'use strict';

import fs from 'node:fs';
import {createHash} from 'node:crypto';
import {decompressPool} from './decompress.js';
import {fuzzyMatch, titleFromUrl, kiwixSearch} from './utils.js';

const HEADER_SIZE = 80;
const NS_CONTENT = 'C';
const NS_METADATA = 'M';
const TITLE_SENTINEL = 0xffffffffffffffffn; // Indicator -> ZIM v6+ archives with no title

const DEFAULT_CLUSTER_CACHE_MAX = 32;
const DEFAULT_CLUSTER_TTL = 60_000;

// --- Fallback search index (only used for archives with no native title listing) ---
const INDEX_SUFFIX = '.searchidx.bin';
const INDEX_MAGIC = 'ZXI1';
const INDEX_HEADER_SIZE = 24;     // magic(4) + staleness key(16) + recordCount(4)
const INDEX_RECORD_SIZE = 20;     // keyOff(4) keyLen(2) urlOff(4) urlLen(2) titleOff(4) titleLen(2) mimetype(2)

/** Native, dependency-light reader for .zim archives. Supports zstd & LZMA cluster compression. */
export class ZimReader {
	#fd = null;
	#header = null;
	#mimeTypes = [];
	#hasTitleListing = false;
	#clusterCache = new Map(); // clusterNumber -> {data, extended, timer}
	#pending = new Map();      // clusterNumber -> Promise, dedupes concurrent misses
	#clusterCacheMax;
	#clusterTTL;

	#indexPath;
	#indexReady = null;   // Promise, awaited by search() before using the fallback index
	#indexFd = null;      // open fd for the fallback index, once loaded/built
	#indexRecordCount = 0;
	#indexTableStart = 0;

	get articleCount() { return this.#header?.articleCount ?? 0; }
	get mediaCount() { return this.#header?.clusterCount ?? 0; }

	/** @param {{clusterCacheMax?: number, clusterTTL?: number}} [opts] clusterTTL in ms; 0/null disables idle eviction. */
	constructor(path, {clusterCacheMax = DEFAULT_CLUSTER_CACHE_MAX, clusterTTL = DEFAULT_CLUSTER_TTL} = {}) {
		this.path = path;
		this.#clusterCacheMax = clusterCacheMax;
		this.#clusterTTL = clusterTTL;
		this.#indexPath = `${path}${INDEX_SUFFIX}`;
	}

	/** Binary search the URL pointer list for namespace+url. For exact-key lookups (readPage, metadata, icons). */
	async #findByUrl(url, namespace) {
		const key = namespace + url;
		let lo = 0, hi = this.#header.articleCount - 1;
		while (lo <= hi) {
			const mid = (lo + hi) >> 1;
			const dirent = await this.#readDirent(await this.#ptr64(this.#header.urlPtrPos, mid));
			const dirKey = dirent.namespace + dirent.url;
			const cmp = key < dirKey ? -1 : key > dirKey ? 1 : 0;
			if (cmp === 0) return dirent;
			if (cmp < 0) hi = mid - 1; else lo = mid + 1;
		}
		return null;
	}

	/** Binary search the ZIM's own title pointer list. O(log n), zero extra storage - the happy path. */
	async #findByTitleBuiltin(title) {
		let lo = 0, hi = this.#header.articleCount - 1;
		while (lo <= hi) {
			const mid = (lo + hi) >> 1;
			const urlIdx = (await this.#read(this.#header.titlePtrPos + mid * 4, 4)).readUInt32LE(0);
			const d = await this.#readDirent(await this.#ptr64(this.#header.urlPtrPos, urlIdx));
			if (d.title === title) return d;
			if (d.title < title) lo = mid + 1; else hi = mid - 1;
		}
		return null;
	}

	/**
	 * Resolves a kiwix-search result (a title, not necessarily a url) to
	 * {url, title, mimetype}. kiwix-search's fulltext index returns titles, and
	 * a title isn't guaranteed to equal its url (unicode normalization,
	 * disambiguation suffixes, punctuation stripping), so:
	 *   1. URL binary search    - matches when title happens to equal url (free to check)
	 *   2. Title pointer list   - when the archive ships one (Wikipedia etc. do)
	 *   3. Persisted fallback index / linear scan - only for archives without (2)
	 */
	async #resolveSearchEntry(name) {
		let dirent = await this.#findByUrl(name, NS_CONTENT);
		if (dirent) return dirent;

		if (this.#hasTitleListing) return this.#findByTitleBuiltin(name);

		if (this.#indexReady) await this.#indexReady;
		const hit = await this.#lookupFallbackIndex(name);
		if (hit) return hit;

