import torch
from torch.utils.data import Dataset
from typing import Callable

class CharRNNDataset(Dataset):
    """Character-level dataset for training RNN language models."""

    def __init__(self, text: str, seq_len: int, text_encoder_fn: Callable[[str], torch.Tensor]):
        """
        Args:
            text (str): Raw text corpus.
            seq_len (int): Length of each training sequence.
            text_encoder_fn (Callable): Function to encode text into a tensor of IDs.
        """
        self.full_sequence = text_encoder_fn(text)
        self.seq_len = seq_len

    def __len__(self) -> int:
        return len(self.full_sequence) - self.seq_len

    def __getitem__(self, idx: int):
        if idx < 0 or idx >= len(self):
            raise IndexError("Index out of range")
        end = idx + self.seq_len
        x = self.full_sequence[idx:end]
        y = self.full_sequence[idx + 1:end + 1]
        return x, y