/**
 * RSS/Atom feed service — fetches and parses RSS/Atom feeds.
 * Uses lightweight XML parsing without heavy dependencies.
 */
import { safeFetch } from '$lib/server/utils/safeFetch.js';

const CACHE_TTL_MS = 15 * 60 * 1000; // 15 minutes
const FETCH_TIMEOUT_MS = 10_000;
const DEFAULT_MAX_ITEMS = 10;

interface CacheEntry {
	readonly data: readonly FeedItem[];
	readonly expiresAt: number;
}

export interface FeedItem {
	readonly title: string;
	readonly link: string;
	readonly pubDate: string;
	readonly summary: string;
}

const cache = new Map<string, CacheEntry>();

function getCached(key: string): readonly FeedItem[] | null {
	const entry = cache.get(key);
	if (!entry) return null;
	if (Date.now() > entry.expiresAt) {
		cache.delete(key);
		return null;
	}
	return entry.data;
}

function setCache(key: string, data: readonly FeedItem[]): void {
	cache.set(key, {
		data,
		expiresAt: Date.now() + CACHE_TTL_MS
	});
}

/**
 * Extract text content between XML tags.
 */
function extractTag(xml: string, tag: string): string {
	// Handle CDATA sections
	const cdataPattern = new RegExp(
		`<${tag}[^>]*>\\s*<!\\[CDATA\\[([\\s\\S]*?)\\]\\]>\\s*</${tag}>`,
		'i'
	);
	const cdataMatch = xml.match(cdataPattern);
	if (cdataMatch) return cdataMatch[1].trim();

	// Handle regular content
	const pattern = new RegExp(`<${tag}[^>]*>([\\s\\S]*?)</${tag}>`, 'i');
	const match = xml.match(pattern);
	if (match) return match[1].trim();

	return '';
}

/**
 * Extract href from Atom link tag.
 */
function extractAtomLink(entryXml: string): string {
	// Look for link with rel="alternate" or no rel
	const altMatch = entryXml.match(/<link[^>]*rel=["']alternate["'][^>]*href=["']([^"']+)["']/i);
	if (altMatch) return altMatch[1];

	const hrefMatch = entryXml.match(/<link[^>]*href=["']([^"']+)["']/i);
	if (hrefMatch) return hrefMatch[1];

	return '';
}

/**
 * Parse RSS 2.0 feed XML.
 */
function parseRss(xml: string, maxItems: number): readonly FeedItem[] {
	const items: FeedItem[] = [];
	const itemRegex = /<item>([\s\S]*?)<\/item>/gi;
	let match: RegExpExecArray | null;

	while ((match = itemRegex.exec(xml)) !== null && items.length < maxItems) {
		const itemXml = match[1];
		items.push({
			title: extractTag(itemXml, 'title') || 'Untitled',
			link: extractTag(itemXml, 'link') || '',
			pubDate: extractTag(itemXml, 'pubDate') || '',
			summary: extractTag(itemXml, 'description') || ''
		});
	}

	return items;
}

/**
 * Parse Atom feed XML.
 */
function parseAtom(xml: string, maxItems: number): readonly FeedItem[] {
	const items: FeedItem[] = [];
	const entryRegex = /<entry>([\s\S]*?)<\/entry>/gi;
	let match: RegExpExecArray | null;

	while ((match = entryRegex.exec(xml)) !== null && items.length < maxItems) {
		const entryXml = match[1];
		items.push({
			title: extractTag(entryXml, 'title') || 'Untitled',
			link: extractAtomLink(entryXml) || '',
			pubDate: extractTag(entryXml, 'published') || extractTag(entryXml, 'updated') || '',
			summary: extractTag(entryXml, 'summary') || extractTag(entryXml, 'content') || ''
		});
	}

	return items;
}

/**
 * Strip HTML tags from a string (for summaries).
 */
function stripHtml(html: string): string {
	return html
		.replace(/<[^>]*>/g, '')
		.replace(/&amp;/g, '&')
		.replace(/&lt;/g, '<')
		.replace(/&gt;/g, '>')
		.replace(/&quot;/g, '"')
		.replace(/&#39;/g, "'")
		.trim();
}

/**
 * Fetch and parse an RSS or Atom feed from a URL.
 */
export async function fetchFeed(feedUrl: string, maxItems?: number): Promise<readonly FeedItem[]> {
	const limit = maxItems ?? DEFAULT_MAX_ITEMS;
	const cacheKey = `${feedUrl}:${limit}`;
	const cached = getCached(cacheKey);
	if (cached) return cached;

	try {
		const response = await safeFetch(feedUrl, {
			timeoutMs: FETCH_TIMEOUT_MS,
			headers: {
				'User-Agent': 'WebAppLauncher/1.0',
				Accept: 'application/rss+xml, application/atom+xml, application/xml, text/xml'
			}
		});

		if (!response.ok) {
			throw new Error(`Feed returned ${response.status}`);
		}

		const xml = await response.text();

		// Detect feed type and parse
		let items: readonly FeedItem[];
		if (xml.includes('<feed') && xml.includes('xmlns="http://www.w3.org/2005/Atom"')) {
			items = parseAtom(xml, limit);
		} else {
			items = parseRss(xml, limit);
		}

		// Strip HTML from summaries
		const cleanItems = items.map((item) => ({
			...item,
			summary: stripHtml(item.summary).substring(0, 500)
		}));

		setCache(cacheKey, cleanItems);
		return cleanItems;
	} catch (err) {
		if (err instanceof DOMException && err.name === 'AbortError') {
			throw new Error('Feed request timed out');
		}
		throw err;
	}
}

/**
 * Clear the RSS feed cache.
 */
export function clearCache(): void {
	cache.clear();
}