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      1 /*	$NetBSD: zopen.c,v 1.16 2022/03/23 11:08:28 andvar Exp $	*/
      2 
      3 /*-
      4  * Copyright (c) 1985, 1986, 1992, 1993
      5  *	The Regents of the University of California.  All rights reserved.
      6  *
      7  * This code is derived from software contributed to Berkeley by
      8  * Diomidis Spinellis and James A. Woods, derived from original
      9  * work by Spencer Thomas and Joseph Orost.
     10  *
     11  * Redistribution and use in source and binary forms, with or without
     12  * modification, are permitted provided that the following conditions
     13  * are met:
     14  * 1. Redistributions of source code must retain the above copyright
     15  *    notice, this list of conditions and the following disclaimer.
     16  * 2. Redistributions in binary form must reproduce the above copyright
     17  *    notice, this list of conditions and the following disclaimer in the
     18  *    documentation and/or other materials provided with the distribution.
     19  * 3. Neither the name of the University nor the names of its contributors
     20  *    may be used to endorse or promote products derived from this software
     21  *    without specific prior written permission.
     22  *
     23  * THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
     24  * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
     25  * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
     26  * ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
     27  * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
     28  * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
     29  * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
     30  * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
     31  * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
     32  * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
     33  * SUCH DAMAGE.
     34  */
     35 
     36 #if defined(LIBC_SCCS) && !defined(lint)
     37 #if 0
     38 static char sccsid[] = "@(#)zopen.c	8.1 (Berkeley) 6/27/93";
     39 #else
     40 static char rcsid[] = "$NetBSD: zopen.c,v 1.16 2022/03/23 11:08:28 andvar Exp $";
     41 #endif
     42 #endif /* LIBC_SCCS and not lint */
     43 
     44 /*-
     45  * fcompress.c - File compression ala IEEE Computer, June 1984.
     46  *
     47  * Compress authors:
     48  *		Spencer W. Thomas	(decvax!utah-cs!thomas)
     49  *		Jim McKie		(decvax!mcvax!jim)
     50  *		Steve Davies		(decvax!vax135!petsd!peora!srd)
     51  *		Ken Turkowski		(decvax!decwrl!turtlevax!ken)
     52  *		James A. Woods		(decvax!ihnp4!ames!jaw)
     53  *		Joe Orost		(decvax!vax135!petsd!joe)
     54  *
     55  * Cleaned up and converted to library returning I/O streams by
     56  * Diomidis Spinellis <dds (at) doc.ic.ac.uk>.
     57  *
     58  * zopen(filename, mode, bits)
     59  *	Returns a FILE * that can be used for read or write.  The modes
     60  *	supported are only "r" and "w".  Seeking is not allowed.  On
     61  *	reading the file is decompressed, on writing it is compressed.
     62  *	The output is compatible with compress(1) with 16 bit tables.
     63  *	Any file produced by compress(1) can be read.
     64  */
     65 
     66 #include <sys/param.h>
     67 #include <sys/stat.h>
     68 
     69 #include <errno.h>
     70 #include <signal.h>
     71 #include <stdio.h>
     72 #include <stdlib.h>
     73 #include <string.h>
     74 #include <unistd.h>
     75 
     76 #define	BITS		16		/* Default bits. */
     77 #define	HSIZE		69001		/* 95% occupancy */
     78 
     79 /* A code_int must be able to hold 2**BITS values of type int, and also -1. */
     80 typedef long code_int;
     81 typedef long count_int;
     82 
     83 typedef u_char char_type;
     84 static char_type magic_header[] =
     85 	{'\037', '\235'};		/* 1F 9D */
     86 
     87 #define	BIT_MASK	0x1f		/* Defines for third byte of header. */
     88 #define	BLOCK_MASK	0x80
     89 
     90 /*
     91  * Masks 0x40 and 0x20 are free.  I think 0x20 should mean that there is
     92  * a fourth header byte (for expansion).
