"""Streaming dataset — sequence packing and validation dataset."""

from typing import Iterator, List, Dict, Optional

import torch
from torch.utils.data import IterableDataset, DataLoader

from llm_lab.config import DataConfig
from .tokenizer import Tokenizer


class PackedStreamingDataset(IterableDataset):
    """Streaming + sequence packing dataset.

    Why sequence packing?
      - Naive approach: truncate each document to max_seq_len with padding → wastes GPU
      - Sequence packing: concatenate multiple documents to fill max_seq_len → 100% utilization

    How it works:
      Doc1 (300 tokens) + Doc2 (1500 tokens) + Doc3 (248 tokens) = 2048 tokens
      → [Doc1][EOS][Doc2][EOS][Doc3][EOS][... no padding, fits exactly]

    Why streaming?
      - FineWeb-Edu 10B samples: tens of GB even when compressed
      - Full download not feasible on Colab disk limit (~200GB)
      - Streaming: reads from the network only as much as needed

    Notes for training:
      - EOS token inserted at document boundaries so the model recognizes end-of-document
      - EOS naturally serves as a boundary marker without cross-attention masking
    """

    def __init__(
        self,
        tokenizer: Tokenizer,
        config: DataConfig,
        split: str = "train",
        seed: int = 42,
    ):
        super().__init__()
        self.tokenizer = tokenizer
        self.config = config
        self.split = split
        self.seed = seed
        self.max_seq_len = config.max_seq_len

    def _load_dataset(self, num_shards: int = 1, shard_index: int = 0):
        """Loads the HuggingFace dataset in streaming mode.

        Args:
            num_shards: Total number of shards (= DataLoader num_workers)
            shard_index: The shard index this worker is responsible for (0 ~ num_shards-1)

        Sharding principle:
            With num_shards=4, the stream is split into 4 equal parts so each worker
            processes a distinct 1/4. Shuffling is applied after sharding so there is
            no document overlap between workers.
        """
        from datasets import load_dataset

        ds = load_dataset(
            self.config.dataset_name,
            name=self.config.dataset_subset,
            split=self.split,
            streaming=True,         # Key: streaming mode
            trust_remote_code=True,
        )

        # Full partitioning (sharding): worker i processes only 1/num_shards of the stream
        # Must be applied before shuffling so each worker has a non-overlapping set of documents
        if num_shards > 1:
            ds = ds.shard(num_shards=num_shards, index=shard_index)

        # Shuffle (approximate buffer-based shuffle in streaming mode)
        ds = ds.shuffle(seed=self.seed, buffer_size=10_000)

        return ds

    def _tokenize_and_pack(self, dataset) -> Iterator[Dict[str, torch.Tensor]]:
        """Tokenizes documents and packs them into sequences.

        Yields:
            {"input_ids": (max_seq_len,), "targets": (max_seq_len,)}

        targets = input_ids shifted by one position:
            input_ids:  [A, B, C, D, E]
            targets:    [B, C, D, E, F]
            → The model sees A and predicts B, sees B and predicts C, ...
        """
        buffer: List[int] = []  # Token buffer

        for example in dataset:
            text = example[self.config.text_column]
            if not text or not text.strip():
                continue

            # Tokenize (without special tokens)
            token_ids = self.tokenizer.encode(text, add_special_tokens=False)

            if not token_ids:
                continue

            # Append EOS token (marks document boundary)
            if self.config.use_eos_separator:
                token_ids.append(self.tokenizer.eos_id)

            # Add to buffer
            buffer.extend(token_ids)

            # Generate sequences once the buffer is full enough
            # +1 is needed to generate targets (input + next token)
            while len(buffer) >= self.max_seq_len + 1:
                # Extract max_seq_len + 1 tokens
                chunk = buffer[: self.max_seq_len + 1]
                buffer = buffer[self.max_seq_len + 1 :]

                # input_ids: from the first to the second-to-last token
                input_ids = torch.tensor(chunk[:-1], dtype=torch.long)
                # targets: from the second to the last token (shifted by one)
                targets = torch.tensor(chunk[1:], dtype=torch.long)

                yield {"input_ids": input_ids, "targets": targets}

    def __iter__(self) -> Iterator[Dict[str, torch.Tensor]]:
        """Iterator called by DataLoader.

