"""Vite manifest reader — maps a source entry to its hashed dist path (Ch05)."""
from __future__ import annotations

import json
from pathlib import Path

from flask import Flask


class ManifestNotFound(RuntimeError):
    """app/static/dist/manifest.json is missing — `npm run build` hasn't been run."""


def _load_manifest(static_dist_dir: Path) -> dict:
    for candidate in (static_dist_dir / ".vite" / "manifest.json", static_dist_dir / "manifest.json"):
        if candidate.exists():
            return json.loads(candidate.read_text())
    raise ManifestNotFound(f"No Vite manifest under {static_dist_dir} — run `npm run build`.")


def register_asset_helper(app: Flask) -> None:
    """Register `asset(name)` as a Jinja global (Chapter 05)."""
    static_dist_dir = Path(app.static_folder) / "dist"

    @app.context_processor
    def inject_asset_helper():
        def asset(name: str) -> str:
            try:
                manifest = _load_manifest(static_dist_dir)
            except ManifestNotFound:
                if app.debug:
                    return f"/static/dist/{name}"  # tolerate an unbuilt dev checkout
                raise
            entry = manifest.get(name)
            if entry is None:
                raise KeyError(f"{name!r} not in Vite manifest.")
            return f"/static/dist/{entry['file']}"

        return {"asset": asset}