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FREEZE / MELT COMPRESSION PROGRAM
This is Alpha version. It is tested under ISC 2.2, Xenix, SunOS.
The following preprocessor symbols control the compilation of Freeze
package:
o BITS The size of hash table (default is 18,
reducing to 14 gives a 50% speeddown).
o COMPAT Turns on backwards compatibility
with Freeze 1.0
o M_XENIX & M_I286 Makes arrays < 65536 bytes each
o BSD4_2 Allow long filenames ( > 14 characters) &
Call setlinebuf(stderr)
o INT_SIG signal is int (*)() unstead of void
o MSDOS Turns off some UNIX-dependencies
and MSDOS' ones - vice versa
!!! It's important !!!
If your MSDOS' C compiler do support
inline functions, define DO_INLINE.
This will cause the main loop to be
replaced with a call of memcmp, which
will be represented as 'repz cmpsb'.
o FAST Forces the Get_Next_Match routine
to be inline. This gives additional
percents of speedup.
o __TURBOC__ For compiling under TURBO C
o __i386__ When compiling under GNU C causes
some fixed register allocations,
which give better code.
o BIN_DEFAULT (For MS-DOS only!) Define, if you
forget to use -i option when freezing
binary files.
Please! If your computer supports the string operations, try to write
"asm" instructions (GNU style) which realize the right-to-left memory
comparison (s1, s2, length) in minimum number of clock cycles.
If a noticeable (5% or more) speedup is gained, please send me a message.
Other preprocessor symbols (DEBUG, GATHER_STAT) are for internal use
only.
The format of frozen (2.X) file is incompatible with that of frozen (1.0),
but if this package is compiled with -DCOMPAT switch, you will able to
unpack frozen (1.0) files, if you have them.
----
The format of a frozen file is as follows:
(version 1.0 had only 2-bytes header)
offset type value comment
0 byte 037 2 byte magic header
1 byte 0237 (version 1.0 - 0236)
2 short X (little endian)
4 byte Y
X = 0 e e e e e d d d d c c c b b a \
> [a-f] are binary digits
Y = 0 0 f f f f f f /
a - number of 1-bit static Huffman codes in the `matching positions'
table (see freeze.1)
bb - number of 2-bit codes,
etc.
The numbers of 7- and 8-bits codes are evaluated from the
conditions: sum(codes) = 62(dec), max code = 1111111(bin).
The default values are: 0 1 1 1 4 10 27 18, what means:
no 1-bit codes,
one 2-bit, 3-bit and 4-bit codes, etc., so Huffman codes are:
00, 010, 0110, 01110, 01111, 10000, .... , 11111111.
------------------- !!!!!!!!!! -----------------
(If you do not deal with compression algorithms, you may skip
until asterisks.)
General format of frozen file is:
magic header - table description - stream of bits
The stream of bits is considered as a sequence of variable length dynamic
Huffman codes (if their values are in the range of 0-255, they mean single
bytes, special value of 256 means EOF, and further values mean the lengths
of matched string.) If we have the value greater than 256, we get a static
Huffman code from the stream (his value is 6 higher bits of matched
string's position in the buffer), and then we get 7 bits literally.
Because buffer length is 8192 bytes and maximum match length is 256 bytes,
the position of matched string cannot be greater than 8192-256, that's why
there is only (8192-256)/2^7 = 62 static codes.
* * *
The default table is tuned for both C texts and executable files (as in
LHARC). If you freeze any other files (databases, images, fonts,
etc.) you can calculate the matching positions distribution using the
`statist' program, which calculates and displays the mentioned
distribution for the given file. It is useful for large (100K or more)
files.
Though the built-in position table is polyvalent, the tuning can increase
the compression rate up to one additional percent. (Or even more, if the
matching strings distribution is very bizarre!)
Usage: statist < sample_file ; you can also see the intermediate values
and watch their changes by pressing INTR key when you wish.
Note: If you use "gensample | statist", remember that INTR influence BOTH
processes !!
You may create the /etc/default/freeze file (if you don't like
/etc/default/ directory, choose another - in MS-DOS it is FREEZE.CNF in
the directory of FREEZE.EXE), which has the following format: name =
``statist's output (8 numbers)'', f.ex.:
---------- cut here -----------
# This is freeze's defaults file
gif = 0 0 0 0 2 60 0 0 # This is NOT! optimal data
# for GIF files
doc=0 0 1 2 7 16 36 0 # The sample was gcc.lp
# End of file
---------- cut here -----------
If you find values which are better THAN DEFAULT both for text (C
programs) and binary (executable) files, please send them to me.
Important note: statist.c is NOT a part of freeze package, it is an
additional feature.
------------------- LINT ----------------------------
Lint complains about MANY `constant in conditional context', but ALL these
contexts aren't `conditional', because they are unconditional (!)
expressions:
#define BITS 18
. . .
#define LEN0 (BITS/3 + (BITS%3 != 0))
. . .
... + (key[0] >> LEN0) ....
Do you think about /*CONDITIONAL*/ pseudo-comment for lint?
Other lint's complaints about `used/declared inconsistently' are (in my
case) due to inconsistencies of /usr/include/* and /usr/lib/llib*ln. It
isn't dangerous.
------------------- BUGS ----------------------------
Please send descriptions of found bugs, incompatibilities, etc. to
[email protected]. MS-DOS version will not be supported in future
versions !! (If they will be :-) )
------------ SPEED & COMPRESSION RATE ---------------
When using 18 bits table (about 600K) and gcc, the speed of freeze is more
than the same of ARJ 1.00, but is less than of LHA 2.05.
Note: the percents mean 'relatively to compressed size', if you want
to have them relatively to original size, divide them to 2-2.5.
Compression rate is *INDEPENDENT* of the hash table size, but may vary
with different static Huffman tables. It is about 2% worse than the same
of ARJ 1.00 and LHA 2.05, but ARJ 2.00 beats Freeze on 8%.
Note: if you see Compress works nearly as Freeze (on some files), this
means the maximum is gained, so LHA and ARJ won't better more than
1-1.5%.
--------------- POSSIBLE IMPROVEMENTS ---------------
The high-level routines (freeze, melt) are almost independent from
low-level routines (Get_Next_Match, Insert/Delete_Node,
Encode/Decode_Char/Position), so if you want the speed and/or compression
rate `a la vogue' you may replace the low-level routines with the homebrew
(f. ex.) ones and enjoy the results.
(I tried to implement splay trees instead of Huffman ones and instead of
static table for position information, but this gives nothing, alas.)
--------- CALGARY COMPRESSION CORPUS RESULTS --------
total 2308
41515 May 9 1990 bib.F
344793 May 9 1990 book1.F
230861 May 9 1990 book2.F
68626 May 9 1990 geo.F
155783 May 9 1990 news.F
10453 May 9 1990 obj1.F
85500 May 9 1990 obj2.F
20021 May 9 1990 paper1.F
32693 May 9 1990 paper2.F
19430 May 9 1990 paper3.F
5771 May 9 1990 paper4.F
5170 May 9 1990 paper5.F
14091 May 9 1990 paper6.F
53291 May 9 1990 pic.F
14143 May 9 1990 progc.F
17064 May 9 1990 progl.F
11686 May 9 1990 progp.F
22861 May 9 1990 trans.F
Average bits/byte on the standard set (except paper3-6) =
1109290 * 8 / 3141622 = 2.825
(With the "-g" flag = 2.892)