     93  */
     94 #define	INIT_BITS 9			/* Initial number of bits/code. */
     95 
     96 #define	MAXCODE(n_bits)	((1 << (n_bits)) - 1)
     97 
     98 struct s_zstate {
     99 	FILE *zs_fp;			/* File stream for I/O */
    100 	char zs_mode;			/* r or w */
    101 	enum {
    102 		S_START, S_MIDDLE, S_EOF
    103 	} zs_state;			/* State of computation */
    104 	int zs_n_bits;			/* Number of bits/code. */
    105 	int zs_maxbits;			/* User settable max # bits/code. */
    106 	code_int zs_maxcode;		/* Maximum code, given n_bits. */
    107 	code_int zs_maxmaxcode;		/* Should NEVER generate this code. */
    108 	count_int zs_htab [HSIZE];
    109 	u_short zs_codetab [HSIZE];
    110 	code_int zs_hsize;		/* For dynamic table sizing. */
    111 	code_int zs_free_ent;		/* First unused entry. */
    112 	/*
    113 	 * Block compression parameters -- after all codes are used up,
    114 	 * and compression rate changes, start over.
    115 	 */
    116 	int zs_block_compress;
    117 	int zs_clear_flg;
    118 	long zs_ratio;
    119 	count_int zs_checkpoint;
    120 	int zs_offset;
    121 	long zs_in_count;		/* Length of input. */
    122 	long zs_bytes_out;		/* Length of compressed output. */
    123 	long zs_out_count;		/* # of codes output (for debugging). */
    124 	char_type zs_buf[BITS];
    125 	union {
    126 		struct {
    127 			long zs_fcode;
    128 			code_int zs_ent;
    129 			code_int zs_hsize_reg;
    130 			int zs_hshift;
    131 		} w;			/* Write parameters */
    132 		struct {
    133 			char_type *zs_stackp;
    134 			int zs_finchar;
    135 			code_int zs_code, zs_oldcode, zs_incode;
    136 			int zs_roffset, zs_size;
    137 			char_type zs_gbuf[BITS];
    138 		} r;			/* Read parameters */
    139 	} u;
    140 };
    141 
    142 /* Definitions to retain old variable names */
    143 #define	fp		zs->zs_fp
    144 #define	zmode		zs->zs_mode
    145 #define	state		zs->zs_state
    146 #define	n_bits		zs->zs_n_bits
    147 #define	maxbits		zs->zs_maxbits
    148 #define	maxcode		zs->zs_maxcode
    149 #define	maxmaxcode	zs->zs_maxmaxcode
    150 #define	htab		zs->zs_htab
    151 #define	codetab		zs->zs_codetab
    152 #define	hsize		zs->zs_hsize
    153 #define	free_ent	zs->zs_free_ent
    154 #define	block_compress	zs->zs_block_compress
    155 #define	clear_flg	zs->zs_clear_flg
    156 #define	ratio		zs->zs_ratio
    157 #define	checkpoint	zs->zs_checkpoint
    158 #define	offset		zs->zs_offset
    159 #define	in_count	zs->zs_in_count
    160 #define	bytes_out	zs->zs_bytes_out
    161 #define	out_count	zs->zs_out_count
    162 #define	buf		zs->zs_buf
    163 #define	fcode		zs->u.w.zs_fcode
    164 #define	hsize_reg	zs->u.w.zs_hsize_reg
    165 #define	ent		zs->u.w.zs_ent
    166 #define	hshift		zs->u.w.zs_hshift
    167 #define	stackp		zs->u.r.zs_stackp
    168 #define	finchar		zs->u.r.zs_finchar
    169 #define	code		zs->u.r.zs_code
    170 #define	oldcode		zs->u.r.zs_oldcode
    171 #define	incode		zs->u.r.zs_incode
    172 #define	roffset		zs->u.r.zs_roffset
    173 #define	size		zs->u.r.zs_size
    174 #define	gbuf		zs->u.r.zs_gbuf
    175 
    176 /*
    177  * To save much memory, we overlay the table used by compress() with those
    178  * used by decompress().  The tab_prefix table is the same size and type as
    179  * the codetab.  The tab_suffix table needs 2**BITS characters.  We get this
    180  * from the beginning of htab.  The output stack uses the rest of htab, and
    181  * contains characters.  There is plenty of room for any possible stack
    182  * (stack used to be 8000 characters).
