Create title index for zims missing them

src/reader.js

@@ -1,6 +1,7 @@
 'use strict';
 
 import fs from 'node:fs';
+import {createHash} from 'node:crypto';
 import {decompressPool} from './decompress.js';
 import {fuzzyMatch, titleFromUrl, kiwixSearch} from './utils.js';
 
@@ -12,6 +13,12 @@ const TITLE_SENTINEL = 0xffffffffffffffffn; // Indicator -> ZIM v6+ archives wit
 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;
@@ -23,6 +30,12 @@ export class ZimReader {
 	#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; }
 
@@ -31,9 +44,10 @@ export class ZimReader {
 		this.path = path;
 		this.#clusterCacheMax = clusterCacheMax;
 		this.#clusterTTL = clusterTTL;
+		this.#indexPath = `${path}${INDEX_SUFFIX}`;
 	}
 
-	/** Binary search the URL pointer list for namespace+url. */
+	/** 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;
@@ -48,18 +62,178 @@ export class ZimReader {
 		return null;
 	}
 
-	/** Binary search the title index for an exact title match. */
-	async #findByTitle(title) {
-		if (!this.#hasTitleListing) return null;
-		const q = title.toLowerCase();
+	/** 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 dirent = await this.#readDirent(await this.#ptr64(this.#header.urlPtrPos, urlIdx));
-			const t = dirent.title.toLowerCase();
-			if (t === q) return dirent;
-			if (t < q) lo = mid + 1; else hi = mid - 1;
+			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;
 	}
@@ -192,15 +366,18 @@ export class ZimReader {
 	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. Safe to call on unopened readers. */
+	/** 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. */
@@ -242,6 +419,14 @@ export class ZimReader {
 			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;
 	}
 
@@ -272,8 +457,8 @@ export class ZimReader {
 		const termList = String(terms).split(',').map(t => t.trim()).filter(Boolean);
 		if (!termList.length) return [];
 
-		const titles = await kiwixSearch(this.path, termList.join(' '));
-		const candidates = (await Promise.all(titles.map(t => this.#findByTitle(t)))).filter(Boolean);
+		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) {