def volume_bars(df, volume_column, threshold):
    """
    Computes volume bars (index)

    :param df: pd.DataFrame()
    :param volume_column: (str) name for the column with volume data
    :param m: (int) threshold for volume

    :return idx: returns list of indices
    """
    t = df[volume_column]
    ts = 0
    idx = []

    for i, x in enumerate(t):
        ts += x
        if ts >= threshold:
            idx.append(i)
            ts = 0

    return idx