    183  */
    184 
    185 #define	htabof(i)	htab[i]
    186 #define	codetabof(i)	codetab[i]
    187 
    188 #define	tab_prefixof(i)	codetabof(i)
    189 #define	tab_suffixof(i)	((char_type *)(htab))[i]
    190 #define	de_stack	((char_type *)&tab_suffixof(1 << BITS))
    191 
    192 #define	CHECK_GAP 10000		/* Ratio check interval. */
    193 
    194 /*
    195  * the next two codes should not be changed lightly, as they must not
    196  * lie within the contiguous general code space.
    197  */
    198 #define	FIRST	257		/* First free entry. */
    199 #define	CLEAR	256		/* Table clear output code. */
    200 
    201 static int	cl_block(struct s_zstate *);
    202 static code_int	getcode(struct s_zstate *);
    203 static int	output(struct s_zstate *, code_int);
    204 static int	zclose(void *);
    205 FILE	       *zopen(const char *, const char *, int);
    206 static int	zread(void *, char *, int);
    207 static int	zwrite(void *, const char *, int);
    208 
    209 /*-
    210  * Algorithm from "A Technique for High Performance Data Compression",
    211  * Terry A. Welch, IEEE Computer Vol 17, No 6 (June 1984), pp 8-19.
    212  *
    213  * Algorithm:
    214  * 	Modified Lempel-Ziv method (LZW).  Basically finds common
    215  * substrings and replaces them with a variable size code.  This is
    216  * deterministic, and can be done on the fly.  Thus, the decompression
    217  * procedure needs no input table, but tracks the way the table was built.
    218  */
    219 
    220 /*-
    221  * compress write
    222  *
    223  * Algorithm:  use open addressing double hashing (no chaining) on the
    224  * prefix code / next character combination.  We do a variant of Knuth's
    225  * algorithm D (vol. 3, sec. 6.4) along with G. Knott's relatively-prime
    226  * secondary probe.  Here, the modular division first probe is gives way
    227  * to a faster exclusive-or manipulation.  Also do block compression with
    228  * an adaptive reset, whereby the code table is cleared when the compression
    229  * ratio decreases, but after the table fills.  The variable-length output
    230  * codes are re-sized at this point, and a special CLEAR code is generated
    231  * for the decompressor.  Late addition:  construct the table according to
    232  * file size for noticeable speed improvement on small files.  Please direct
    233  * questions about this implementation to ames!jaw.
    234  */
    235 static int
    236 zwrite(void *cookie, const char *wbp, int num)
    237 {
    238 	code_int i;
    239 	int c, disp;
    240 	struct s_zstate *zs;
    241 	const u_char *bp;
    242 	u_char tmp;
    243 	int count;
    244 
    245 	if (num == 0)
    246 		return (0);
    247 
    248 	zs = cookie;
    249 	count = num;
    250 	bp = (const u_char *)wbp;
    251 	if (state == S_MIDDLE)
    252 		goto middle;
    253 	state = S_MIDDLE;
    254 
    255 	maxmaxcode = 1L << maxbits;
    256 	if (fwrite(magic_header,
    257 	    sizeof(char), sizeof(magic_header), fp) != sizeof(magic_header))
    258 		return (-1);
    259 	tmp = (u_char)(maxbits | block_compress);
    260 	if (fwrite(&tmp, sizeof(char), sizeof(tmp), fp) != sizeof(tmp))
    261 		return (-1);
    262 
    263 	offset = 0;
    264 	bytes_out = 3;		/* Includes 3-byte header mojo. */
    265 	out_count = 0;
    266 	clear_flg = 0;
    267 	ratio = 0;
    268 	in_count = 1;
    269 	checkpoint = CHECK_GAP;
