compress.c

这是一个同样来自贝尔实验室的和UNIX有着渊源的操作系统, 其简洁的设计和实现易于我们学习和理解

		if ((getchar()!=(magic_header[0] & 0xFF))
		 || (getchar()!=(magic_header[1] & 0xFF))) {
			fprintf(stderr, "stdin: not in compressed format\n");
			exit(1);
		}
		maxbits = getchar();	/* set -b from file */
		block_compress = maxbits & BLOCK_MASK;
		maxbits &= BIT_MASK;
		maxmaxcode = 1 << maxbits;
		fsize = 100000;		/* assume stdin large for USERMEM */
		if(maxbits > BITS) {
			fprintf(stderr,
			"stdin: compressed with %d bits, can only handle %d bits\n",
			maxbits, BITS);
			exit(1);
		}
		}
#ifndef DEBUG
		decompress();
#else
		if (debug == 0)
			decompress();
		else
			printcodes();
		if (verbose)
			dump_tab();
#endif						/* DEBUG */
	}
	}
	exit(exit_stat);
}

static int offset;
long in_count = 1;		/* length of input */
long bytes_out;			/* length of compressed output */
long out_count = 0;		/* # of codes output (for debugging) */

/*
 * compress stdin to stdout
 *
 * Algorithm:  use open addressing double hashing (no chaining) on the
 * prefix code / next character combination.  We do a variant of Knuth's
 * algorithm D (vol. 3, sec. 6.4) along with G. Knott's relatively-prime
 * secondary probe.  Here, the modular division first probe is gives way
 * to a faster exclusive-or manipulation.  Also do block compression with
 * an adaptive reset, whereby the code table is cleared when the compression
 * ratio decreases, but after the table fills.  The variable-length output
 * codes are re-sized at this point, and a special CLEAR code is generated
 * for the decompressor.  Late addition:  construct the table according to
 * file size for noticeable speed improvement on small files.  Please direct
 * questions about this implementation to ames!jaw.
 */
compress()
{
	code_int ent, hsize_reg;
	code_int i = 0;
	int c, disp, hshift;
	long fcode;

	if (nomagic == 0) {
		putchar(magic_header[0]);
		putchar(magic_header[1]);
		putchar((char)(maxbits | block_compress));
		if(ferror(stdout))
			writeerr();
	}
	offset = 0;
	bytes_out = 3;		/* includes 3-byte header mojo */
	out_count = 0;
	clear_flg = 0;
	ratio = 0;
	in_count = 1;
	checkpoint = CHECK_GAP;
	maxcode = MAXCODE(n_bits = INIT_BITS);
	free_ent = (block_compress? FIRST: 256);

	ent = getchar ();

	hshift = 0;
	for (fcode = (long)hsize;  fcode < 65536L; fcode *= 2)
		hshift++;
	hshift = 8 - hshift;		/* set hash code range bound */

	hsize_reg = hsize;
	cl_hash( (count_int) hsize_reg);		/* clear hash table */

	while ((c = getchar()) != EOF) {
		in_count++;
		fcode = (long) (((long) c << maxbits) + ent);
	 	i = ((c << hshift) ^ ent);	/* xor hashing */

		if (htabof (i) == fcode) {
			ent = codetabof(i);
			continue;
		} else if ((long)htabof(i) < 0 )	/* empty slot */
			goto nomatch;
	 	disp = hsize_reg - i;	/* secondary hash (after G. Knott) */
		if (i == 0)
			disp = 1;
probe:
		if ((i -= disp) < 0)
			i += hsize_reg;

		if (htabof (i) == fcode) {
			ent = codetabof(i);
			continue;
		}
		if ((long)htabof(i) > 0)
			goto probe;
nomatch:
		output((code_int)ent);
		out_count++;
	 	ent = c;
		if (free_ent < maxmaxcode) {
	 		codetabof(i) = free_ent++;	/* code -> hashtable */
			htabof(i) = fcode;
		} else if ((count_int)in_count >= checkpoint && block_compress)
			cl_block ();
	}
	/*
	* Put out the final code.
	*/
	output( (code_int)ent );
	out_count++;
	output( (code_int)-1 );

