LHA (file format)

LHA
Filename extension .lzh, .lha
Internet media type application/x-lzh-compressed
Type code "LHA " (L-H-A-SPACE)
Uniform Type Identifier (UTI) public.archive.lha
Developed by Haruyasu Yoshizaki (Yoshi)
Type of format Data compression

LHA is a freeware compression utility and associated file format. It was created in 1988 by Haruyasu Yoshizaki (吉崎栄泰 Yoshizaki Haruyasu), and originally named LHarc. A complete rewrite of LHarc, tentatively named LHx, was eventually released as LH. It was then renamed to LHA to avoid conflicting with the then-new MS-DOS 5.0 LH ("load high") command. According to early documentation, LHA is pronounced like La.

Although no longer much used in the West, LHA remains popular in Japan. It was used by id Software to compress installation files for their earlier games, including Doom and Quake. LHA has been ported to many operating systems, and is still the main archiving format used on the Amiga computer, although it competed with LZX in the mid 1990s. This was due to Aminet, the world's largest archive of Amiga related software and files, standardising on Stefan Boberg's implementation of LHA for the Amiga. Microsoft has released a Windows XP add-on, Microsoft Compressed (LZH) Folder Add-on, designed for the Japanese version of the operating system.[1][2] The Japanese version of Windows 7 ships with the LZH folder add-on built-in.[3] Users of non-Japanese versions of Windows 7 Enterprise and Ultimate can also install the LZH folder add-on by installing the optional Japanese language pack from Windows Update.

Compression methods

In an LZH archive, the compression method is stored as a five-byte text string. These are the third through seventh bytes of the file.

Canonical LZH

LHarc compresses files using an algorithm from Yoshizaki's earlier LZHUF product, which was modified from LZARI developed by Haruhiko Okumura (奥村晴彦 Okumura Haruhiko), but uses Huffman coding instead of arithmetic coding. LZARI uses Lempel–Ziv–Storer–Szymanski with arithmetic coding.

lh0

No compression method is applied to the source data.

lh1

This method is introduced in LHarc version 1.

It supports 4KiByte sliding window, with support of maximum 60 bytes of matching length. Dynamic Huffman encoding is used.

lh2

This method supports 8KiByte sliding window, with support of maximum 256 bytes of matching length. Dynamic Huffman encoding is used.

lh4, lh5, lh6, lh7

Methods 4, 5, 6, 7 support 4, 8, 32, 64 KiByte sliding window respectively, with support of maximum 256 bytes of matching length. Static Huffman encoding is used. lh5 is first introduced in LHarc 2, followed by lh6 in LHA 2.66 (MSDOS), lh7 in LHA 2.67 beta (MSDOS). LHA itself never compresses into lh4.

lhd

Technically it is not a compression method, but it is used in .LZH archive to indicate the compressed object is an empty directory.

Joe Jared extensions

Joe Jared extended LZSS to use larger dictionaries.

lh8, lh9, lha, lhb, lhc, lhe

Dictionary sizes are 64, 128, 256, 512, 1024, 2048 Ki bytes respectively.

UNLHA32 extensions

UNLHA32.DLL uses its own method for testing purposes.

lhx

It uses 128-256 Ki bytes dictionary.

PMarc extensions

These compression methods are created by PMarc, an CP/M archiver created by Miyo. The archive usually has a .PMA extension.

pc1

PopCom compressed executable archive.

pm0

No compression method is applied to the source data.

pm1
pm2
pms

Used to indicate PMarc self-extracting archive.

LArc extensions

LArc uses the same file format as .LZH, but was written by Kazuhiko Miki, Haruhiko Okumura, Ken Masuyama, with extension name '.LZS'.

lzs

It supports 2KiByte sliding window, with support of maximum 17 bytes of matching length.

lz2

It is similar to lzs, except dictionary size and match length can be changed.

lz3
lz4

No compression method is applied to the source data.

lz5

It supports 4KiByte sliding window, with support of maximum 17 bytes of matching length.

lz7
lz8

Issues

LHICE/ICE

There are copies of LHICE marked as version 1.14. According to Okumura, LHICE is not written by Yoshi.[4]

y2k11 bug

Because of a bug, timestamps since the year 2011 will be set to 1980 - meaning that some utilities need to be patched.[5][6]

See also

References

  1. ↑ http://support.microsoft.com/kb/896132/JA/
  2. ↑ http://www.microsoft.com/globaldev/outreach/dnloads/LZH_add-on.mspx
  3. ↑ You cannot install the Microsoft Compressed (LZH) Folder add-in in Windows 7
  4. ↑ http://oku.edu.mie-u.ac.jp/~okumura/compression/history.html
  5. ↑ http://aminet.net/package/util/arc/lha138pch
  6. ↑ http://aminet.net/package/util/arc/lha_68k

External links

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