    270 	maxcode = MAXCODE(n_bits = INIT_BITS);
    271 	free_ent = ((block_compress) ? FIRST : 256);
    272 
    273 	ent = *bp++;
    274 	--count;
    275 
    276 	hshift = 0;
    277 	for (fcode = (long)hsize; fcode < 65536L; fcode *= 2L)
    278 		hshift++;
    279 	hshift = 8 - hshift;	/* Set hash code range bound. */
    280 
    281 	hsize_reg = hsize;
    282 	memset(htab, 0xff, hsize_reg * sizeof(count_int));
    283 
    284 middle:	while (count--) {
    285 		c = *bp++;
    286 		in_count++;
    287 		fcode = (long)(((long)c << maxbits) + ent);
    288 		i = ((c << hshift) ^ ent);	/* Xor hashing. */
    289 
    290 		if (htabof(i) == fcode) {
    291 			ent = codetabof(i);
    292 			continue;
    293 		} else if ((long)htabof(i) < 0)	/* Empty slot. */
    294 			goto nomatch;
    295 		disp = hsize_reg - i;	/* Secondary hash (after G. Knott). */
    296 		if (i == 0)
    297 			disp = 1;
    298 probe:		if ((i -= disp) < 0)
    299 			i += hsize_reg;
    300 
    301 		if (htabof(i) == fcode) {
    302 			ent = codetabof(i);
    303 			continue;
    304 		}
    305 		if ((long)htabof(i) >= 0)
    306 			goto probe;
    307 nomatch:	if (output(zs, (code_int) ent) == -1)
    308 			return (-1);
    309 		out_count++;
    310 		ent = c;
    311 		if (free_ent < maxmaxcode) {
    312 			codetabof(i) = free_ent++;	/* code -> hashtable */
    313 			htabof(i) = fcode;
    314 		} else if ((count_int)in_count >=
    315 		    checkpoint && block_compress) {
    316 			if (cl_block(zs) == -1)
    317 				return (-1);
    318 		}
    319 	}
    320 	return (num);
    321 }
    322 
    323 static int
    324 zclose(void *cookie)
    325 {
    326 	struct s_zstate *zs;
    327 	int rval;
    328 
    329 	zs = cookie;
    330 	if (zmode == 'w') {		/* Put out the final code. */
    331 		if (output(zs, (code_int) ent) == -1) {
    332 			(void)fclose(fp);
    333 			free(zs);
    334 			return (-1);
    335 		}
    336 		out_count++;
    337 		if (output(zs, (code_int) - 1) == -1) {
    338 			(void)fclose(fp);
    339 			free(zs);
    340 			return (-1);
    341 		}
    342 	}
    343 	rval = fclose(fp) == EOF ? -1 : 0;
    344 	free(zs);
    345 	return (rval);
    346 }
    347 
    348 /*-
    349  * Output the given code.
    350  * Inputs:
    351  * 	code:	A n_bits-bit integer.  If == -1, then EOF.  This assumes
    352  *		that n_bits =< (long)wordsize - 1.
    353  * Outputs:
    354  * 	Outputs code to the file.
    355  * Assumptions:
    356  *	Chars are 8 bits long.
    357  * Algorithm:
    358  * 	Maintain a BITS character long buffer (so that 8 codes will
    359  * fit in it exactly).  Use the VAX insv instruction to insert each
    360  * code in turn.  When the buffer fills up empty it and start over.
    361  */
    362 
    363 static char_type lmask[9] =
    364 	{0xff, 0xfe, 0xfc, 0xf8, 0xf0, 0xe0, 0xc0, 0x80, 0x00};
    365 static char_type rmask[9] =
    366 	{0x00, 0x01, 0x03, 0x07, 0x0f, 0x1f, 0x3f, 0x7f, 0xff};
    367 
    368 static int
    369 output(struct s_zstate *zs, code_int ocode)
    370 {
    371 	int bits, r_off;
    372 	char_type *bp;
    373 
    374 	r_off = offset;
    375 	bits = n_bits;
    376 	bp = buf;
    377 	if (ocode >= 0) {
    378 		/* Get to the first byte. */
    379 		bp += (r_off >> 3);
    380 		r_off &= 7;
    381 		/*
    382 		 * Since ocode is always >= 8 bits, only need to mask the first
    383 		 * hunk on the left.