        Multi-worker support (full partitioning approach):
          - Previous: all workers read the same stream with different seeds → possible document duplication
          - Improved: ds.shard() splits the stream into num_workers parts → no document overlap between workers

          Example (num_workers=4, total N documents):
            Worker 0: docs 0, 4, 8,  12, ...  (N/4 docs)
            Worker 1: docs 1, 5, 9,  13, ...  (N/4 docs)
            Worker 2: docs 2, 6, 10, 14, ...  (N/4 docs)
            Worker 3: docs 3, 7, 11, 15, ...  (N/4 docs)
        """
        worker_info = torch.utils.data.get_worker_info()

        if worker_info is not None:
            # Full partitioning: assign a shard per worker + independent shuffle seed
            num_shards = worker_info.num_workers
            shard_index = worker_info.id
            worker_seed = self.seed + worker_info.id
        else:
            # Single process: process the full stream without sharding
            num_shards = 1
            shard_index = 0
            worker_seed = self.seed

        self.seed = worker_seed
        dataset = self._load_dataset(num_shards=num_shards, shard_index=shard_index)

        return self._tokenize_and_pack(dataset)


class ValidationDataset:
    """Validation dataset.

    Pre-fetches a fixed amount of data from the streaming dataset and stores it in memory.
    Consistent data across evaluations is necessary for meaningful comparisons between epochs.
    """

    def __init__(
        self,
        tokenizer: Tokenizer,
        config: DataConfig,
        num_samples: int = 100,
        seed: int = 9999,
    ):
        self.tokenizer = tokenizer
        self.config = config
        self.num_samples = num_samples
        self.samples: List[Dict[str, torch.Tensor]] = []

        self._prepare(seed)

    def _prepare(self, seed: int):
        """Pre-extracts validation samples from the dataset."""
        from datasets import load_dataset

        print(f"[Validation] Preparing {self.num_samples} validation samples...")

        ds = load_dataset(
            self.config.dataset_name,
            name=self.config.dataset_subset,
            split=self.config.dataset_split,
            streaming=True,
            trust_remote_code=True,
        )
        # Use a different seed and skip the beginning to avoid overlap with training data
        ds = ds.shuffle(seed=seed, buffer_size=5_000)

        buffer: List[int] = []
        count = 0

        for example in ds:
            if count >= self.num_samples:
                break

            text = example[self.config.text_column]
            if not text or not text.strip():
                continue

            token_ids = self.tokenizer.encode(text, add_special_tokens=False)
            if not token_ids:
                continue

            token_ids.append(self.tokenizer.eos_id)
            buffer.extend(token_ids)

            while len(buffer) >= self.config.max_seq_len + 1 and count < self.num_samples:
                chunk = buffer[: self.config.max_seq_len + 1]
                buffer = buffer[self.config.max_seq_len + 1 :]

                self.samples.append({
                    "input_ids": torch.tensor(chunk[:-1], dtype=torch.long),
                    "targets": torch.tensor(chunk[1:], dtype=torch.long),
                })
                count += 1

        print(f"[Validation] {len(self.samples)} samples ready")

    def get_dataloader(self, batch_size: int) -> DataLoader:
        """Returns a validation DataLoader."""
        return DataLoader(
            self.samples,
            batch_size=batch_size,
            shuffle=False,
            num_workers=0,
            collate_fn=_collate_fn,
        )


class MixedStreamingDataset(IterableDataset):
    """Interleaves multiple streaming datasets by sampling weight.

    Used for Continual Pre-Training (CPT) to mix domain-specific data
    (e.g., code) with general data to prevent catastrophic forgetting.

    How it works:
      - Maintains one PackedStreamingDataset per source
      - At each yield, randomly selects a source according to mix_weights
      - Each source independently packs and tokenizes its own stream

    Example (Code CPT):
      sources = [FineWeb-Edu (general), StarCoder (Python)]
      weights = [0.2, 0.8]  →  20% general, 80% code
    """

    def __init__(
        self,
        tokenizer: "Tokenizer",
        config: "DataConfig",
        seed: int = 42,
    ):
        super().__init__()
        self.tokenizer = tokenizer
        self.config = config
        self.seed = seed
        self.max_seq_len = config.max_seq_len

