def healpix_i2id(ringi, nside):
    """Converts ringi to idash.

    Parameters
    ----------
    ringi : int
        Pixel ring index.
    nside : int
        Healpix nside.

    Returns
    -------
    idash : int
        Alternate pixel index along each ring. This is for pixel transformations
        as this maps exactly to healpix y without a factor.
    """
    idash = nside - ringi/2
    return idash