    384 		 */
    385 		*bp = (*bp & rmask[r_off]) | ((ocode << r_off) & lmask[r_off]);
    386 		bp++;
    387 		bits -= (8 - r_off);
    388 		ocode >>= 8 - r_off;
    389 		/* Get any 8 bit parts in the middle (<=1 for up to 16 bits). */
    390 		if (bits >= 8) {
    391 			*bp++ = ocode;
    392 			ocode >>= 8;
    393 			bits -= 8;
    394 		}
    395 		/* Last bits. */
    396 		if (bits)
    397 			*bp = ocode;
    398 		offset += n_bits;
    399 		if (offset == (n_bits << 3)) {
    400 			bp = buf;
    401 			bits = n_bits;
    402 			bytes_out += bits;
    403 			if (fwrite(bp, sizeof(char), bits, fp) != (size_t)bits)
    404 				return (-1);
    405 			bp += bits;
    406 			bits = 0;
    407 			offset = 0;
    408 		}
    409 		/*
    410 		 * If the next entry is going to be too big for the ocode size,
    411 		 * then increase it, if possible.
    412 		 */
    413 		if (free_ent > maxcode || (clear_flg > 0)) {
    414 		       /*
    415 			* Write the whole buffer, because the input side won't
    416 			* discover the size increase until after it has read it.
    417 			*/
    418 			if (offset > 0) {
    419 				if (fwrite(buf, 1, n_bits, fp) != (size_t)n_bits)
    420 					return (-1);
    421 				bytes_out += n_bits;
    422 			}
    423 			offset = 0;
    424 
    425 			if (clear_flg) {
    426 				maxcode = MAXCODE(n_bits = INIT_BITS);
    427 				clear_flg = 0;
    428 			} else {
    429 				n_bits++;
    430 				if (n_bits == maxbits)
    431 					maxcode = maxmaxcode;
    432 				else
    433 					maxcode = MAXCODE(n_bits);
    434 			}
    435 		}
    436 	} else {
    437 		/* At EOF, write the rest of the buffer. */
    438 		if (offset > 0) {
    439 			offset = (offset + 7) / 8;
    440 			if (fwrite(buf, 1, offset, fp) != (size_t)offset)
    441 				return (-1);
    442 			bytes_out += offset;
    443 		}
    444 		offset = 0;
    445 	}
    446 	return (0);
    447 }
    448 
    449 /*
    450  * Decompress read.  This routine adapts to the codes in the file building
    451  * the "string" table on-the-fly; requiring no table to be stored in the
    452  * compressed file.  The tables used herein are shared with those of the
    453  * compress() routine.  See the definitions above.
