// ---------------------------------------------------------------------------
// Ordner: lib
// Datei: storage.js
// Relativer Pfad: lib/storage.js
// ---------------------------------------------------------------------------
// lib/storage.js
// -----------------------------------------------------------------------------
// Central abstraction layer for reading files and directories from the NAS
// share mounted at `NAS_ROOT_PATH` (e.g. `/mnt/niederlassungen`).
//
// All access to the branch/year/month/day/PDF structure should go through
// these functions instead of using `fs` directly in route handlers.
//
// - Read-only: no write/delete operations here.
// - Async only: uses `fs/promises` + async/await to avoid blocking the event loop.
//
// RHL-006 (Caching / Freshness):
// - We add a small process-local TTL micro-cache for directory listings.
// - Goal: reduce filesystem load while keeping freshness predictable.
// - Security note: RBAC is enforced in API routes BEFORE calling storage helpers,
//   therefore caching here does not bypass auth/permissions.
// -----------------------------------------------------------------------------

import fs from "node:fs/promises"; // Promise-based filesystem API
import path from "node:path"; // Safe path utilities (handles separators)

// Root directory of the NAS share, injected via environment variable.
// On the Linux app server, this is typically `/mnt/niederlassungen`.
// Do NOT cache process.env.NAS_ROOT_PATH at module load time.
// Instead, resolve it on demand so tests (and runtime) can change it.
function getRoot() {
	const root = process.env.NAS_ROOT_PATH;

	if (!root) {
		throw new Error("NAS_ROOT_PATH environment variable is not set");
	}

	return root;
}

// Build an absolute path below the NAS root from a list of segments.
function fullPath(...segments) {
	const root = getRoot();
	return path.join(root, ...segments.map(String));
}

// Compare strings that represent numbers in a numeric way.
// This ensures "2" comes before "10" (2 < 10), not after.
function sortNumericStrings(a, b) {
	const na = parseInt(a, 10);
	const nb = parseInt(b, 10);

	if (!Number.isNaN(na) && !Number.isNaN(nb)) {
		return na - nb;
	}

	// Fallback to localeCompare if parsing fails
	return a.localeCompare(b, "en");
}

// -----------------------------------------------------------------------------
// RHL-006: Storage micro-cache (process-local TTL cache)
// -----------------------------------------------------------------------------
//
// Why a cache here (and not Next route caching)?
// - We want to avoid any risk of shared caching across users/sessions.
// - Next route caching / ISR-style caching is powerful but easy to misuse with auth.
// - A micro-cache AFTER RBAC checks is safe and predictable.
//
// Important constraints:
// - Process-local only: if we ever run multiple instances, caches are not shared.
// - Short TTL only: we accept a small window where newly scanned PDFs might not
//   appear immediately, but they will appear after TTL expires.
// - Failure-safe: if a filesystem read throws, we do NOT keep a "poisoned" cache entry.
// -----------------------------------------------------------------------------

// TTLs chosen in the design (accepted by you):
// - Branches/Years change rarely -> 60 seconds
// - Months/Days/Files can change frequently -> 15 seconds
const TTL_BRANCHES_MS = 60_000;
const TTL_YEARS_MS = 60_000;
const TTL_MONTHS_MS = 15_000;
const TTL_DAYS_MS = 15_000;
const TTL_FILES_MS = 15_000;

// Internal cache store:
// key -> { expiresAt, value } OR { expiresAt, promise }
// - value: resolved cache value
// - promise: in-flight load promise to collapse concurrent reads
const __storageCache = new Map();

/**
 * Build a stable cache key for a given listing type.
 *
 * We include NAS_ROOT_PATH in the key so tests that change the env var do not
 * accidentally reuse data from a previous test run.
 *
 * @param {string} type
 * @param  {...string} parts
 * @returns {string}
 */
function buildCacheKey(type, ...parts) {
	const root = getRoot();
	return [type, root, ...parts.map(String)].join("|");
}

/**
 * Generic TTL-cache wrapper.
 *
 * Behavior:
 * 1) If a load is already in-flight (promise exists), reuse it.
 * 2) If a cached value exists and is not expired, return it.
 * 3) Otherwise run loader(), store the result, and return it.
 *
 * Failure policy:
 * - If loader() throws, the cache entry is removed so later calls can retry.
 *
 * @template T
 * @param {string} key
 * @param {number} ttlMs
 * @param {() => Promise<T>} loader
 * @returns {Promise<T>}
 */
async function withTtlCache(key, ttlMs, loader) {
	const now = Date.now();
	const existing = __storageCache.get(key);

	// 1) Collapsing concurrent calls:
	// If another request already triggered the same filesystem read,
	// we reuse the same promise to avoid redundant fs operations.
	if (existing?.promise) {
		return existing.promise;
	}

	// 2) Serve cached values while still fresh:
	if (existing && existing.value !== undefined && existing.expiresAt > now) {
		return existing.value;
	}

	// 3) Cache miss or expired: start a new load.
	const promise = (async () => {
		try {
			const value = await loader();

			// Store resolved value with a fresh expiry timestamp.
			__storageCache.set(key, {
				value,
				expiresAt: Date.now() + ttlMs,
			});

			return value;
		} catch (err) {
			// Do not keep failed results in cache.
			__storageCache.delete(key);
			throw err;
		}
	})();

