summaryrefslogtreecommitdiff
path: root/archival/libunarchive/decompress_bunzip2.c
blob: 8694a32cdd63a30bd40744e977780d377d45ac6e (plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
/* vi: set sw=4 ts=4: */
/*	Small bzip2 deflate implementation, by Rob Landley (rob@landley.net).

	Based on bzip2 decompression code by Julian R Seward (jseward@acm.org),
	which also acknowledges contributions by Mike Burrows, David Wheeler,
	Peter Fenwick, Alistair Moffat, Radford Neal, Ian H. Witten,
	Robert Sedgewick, and Jon L. Bentley.

	This code is licensed under the LGPLv2:
		LGPL http://www.gnu.org/copyleft/lgpl.html
*/

/*
	Size and speed optimizations by Manuel Novoa III  (mjn3@codepoet.org).

	More efficient reading of Huffman codes, a streamlined read_bunzip()
	function, and various other tweaks.  In (limited) tests, approximately
	20% faster than bzcat on x86 and about 10% faster on arm.

	Note that about 2/3 of the time is spent in read_unzip() reversing
	the Burrows-Wheeler transformation.  Much of that time is delay
	resulting from cache misses.

	I would ask that anyone benefiting from this work, especially those
	using it in commercial products, consider making a donation to my local
	non-profit hospice organization (www.hospiceacadiana.com) in the name of
   	the woman I loved, Toni W. Hagan, who passed away Feb. 12, 2003.

	Manuel
 */

#include <setjmp.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <limits.h>

#include "libbb.h"

/* Constants for Huffman coding */
#define MAX_GROUPS			6
#define GROUP_SIZE			50		/* 64 would have been more efficient */
#define MAX_HUFCODE_BITS	20		/* Longest Huffman code allowed */
#define MAX_SYMBOLS			258		/* 256 literals + RUNA + RUNB */
#define SYMBOL_RUNA			0
#define SYMBOL_RUNB			1

/* Status return values */
#define RETVAL_OK						0
#define RETVAL_LAST_BLOCK				(-1)
#define RETVAL_NOT_BZIP_DATA			(-2)
#define RETVAL_UNEXPECTED_INPUT_EOF		(-3)
#define RETVAL_UNEXPECTED_OUTPUT_EOF	(-4)
#define RETVAL_DATA_ERROR				(-5)
#define RETVAL_OUT_OF_MEMORY			(-6)
#define RETVAL_OBSOLETE_INPUT			(-7)

/* Other housekeeping constants */
#define IOBUF_SIZE			4096

/* This is what we know about each Huffman coding group */
struct group_data {
	/* We have an extra slot at the end of limit[] for a sentinal value. */
	int limit[MAX_HUFCODE_BITS+1],base[MAX_HUFCODE_BITS],permute[MAX_SYMBOLS];
	int minLen, maxLen;
};

/* Structure holding all the housekeeping data, including IO buffers and
   memory that persists between calls to bunzip */

typedef struct {
	/* State for interrupting output loop */
	
	int writeCopies,writePos,writeRunCountdown,writeCount,writeCurrent;

	/* I/O tracking data (file handles, buffers, positions, etc.) */

	int in_fd,out_fd,inbufCount,inbufPos /*,outbufPos*/;
	unsigned char *inbuf /*,*outbuf*/;
	unsigned int inbufBitCount, inbufBits;

	/* The CRC values stored in the block header and calculated from the data */

	unsigned int crc32Table[256],headerCRC, totalCRC, writeCRC;

	/* Intermediate buffer and its size (in bytes) */

	unsigned int *dbuf, dbufSize;

	/* These things are a bit too big to go on the stack */

	unsigned char selectors[32768];			/* nSelectors=15 bits */
	struct group_data groups[MAX_GROUPS];	/* Huffman coding tables */

	/* For I/O error handling */

	jmp_buf jmpbuf;
} bunzip_data;

/* Return the next nnn bits of input.  All reads from the compressed input
   are done through this function.  All reads are big endian */

static unsigned int get_bits(bunzip_data *bd, char bits_wanted)
{
	unsigned int bits=0;

	/* If we need to get more data from the byte buffer, do so.  (Loop getting
	   one byte at a time to enforce endianness and avoid unaligned access.) */

	while (bd->inbufBitCount<bits_wanted) {

		/* If we need to read more data from file into byte buffer, do so */

		if(bd->inbufPos==bd->inbufCount) {
			if((bd->inbufCount = read(bd->in_fd, bd->inbuf, IOBUF_SIZE)) <= 0)
				longjmp(bd->jmpbuf,RETVAL_UNEXPECTED_INPUT_EOF);
			bd->inbufPos=0;
		}