    454  */
    455 static int
    456 zread(void *cookie, char *rbp, int num)
    457 {
    458 	u_int count;
    459 	struct s_zstate *zs;
    460 	u_char *bp, header[3];
    461 
    462 	if (num == 0)
    463 		return (0);
    464 
    465 	zs = cookie;
    466 	count = num;
    467 	bp = (u_char *)rbp;
    468 	switch (state) {
    469 	case S_START:
    470 		state = S_MIDDLE;
    471 		break;
    472 	case S_MIDDLE:
    473 		goto middle;
    474 	case S_EOF:
    475 		goto eof;
    476 	}
    477 
    478 	/* Check the magic number */
    479 	if (fread(header,
    480 	    sizeof(char), sizeof(header), fp) != sizeof(header) ||
    481 	    memcmp(header, magic_header, sizeof(magic_header)) != 0) {
    482 		errno = EFTYPE;
    483 		return (-1);
    484 	}
    485 	maxbits = header[2];	/* Set -b from file. */
    486 	block_compress = maxbits & BLOCK_MASK;
    487 	maxbits &= BIT_MASK;
    488 	maxmaxcode = 1L << maxbits;
    489 	if (maxbits > BITS || maxbits < 12) {
    490 		errno = EFTYPE;
    491 		return (-1);
    492 	}
    493 	/* As above, initialize the first 256 entries in the table. */
    494 	maxcode = MAXCODE(n_bits = INIT_BITS);
    495 	for (code = 255; code >= 0; code--) {
    496 		tab_prefixof(code) = 0;
    497 		tab_suffixof(code) = (char_type) code;
    498 	}
    499 	free_ent = block_compress ? FIRST : 256;
    500 	oldcode = -1;
    501 	stackp = de_stack;
    502 
    503 	while ((code = getcode(zs)) > -1) {
    504 
    505 		if ((code == CLEAR) && block_compress) {
    506 			for (code = 255; code >= 0; code--)
    507 				tab_prefixof(code) = 0;
    508 			clear_flg = 1;
    509 			free_ent = FIRST;
    510 			oldcode = -1;
    511 			continue;
    512 		}
    513 		incode = code;
    514 
    515 		/* Special case for kWkWk string. */
    516 		if (code >= free_ent) {
    517 			if (code > free_ent || oldcode == -1) {
    518 				/* Bad stream. */
    519 				errno = EINVAL;
    520 				return (-1);
    521 			}
    522 			*stackp++ = finchar;
    523 			code = oldcode;
    524 		}
    525 		/*
    526 		 * The above condition ensures that code < free_ent.
    527 		 * The construction of tab_prefixof in turn guarantees that
    528 		 * each iteration decreases code and therefore stack usage is
    529 		 * bound by 1 << BITS - 256.
    530 		 */
    531 
    532 		/* Generate output characters in reverse order. */
    533 		while (code >= 256) {
    534 			*stackp++ = tab_suffixof(code);
    535 			code = tab_prefixof(code);
    536 		}
    537 		*stackp++ = finchar = tab_suffixof(code);
    538 
    539 		/* And put them out in forward order.  */
    540 middle:		do {
    541 			if (count-- == 0)
    542 				return (num);
    543 			*bp++ = *--stackp;
    544 		} while (stackp > de_stack);
    545 
    546 		/* Generate the new entry. */
    547 		if ((code = free_ent) < maxmaxcode && oldcode != -1) {
    548 			tab_prefixof(code) = (u_short) oldcode;
    549 			tab_suffixof(code) = finchar;
    550 			free_ent = code + 1;
    551 		}
    552 
    553 		/* Remember previous code. */
    554 		oldcode = incode;
    555 	}
    556 	state = S_EOF;
    557 eof:	return (num - count);
    558 }
    559 
    560 /*-
    561  * Read one code from the standard input.  If EOF, return -1.
    562  * Inputs:
    563  * 	stdin
    564  * Outputs:
    565  * 	code or -1 is returned.
    566  */
    567 static code_int
    568 getcode(struct s_zstate *zs)
    569 {
    570 	code_int gcode;
    571 	int r_off, bits;
    572 	char_type *bp;
    573 
    574 	bp = gbuf;
    575 	if (clear_flg > 0 || roffset >= size || free_ent > maxcode) {
    576 		/*
    577 		 * If the next entry will be too big for the current gcode
    578 		 * size, then we must increase the size.  This implies reading
    579 		 * a new buffer full, too.
