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早期freebsd实现

Compress version 4.0 improvements:
	o compress() speedup (10-50%) by changing division hash to xor
	o decompress() speedup (5-10%)
	o Memory requirements reduced (3-30%)
	o Stack requirements reduced to less than 4kb
	o Removed 'Big+Fast' compress code (FBITS) because of compress speedup
    	o Portability mods for Z8000 and PC/XT (but not zeus 3.2)
	o Default to 'quiet' mode
	o Unification of 'force' flags
	o Manual page overhaul
	o Portability enhancement for M_XENIX
	o Removed text on #else and #endif
	o Added "-V" switch to print version and options
	o Added #defines for SIGNED_COMPARE_SLOW
	o Added Makefile and "usermem" program
	o Removed all floating point computations
	o New programs:
		compressdir - compress all files on a directory
		uncompressdir - uncompress all files on a directory
		zcmp - cmp compressed files
		zdiff - diff compressed files
	  The following are with thanks to philabs!per:
		btoa - convert binary to ascii for mailing
		atob - convert ascii to binary with checksum
		tarmail - tar, compress, btoa, and mail files
		untarmail - restore "tarmail" files

		WARNING: These last few programs are not compatible 
		with the original ones from the net.  The encoding
		has changed.  See btoa.c for more info.

The "usermem" script attempts to determine the maximum process size.  Some
editing of the script may be necessary (see the comments).  If you can't get
it to work at all, just create file "USERMEM" containing the maximum process
size in decimal.

The following preprocessor symbols control the compilation of "compress.c":

	o USERMEM		Maximum process memory on the system
	o SACREDMEM		Amount to reserve for other proceses
	o SIGNED_COMPARE_SLOW	Unsigned compare instructions are faster
	o NO_UCHAR		Don't use "unsigned char" types
	o BITS			Overrules default set by USERMEM-SACREDMEM
	o vax			Generate inline assembler
	o interdata		Defines SIGNED_COMPARE_SLOW
	o M_XENIX		Makes arrays < 65536 bytes each
	o pdp11			BITS=12, NO_UCHAR
	o z8000			BITS=12
	o pcxt			BITS=12
	o BSD4_2		Allow long filenames ( > 14 characters) &
				Call setlinebuf(stderr)

The difference "usermem-sacredmem" determines the maximum BITS that can be
specified with the "-b" flag.

memory: at least		BITS
------  -- -----                ----
     433,484			 16
     229,600			 15
     127,536			 14
      73,464			 13
           0			 12

The default is BITS=16.

The maximum bits can be overrulled by specifying "-DBITS=bits" at
compilation time.

WARNING: files compressed on a large machine with more bits than allowed by 
a version of compress on a smaller machine cannot be decompressed!  Use the
"-b12" flag to generate a file on a large machine that can be uncompressed 
on a 16-bit machine.

The output of compress 4.0 is fully compatible with that of compress 3.0.
In other words, the output of compress 4.0 may be fed into uncompress 3.0 or
the output of compress 3.0 may be fed into uncompress 4.0.

The output of compress 4.0 not compatable with that of
compress 2.0.  However, compress 4.0 still accepts the output of
compress 2.0.  To generate output that is compatable with compress
2.0, use the undocumented "-C" flag.

Check the Makefile, then "make".