		// Index unavailable (build failed - unwritable disk, etc.) or genuinely no match.
		for (let i = 0; i < this.#header.articleCount; i++) {
			const d = await this.#readDirent(await this.#ptr64(this.#header.urlPtrPos, i));
			if (d.namespace === NS_CONTENT && (d.title === name || titleFromUrl(d.url) === name)) return d;
		}
		return null;
	}

	/**
	 * A cheap, stable fingerprint for "is the cached index still valid for this file".
	 * ZIM archives end with a 16-byte MD5 of their own contents (the same trailer
	 * `zimcheck` validates against), so this is a single 16-byte read regardless of
	 * archive size - no need to hash a multi-hundred-GB file. It's also
	 * content-based rather than path/mtime-based, so moving or redownloading an
	 * identical archive doesn't invalidate the cache. Falls back to a tiny
	 * size+mtime hash only if the file is too short to have a real trailer.
	 */
	async #stalenessKey() {
		const {size} = await fs.promises.stat(this.path);
		if (size >= 16) return this.#read(size - 16, 16);
		const stat = await fs.promises.stat(this.path);
		return createHash('md5').update(`${stat.size}:${stat.mtimeMs}`).digest();
	}

	/** Walks every content dirent once, resolving redirects, keyed by both its title and its url-derived title. */
	async #collectFallbackEntries() {
		const map = new Map(); // key -> {url, title, mimetype}
		for (let i = 0; i < this.#header.articleCount; i++) {
			let dirent = await this.#readDirent(await this.#ptr64(this.#header.urlPtrPos, i));
			if (dirent.namespace !== NS_CONTENT) continue;
			dirent = await this.#resolveRedirect(dirent);
			if (dirent.namespace !== NS_CONTENT) continue; // redirected outside content namespace

			const entry = {url: dirent.url, title: dirent.title, mimetype: dirent.mimetype};
			if (!map.has(dirent.title)) map.set(dirent.title, entry);
			const urlTitle = titleFromUrl(dirent.url);
			if (urlTitle !== dirent.title && !map.has(urlTitle)) map.set(urlTitle, entry);
		}
		return map;
	}

	/** Serializes the fallback index to disk: header, sorted fixed-width records, then a string table. */
	async #buildFallbackIndex(stalenessKey) {
		const map = await this.#collectFallbackEntries();
		const keys = [...map.keys()].sort();

		const records = Buffer.alloc(keys.length * INDEX_RECORD_SIZE);
		const strings = [];
		let tableOffset = 0;
		keys.forEach((key, i) => {
			const {url, title, mimetype} = map.get(key);
			const keyBuf = Buffer.from(key, 'utf8');
			const urlBuf = Buffer.from(url, 'utf8');
			const titleBuf = Buffer.from(title, 'utf8');
			const base = i * INDEX_RECORD_SIZE;

			records.writeUInt32LE(tableOffset, base); records.writeUInt16LE(keyBuf.length, base + 4);
			tableOffset += keyBuf.length;
			records.writeUInt32LE(tableOffset, base + 6); records.writeUInt16LE(urlBuf.length, base + 10);
			tableOffset += urlBuf.length;
			records.writeUInt32LE(tableOffset, base + 12); records.writeUInt16LE(titleBuf.length, base + 16);
			tableOffset += titleBuf.length;
			records.writeUInt16LE(mimetype, base + 18);

			strings.push(keyBuf, urlBuf, titleBuf);
		});

		const header = Buffer.alloc(INDEX_HEADER_SIZE);
		header.write(INDEX_MAGIC, 0, 'ascii');
		stalenessKey.copy(header, 4);
		header.writeUInt32LE(keys.length, 20);

		const tmpPath = `${this.#indexPath}.tmp-${process.pid}`;
		await fs.promises.writeFile(tmpPath, Buffer.concat([header, records, ...strings]));
		await fs.promises.rename(tmpPath, this.#indexPath);
	}