	/*
	* Print out stats on stderr
	*/
	if(zcat_flg == 0 && !quiet) {
#ifdef DEBUG
	fprintf( stderr,
		"%ld chars in, %ld codes (%ld bytes) out, compression factor: ",
		in_count, out_count, bytes_out );
	prratio( stderr, in_count, bytes_out );
	fprintf( stderr, "\n");
	fprintf( stderr, "\tCompression as in compact: " );
	prratio( stderr, in_count-bytes_out, in_count );
	fprintf( stderr, "\n");
	fprintf( stderr, "\tLargest code (of last block) was %d (%d bits)\n",
		free_ent - 1, n_bits );
#else /* !DEBUG */
	fprintf( stderr, "Compression: " );
	prratio( stderr, in_count-bytes_out, in_count );
#endif /* DEBUG */
	}
	if(bytes_out > in_count)	/* exit(2) if no savings */
		exit_stat = 2;
}

/*
 * TAG( output )
 *
 * Output the given code.
 * Inputs:
 * 	code:	A n_bits-bit integer.  If == -1, then EOF.  This assumes
 *		that n_bits =< (long)wordsize - 1.
 * Outputs:
 * 	Outputs code to the file.
 * Assumptions:
 *	Chars are 8 bits long.
 * Algorithm:
 * 	Maintain a BITS character long buffer (so that 8 codes will
 * fit in it exactly).  When the buffer fills up empty it and start over.
 */

static char buf[BITS];

uchar lmask[9] = {0xff, 0xfe, 0xfc, 0xf8, 0xf0, 0xe0, 0xc0, 0x80, 0x00};
uchar rmask[9] = {0x00, 0x01, 0x03, 0x07, 0x0f, 0x1f, 0x3f, 0x7f, 0xff};

output( code )
code_int  code;
{
#ifdef DEBUG
	static int col = 0;
#endif
	int r_off = offset, bits= n_bits;
	char *bp = buf;

#ifdef DEBUG
	if (verbose)
		fprintf(stderr, "%5d%c", code,
			(col+=6) >= 74? (col = 0, '\n'): ' ');
#endif
	if (code >= 0) {
		/*
		 * byte/bit numbering on the VAX is simulated by the
		 * following code
		 */
		/*
		 * Get to the first byte.
		 */
		bp += (r_off >> 3);
		r_off &= 7;
		/*
		 * Since code is always >= 8 bits, only need to mask the first
		 * hunk on the left.
		 */
		*bp = (*bp & rmask[r_off]) | (code << r_off) & lmask[r_off];
		bp++;
		bits -=  8 - r_off;
		code >>= 8 - r_off;
		/* Get any 8 bit parts in the middle (<=1 for up to 16 bits). */
		if ( bits >= 8 ) {
			*bp++ = code;
			code >>= 8;
			bits -= 8;
		}
		/* Last bits. */
		if(bits)
			*bp = code;

		offset += n_bits;
		if ( offset == (n_bits << 3) ) {
			bp = buf;
			bits = n_bits;
			bytes_out += bits;
			do {
				putchar(*bp++);
			} while(--bits);
			offset = 0;
		}

		/*
		 * If the next entry is going to be too big for the code size,
		 * then increase it, if possible.
		 */
		if ( free_ent > maxcode || (clear_flg > 0)) {
			/*
			* Write the whole buffer, because the input side won't
			* discover the size increase until after it has read it.
			*/
			if ( offset > 0 ) {
				if( fwrite( buf, 1, n_bits, stdout ) != n_bits)
					writeerr();
				bytes_out += n_bits;
			}
			offset = 0;

			if ( clear_flg ) {
				maxcode = MAXCODE (n_bits = INIT_BITS);
				clear_flg = 0;
			} else {
				n_bits++;
				if ( n_bits == maxbits )
					maxcode = maxmaxcode;
				else
					maxcode = MAXCODE(n_bits);
			}
#ifdef DEBUG
			if ( debug ) {
				fprintf(stderr,
					"\nChange to %d bits\n", n_bits);
				col = 0;
			}
#endif
		}
	} else {
		/*
		 * At EOF, write the rest of the buffer.
		 */
		if ( offset > 0 )
			fwrite( buf, 1, (offset + 7) / 8, stdout );
		bytes_out += (offset + 7) / 8;
		offset = 0;
		fflush( stdout );
#ifdef DEBUG
		if ( verbose )
			fprintf( stderr, "\n" );
#endif
		if( ferror( stdout ) )
			writeerr();
	}
}

