kwaj-lzh

Zero-dependency decompressor for the Microsoft KWAJ and SZDD container formats — the COMPRESS.EXE / EXPAND.EXE family used for the compressed files on old MS-DOS and 16-bit Windows install media (the ones with a trailing underscore in the extension, e.g. SETUP.EX_, DRIVER.DL_, HELP.HL_).

Includes KWAJ compression method 3 (LZH) — LZSS with static Huffman coding — which most other JavaScript libraries do not implement.

• Pure TypeScript, no dependencies
• Runs in the browser and Node.js — every function takes and returns a Uint8Array
• Ships ESM with type definitions

Install

npm install kwaj-lzh

Usage

Auto-detect the container by its signature:

import { decompress } from 'kwaj-lzh';

const compressed = new Uint8Array(/* bytes of a .EX_ / SZDD / KWAJ file */);
const original = decompress(compressed); // Uint8Array

Node — read a compressed file and write the expanded one:

import { readFile, writeFile } from 'node:fs/promises';
import { decompress } from 'kwaj-lzh';

const input = await readFile('SETUP.EX_');
await writeFile('SETUP.EXE', decompress(input));

Browser — let the user drop a file in:

import { decompress } from 'kwaj-lzh';

const buf = await file.arrayBuffer();
const original = decompress(new Uint8Array(buf));

If you already know the format, call it directly and skip detection:

import { decompressKwaj, decompressSzdd, readKwajHeader } from 'kwaj-lzh';

const header = readKwajHeader(bytes); // { method, dataOffset, flags, length? }
const out = decompressKwaj(bytes);

Supported formats

| Container | Method | Supported | | --------- | ----------------- | --------- | | KWAJ | 0 — none (store) | ✅ | | KWAJ | 1 — XOR | ✅ | | KWAJ | 2 — SZDD (LZSS) | ✅ | | KWAJ | 3 — LZH | ✅ | | KWAJ | 4 — MSZIP | ❌ (throws UnsupportedCompressionError) | | SZDD | LZSS (mode A) | ✅ |

API

• decompress(bytes: Uint8Array): Uint8Array — detect KWAJ or SZDD and decompress.
• decompressKwaj(bytes): Uint8Array / readKwajHeader(bytes): KwajHeader / hasKwajSignature(bytes): boolean
• decompressSzdd(bytes): Uint8Array / readSzddHeader(bytes): SzddHeader / hasSzddSignature(bytes): boolean
• lzssDecompress(bytes, start, mode) and the LZSS_MODE_* constants, for raw LZSS streams.
• Errors: FormatError (bad signature / malformed input), UnsupportedCompressionError (recognized but unimplemented method).

Notes

• KWAJ MSZIP (method 4) is deflate-based and is not implemented yet.
• SZDD output is truncated to the exact length recorded in the header.

Credits

This is an original, clean-room TypeScript port of the publicly documented KWAJ and SZDD wire formats. The reference used for the format details is libmspack by Stuart Caie — specifically its mspack/kwajd.c (KWAJ, incl. the method-3 LZH codec by Jeff Johnson) and mspack/szddd.c decompressors:

• Source repo: https://github.com/kyz/libmspack
• Project page: https://www.cabextract.org.uk/libmspack/

libmspack is licensed LGPL-2.1. Only its wire format was reproduced here — no libmspack source code is copied or machine-translated into this project, which is an independent implementation licensed under MIT.

License

MIT