		/* Avoid 32-bit overflow (dump bit buffer to top of output) */

		if(bd->inbufBitCount>=24) {
			bits=bd->inbufBits&((1<<bd->inbufBitCount)-1);
			bits_wanted-=bd->inbufBitCount;
			bits<<=bits_wanted;
			bd->inbufBitCount=0;
		}

		/* Grab next 8 bits of input from buffer. */

		bd->inbufBits=(bd->inbufBits<<8)|bd->inbuf[bd->inbufPos++];
		bd->inbufBitCount+=8;
	}

	/* Calculate result */

	bd->inbufBitCount-=bits_wanted;
	bits|=(bd->inbufBits>>bd->inbufBitCount)&((1<<bits_wanted)-1);

	return bits;
}

/* Unpacks the next block and sets up for the inverse burrows-wheeler step. */

static int get_next_block(bunzip_data *bd)
{
	struct group_data *hufGroup;
	int dbufCount,nextSym,dbufSize,groupCount,*base,*limit,selector,
		i,j,k,t,runPos,symCount,symTotal,nSelectors,byteCount[256];
	unsigned char uc, symToByte[256], mtfSymbol[256], *selectors;
	unsigned int *dbuf,origPtr;

	dbuf=bd->dbuf;
	dbufSize=bd->dbufSize;
	selectors=bd->selectors;
	
	/* Reset longjmp I/O error handling */
	
	i=setjmp(bd->jmpbuf);
	if(i) return i;
	
	/* Read in header signature and CRC, then validate signature.
	   (last block signature means CRC is for whole file, return now) */
	
	i = get_bits(bd,24);
	j = get_bits(bd,24);
	bd->headerCRC=get_bits(bd,32);
	if ((i == 0x177245) && (j == 0x385090)) return RETVAL_LAST_BLOCK;
	if ((i != 0x314159) || (j != 0x265359)) return RETVAL_NOT_BZIP_DATA;
	
	/* We can add support for blockRandomised if anybody complains.  There was
	   some code for this in busybox 1.0.0-pre3, but nobody ever noticed that
	   it didn't actually work. */
	
	if(get_bits(bd,1)) return RETVAL_OBSOLETE_INPUT;
	if((origPtr=get_bits(bd,24)) > dbufSize) return RETVAL_DATA_ERROR;
	
	/* mapping table: if some byte values are never used (encoding things
	   like ascii text), the compression code removes the gaps to have fewer
	   symbols to deal with, and writes a sparse bitfield indicating which
	   values were present.  We make a translation table to convert the symbols
	   back to the corresponding bytes. */
	
	t=get_bits(bd, 16);
	symTotal=0;
	for (i=0;i<16;i++) {
		if(t&(1<<(15-i))) {
			k=get_bits(bd,16);
			for(j=0;j<16;j++)
				if(k&(1<<(15-j))) symToByte[symTotal++]=(16*i)+j;
		}
	}
	
	/* How many different Huffman coding groups does this block use? */
	
	groupCount=get_bits(bd,3);
	if (groupCount<2 || groupCount>MAX_GROUPS) return RETVAL_DATA_ERROR;
	
	/* nSelectors: Every GROUP_SIZE many symbols we select a new Huffman coding
	   group.  Read in the group selector list, which is stored as MTF encoded
	   bit runs.  (MTF=Move To Front, as each value is used it's moved to the
	   start of the list.) */
	
	if(!(nSelectors=get_bits(bd, 15))) return RETVAL_DATA_ERROR;
	for(i=0; i<groupCount; i++) mtfSymbol[i] = i;
	for(i=0; i<nSelectors; i++) {
		
		/* Get next value */
		
		for(j=0;get_bits(bd,1);j++) if (j>=groupCount) return RETVAL_DATA_ERROR;
		
		/* Decode MTF to get the next selector */
		
		uc = mtfSymbol[j];
		for(;j;j--) mtfSymbol[j] = mtfSymbol[j-1];
		mtfSymbol[0]=selectors[i]=uc;
	}
	
