Optional[tf.nn.activation]): Optional tf.nn
                activation function. Should be relu for GTrXL.
            **kwargs:
        """
        super().__init__(**kwargs)

        # No bias or non-linearity.
        self._num_heads = num_heads
        self._head_dim = head_dim
        # 3=Query, key, and value inputs.
        self._qkv_layer = tf.keras.layers.Dense(
            3 * num_heads * head_dim, use_bias=False)
        self._linear_layer = tf.keras.layers.TimeDistributed(
            tf.keras.layers.Dense(
                out_dim, use_bias=False, activation=output_activation))

        self._uvar = self.add_weight(shape=(num_heads, head_dim))
        self._vvar = self.add_weight(shape=(num_heads, head_dim))

        self._pos_proj = tf.keras.layers.Dense(
            num_heads * head_dim, use_bias=False)
        self._rel_pos_encoder = rel_pos_encoder

        self._input_layernorm = None
        if input_layernorm:
            self._input_layernorm = tf.keras.layers.LayerNormalization(axis=-1)

    def call(self, inputs, memory=None):
        T = tf.shape(inputs)[1]  # length of segment (time)
        H = self._num_heads  # number of attention heads
        d = self._head_dim  # attention head dimension

        # Add previous memory chunk (as const, w/o gradient) to input.
        # Tau (number of (prev) time slices in each memory chunk).
        Tau = memory.shape.as_list()[1] if memory is not None else 0
        if memory is not None:
            inputs = tf.concat((tf.stop_gradient(memory), inputs), axis=1)

        # Apply the Layer-Norm.
        if self._input_layernorm is not None:
            inputs = self._input_layernorm(inputs)

        qkv = self._qkv_layer(inputs)

        queries, keys, values = tf.split(qkv, 3, -1)
        # Cut out Tau memory timesteps from query.
        queries = queries[:, -T:]

        queries = tf.reshape(queries, [-1, T, H, d])
        keys = tf.reshape(keys, [-1, T + Tau, H, d])
        values = tf.reshape(values, [-1, T + Tau, H, d])

        R = self._pos_proj(self._rel_pos_encoder)
        R = tf.reshape(R, [T + Tau, H, d])

        # b=batch
        # i and j=time indices (i=max-timesteps (inputs); j=Tau memory space)
        # h=head
        # d=head-dim (over which we will reduce-sum)
        score = tf.einsum("bihd,bjhd->bijh", queries + self._uvar, keys)
        pos_score = tf.einsum("bihd,jhd->bijh", queries + self._vvar, R)
        score = score + self.rel_shift(pos_score)
        score = score / d**0.5

        # causal mask of the same length as the sequence
        mask = tf.sequence_mask(
            tf.range(Tau + 1, T + Tau + 1), dtype=score.dtype)
        mask = mask[None, :, :, None]

        masked_score = score * mask + 1e30 * (mask - 1.)
        wmat = tf.nn.softmax(masked_score, axis=2)

        out = tf.einsum("bijh,bjhd->bihd", wmat, values)
        out = tf.reshape(out, tf.concat((tf.shape(out)[:2], [H * d]), axis=0))
        return self._linear_layer(out)

    @staticmethod
    def rel_shift(x):
        # Transposed version of the shift approach described in [3].
        # https://github.com/kimiyoung/transformer-xl/blob/
        # 44781ed21dbaec88b280f74d9ae2877f52b492a5/tf/model.py#L31
        x_size = tf.shape(x)

        x = tf.pad(x, [[0, 0], [0, 0], [1, 0], [0, 0]])
        x = tf.reshape(x, [x_size[0], x_size[2] + 1, x_size[1], x_size[3]])
        x = x[:, 1:, :, :]
        x = tf.reshape(x, x_size)

        return x