"""
Navigation resolution utilities.

This module resolves a ``NavSpec`` against the filesystem by expanding glob
patterns and validating that referenced documentation files exist.

---

Notes:
    - Glob resolution is recursive and returns paths in sorted order.
    - Unmatched patterns raise ``FileNotFoundError`` to fail fast on typos.

---
"""

import glob
from collections.abc import Iterable
from pathlib import Path

from docforge.nav.spec import NavSpec


class ResolvedNav:
    """
    Resolved navigation structure.

    A ``ResolvedNav`` represents navigation data after glob patterns have been
    expanded and paths validated against the filesystem.

    Attributes:
        home: Relative path to the documentation home page.
        groups: Mapping of navigation group titles to lists of resolved
            documentation file paths.
    """

    def __init__(
        self,
        home: str | None,
        groups: dict[str, list[Path]],
        docs_root: Path | None = None,
    ) -> None:
        """
        Initialize a ResolvedNav instance.

        Args:
            home (str | None):
                Relative path to the home page within the documentation root.
            groups (dict[str, list[Path]]):
                Mapping of group titles to resolved documentation file paths.
            docs_root (Path | None):
                Root directory of the documentation source files.
        """
        self.home = home
        self.groups = groups
        self._docs_root = docs_root

    def all_files(self) -> Iterable[Path]:
        """
        Iterate over all files referenced by the navigation structure.

        Yields:
            Path:
                A documentation file referenced by the navigation, including
                the home page when defined.

        Raises:
            RuntimeError: If the home page is defined but the documentation
                root is not available for resolution.
        """
        if self.home:
            if self._docs_root is None:
                raise RuntimeError("docs_root is required to resolve home path")
            yield self._docs_root / self.home

        for paths in self.groups.values():
            yield from paths


def resolve_nav(
    spec: NavSpec,
    docs_root: Path,
) -> ResolvedNav:
    """
    Resolve a navigation specification against the filesystem.

    The function expands glob patterns defined in a ``NavSpec`` and verifies
    that referenced documentation files exist within the documentation root.

    Args:
        spec (NavSpec):
            Navigation specification describing documentation layout.
        docs_root (Path):
            Root directory containing documentation Markdown files.

    Returns:
        ResolvedNav:
            A `ResolvedNav` instance containing validated navigation paths.

    Raises:
        FileNotFoundError: If the documentation root does not exist or a
            navigation pattern does not match any files.
    """
    if not docs_root.exists():
        raise FileNotFoundError(docs_root)

    def resolve_pattern(pattern: str) -> list[Path]:
        """
        Resolve a glob pattern relative to the documentation root.

        Args:
            pattern (str):
                Glob pattern used to match documentation files.

        Returns:
            list[Path]:
                A sorted list of matching `Path` objects.

        Raises:
            FileNotFoundError: If the pattern does not match any files.
        """
        full = docs_root / pattern
        matches = sorted(Path(p) for p in glob.glob(str(full), recursive=True))

        if not matches:
            raise FileNotFoundError(pattern)

        return matches

    # Resolve home page
    home: str | None = None
    if spec.home:
        home_path = docs_root / spec.home
        if not home_path.exists():
            raise FileNotFoundError(spec.home)
        home = spec.home

    # Resolve navigation groups
    resolved_groups: dict[str, list[Path]] = {}

    for group, patterns in spec.groups.items():
        files: list[Path] = []
        for pattern in patterns:
            files.extend(resolve_pattern(pattern))
        resolved_groups[group] = files

    return ResolvedNav(
        home=home,
        groups=resolved_groups,
        docs_root=docs_root,
    )