    580 		 */
    581 		if (free_ent > maxcode) {
    582 			n_bits++;
    583 			if (n_bits == maxbits)	/* Won't get any bigger now. */
    584 				maxcode = maxmaxcode;
    585 			else
    586 				maxcode = MAXCODE(n_bits);
    587 		}
    588 		if (clear_flg > 0) {
    589 			maxcode = MAXCODE(n_bits = INIT_BITS);
    590 			clear_flg = 0;
    591 		}
    592 		size = fread(gbuf, 1, n_bits, fp);
    593 		if (size <= 0)			/* End of file. */
    594 			return (-1);
    595 		roffset = 0;
    596 		/* Round size down to integral number of codes. */
    597 		size = (size << 3) - (n_bits - 1);
    598 	}
    599 	r_off = roffset;
    600 	bits = n_bits;
    601 
    602 	/* Get to the first byte. */
    603 	bp += (r_off >> 3);
    604 	r_off &= 7;
    605 
    606 	/* Get first part (low order bits). */
    607 	gcode = (*bp++ >> r_off);
    608 	bits -= (8 - r_off);
    609 	r_off = 8 - r_off;	/* Now, roffset into gcode word. */
    610 
    611 	/* Get any 8 bit parts in the middle (<=1 for up to 16 bits). */
    612 	if (bits >= 8) {
    613 		gcode |= *bp++ << r_off;
    614 		r_off += 8;
    615 		bits -= 8;
    616 	}
    617 
    618 	/* High order bits. */
    619 	gcode |= (*bp & rmask[bits]) << r_off;
    620 	roffset += n_bits;
    621 
    622 	return (gcode);
    623 }
    624 
    625 static int
    626 cl_block(struct s_zstate *zs)		/* Table clear for block compress. */
    627 {
    628 	long rat;
    629 
    630 	checkpoint = in_count + CHECK_GAP;
    631 
    632 	if (in_count > 0x007fffff) {	/* Shift will overflow. */
    633 		rat = bytes_out >> 8;
    634 		if (rat == 0)		/* Don't divide by zero. */
    635 			rat = 0x7fffffff;
    636 		else
    637 			rat = in_count / rat;
    638 	} else
    639 		rat = (in_count << 8) / bytes_out;	/* 8 fractional bits. */
    640 	if (rat > ratio)
    641 		ratio = rat;
    642 	else {
    643 		ratio = 0;
    644 		memset(htab, 0xff, hsize * sizeof(count_int));
    645 		free_ent = FIRST;
    646 		clear_flg = 1;
    647 		if (output(zs, (code_int) CLEAR) == -1)
    648 			return (-1);
    649 	}
    650 	return (0);
    651 }
    652 
    653 FILE *
    654 zopen(const char *fname, const char *mode, int bits)
    655 {
    656 	struct s_zstate *zs;
    657 
    658 	if ((mode[0] != 'r' && mode[0] != 'w') || mode[1] != '\0' ||
    659 	    bits < 0 || bits > BITS) {
    660 		errno = EINVAL;
    661 		return (NULL);
    662 	}
    663 
    664 	if ((zs = calloc(1, sizeof(struct s_zstate))) == NULL)
    665 		return (NULL);
    666 
    667 	maxbits = bits ? bits : BITS;	/* User settable max # bits/code. */
    668 	maxmaxcode = 1 << maxbits;	/* Should NEVER generate this code. */
    669 	hsize = HSIZE;			/* For dynamic table sizing. */
    670 	free_ent = 0;			/* First unused entry. */
    671 	block_compress = BLOCK_MASK;
    672 	clear_flg = 0;
    673 	ratio = 0;
    674 	checkpoint = CHECK_GAP;
    675 	in_count = 1;			/* Length of input. */
    676 	out_count = 0;			/* # of codes output (for debugging). */
    677 	state = S_START;
    678 	roffset = 0;
    679 	size = 0;
    680 
    681 	/*
    682 	 * Layering compress on top of stdio in order to provide buffering,
    683 	 * and ensure that reads and write work with the data specified.
    684 	 */
    685 	if ((fp = fopen(fname, mode)) == NULL) {
    686 		free(zs);
    687 		return (NULL);
    688 	}
    689 	switch (*mode) {
    690 	case 'r':
    691 		zmode = 'r';
    692 		return (funopen(zs, zread, NULL, NULL, zclose));
    693 	case 'w':
    694 		zmode = 'w';
    695 		return (funopen(zs, NULL, zwrite, NULL, zclose));
    696 	}
    697 	/* NOTREACHED */
    698 	return (NULL);
    699 }
    700