Return frame from the buffer

def get_frame(self, idx, history_length=1):
        """ Return frame from the buffer """
        if idx >= self.current_size:
            raise VelException("Requested frame beyond the size of the buffer")

        if history_length > 1:
            assert self.state_buffer.shape[-1] == 1, \
                "State buffer must have last dimension of 1 if we want frame history"

        accumulator = []

        last_frame = self.state_buffer[idx]
        accumulator.append(last_frame)

        for i in range(history_length - 1):
            prev_idx = (idx - 1) % self.buffer_capacity

            if prev_idx == self.current_idx:
                raise VelException("Cannot provide enough history for the frame")
            elif self.dones_buffer[prev_idx]:
                # If previous frame was done - just append zeroes
                accumulator.append(np.zeros_like(last_frame))
            else:
                idx = prev_idx
                accumulator.append(self.state_buffer[idx])

        # We're pushing the elements in reverse order
        return np.concatenate(accumulator[::-1], axis=-1)