	/* Read the Huffman coding tables for each group, which code for symTotal
	   literal symbols, plus two run symbols (RUNA, RUNB) */
	
	symCount=symTotal+2;
	for (j=0; j<groupCount; j++) {
		unsigned char length[MAX_SYMBOLS],temp[MAX_HUFCODE_BITS+1];
		int	minLen,	maxLen, pp;
		
		/* Read Huffman code lengths for each symbol.  They're stored in
		   a way similar to mtf; record a starting value for the first symbol,
		   and an offset from the previous value for everys symbol after that.
		   (Subtracting 1 before the loop and then adding it back at the end is
		   an optimization that makes the test inside the loop simpler: symbol
		   length 0 becomes negative, so an unsigned inequality catches it.) */
		
		t=get_bits(bd, 5)-1;
		for (i = 0; i < symCount; i++) {
			for(;;) {
				if (((unsigned)t) > (MAX_HUFCODE_BITS-1))
					return RETVAL_DATA_ERROR;
				
				/* If first bit is 0, stop.  Else second bit indicates whether
				   to increment or decrement the value.  Optimization: grab 2
				   bits and unget the second if the first was 0. */
				
				k = get_bits(bd,2);
				if (k < 2) {
					bd->inbufBitCount++;
					break;
				}
				
				/* Add one if second bit 1, else subtract 1.  Avoids if/else */
				
				t+=(((k+1)&2)-1);
			}
			
			/* Correct for the initial -1, to get the final symbol length */
			
			length[i]=t+1;
		}
		
		/* Find largest and smallest lengths in this group */
		
		minLen=maxLen=length[0];
		for(i = 1; i < symCount; i++) {
			if(length[i] > maxLen) maxLen = length[i];
			else if(length[i] < minLen) minLen = length[i];
		}
		
		/* Calculate permute[], base[], and limit[] tables from length[].
		 *
		 * permute[] is the lookup table for converting Huffman coded symbols
		 * into decoded symbols.  base[] is the amount to subtract from the
		 * value of a Huffman symbol of a given length when using permute[].
		 *
		 * limit[] indicates the largest numerical value a symbol with a given
		 * number of bits can have.  This is how the Huffman codes can vary in
		 * length: each code with a value>limit[length] needs another bit.
		 */
		
		hufGroup=bd->groups+j;
		hufGroup->minLen = minLen;
		hufGroup->maxLen = maxLen;
		
		/* Note that minLen can't be smaller than 1, so we adjust the base
		   and limit array pointers so we're not always wasting the first
		   entry.  We do this again when using them (during symbol decoding).*/
		
		base=hufGroup->base-1;
		limit=hufGroup->limit-1;
		
		/* Calculate permute[].  Concurently, initialize temp[] and limit[]. */
		
		pp=0;
		for(i=minLen;i<=maxLen;i++) {
			temp[i]=limit[i]=0;
			for(t=0;t<symCount;t++)
				if(length[t]==i) hufGroup->permute[pp++] = t;
		}
		
		/* Count symbols coded for at each bit length */
		
		for (i=0;i<symCount;i++) temp[length[i]]++;
		
		/* Calculate limit[] (the largest symbol-coding value at each bit
		 * length, which is (previous limit<<1)+symbols at this level), and
		 * base[] (number of symbols to ignore at each bit length, which is
		 * limit minus the cumulative count of symbols coded for already). */
		
		pp=t=0;
		for (i=minLen; i<maxLen; i++) {
			pp+=temp[i];
			
			/* We read the largest possible symbol size and then unget bits
			   after determining how many we need, and those extra bits could
			   be set to anything.  (They're noise from future symbols.)  At
			   each level we're really only interested in the first few bits,
			   so here we set all the trailing to-be-ignored bits to 1 so they
			   don't affect the value>limit[length] comparison. */
			
			limit[i]= (pp << (maxLen - i)) - 1;
			pp<<=1;
			base[i+1]=pp-(t+=temp[i]);
		}
		limit[maxLen+1] = INT_MAX; /* Sentinal value for reading next sym. */
		limit[maxLen]=pp+temp[maxLen]-1;
		base[minLen]=0;
	}
	
	/* We've finished reading and digesting the block header.  Now read this
	   block's Huffman coded symbols from the file and undo the Huffman coding
	   and run length encoding, saving the result into dbuf[dbufCount++]=uc */

