Decompression caching and preview links

src/decompress-worker.js

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+'use strict';
+
+import {parentPort} from 'node:worker_threads';
+import {decompress as lzmaDecompress} from 'lzma1';
+import {ZstdCodec} from 'zstd-codec';
+
+let zstdStreamingPromise = null;
+
+function getZstd() {
+	if (!zstdStreamingPromise) {
+		zstdStreamingPromise = new Promise(resolve => ZstdCodec.run(zstd => resolve(new zstd.Streaming())));
+	}
+	return zstdStreamingPromise;
+}
+
+/** Rebuilds a full 13-byte "alone" LZMA header from libzim's truncated 5-byte one (unknown size). */
+function toLzmaAloneStream(body) {
+	const header = Buffer.concat([body.subarray(0, 5), Buffer.alloc(8, 0xff)]);
+	return Buffer.concat([header, body.subarray(5)]);
+}
+
+parentPort.on('message', async ({id, compType, body}) => {
+	try {
+		const buf = Buffer.from(body);
+		const data = compType === 4
+			? Buffer.from(lzmaDecompress(toLzmaAloneStream(buf)))
+			: Buffer.from((await getZstd()).decompress(new Uint8Array(buf)));
+		parentPort.postMessage({id, data}, [data.buffer]);
+	} catch (e) {
+		parentPort.postMessage({id, error: e.message});
+	}
+});