Build an indexer mapping ``self.sids`` to ``assets``.

        Parameters
        ----------
        assets : list[int]
            List of assets requested by a caller of ``load_raw_arrays``.

        Returns
        -------
        index : np.array[int64]
            Index array containing the index in ``self.sids`` for each location
            in ``assets``. Entries in ``assets`` for which we don't have a sid
            will contain -1. It is caller's responsibility to handle these
            values correctly.

def _make_sid_selector(self, assets):
        """
        Build an indexer mapping ``self.sids`` to ``assets``.

        Parameters
        ----------
        assets : list[int]
            List of assets requested by a caller of ``load_raw_arrays``.

        Returns
        -------
        index : np.array[int64]
            Index array containing the index in ``self.sids`` for each location
            in ``assets``. Entries in ``assets`` for which we don't have a sid
            will contain -1. It is caller's responsibility to handle these
            values correctly.
        """
        assets = np.array(assets)
        sid_selector = self.sids.searchsorted(assets)
        unknown = np.in1d(assets, self.sids, invert=True)
        sid_selector[unknown] = -1
        return sid_selector