	/** Opens the fallback index file and caches its header fields for querying. */
	async #openFallbackIndex() {
		const fd = await fs.promises.open(this.#indexPath, 'r');
		const header = Buffer.alloc(INDEX_HEADER_SIZE);
		await fd.read(header, 0, INDEX_HEADER_SIZE, 0);
		this.#indexFd = fd;
		this.#indexRecordCount = header.readUInt32LE(20);
		this.#indexTableStart = INDEX_HEADER_SIZE + this.#indexRecordCount * INDEX_RECORD_SIZE;
	}

	/**
	 * Validates the on-disk fallback index against the archive's current staleness
	 * key and (re)builds it if missing/stale. Failures are swallowed - callers see
	 * #indexFd stay null and fall through to the linear-scan last resort.
	 */
	async #loadOrBuildFallbackIndex() {
		const stalenessKey = await this.#stalenessKey();

		try {
			const fd = await fs.promises.open(this.#indexPath, 'r');
			const header = Buffer.alloc(INDEX_HEADER_SIZE);
			await fd.read(header, 0, INDEX_HEADER_SIZE, 0);
			const fresh = header.toString('ascii', 0, 4) === INDEX_MAGIC && header.subarray(4, 20).equals(stalenessKey);
			if (fresh) {
				this.#indexFd = fd;
				this.#indexRecordCount = header.readUInt32LE(20);
				this.#indexTableStart = INDEX_HEADER_SIZE + this.#indexRecordCount * INDEX_RECORD_SIZE;
				return;
			}
			await fd.close();
		} catch { /* missing, corrupt, or unreadable -> rebuild below */ }

		await this.#buildFallbackIndex(stalenessKey);
		await this.#openFallbackIndex();
	}

	async #indexRead(pos, length) {
		const buf = Buffer.alloc(length);
		await this.#indexFd.read(buf, 0, length, pos);
		return buf;
	}

	async #indexString(offset, length) {
		if (!length) return '';
		return (await this.#indexRead(this.#indexTableStart + offset, length)).toString('utf8');
	}

	/** Binary search over the on-disk record array. Never loads the full index into memory. */
	async #lookupFallbackIndex(name) {
		if (!this.#indexFd) return null;
		let lo = 0, hi = this.#indexRecordCount - 1;
		while (lo <= hi) {
			const mid = (lo + hi) >> 1;
			const rec = await this.#indexRead(INDEX_HEADER_SIZE + mid * INDEX_RECORD_SIZE, INDEX_RECORD_SIZE);
			const keyOff = rec.readUInt32LE(0), keyLen = rec.readUInt16LE(4);
			const key = await this.#indexString(keyOff, keyLen);
			if (key === name) {
				const urlOff = rec.readUInt32LE(6), urlLen = rec.readUInt16LE(10);
				const titleOff = rec.readUInt32LE(12), titleLen = rec.readUInt16LE(16);
				const [url, title] = await Promise.all([this.#indexString(urlOff, urlLen), this.#indexString(titleOff, titleLen)]);
				return {url, title, mimetype: rec.readUInt16LE(18)};
			}
			if (key < name) lo = mid + 1; else hi = mid - 1;
		}
		return null;
	}

	/** Resets a cluster's idle-eviction timer. No-op when TTL disabled. */
	#touch(clusterNumber, entry) {
		if (!this.#clusterTTL) return;
		clearTimeout(entry.timer);
		entry.timer = setTimeout(() => this.#clusterCache.delete(clusterNumber), this.#clusterTTL).unref();
	}

	/** Fetches + decompresses a cluster exactly once, offloading decompression to the worker pool. */
	async #loadCluster(clusterNumber) {
		const start = await this.#ptr64(this.#header.clusterPtrPos, clusterNumber);
		const isLast = clusterNumber === this.#header.clusterCount - 1;
		const end = isLast
			? (await fs.promises.stat(this.path)).size
			: await this.#ptr64(this.#header.clusterPtrPos, clusterNumber + 1);

		const raw = await this.#read(start, end - start);
		const compType = raw[0] & 0x0f;
		const extended = (raw[0] & 0x10) !== 0;
		const body = raw.subarray(1);

		let data;
		if (compType <= 1) data = Buffer.from(body);
		else if (compType === 4 || compType === 5) data = await decompressPool.run({compType, body});
		else throw new Error(`Unsupported cluster compression type: ${compType}`);
		if (!data) throw new Error(`Cluster ${clusterNumber} failed to decompress (compType ${compType})`);

