def find_nearest_neighbor(query, vectors, ban_set, cossims=None):
    """
    query is a 1d numpy array corresponding to the vector to which you want to
    find the closest vector
    vectors is a 2d numpy array corresponding to the vectors you want to consider
    ban_set is a set of indicies within vectors you want to ignore for nearest match
    cossims is a 1d numpy array of size len(vectors), which can be passed for efficiency

    returns the index of the closest match to query within vectors

    """
    if cossims is None:
        cossims = np.matmul(vectors, query, out=cossims)
    else:
        np.matmul(vectors, query, out=cossims)
    rank = len(cossims) - 1
    result_i = np.argpartition(cossims, rank)[rank]
    while result_i in ban_set:
        rank -= 1
        result_i = np.argpartition(cossims, rank)[rank]
    return result_i