'use strict';

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

const INDEX_VERSION = 1;
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;

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

	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;
	}

	/** Full O(n) scan over the URL pointer list, used when there's no title index. */
	async #allDirents() {
		const dirents = [];
		for (let i = 0; i < this.#header.articleCount; i++) {
			dirents.push(await this.#readDirent(await this.#ptr64(this.#header.urlPtrPos, i)));
		}
		return dirents;
	}

	/** Binary search the URL pointer list for namespace+url. */
	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;
	}

	/** 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));
	}

	/** Narrows to dirents near the term's alphabetical position in the title index. */
	async #titleIndexCandidates(term) {
		const q = term.toLowerCase();
		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 dirent = await this.#readDirent(await this.#ptr64(this.#header.urlPtrPos, urlIdx));
			if (dirent.title.toLowerCase() < q) lo = mid + 1; else hi = mid;
		}
		// Widen around the prefix match since fuzzy scoring isn't purely alphabetical.
		const start = Math.max(0, lo - 50), end = Math.min(this.#header.articleCount, lo + 200);
		const dirents = [];
		for (let i = start; i < end; i++) {
			const urlIdx = (await this.#read(this.#header.titlePtrPos + i * 4, 4)).readUInt32LE(0);
			dirents.push(await this.#readDirent(await this.#ptr64(this.#header.urlPtrPos, urlIdx)));
		}
		return dirents;
	}

	/** Path of the cached word index, kept in an `index/` subdirectory next to the archive. */
	#indexPath() {
		return path.join(path.dirname(this.path), 'index', `${path.basename(this.path)}.idx.json`);
	}

	/** Builds (or loads a cached) word -> entry index, avoiding a re-scan on every search. */
	async #wordIndex() {
		if (this.#index) return this.#index;
		const cachePath = this.#indexPath();
		const stat = await fs.promises.stat(this.path);

		if (fs.existsSync(cachePath)) {
			try {
				const cached = JSON.parse(await fs.promises.readFile(cachePath, 'utf8'));
				// Rebuild if stale: index predates the archive's current mtime (e.g. a re-download).
				if (cached.version === INDEX_VERSION && cached.mtimeMs >= stat.mtimeMs) {
					return (this.#index = cached);
				}
			} catch {}
		}

		// Expensive full scan — only happens once per archive (or after it changes).
		const dirents = await this.#allDirents();
		const entries = [];
		const words = new Map(); // word -> [entryIndex, ...]
		for (const d of dirents) {
			if (d.namespace !== NS_CONTENT) continue;
			const idx = entries.length;
			entries.push({url: d.url, title: d.title, mimetype: d.mimetype});
			const text = `${d.title} ${titleFromUrl(d.url)}`.toLowerCase();
			for (const w of text.split(/\W+/).filter(Boolean)) {
				if (!words.has(w)) words.set(w, []);
				words.get(w).push(idx);
			}
		}

		const index = {version: INDEX_VERSION, size: stat.size, mtimeMs: stat.mtimeMs, entries, words: Object.fromEntries(words)};
		await fs.promises.mkdir(path.dirname(cachePath), {recursive: true});
		await fs.promises.writeFile(cachePath, JSON.stringify(index));
		return (this.#index = index);
	}

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

	/** Deletes the zim archive and its cached index (if any). Safe to call on unopened readers. */
	async delete() {
		await this.close();
		this.#index = null;
		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');
		await this.#readHeader();
		await this.#readMimeTypes();
		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 the sorted title index when present (binary search
	 * narrows the candidate window); falls back to a full linear scan otherwise
	 * (common on ZIM v6+/zimit-generated archives with no title index).
	 */
	async search(terms, {limit = 20, htmlOnly = true} = {}) {
		const termList = String(terms).split(',').map(t => t.trim()).filter(Boolean);
		if (!termList.length) return [];

		let candidates;
		if (this.#hasTitleListing && this.#header.articleCount > 5000) {
			candidates = await this.#titleIndexCandidates(termList[0]);
		} else {
			const {entries, words} = await this.#wordIndex();
			// Pull candidates from postings of any word that starts with (or contains) the search term.
			const q = termList[0].toLowerCase();
			const idxSet = new Set();
			for (const [word, postings] of Object.entries(words)) {
				if (word.includes(q) || q.includes(word)) postings.forEach(i => idxSet.add(i));
			}
			candidates = [...idxSet].map(i => ({...entries[i], namespace: NS_CONTENT}));
		}

		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}));
	}
}