import logging
from pathlib import Path
from typing import Dict
from torch.optim import Optimizer

import torch
import torch.nn as nn
from torch.utils.data import DataLoader
from torch.utils.data import TensorDataset
import pandas as pd

logger = logging.getLogger(__name__)


class PyTorchModelTrainer:
    def __init__(
            self,
            model: nn.Module,
            optimizer: Optimizer,
            criterion: nn.Module,
            device: str,
            batch_size: int,
            max_iters: int,
            eval_iters: int,
            init_model: Dict
    ):
        self.model = model
        self.optimizer = optimizer
        self.criterion = criterion
        self.device = device
        self.max_iters = max_iters
        self.batch_size = batch_size
        self.eval_iters = eval_iters

        if init_model:
            self.load_from_checkpoint(init_model)

    def fit(self, data_dictionary: Dict[str, pd.DataFrame]):
        data_loaders_dictionary = self.create_data_loaders_dictionary(data_dictionary)
        epochs = self.calc_n_epochs(
            n_obs=len(data_dictionary['train_features']),
            batch_size=self.batch_size,
            n_iters=self.max_iters
        )
        for epoch in range(epochs):
            # evaluation
            losses = self.estimate_loss(data_loaders_dictionary, data_dictionary)
            logger.info(
                f"epoch ({epoch}/{epochs}):"
                f" train loss {losses['train']:.4f} ; test loss {losses['test']:.4f}"
            )
            # training
            for batch_data in data_loaders_dictionary['train']:
                xb, yb = batch_data
                xb = xb.to(self.device)
                yb = yb.to(self.device)
                yb_pred = self.model(xb)
                loss = self.criterion(yb_pred, yb)

                self.optimizer.zero_grad(set_to_none=True)
                loss.backward()
                self.optimizer.step()

    @torch.no_grad()
    def estimate_loss(
            self,
            data_loader_dictionary: Dict[str, DataLoader],
            data_dictionary: Dict[str, pd.DataFrame]
    ) -> Dict[str, float]:

        self.model.eval()
        epochs = self.calc_n_epochs(
            n_obs=len(data_dictionary['test_features']),
            batch_size=self.batch_size,
            n_iters=self.eval_iters
        )
        loss_dictionary = {}
        for split in ['train', 'test']:
            losses = torch.zeros(epochs)
            for i, batch in enumerate(data_loader_dictionary[split]):
                xb, yb = batch
                xb = xb.to(self.device)
                yb = yb.to(self.device)
                yb_pred = self.model(xb)
                loss = self.criterion(yb_pred, yb)
                losses[i] = loss.item()

            loss_dictionary[split] = losses.mean().item()

        self.model.train()
        return loss_dictionary

    def create_data_loaders_dictionary(
            self,
            data_dictionary: Dict[str, pd.DataFrame]
    ) -> Dict[str, DataLoader]:
        data_loader_dictionary = {}
        for split in ['train', 'test']:
            labels_shape = data_dictionary[f'{split}_labels'].shape
            labels_view = labels_shape[0] if labels_shape[1] == 1 else labels_shape
            dataset = TensorDataset(
                torch.from_numpy(data_dictionary[f'{split}_features'].values).float(),
                torch.from_numpy(data_dictionary[f'{split}_labels'].astype(float).values)
                .long()
                .view(labels_view)
            )
            # todo currently assuming class labels are strings ['0.0', '1.0' .. n_classes].
            #  need to resolve it per ClassifierModel

            data_loader = DataLoader(
                dataset,
                batch_size=self.batch_size,
                shuffle=True,
                drop_last=True,
                num_workers=0,
            )
            data_loader_dictionary[split] = data_loader

        return data_loader_dictionary

    @staticmethod
    def calc_n_epochs(n_obs: int, batch_size: int, n_iters: int) -> int:
        n_batches = n_obs // batch_size
        epochs = n_iters // n_batches
        return epochs

    def save(self, path: Path):
        torch.save({
            'model_state_dict': self.model.state_dict(),
            'optimizer_state_dict': self.optimizer.state_dict(),
        }, path)

    def load_from_file(self, path: Path):
        checkpoint = torch.load(path)
        return self.load_from_checkpoint(checkpoint)

    def load_from_checkpoint(self, checkpoint: Dict):
        self.model.load_state_dict(checkpoint['model_state_dict'])
        self.optimizer.load_state_dict(checkpoint['optimizer_state_dict'])
        return self