		return {data, extended};
	}

	async #getBlob(clusterNumber, blobNumber) {
		let entry = this.#clusterCache.get(clusterNumber);
		if (!entry) {
			let pending = this.#pending.get(clusterNumber);
			if (!pending) {
				pending = this.#loadCluster(clusterNumber);
				this.#pending.set(clusterNumber, pending);
			}
			entry = await pending;
			this.#pending.delete(clusterNumber);
			this.#clusterCache.set(clusterNumber, entry);
			if (this.#clusterCache.size > this.#clusterCacheMax) {
				const oldestKey = this.#clusterCache.keys().next().value;
				clearTimeout(this.#clusterCache.get(oldestKey)?.timer);
				this.#clusterCache.delete(oldestKey);
			}
		}
		this.#touch(clusterNumber, entry);

		const readPtr = i => entry.extended ? Number(entry.data.readBigUInt64LE(i * 8)) : entry.data.readUInt32LE(i * 4);
		return entry.data.subarray(readPtr(blobNumber), readPtr(blobNumber + 1));
	}

	async #icon(size = 48) {
		const page = await this.readPage(`Illustration_${size}x${size}@1`, NS_METADATA)
			|| await this.readPage('Favicon', NS_METADATA);
		if (!page) return null;
		return `data:${page.mimetype};base64,${page.data.toString('base64')}`;
	}

	async #ptr64(base, index) {
		return Number((await this.#read(base + index * 8, 8)).readBigUInt64LE(0));
	}

	async #read(pos, length) {
		const buf = Buffer.alloc(length);
		await this.#fd.read(buf, 0, length, pos);
		return buf;
	}

	async #readHeader() {
		const b = await this.#read(0, HEADER_SIZE);
		const titlePtrRaw = b.readBigUInt64LE(40);
		this.#hasTitleListing = titlePtrRaw !== TITLE_SENTINEL;
		this.#header = {
			articleCount: b.readUInt32LE(24),
			clusterCount: b.readUInt32LE(28),
			urlPtrPos: Number(b.readBigUInt64LE(32)),
			titlePtrPos: this.#hasTitleListing ? Number(titlePtrRaw) : null,
			clusterPtrPos: Number(b.readBigUInt64LE(48)),
			mimeListPos: Number(b.readBigUInt64LE(56)),
			mainPage: b.readUInt32LE(64),
		};
	}

	async #readMimeTypes() {
		let pos = this.#header.mimeListPos, str = '';
		for (;;) {
			str += (await this.#read(pos, 1024)).toString('binary');
			const end = str.indexOf('\0\0');
			if (end !== -1) { str = str.slice(0, end + 1); break; }
			pos += 1024;
		}
		this.#mimeTypes = str.split('\0').filter(Boolean);
	}

	/** Directory entry (article record) at byte `offset`, growing the read window as needed. */
	async #readDirent(offset) {
		for (let size = 512; ; size *= 2) {
			const buf = await this.#read(offset, size);
			let o = 0;
			const mimetype = buf.readUInt16LE(o); o += 2;
			o += 1; // extraLen, unused
			const namespace = String.fromCharCode(buf.readUInt8(o)); o += 1;
			o += 4; // revision, unused

			let redirectIndex = null, cluster = null, blob = null;
			if (mimetype === 0xffff) { redirectIndex = buf.readUInt32LE(o); o += 4; }
			else { cluster = buf.readUInt32LE(o); o += 4; blob = buf.readUInt32LE(o); o += 4; }

			const urlEnd = buf.indexOf(0, o);
			if (urlEnd === -1) continue;
			const titleEnd = buf.indexOf(0, urlEnd + 1);
			if (titleEnd === -1) continue;

			const url = buf.toString('utf8', o, urlEnd);
			const title = buf.toString('utf8', urlEnd + 1, titleEnd) || url;
			return {mimetype, namespace, redirectIndex, cluster, blob, url, title};
		}
	}

	async #resolveRedirect(dirent) {
		if (dirent.mimetype !== 0xffff) return dirent;
		return this.#readDirent(await this.#ptr64(this.#header.urlPtrPos, dirent.redirectIndex));
	}

	async close() {
		if (this.#fd) await this.#fd.close();
		this.#fd = null;
		if (this.#indexFd) await this.#indexFd.close();
		this.#indexFd = null;
		for (const entry of this.#clusterCache.values()) clearTimeout(entry.timer);
		this.#clusterCache.clear();
		this.#pending.clear();
	}

