home
***
CD-ROM
|
disk
|
FTP
|
other
***
search
/
Amiga MA Magazine 1998 #6
/
amigamamagazinepolishissue1998.iso
/
packery
/
bwt
/
src
/
bwta.cpp
< prev
next >
Wrap
C/C++ Source or Header
|
1996-04-20
|
10KB
|
267 lines
//
// BWTA.CPP
//
// Mark Nelson
// March 8, 1996
// http://web2.airmail.net/markn
//
// DESCRIPTION
// -----------
//
// This program performs a Burrows-Wheeler transform on an input
// file or stream, and sends the result to an output file or stream.
//
// While this program can be compiled in 16 bit mode, it will suffer
// greatly by virtue of the fact that it will need to drop its
// block size tremendously.
//
// This program takes two arguments: an input file and an output
// file. You can leave off one argument and send your output to
// stdout. Leave off two arguments and read your input from stdin
// as well. You can also specify "-d" as the first argument to get
// a debug dump as well. However, the debug listing can be a little
// overwhelming if you are working on a large file!
//
// The output consists of a series of blocks that look like this:
//
// long byte_count | ...data... | long first | long last
//
// The byte_count refers to the number of data bytes. The data
// itself is the "L" column from the sorted data. "first" is the
// index where the first character from the buffer appears in the
// sorted output. "last" is where the end-of-buffer special byte
// appears in the output buffer. These blocks are repeated until
// I'm out of data.
//
// This program accompanies my article "Data Compression with the
// Burrows-Wheeler Transform." There is one major deviation from
// the text of the article in this implementation. To simplify the
// sorting, I append a special end-of-buffer character to the end
// of the input buffer. The end-of-buffer character isn't found
// in the buffer, which means I no longer have to wrap around to
// the start of the buffer when performing comparisons. Instead,
// I'm guaranteed that a memcmp() will terminate at or before the
// last character in the buffer.
//
// One problem, though. Since I can handle any kind of binary input,
// what character is guaranteed to never appear in the buffer? None,
// so instead I do a special hack and make sure I never *really*
// look at that last position when comparing. Instead, I only compare
// until one or the other string gets to the end, then award the
// comparison to whoever hit the end first.
//
// This special character means the output has N+1 characters. I just
// output a '?' when I hit that special end-of-buffer character, but
// I also have to pass along the information about the end-of-buffer
// character's position to the decoder, so I append it to the end
// of each data block.
//
// The sorting for this routine is done by inserting pointers into
// the buffer into an STL set. There are two good things about this.
// First, I can create a templated comparison function which is then
// called in-line during the insertion. Second, since I insert each
// string one at a time, I can provide some indication to the end user
// of how far along in the process I am. With a function like qsort(),
// it's hard to know how far along you are.
//
// If you don't have an STL capable compiler, you are going to have to
// use the less-sexy version of this program, BWTA.CPP.
//
// Build Instructions
// ------------------
//
// Borland C++ 4.5 16 bit : bcc -w -ml bwt.cpp //Yes, large model!
// Borland C++ 4.5 32 bit : bcc32 -w bwt.cpp
// Microsoft Visual C++ 4.0 : ???? haven't had a chance to test it
//
// This code has not been tested under UNIX. I don't have a g++
// testbed that supports the STL.
//
// Typical Use
// -----------
//
// rle < raw-file | bwta | mtf | rle | ari > compressed-file
//
//
// Borland STL hack
//
#define __MINMAX_DEFINED
//
// set.h is an STL file. If you don't have the STL on your system,
// you can expect an error here.
//
#include <set.h>
#include <stdio.h>
#include <ctype.h>
#include <string.h>
#include <fcntl.h>
#include <io.h>
#include <limits.h>
#if ( INT_MAX == 32767 )
#define BLOCK_SIZE 20000
#else
#define BLOCK_SIZE 200000
#endif
//
// length has the number of bytes presently read into the buffer,
// buffer contains the data itself.
//
long length;
unsigned char buffer[ BLOCK_SIZE ];
//
// This is the special comparison function used when inserting
// strings into the sorted set. Remember that the character
// at buffer+length doesn't really exist, but it is assumed to
// be the special end-of-buffer character, which is bigger than
// any character found in the input buffer. So I terminate the
// comparison at the end of the buffer.
