"""This is a slightly modified version of https://github.com/ufoym/imbalanced-dataset-sampler."""

from typing import Callable, List, Optional

import pandas as pd
import torch
import torch.utils.data


class ImbalancedDatasetSampler(torch.utils.data.sampler.Sampler):
    """Samples elements randomly from a given list of indices for imbalanced dataset.

    Arguments:
        indices: a list of indices
        num_samples: number of samples to draw
        callback_get_label: a callback-like function which takes one argument - the dataset
    """

    def __init__(
        self,
        dataset,
        labels: Optional[List] = None,
        indices: Optional[List] = None,
        num_samples: Optional[int] = None,
        callback_get_label: Optional[Callable] = None,
    ):
        # if indices is not provided, all elements in the dataset will be considered
        self.indices = list(range(len(dataset))) if indices is None else indices

        # define custom callback
        self.callback_get_label = callback_get_label

        # if num_samples is not provided, draw `len(indices)` samples in each iteration
        self.num_samples = len(self.indices) if num_samples is None else num_samples

        # distribution of classes in the dataset
        df = pd.DataFrame()
        df["label"] = self._get_labels(dataset) if labels is None else labels
        df.index = self.indices
        df = df.sort_index()

        label_to_count = df["label"].value_counts()

        weights = 1.0 / label_to_count[df["label"]]

        self.weights = torch.DoubleTensor(weights.to_list())

    def _get_labels(self, dataset):
        if self.callback_get_label:
            return self.callback_get_label(dataset)
        elif isinstance(dataset, torch.utils.data.TensorDataset):
            return dataset.tensors[1]
        elif isinstance(dataset, torch.utils.data.Subset):
            return dataset.dataset.imgs[:][1]
        elif isinstance(dataset, torch.utils.data.Dataset):
            return dataset.get_labels()
        else:
            raise NotImplementedError

    def __iter__(self):
        return (
            self.indices[i]
            for i in torch.multinomial(self.weights, self.num_samples, replacement=True)
        )

    def __len__(self):
        return self.num_samples