def combine(in_root, out_root):

    all_sessions = os.listdir(in_root)
    map_args = list(map(lambda x: (out_root, x, in_root), all_sessions))

    with multiprocessing.Pool(10) as p:
        p.map(combine_in_session, map_args)

def combine_in_session(args):

    out_root = args[0]
    session = args[1]
    in_root = args[2]

    store_dir = os.path.join(out_root, session)
    os.makedirs(store_dir, exist_ok=True)

    all_parts = glob.glob(os.path.join(in_root, session, '*.wav'))

    current_len = 0
    current_batch_num = 0
    current_max_len = np.random.randint(low=20, high=30)
    current_batch = []

    for part in all_parts:
        if current_len >= current_max_len:
            sf.write(file=os.path.join(store_dir, 'split_{}.wav'.format(current_batch_num)),
                     data=current_batch, samplerate=8000)
            current_batch = []
            current_batch_num += 1
            current_len = 0

        data, rate = sf.read(part)
        time_len = data.shape[-1] / 8000

        current_batch += data.tolist()
        current_len += time_len

    if len(current_batch) > 0:
        sf.write(file=os.path.join(store_dir, 'split_{}.wav'.format(current_batch_num)),
                 data=current_batch, samplerate=8000)