	/* Initialize symbol occurrence counters and symbol Move To Front table */
	
	for(i=0;i<256;i++) {
		byteCount[i] = 0;
		mtfSymbol[i]=(unsigned char)i;
	}
	
	/* Loop through compressed symbols. */
	
	runPos=dbufCount=selector=0;
	for(;;) {
		
		/* fetch next Huffman coding group from list. */
		
		symCount=GROUP_SIZE-1;
		if(selector>=nSelectors) return RETVAL_DATA_ERROR;
		hufGroup=bd->groups+selectors[selector++];
		base=hufGroup->base-1;
		limit=hufGroup->limit-1;
continue_this_group:
		
		/* Read next Huffman-coded symbol. */
		
		/* Note: It is far cheaper to read maxLen bits and back up than it is
		   to read minLen bits and then an additional bit at a time, testing
		   as we go.  Because there is a trailing last block (with file CRC),
		   there is no danger of the overread causing an unexpected EOF for a
		   valid compressed file.  As a further optimization, we do the read
		   inline (falling back to a call to get_bits if the buffer runs
		   dry).  The following (up to got_huff_bits:) is equivalent to
		   j=get_bits(bd,hufGroup->maxLen);
		 */
		
		while (bd->inbufBitCount<hufGroup->maxLen) {
			if(bd->inbufPos==bd->inbufCount) {
				j = get_bits(bd,hufGroup->maxLen);
				goto got_huff_bits;
			}
			bd->inbufBits=(bd->inbufBits<<8)|bd->inbuf[bd->inbufPos++];
			bd->inbufBitCount+=8;
		};
		bd->inbufBitCount-=hufGroup->maxLen;
		j = (bd->inbufBits>>bd->inbufBitCount)&((1<<hufGroup->maxLen)-1);
		
got_huff_bits:
		
		/* Figure how how many bits are in next symbol and unget extras */
		
		i=hufGroup->minLen;
		while(j>limit[i]) ++i;
		bd->inbufBitCount += (hufGroup->maxLen - i);
		
		/* Huffman decode value to get nextSym (with bounds checking) */
		
		if ((i > hufGroup->maxLen)
			|| (((unsigned)(j=(j>>(hufGroup->maxLen-i))-base[i]))
				>= MAX_SYMBOLS))
			return RETVAL_DATA_ERROR;
		nextSym = hufGroup->permute[j];
		
		/* We have now decoded the symbol, which indicates either a new literal
		   byte, or a repeated run of the most recent literal byte.  First,
		   check if nextSym indicates a repeated run, and if so loop collecting
		   how many times to repeat the last literal. */
		
		if (((unsigned)nextSym) <= SYMBOL_RUNB) { /* RUNA or RUNB */
			
			/* If this is the start of a new run, zero out counter */
			
			if(!runPos) {
				runPos = 1;
				t = 0;
			}
			
			/* Neat trick that saves 1 symbol: instead of or-ing 0 or 1 at
			   each bit position, add 1 or 2 instead.  For example,
			   1011 is 1<<0 + 1<<1 + 2<<2.  1010 is 2<<0 + 2<<1 + 1<<2.
			   You can make any bit pattern that way using 1 less symbol than
			   the basic or 0/1 method (except all bits 0, which would use no
			   symbols, but a run of length 0 doesn't mean anything in this
			   context).  Thus space is saved. */
			
			t += (runPos << nextSym); /* +runPos if RUNA; +2*runPos if RUNB */
			runPos <<= 1;
			goto end_of_huffman_loop;
		}
		
		/* When we hit the first non-run symbol after a run, we now know
		   how many times to repeat the last literal, so append that many
		   copies to our buffer of decoded symbols (dbuf) now.  (The last
		   literal used is the one at the head of the mtfSymbol array.) */
		
		if(runPos) {
			runPos=0;
			if(dbufCount+t>=dbufSize) return RETVAL_DATA_ERROR;

			uc = symToByte[mtfSymbol[0]];
			byteCount[uc] += t;
			while(t--) dbuf[dbufCount++]=uc;
		}
		
		/* Is this the terminating symbol? */
		
		if(nextSym>symTotal) break;
		
		/* At this point, nextSym indicates a new literal character.  Subtract
		   one to get the position in the MTF array at which this literal is
		   currently to be found.  (Note that the result can't be -1 or 0,
		   because 0 and 1 are RUNA and RUNB.  But another instance of the
		   first symbol in the mtf array, position 0, would have been handled
		   as part of a run above.  Therefore 1 unused mtf position minus
		   2 non-literal nextSym values equals -1.) */
		
		if(dbufCount>=dbufSize) return RETVAL_DATA_ERROR;
		i = nextSym - 1;
		uc = mtfSymbol[i];
		