/*
 * Decompress stdin to stdout.  This routine adapts to the codes in the
 * file building the "string" table on-the-fly; requiring no table to
 * be stored in the compressed file.  The tables used herein are shared
 * with those of the compress() routine.  See the definitions above.
 */
decompress()
{
	int finchar;
	code_int code, oldcode, incode;
	uchar *stackp;

	/*
	* As above, initialize the first 256 entries in the table.
	*/
	maxcode = MAXCODE(n_bits = INIT_BITS);
	for (code = 255; code >= 0; code--) {
		tab_prefixof(code) = 0;
		tab_suffixof(code) = (uchar)code;
	}
	free_ent = (block_compress? FIRST: 256);

	finchar = oldcode = getcode();
	if(oldcode == -1)		/* EOF already? */
		return;			/* Get out of here */
	putchar((char)finchar);		/* first code must be 8 bits = char */
	if(ferror(stdout))		/* Crash if can't write */
		writeerr();
	stackp = de_stack;

	while ((code = getcode()) > -1) {
		if ((code == CLEAR) && block_compress) {
			for (code = 255; code >= 0; code--)
				tab_prefixof(code) = 0;
			clear_flg = 1;
			free_ent = FIRST - 1;
			if ((code = getcode()) == -1)	/* O, untimely death! */
				break;
		}
		incode = code;
		/*
		 * Special case for KwKwK string.
		 */
		if (code >= free_ent) {
			*stackp++ = finchar;
			code = oldcode;
		}

		/*
		 * Generate output characters in reverse order
		 */
		while (code >= 256) {
			*stackp++ = tab_suffixof(code);
			code = tab_prefixof(code);
		}
		*stackp++ = finchar = tab_suffixof(code);

		/*
		 * And put them out in forward order
		 */
		do {
			putchar(*--stackp);
		} while (stackp > de_stack);

		/*
		 * Generate the new entry.
		 */
		if ( (code=free_ent) < maxmaxcode ) {
			tab_prefixof(code) = (ushort)oldcode;
			tab_suffixof(code) = finchar;
			free_ent = code+1;
		}
		/*
		 * Remember previous code.
		 */
		oldcode = incode;
	}
	fflush(stdout);
	if(ferror(stdout))
		writeerr();
}

/*
 * TAG( getcode )
 *
 * Read one code from the standard input.  If EOF, return -1.
 * Inputs:
 * 	stdin
 * Outputs:
 * 	code or -1 is returned.
 */
code_int
getcode()
{
	int r_off, bits;
	code_int code;
	static int offset = 0, size = 0;
	static uchar buf[BITS];
	uchar *bp = buf;

	if ( clear_flg > 0 || offset >= size || free_ent > maxcode ) {
		/*
		 * If the next entry will be too big for the current code
		 * size, then we must increase the size.  This implies reading
		 * a new buffer full, too.
		 */
		if ( free_ent > maxcode ) {
			n_bits++;
			if ( n_bits == maxbits )
				maxcode = maxmaxcode; /* won't get any bigger now */
			else
				maxcode = MAXCODE(n_bits);
		}
		if ( clear_flg > 0) {
			maxcode = MAXCODE(n_bits = INIT_BITS);
			clear_flg = 0;
		}
		size = fread(buf, 1, n_bits, stdin);
		if (size <= 0)
			return -1;			/* end of file */
		offset = 0;
		/* Round size down to integral number of codes */
		size = (size << 3) - (n_bits - 1);
	}
	r_off = offset;
	bits = n_bits;
	/*
	 * Get to the first byte.
	 */
	bp += (r_off >> 3);
	r_off &= 7;
	/* Get first part (low order bits) */
	code = (*bp++ >> r_off);
	bits -= (8 - r_off);
	r_off = 8 - r_off;		/* now, offset into code word */
	/* Get any 8 bit parts in the middle (<=1 for up to 16 bits). */
	if (bits >= 8) {
		code |= *bp++ << r_off;
		r_off += 8;