def compute_asset_lifetimes(frames):
    """
    Parameters
    ----------
    frames : dict[str, pd.DataFrame]
        A dict mapping each OHLCV field to a dataframe with a row for
        each date and a column for each sid, as passed to write().

    Returns
    -------
    start_date_ixs : np.array[int64]
        The index of the first date with non-nan values, for each sid.
    end_date_ixs : np.array[int64]
        The index of the last date with non-nan values, for each sid.
    """
    # Build a 2D array (dates x sids), where an entry is True if all
    # fields are nan for the given day and sid.
    is_null_matrix = np.logical_and.reduce(
        [frames[field].isnull().values for field in FIELDS],
    )
    if not is_null_matrix.size:
        empty = np.array([], dtype='int64')
        return empty, empty.copy()

    # Offset of the first null from the start of the input.
    start_date_ixs = is_null_matrix.argmin(axis=0)
    # Offset of the last null from the **end** of the input.
    end_offsets = is_null_matrix[::-1].argmin(axis=0)
    # Offset of the last null from the start of the input
    end_date_ixs = is_null_matrix.shape[0] - end_offsets - 1

    return start_date_ixs, end_date_ixs