@4bitlabs/codecs

A collection of decoders (and eventually encoders) for working with Sierra On-line SCI-engine assets.

Supported Codecs

Name	Decoder	Encoder
Huffman	✅
Lempel–Ziv–Welch	✅
COMP3	✅

Huffman

Example decoding bytes with Huffman:

import { Huffman } from '@4bitlabs/codecs';

const encodedBytes = Uint8Array.of(/* encoded data */);
const bytes = Huffman.unpack(encodedBytes);

Lempel–Ziv–Welch

Example decoding bytes with Lempel-Ziv-Welch:

import { Lzw } from '@4bitlabs/codecs';

const encodedBytes = Uint8Array.of(/* encoded data */);
const bytes = Lzw.unpack(encodedBytes);

Custom LZW Options

By default, most-significant bit ordering is used. You can change the encoded byte ordering of the decoder with the options parameter. To use least-significant bit ordering:

const bytes = Lzw.unpack(encodedBytes, { order: 'lsb' });

The default code-width it uses is 8, this can also be adjusted. To use a 7-bit code width:

const bytes = Lzw.unpack(encodedBytes, { literalWidth: 7 });

Also, an entirely custom LZW dictionaries can be used for decoding:

import { Lzw } from '@4bitlabs/codecs';

const dictionary = [
  Lzw.EOF_MARKER,
  0x41, // A
  0x42, // B
  0x43, // C
  0x44, // D
];

const bytes = Lzw.unpack(encodedBytes, { dictionary });

Longer codings can be encoded in the dictionary by using either an array of numbers of with a Uint8Array:

import { Lzw, EOF_MARKER } from '@4bitlabs/codecs';

const dictionary = [
  Lzw.EOF_MARKER,
  Uint8Array.of(0x47, 0x41, 0x54, 0x41), // GATA
  Uint8Array.of(0x41, 0x54, 0x54, 0x41), // ATTA
  Uint8Array.of(0x43, 0x47, 0x41, 0x54), // CGAT
  Uint8Array.of(0x41, 0x43, 0x41, 0x47), // ACAG
];

const bytes = Lzw.unpack(encodedBytes, { dictionary });

SCI01/SCI1-engine COMP3

COMP3 compression is used in SCI01/SCI1 engine games, it is a 9–12 bit variable length encoding.

Example decoding bytes with COMP3 compression:

import { Comp3 } from '@4bitlabs/codecs';

const encodedBytes = Uint8Array.of(/* encoded data */);
const bytes = Comp3.unpack(encodedBytes);