The LHA format

  Disclaimer: The description below is a mere extrapolation from the files and
programs I have encountered.

  LHA is used to compress multiple files into a single archive.  There  is  no
signature at the beginning of the archives but you can search  for  the  usual
"-lh?-" compression method signature - the archive starts two bytes before the
first instance of this string.

  Normally archives start at the beginning  of  the  file  but  there  can  be
different extractors prepended to the archive therefore you should not rely on
the start offset of the archive being constant. However, the standard C64/C128
LHA Extractor by ((C) by Chris Smeets, 1990) is 3721 (hexadecimal $0E89) bytes
long and it relocates itself to be able to  work  on  any  Commodore  machine,
independently of the address it was loaded to in the memory.

  The archives contain one or more blocks that consist of a  file  header  and
the compressed data of the file. The original file data  is  compressed  using
LZW compression bundled with a dynamic Huffman algorithm and is protected with
a 16-bit CRC checksum.

  The file header has the following structure:

  POSITION      DESCRIPTION
  $00           Length of the header, without the header checksum (LEN)
  $01           Header checksum
  $02-$06       Compression method signature (the ASCII string "-lh?-",  where
                "?" is a number showing the method)
  $07-$0A       Size of the packed file data
  $0B-$0E       Size of the original file data
  $0F-$12       Original time stamp of the file (MS-DOS format)
  $13-$14       Original attributes of the file (MS-DOS attributes along  with
                some extended attributes;  usually  $0020,  only  the  Archive
                attribute is set)
  $15-(LEN-1)   Original name of the file
  LEN-(LEN+1)   CRC checksum of the file

  The header checksum is computed by simply adding the  bytes  in  the  header
(bytes $02-(LEN+1), from the compression method signature to the CRC checksum)
without carry.

  The number in compression method signature tells the method used to compress
that file: "0" means that the file is stored without any compression,  "1"  is
the normal compression for Commodore LHA archivers and from "2" on compression
methods are only used in the 2.xx releases of LHA for DOS but not in Commodore
LHA archivers. The old version 1.xx compression routines of LHA are absolutely
compatible with Commodore LHA archivers: the compressed file is the  same  or,
if not, it is still decompressed by the C64/C128 LHA Extractor. The number  is
related to the size of the string dictionary window of LZW compression and  on
Commodore machines the window size cannot be as big as on computers with  more
memory.

  The MS-DOS time stamp format packs the last modification date and time  into
a long integer:

  POSITION      DESCRIPTION
  $00-$01       Time of last modification:
                BITS  0- 4: Seconds divided by 2 (0-58, only even numbers)
                BITS  5-10: Minutes (0-59)
                BITS 11-15: Hours (0-23, no AM or PM)
  $02-$03       Date of last modification:
                BITS  0- 4: Day (1-31)
                BITS  5- 9: Month (1-12)
                BITS 10-15: Year minus 1980

  The file name is a Pascal-style ASCII string, its  length  being  its  first
character. Commodore LHA archives append a zero byte and the  filetype  (ASCII
uppercase character D, P, S  or  U)  to  the  file  name  in  the  headers  of
non-program files and sometimes in those of programs, as well. Note  that  LHA
archives are not designed to store relative files.

  You can read through an LHA archive with the following algorithm:

  1. Determine the start of the archive, e.g. by searching for  the  signature
     "-lh?-".

  2. Read in one byte from the file. It tells you the length of the  following
     header (LEN). If it is 0, then you have reached the end of  the  archive,
     stop.

  3. Read in the checksum and the header (LEN+1 bytes). Now  you  can  process
     the header and the file.

  4. Add the length of the packed file size to the current file  position.  By
     seeking there, you get to the next header, go to step 2.