        # Build dataset specs: primary + secondary datasets
        self.dataset_specs = [
            {
                "name": config.dataset_name,
                "subset": config.dataset_subset,
                "split": config.dataset_split,
                "text_column": config.text_column,
            }
        ] + list(config.mix_datasets)

        self.weights = config.mix_weights
        assert len(self.weights) == len(self.dataset_specs), (
            f"mix_weights length ({len(self.weights)}) must match "
            f"number of datasets ({len(self.dataset_specs)})"
        )
        assert abs(sum(self.weights) - 1.0) < 1e-6, (
            f"mix_weights must sum to 1.0, got {sum(self.weights)}"
        )

    def _load_single_dataset(self, spec: dict, num_shards: int, shard_index: int, seed: int):
        """Loads a single HuggingFace dataset in streaming mode."""
        from datasets import load_dataset

        ds = load_dataset(
            spec["name"],
            name=spec["subset"],
            split=spec["split"],
            streaming=True,
            trust_remote_code=True,
        )
        if num_shards > 1:
            ds = ds.shard(num_shards=num_shards, index=shard_index)
        ds = ds.shuffle(seed=seed, buffer_size=10_000)
        return ds

    def _token_stream(self, dataset, text_column: str) -> Iterator[int]:
        """Yields tokens one-by-one from a dataset stream (with EOS separators)."""
        for example in dataset:
            text = example[text_column]
            if not text or not text.strip():
                continue
            token_ids = self.tokenizer.encode(text, add_special_tokens=False)
            if not token_ids:
                continue
            if self.config.use_eos_separator:
                token_ids.append(self.tokenizer.eos_id)
            yield from token_ids

    def _pack_from_stream(self, token_iter: Iterator[int]) -> Iterator[List[int]]:
        """Packs token stream into max_seq_len+1 chunks."""
        buffer: List[int] = []
        for tok in token_iter:
            buffer.append(tok)
            if len(buffer) >= self.max_seq_len + 1:
                yield buffer[: self.max_seq_len + 1]
                buffer = buffer[self.max_seq_len + 1 :]

    def __iter__(self) -> Iterator[Dict[str, torch.Tensor]]:
        import random

        worker_info = torch.utils.data.get_worker_info()
        if worker_info is not None:
            num_shards = worker_info.num_workers
            shard_index = worker_info.id
            worker_seed = self.seed + worker_info.id
        else:
            num_shards = 1
            shard_index = 0
            worker_seed = self.seed

        rng = random.Random(worker_seed)

        # Create a packed-sequence iterator for each dataset source
        source_iters = []
        for i, spec in enumerate(self.dataset_specs):
            ds = self._load_single_dataset(
                spec, num_shards, shard_index, seed=worker_seed + i * 1000
            )
            token_iter = self._token_stream(ds, spec["text_column"])
            pack_iter = self._pack_from_stream(token_iter)
            source_iters.append(pack_iter)

        # Pre-fetch one chunk from each source (None = exhausted)
        buffers: List[Optional[List[int]]] = [None] * len(source_iters)
        for i, it in enumerate(source_iters):
            try:
                buffers[i] = next(it)
            except StopIteration:
                buffers[i] = None

        # Weighted round-robin sampling
        while any(b is not None for b in buffers):
            # Build active weights (zero out exhausted sources)
            active_weights = [
                w if buffers[i] is not None else 0.0
                for i, w in enumerate(self.weights)
            ]
            total = sum(active_weights)
            if total == 0:
                break

            # Weighted random selection
            r = rng.random() * total
            cumulative = 0.0
            chosen = 0
            for i, w in enumerate(active_weights):
                cumulative += w
                if r <= cumulative:
                    chosen = i
                    break

            chunk = buffers[chosen]
            input_ids = torch.tensor(chunk[:-1], dtype=torch.long)
            targets = torch.tensor(chunk[1:], dtype=torch.long)
            yield {"input_ids": input_ids, "targets": targets}

            # Refill the chosen source
            try:
                buffers[chosen] = next(source_iters[chosen])
            except StopIteration:
                buffers[chosen] = None


def _collate_fn(batch: List[Dict[str, torch.Tensor]]) -> Dict[str, torch.Tensor]:
    """Combines samples in a batch into a single tensor.

    Because of sequence packing, all samples have the same length (max_seq_len),
    so no additional padding is needed.
    """
    return {
        "input_ids": torch.stack([s["input_ids"] for s in batch]),
        "targets": torch.stack([s["targets"] for s in batch]),
    }