	/** Deletes the zim archive and its cached fallback index (if any). Safe to call on unopened readers. */
	async delete() {
		await this.close();
		await fs.promises.rm(this.path, {force: true});
		await fs.promises.rm(this.#indexPath, {force: true});
	}

	/** Reads an 'M' namespace metadata value (e.g. Name, Date, Title). Returns null if missing. */
	async metadata(key) {
		const get = async key => {
			const page = await this.readPage(key, NS_METADATA);
			return page ? page.data.toString('utf8') : null;
		};
		if(key) return get(key);

		const [title, creator, publisher, date, description, language, name, tags] = await Promise.all(
			['Title', 'Creator', 'Publisher', 'Date', 'Description', 'Language', 'Name', 'Tags'].map(get)
		);
		return {
			title,
			updated: date ? new Date(date) : null,
			summary: description,
			language,
			name,
			category: tags ? tags.split(';')[0] || '' : '',
			tags: tags ? tags.split(';') : [],
			author: creator,
			publisher,
			articleCount: this.articleCount,
			mediaCount: this.mediaCount,
			sizeMb: +((await fs.promises.stat(this.path)).size / 1024 / 1024).toFixed(1),
			icon: await this.#icon(),
		};
	}

	/** Opens the archive and parses its header + mimetype list. */
	async open() {
		this.#fd = await fs.promises.open(this.path, 'r');
		try {
			await this.#readHeader();
			await this.#readMimeTypes();
		} catch (e) {
			await this.#fd.close();
			this.#fd = null;
			throw e;
		}

		// The fallback index is only needed when the archive has no native title
		// pointer list - #findByTitleBuiltin already covers that case in O(log n)
		// with zero extra storage. Well-maintained archives (Wikipedia etc.)
		// ship a title listing, so this path is expected to be rare in practice.
		if (!this.#hasTitleListing) {
			this.#indexReady = this.#loadOrBuildFallbackIndex().catch(() => {});
		}
		return this;
	}

	/** Read a page's content by URL. Returns `{mimetype, data}` or `null` if not found. */
	async readPage(url, namespace = NS_CONTENT) {
		let dirent = await this.#findByUrl(url, namespace);
		if (!dirent) return null;
		dirent = await this.#resolveRedirect(dirent);
		const data = await this.#getBlob(dirent.cluster, dirent.blob);
		return {mimetype: this.#mimeTypes[dirent.mimetype] || 'application/octet-stream', data};
	}

	/** Reads the archive's designated main/landing page, if one is set. */
	async mainPage() {
		if (this.#header.mainPage === 0xffffffff) return null;
		let dirent = await this.#readDirent(await this.#ptr64(this.#header.urlPtrPos, this.#header.mainPage));
		dirent = await this.#resolveRedirect(dirent);
		const data = await this.#getBlob(dirent.cluster, dirent.blob);
		return {mimetype: this.#mimeTypes[dirent.mimetype] || 'application/octet-stream', data, url: dirent.url, namespace: dirent.namespace};
	}

	/**
	 * Fuzzy-ranked title search. Accepts comma-separated `terms` the same way the
	 * catalog search does. Uses kiwix-search's embedded fulltext index as a prefilter
	 * to narrow candidates before fuzzy scoring.
	 */
	async search(terms, {limit = 20, htmlOnly = true} = {}) {
		const termList = String(terms).split(',').map(t => t.trim()).filter(Boolean);
		if (!termList.length) return [];

		const names = await kiwixSearch(this.path, termList.join(' '));
		const candidates = (await Promise.all(names.map(n => this.#resolveSearchEntry(n)))).filter(Boolean);

		const scored = [];
		for (const dirent of candidates) {
			if (htmlOnly && !(this.#mimeTypes[dirent.mimetype] || '').startsWith('text/html')) continue;
			const urlTitle = titleFromUrl(dirent.url);
			const titleScore = fuzzyMatch(dirent.title, ...termList).max;
			const urlScore = fuzzyMatch(urlTitle, ...termList).max;
			scored.push({url: dirent.url, title: dirent.title.length > urlTitle.length ? dirent.title : urlTitle, namespace: NS_CONTENT, score: Math.max(titleScore, urlScore)});
		}
		const {summary, mediaCount, articleCount, sizeMb, ...meta} = await this.metadata();
		return scored.filter(a => a.score > 0).toSorted((a, b) => b.score - a.score).slice(0, limit).map(a => ({...meta, ...a}));
	}
}