//
class BoundedCompare {
public :
operator()( const unsigned char *p1,
const unsigned char *p2 ) const
{
unsigned int l1 = (unsigned int) (( buffer - p1 ) + length );
unsigned int l2 = (unsigned int) (( buffer - p2 ) + length );
int result = memcmp( p1, p2, min( l1, l2 ) );
if ( result < 0 )
return 1;
if ( result > 0 )
return 0;
return l1 > l2;
}
};
main( int argc, char *argv[] )
{
int debug = 0;
if ( argc > 1 && strcmp( argv[ 1 ], "-d" ) == 0 ) {
debug = 1;
argv++;
argc--;
}
fprintf( stderr, "Performing BWT on " );
if ( argc > 1 ) {
freopen( argv[ 1 ], "rb", stdin );
fprintf( stderr, "%s", argv[ 1 ] );
} else
fprintf( stderr, "stdin" );
fprintf( stderr, " to " );
if ( argc > 2 ) {
freopen( argv[ 2 ], "wb", stdout );
fprintf( stderr, "%s", argv[ 2 ] );
} else
fprintf( stderr, "stdout" );
fprintf( stderr, "\n" );
setmode( fileno( stdin ), O_BINARY );
setmode( fileno( stdout ), O_BINARY );
//
// This is the start of the giant outer loop. Each pass
// through the loop compresses up to BLOCK_SIZE characters.
// When an fread() operation finally reads in 0 characters,
// we break out of the loop and are done.
//
for ( ; ; ) {
//
// After reading in the data into the buffer, I do some
// UI stuff, then write the length out to the output
// stream.
//
length = fread( buffer, 1, BLOCK_SIZE, stdin );
if ( length == 0 )
break;
fprintf( stderr, "Performing BWT on %ld bytes\n", length );
long l = length + 1;
fwrite( &l, 1, sizeof( long ), stdout );
//
// Sorting the input strings is simply a matter of inserting
// a pointer to each string into an STL set<> container.
// The sorting is done by operator()() in the BoundedCompare
// class. Note that I insert N+1 pointers. The last pointer
// points one past the end of the buffer, which is where the
// imaginary end-of-buffer character resides. Sort of.
//
int i;
int ticker = 0;
set< unsigned char *, BoundedCompare > p;
for ( i = 0 ; i <= length ; i++ ) {
if ( ( ticker++ % 1024 ) == 0 )
fprintf( stderr, "." );
p.insert( buffer + i );
}
fprintf( stderr, "\n" );
set< unsigned char *, BoundedCompare >::iterator ii;
//
// If the debug flag was turned on, I print out the sorted
// strings, along with their prefix characters. This is
// not a very good idea when you are compressing a giant
// binary file, but it can be real helpful when debugging.
//
if ( debug ) {
for ( ii = p.begin(), i = 0 ; ii != p.end() ; ii++, i++ ) {
unsigned char *s = *ii;
fprintf( stderr, "%d : " );
unsigned char prefix;
if ( s == buffer )
prefix = '?';
else
prefix = s[ -1 ];
if ( isprint( prefix ) )
fprintf( stderr, "%c", prefix );
else
fprintf( stderr, "<%d>", prefix );
fprintf( stderr, ": " );
int stop = (int)( ( buffer - s ) + length );
if ( stop > 30 )
stop = 30;
for ( int j = 0 ; j < stop ; j++ ) {
if ( isprint( *s ) )
fprintf( stderr, "%c", *s );
else
fprintf( stderr, "<%d>", *s );
s++;
}
fprintf( stderr, "\n" );
}
}
//
// Finally, I write out column L. Column L consists of all
// the prefix characters to the sorted strings, in order.
// It's easy to get the prefix character, but I have to
// handle S0 with care, since its prefix character is the
// imaginary end-of-buffer character. I also have to spot
// the positions in L of the end-of-buffer character and
// the first character, so I can write them out at the end
// for transmission to the output stream.
//
long first;
long last;
for ( i = 0, ii = p.begin() ; ii != p.end() ; i++, ii++ ) {
if ( *ii == ( buffer + 1 ) )
first = i;
if ( *ii == buffer ) {
last = i;
fputc( '?', stdout );
} else
fputc( (*ii)[ -1 ], stdout );
}
p.erase( p.begin(), p.end() );
fprintf( stderr,
"first = %ld"
" last = %ld\n",
first,
last );
fwrite( &first, 1, sizeof( long ), stdout );
fwrite( &last, 1, sizeof( long ), stdout );
}
return 0;
}