"""Dataset registry: decorator registration + name lookup with suggestions.

Registration happens at import time ("import = register"). The registry maps
name -> DatasetProvider; a Dataset is only *materialized* on first use.
"""

import difflib
from typing import Callable, Dict, List, Optional, Union

from .spec import DatasetSpec, FieldSpec

# A provider factory: called once, returns how records become Samples.
ProviderFactory = Callable[[], Union[FieldSpec, Callable, None]]


class DatasetProvider:
    """A registered dataset: metadata + the recipe for converting records."""

    def __init__(self, spec: DatasetSpec, factory: Optional[ProviderFactory] = None):
        self.spec = spec
        self._factory = factory
        self._record_fn = None
        self._resolved = False

    def resolve_record_fn(self) -> Callable:
        """Resolve the record->Sample converter, lazily and exactly once."""
        if not self._resolved:
            result = self._factory() if self._factory else None
            if isinstance(result, FieldSpec):
                from .loader import field_spec_to_record_fn

                self._record_fn = field_spec_to_record_fn(result)
            elif callable(result):
                self._record_fn = result
            elif result is None:
                self._record_fn = field_spec_to_record_fn(FieldSpec())
            else:
                raise TypeError(f'{self.spec.name}: factory must return FieldSpec or callable, got {type(result)}')
            self._resolved = True
        return self._record_fn


class Registry:
    """Minimal dict-like registry with duplicate protection and suggestions."""

    def __init__(self, kind: str):
        self.kind = kind
        self._items: Dict[str, DatasetProvider] = {}

    def register(self, name: str, item: DatasetProvider) -> DatasetProvider:
        if name in self._items:
            raise ValueError(f'{self.kind} {name!r} is already registered')
        self._items[name] = item
        return item

    def get(self, name: str) -> DatasetProvider:
        if name not in self._items:
            suggestions = difflib.get_close_matches(name, self._items.keys(), n=3)
            hint = f" Did you mean: {', '.join(suggestions)}?" if suggestions else ''
            raise KeyError(f'unknown {self.kind} {name!r}.{hint}')
        return self._items[name]

    def names(self) -> List[str]:
        return sorted(self._items)

    def __contains__(self, name: str) -> bool:
        return name in self._items

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


DATASET_REGISTRY = Registry('dataset')


def register_dataset(spec: DatasetSpec):
    """Decorator: register a dataset plugin.

    Usage:
        @register_dataset(DatasetSpec(name='gsm8k', source=...))
        def gsm8k():
            return lambda record: Sample(...)   # or FieldSpec(...), or None
    """

    def decorator(factory: ProviderFactory) -> ProviderFactory:
        DATASET_REGISTRY.register(spec.name, DatasetProvider(spec, factory))
        return factory

    return decorator


def get_dataset_provider(name: str) -> DatasetProvider:
    return DATASET_REGISTRY.get(name)