import torch
import torch.nn as nn
import traceback
from transformers import BartTokenizer, BartForConditionalGeneration
from tqdm import tqdm


class BARTScorer:
    def __init__(self, device='cuda:0', max_length=1024, checkpoint='facebook/bart-large-cnn'):
        # Set up model
        self.device = device
        self.max_length = max_length
        self.tokenizer = BartTokenizer.from_pretrained(checkpoint)
        self.model = BartForConditionalGeneration.from_pretrained(checkpoint)
        self.model.eval()
        self.model.to(device)

        # Set up loss
        self.loss_fct = nn.NLLLoss(reduction='none', ignore_index=self.model.config.pad_token_id)
        self.lsm = nn.LogSoftmax(dim=1)

    def load(self, path=None):
        """ Load model from paraphrase finetuning """
        if path is None:
            path = 'models/bart.pth'
        self.model.load_state_dict(torch.load(path, map_location=self.device))

    def score(self, srcs, tgts, batch_size=4):
        """ Score a batch of examples """
        score_list = []
        for i in tqdm(range(0, len(srcs), batch_size)):
            src_list = srcs[i: i + batch_size]
            tgt_list = tgts[i: i + batch_size]
            try:
                with torch.no_grad():
                    encoded_src = self.tokenizer(
                        src_list,
                        max_length=self.max_length,
                        truncation=True,
                        padding=True,
                        return_tensors='pt'
                    )
                    encoded_tgt = self.tokenizer(
                        tgt_list,
                        max_length=self.max_length,
                        truncation=True,
                        padding=True,
                        return_tensors='pt'
                    )
                    src_tokens = encoded_src['input_ids'].to(self.device)
                    src_mask = encoded_src['attention_mask'].to(self.device)

                    tgt_tokens = encoded_tgt['input_ids'].to(self.device)
                    tgt_mask = encoded_tgt['attention_mask']
                    tgt_len = tgt_mask.sum(dim=1).to(self.device)

                    output = self.model(
                        input_ids=src_tokens,
                        attention_mask=src_mask,
                        labels=tgt_tokens
                    )
                    logits = output.logits.view(-1, self.model.config.vocab_size)
                    loss = self.loss_fct(self.lsm(logits), tgt_tokens.view(-1))
                    loss = loss.view(tgt_tokens.shape[0], -1)
                    loss = loss.sum(dim=1) / tgt_len
                    curr_score_list = [-x.item() for x in loss]
                    score_list += curr_score_list

            except RuntimeError:
                traceback.print_exc()
                print(f'source: {src_list}')
                print(f'target: {tgt_list}')
                exit(0)
        return score_list

    def test(self, batch_size=3):
        """ Test """
        src_list = [
            'This is a very good idea. Although simple, but very insightful.',
            'Can I take a look?',
            'Do not trust him, he is a liar.'
        ]

        tgt_list = [
            "That's stupid.",
            "What's the problem?",
            'He is trustworthy.'
        ]

        print(self.score(src_list, tgt_list, batch_size))
