compress.txt

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COMPRESSION IN WINHELP FILES
============================

In Windows 3.1 helpfiles, the actual help text (stored in the |TOPIC file)
is usually compressed two ways:  first with phrase replacement, then with
a simplified LZ77 algorithm.  Note that the level of compression used in
a help file is given in the |SYSTEM internal file; see system.txt.

Phrase Replacement
------------------

When reading the help text in the |TOPIC file, if WinHelp sees a byte in the
range 0x01 to 0x0E, it is interpreted as the first byte of a two byte
phrase index.  The phrase index refers to one of the phrases in the |Phrases
internal file (see phrases.txt).  However, the index of the phrase to be
substituted is stored in an unusual way: high-byte first, then low-byte, not
using the last bit. That is, the formula for the index of the phrase given
the two-byte code X,Y is:   (((X-1)*265)+Y)/2.  Because of the restriction on
the first byte mentioned above, this implies a maximum of 1,280 phrases in
the |Phrases table.

In addition, the last bit has a special meaning:  if the last bit is one
(that is, if Y is odd) then a space is to be inserted immediately after
the phrase to be substituted.  Otherwise, no space.

Compression a-la LZ77
---------------------

Microsoft's version of the LZ77 compression algorithm is fairly simple; the
following description is from the decompresser's point of view.

A compressed block of data is divided into smaller blocks of between 9 and
17 bytes.  The first byte of data is a bitmask saying which of the rest of
the bytes are plain data and which are two-byte codes.  Bit 0 of the bitmask
refers to the byte immediately following the bitmask.  In each bit, 0 means
a plain byte while 1 means a code.

A two-byte code is interpreted as a reference to a string of bytes that
occur somewhere in the preceeding uncompressed text.  The code has the
following structure:
	Bits		Meaning
	-----------------------
	0-11		Distance back in the uncompressed text, minus 1
	12-15		Length of the string to substitute, minus 3
	
The 'minuses' are there because a zero distance would be meaningless, and
referring to a length 2 string would be pointless.

For example if a block of compressed text began as follows:

  00 46 69 72 73 74 20 48 65 08 6C 70 20 0A 20 .......
  ^^                         ^^          ^^^^^
  bitmask                    bitmask     code
  
Since the first byte is '00', the next 8 bytes are plain text. (ASCII for
"First He", in this case)  The next byte after that is '08', so '6C', '70',
and '20' are plain bytes while '0A 20' is a code.  Translated, '0A 20' means
11 places back and a length of 5, so the text "First" would be inserted at
that point.