"""Dataset loading and PyTorch dataset classes for English-Chinese translation."""

from __future__ import annotations

from pathlib import Path
from typing import Mapping, Sequence

import torch
from torch.utils.data import Dataset

from easytranslate.data.preprocessing import preprocess_pipeline
from easytranslate.data.tokenizer import TokenizerWrapper


class TranslationDataset(Dataset):
    """PyTorch dataset that builds seq2seq inputs for teacher forcing."""

    def __init__(
        self,
        src_texts: Sequence[str],
        tgt_texts: Sequence[str],
        tokenizer: TokenizerWrapper | None = None,
        src_tokenizer: TokenizerWrapper | None = None,
        tgt_tokenizer: TokenizerWrapper | None = None,
        max_src_len: int = 256,
        max_tgt_len: int = 256,
    ):
        if len(src_texts) != len(tgt_texts):
            raise ValueError("src_texts and tgt_texts must have the same length")

        if tokenizer is not None:
            src_tokenizer = src_tokenizer or tokenizer
            tgt_tokenizer = tgt_tokenizer or tokenizer
        if src_tokenizer is None or tgt_tokenizer is None:
            raise ValueError("Provide tokenizer or both src_tokenizer and tgt_tokenizer")

        self.src_texts = list(src_texts)
        self.tgt_texts = list(tgt_texts)
        self.src_tokenizer = src_tokenizer
        self.tgt_tokenizer = tgt_tokenizer
        self.max_src_len = max_src_len
        self.max_tgt_len = max_tgt_len

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

    def __getitem__(self, index: int) -> dict[str, torch.Tensor]:
        src_ids = self.src_tokenizer.encode(
            self.src_texts[index],
            add_special_tokens=True,
            max_length=self.max_src_len,
        )

        target_core = self.tgt_tokenizer.encode(
            self.tgt_texts[index],
            add_special_tokens=False,
            max_length=max(1, self.max_tgt_len - 1),
        )
        tgt_input_ids = [self.tgt_tokenizer.bos_token_id] + target_core
        labels = target_core + [self.tgt_tokenizer.eos_token_id]

        return {
            "src_ids": torch.tensor(src_ids, dtype=torch.long),
            "tgt_input_ids": torch.tensor(tgt_input_ids, dtype=torch.long),
            "labels": torch.tensor(labels, dtype=torch.long),
            "src_len": torch.tensor(len(src_ids), dtype=torch.long),
            "tgt_len": torch.tensor(len(labels), dtype=torch.long),
        }


def _translation_to_columns(dataset, src_lang: str, tgt_lang: str):
    def convert(example):
        translation = example["translation"]
        return {"src": translation[src_lang], "tgt": translation[tgt_lang]}

    return dataset.map(convert, remove_columns=dataset.column_names)


def load_wmt_dataset(
    year: str = "19",
    language_pair: str = "zh-en",
    src_lang: str = "en",
    tgt_lang: str = "zh",
    split: str | None = None,
    cache_dir: str | None = None,
):
    """Load WMT zh-en and normalize rows to {'src', 'tgt'}."""
    from datasets import load_dataset

    dataset_name = f"wmt/wmt{year}"
    try:
        dataset = load_dataset(dataset_name, language_pair, split=split, cache_dir=cache_dir)
    except Exception:
        dataset = load_dataset(f"wmt{year}", language_pair, split=split, cache_dir=cache_dir)

    if split is not None:
        return _translation_to_columns(dataset, src_lang, tgt_lang)

    return dataset.map(
        lambda example: {"src": example["translation"][src_lang], "tgt": example["translation"][tgt_lang]},
        remove_columns=next(iter(dataset.values())).column_names,
    )


def load_opus_dataset(
    subset: str = "en-zh",
    src_lang: str = "en",
    tgt_lang: str = "zh",
    split: str | None = None,
    cache_dir: str | None = None,
):
    """Load OPUS-100 and normalize rows to {'src', 'tgt'}."""
    from datasets import load_dataset

    dataset = load_dataset("Helsinki-NLP/opus-100", subset, split=split, cache_dir=cache_dir)
    if split is not None:
        return _translation_to_columns(dataset, src_lang, tgt_lang)

    return dataset.map(
        lambda example: {"src": example["translation"][src_lang], "tgt": example["translation"][tgt_lang]},
        remove_columns=next(iter(dataset.values())).column_names,
    )


def _read_lines(path: str | Path) -> list[str]:
    with Path(path).open("r", encoding="utf-8") as f:
        return [line.rstrip("\n") for line in f]


def load_custom_dataset(
    train_src: str | Path,
    train_tgt: str | Path,
    val_src: str | Path | None = None,
    val_tgt: str | Path | None = None,
    test_src: str | Path | None = None,
    test_tgt: str | Path | None = None,
    preprocessing_config: Mapping | None = None,
) -> dict[str, dict[str, list[str]]]:
    """Load parallel text files and return split dictionaries."""
    preprocessing_config = dict(preprocessing_config or {})

    def load_split(src_path: str | Path, tgt_path: str | Path) -> dict[str, list[str]]:
        src_texts, tgt_texts = preprocess_pipeline(
            _read_lines(src_path),
            _read_lines(tgt_path),
            **preprocessing_config,
        )
        return {"src": src_texts, "tgt": tgt_texts}

    result = {"train": load_split(train_src, train_tgt)}

    if val_src and val_tgt:
        result["validation"] = load_split(val_src, val_tgt)
    if test_src and test_tgt:
        result["test"] = load_split(test_src, test_tgt)

    return result