ARC’s “Trimmed” compression scheme

ARC is a file compression and archiving utility that was in use from the mid-1980s to the early 1990s, mainly on DOS computers. It was developed by a small company named System Enhancement Associates. The last major version of ARC was 7.x, first released in late 1989 or early 1990. In v7, ARC became part …

Making an uncompressed JPEG 2000 file

Challenge: Construct a JPEG 2000 image file that isn't compressed. Also, try to do it without spending any money. Overview The flagship feature of the JPEG 2000 suite of graphics formats is the wavelet-based "JPEG 2000 codestream" compression format. This challenge is not to figure out how to make a degenerate form of that format …

Encoding Huffman codebooks

This post will assume you have a basic knowledge of the data compression technique known as Huffman coding. Though maybe, since I'm only concerned about decompression, I should call it something like "bit-oriented prefix codes". Huffman coding is really just one of the algorithms that can produce such a code, but it's the term everybody …

Notes on LHARK compression format

LHARK (with a K) is an old compression/archiver utility for DOS. It is related to the popular utility named LHA (formerly LHarc), but should not be confused with it. You should be able to find a copy of LHARK by searching the web for "LHARK04D". LHARK was developed by Kerwin F. Medina around 1995/1996. It …

The blocksize field in LHA compression format

This post is about the data compression format I'll call "lh5". It is actually a family of formats that includes the compression methods often named lh{4, 5, 6, 7, 8}. It was most notably used by version 2.x of the old LHA/LZH/LHArc compressed archive format. It was used, often in modified form, in a number …

LZ77 compression prehistory

LZ77 is a widely-used class of data compression algorithms. I'll start with a quick overview of it. Assuming you're compressing a stream of bytes (a "file"), your LZ77 compressed data, at a high level, would contain two possible kinds of instructions for the decompressor: Emit literal: {byte value=A}Copy from history: {match-offset=B, match-length=C} The match-offset may …

An obscure polyglot file: LHA/CAR

LHA is a compressed archive file format and compression utility that was, for a long time, a competitor of ZIP. It's also known as LZH format, or LHarc format, but I'll call it LHA. In the course of researching it, I came across an obscure lookalike format created by a program named CAR. CAR is …

Thoughts on timestamps of computer files

Computer files can have a number of different kinds of timestamps. Some of them are stored in the file's external metadata, alongside the file's name -- I'll call these external timestamps. Others are stored inside the file itself -- I'll call these internal timestamps. I use the term "timestamp" loosely. When something called a "timestamp" …

PKZIP Implode bug #3

I've already written about two PKZIP bugs related to the "Implode" compression method. Now I've come across another one, so I guess I'll investigate it as well. Here are the first two: Bug #1 (MML)Bug #2 (v1.01 literal tree issue) There's an old collection of files called the Pier 1 Shareware CDROM (#1). On it …

Notes on WinHelp format, part 3

This post is part of a series about WinHelp file format. Please read the other parts first: Part 1Part 2 With what we learned previously, we can decompress the TOPIC blocks, locate the TOPICLINKs, and stitch each TOPICLINK's fragments together to make each TOPICLINK a contiguous blob of bytes: A defragmented TOPICLINK is composed of …