import numpy as np
import torch
from torch.nn.utils.rnn import pack_padded_sequence, pad_sequence

def pack_lstm(items, lstm):
    N = len(items)
    reorder_args = np.argsort([len(it) for it in items])[::-1]
    origin_args = torch.from_numpy(np.argsort(reorder_args))
    ordered = [items[i] for i in reorder_args]
    packed_items = pack_padded_sequence(pad_sequence(ordered, batch_first=True), [len(od) for od in ordered], batch_first=True)
    _, (hn, _) = lstm(packed_items)
    by_inst_repr = hn.transpose(0,1).reshape(N,-1)
    # Now untwizzle
    return torch.index_select(by_inst_repr, 0, origin_args)