	// Store in-flight promise immediately so concurrent calls reuse it.
	__storageCache.set(key, {
		promise,
		expiresAt: now + ttlMs,
	});

	return promise;
}

/**
 * TEST-ONLY helper: clear the micro-cache.
 *
 * Why this exists:
 * - Unit tests often mutate the filesystem fixture after calling list*() once.
 * - Without a cache reset, tests could observe stale values.
 *
 * We intentionally export this with a loud name to discourage production usage.
 */
export function __clearStorageCacheForTests() {
	__storageCache.clear();
}

// -----------------------------------------------------------------------------
// 1. Branches (NL01, NL02, ...)
// Path pattern: `${ROOT}/NLxx`
// -----------------------------------------------------------------------------

export async function listBranches() {
	// RHL-006: cache directory listing for 60 seconds (branches change rarely).
	return withTtlCache(buildCacheKey("branches"), TTL_BRANCHES_MS, async () => {
		// Read the root directory of the NAS share.
		// `withFileTypes: true` returns `Dirent` objects so we can call `isDirectory()`
		// without extra stat() calls, which is more efficient.
		const entries = await fs.readdir(fullPath(), { withFileTypes: true });

		return (
			entries
				.filter(
					(entry) =>
						entry.isDirectory() && // only directories
						entry.name !== "@Recently-Snapshot" && // ignore QNAP snapshot folder
						/^NL\d+$/i.test(entry.name) // keep only names like "NL01", "NL02", ...
				)
				.map((entry) => entry.name)
				// Sort by numeric branch number: NL1, NL2, ..., NL10
				.sort((a, b) =>
					sortNumericStrings(a.replace("NL", ""), b.replace("NL", ""))
				)
		);
	});
}

// -----------------------------------------------------------------------------
// 2. Years (2023, 2024, ...)
// Path pattern: `${ROOT}/${branch}/${year}`
// -----------------------------------------------------------------------------

export async function listYears(branch) {
	// RHL-006: cache directory listing for 60 seconds (years change rarely).
	return withTtlCache(
		buildCacheKey("years", branch),
		TTL_YEARS_MS,
		async () => {
			const dir = fullPath(branch);
			const entries = await fs.readdir(dir, { withFileTypes: true });

			return entries
				.filter(
					(entry) => entry.isDirectory() && /^\d{4}$/.test(entry.name) // exactly 4 digits → year folders like "2024"
				)
				.map((entry) => entry.name)
				.sort(sortNumericStrings);
		}
	);
}

// -----------------------------------------------------------------------------
// 3. Months (01–12)
// Path pattern: `${ROOT}/${branch}/${year}/${month}`
// -----------------------------------------------------------------------------

export async function listMonths(branch, year) {
	// RHL-006: cache directory listing for 15 seconds (months can change occasionally).
	return withTtlCache(
		buildCacheKey("months", branch, year),
		TTL_MONTHS_MS,
		async () => {
			const dir = fullPath(branch, year);
			const entries = await fs.readdir(dir, { withFileTypes: true });

			return (
				entries
					.filter(
						(entry) => entry.isDirectory() && /^\d{1,2}$/.test(entry.name) // supports "1" or "10", we normalize below
					)
					// Normalize to two digits so the UI shows "01", "02", ..., "12"
					.map((entry) => entry.name.trim().padStart(2, "0"))
					.sort(sortNumericStrings)
			);
		}
	);
}

// -----------------------------------------------------------------------------
// 4. Days (01–31)
// Path pattern: `${ROOT}/${branch}/${year}/${month}/${day}`
// -----------------------------------------------------------------------------

export async function listDays(branch, year, month) {
	// RHL-006: cache directory listing for 15 seconds (days change frequently with new scans).
	return withTtlCache(
		buildCacheKey("days", branch, year, month),
		TTL_DAYS_MS,
		async () => {
			const dir = fullPath(branch, year, month);
			const entries = await fs.readdir(dir, { withFileTypes: true });

			return entries
				.filter(
					(entry) => entry.isDirectory() && /^\d{1,2}$/.test(entry.name) // supports "1" or "23"
				)
				.map((entry) => entry.name.trim().padStart(2, "0"))
				.sort(sortNumericStrings);
		}
	);
}

// -----------------------------------------------------------------------------
// 5. Files (PDFs) for a given day
// Path pattern: `${ROOT}/${branch}/${year}/${month}/${day}/<file>.pdf`
// -----------------------------------------------------------------------------

export async function listFiles(branch, year, month, day) {
	// RHL-006: cache file listing for 15 seconds (new PDFs can appear at any time).
	return withTtlCache(
		buildCacheKey("files", branch, year, month, day),
		TTL_FILES_MS,
		async () => {
			const dir = fullPath(branch, year, month, day);
			const entries = await fs.readdir(dir);

			return (
				entries
					// We only care about PDF files at the moment
					.filter((name) => name.toLowerCase().endsWith(".pdf"))
					.sort((a, b) => a.localeCompare(b, "en"))
					.map((name) => ({
						// Just the file name, e.g. "Stapel-1_Seiten-1_Zeit-1048.pdf"
						name,
						// Relative path from the NAS root, used for download URLs etc.
						// Example: "NL01/2024/10/23/Stapel-1_Seiten-1_Zeit-1048.pdf"
						relativePath: `${branch}/${year}/${month}/${day}/${name}`,
					}))
			);
		}
	);
}
// ---------------------------------------------------------------------------