		/* Adjust the MTF array.  Since we typically expect to move only a
		 * small number of symbols, and are bound by 256 in any case, using
		 * memmove here would typically be bigger and slower due to function
		 * call overhead and other assorted setup costs. */
		
		do {
			mtfSymbol[i] = mtfSymbol[i-1];
		} while (--i);
		mtfSymbol[0] = uc;
		uc=symToByte[uc];
		
		/* We have our literal byte.  Save it into dbuf. */
		
		byteCount[uc]++;
		dbuf[dbufCount++] = (unsigned int)uc;
		
		/* Skip group initialization if we're not done with this group.  Done
		 * this way to avoid compiler warning. */
		
end_of_huffman_loop:
		if(symCount--) goto continue_this_group;
	}

	/* At this point, we've read all the Huffman-coded symbols (and repeated
       runs) for this block from the input stream, and decoded them into the
	   intermediate buffer.  There are dbufCount many decoded bytes in dbuf[].
	   Now undo the Burrows-Wheeler transform on dbuf.
	   See http://dogma.net/markn/articles/bwt/bwt.htm
	 */
	
	/* Turn byteCount into cumulative occurrence counts of 0 to n-1. */

	j=0;
	for(i=0;i<256;i++) {
		k=j+byteCount[i];
		byteCount[i] = j;
		j=k;
	}

	/* Figure out what order dbuf would be in if we sorted it. */

	for (i=0;i<dbufCount;i++) {
		uc=(unsigned char)(dbuf[i] & 0xff);
		dbuf[byteCount[uc]] |= (i << 8);
		byteCount[uc]++;
	}

	/* Decode first byte by hand to initialize "previous" byte.  Note that it
	   doesn't get output, and if the first three characters are identical
	   it doesn't qualify as a run (hence writeRunCountdown=5). */
	
	if(dbufCount) {
		if(origPtr>=dbufCount) return RETVAL_DATA_ERROR;
		bd->writePos=dbuf[origPtr];
	    bd->writeCurrent=(unsigned char)(bd->writePos&0xff);
		bd->writePos>>=8;
		bd->writeRunCountdown=5;
	}
	bd->writeCount=dbufCount;

	return RETVAL_OK;
}

/* Undo burrows-wheeler transform on intermediate buffer to produce output.
   If start_bunzip was initialized with out_fd=-1, then up to len bytes of
   data are written to outbuf.  Return value is number of bytes written or
   error (all errors are negative numbers).  If out_fd!=-1, outbuf and len
   are ignored, data is written to out_fd and return is RETVAL_OK or error.
*/

static int read_bunzip(bunzip_data *bd, char *outbuf, int len)
{
	const unsigned int *dbuf;
	int pos,current,previous,gotcount;

	/* If last read was short due to end of file, return last block now */
	if(bd->writeCount<0) return bd->writeCount;

	gotcount = 0;
	dbuf=bd->dbuf;
	pos=bd->writePos;
	current=bd->writeCurrent;

	/* We will always have pending decoded data to write into the output
	   buffer unless this is the very first call (in which case we haven't
	   Huffman-decoded a block into the intermediate buffer yet). */

	if (bd->writeCopies) {

		/* Inside the loop, writeCopies means extra copies (beyond 1) */

		--bd->writeCopies;

		/* Loop outputting bytes */

		for(;;) {

			/* If the output buffer is full, snapshot state and return */

			if(gotcount >= len) {
				bd->writePos=pos;
				bd->writeCurrent=current;
				bd->writeCopies++;
				return len;
			}

			/* Write next byte into output buffer, updating CRC */

			outbuf[gotcount++] = current;
			bd->writeCRC=(((bd->writeCRC)<<8)
						  ^bd->crc32Table[((bd->writeCRC)>>24)^current]);

			/* Loop now if we're outputting multiple copies of this byte */

			if (bd->writeCopies) {
				--bd->writeCopies;
				continue;
			}
decode_next_byte:
			if (!bd->writeCount--) break;
			/* Follow sequence vector to undo Burrows-Wheeler transform */
			previous=current;
			pos=dbuf[pos];
			current=pos&0xff;
			pos>>=8;

			/* After 3 consecutive copies of the same byte, the 4th is a repeat
			   count.  We count down from 4 instead
			 * of counting up because testing for non-zero is faster */

			if(--bd->writeRunCountdown) {
				if(current!=previous) bd->writeRunCountdown=4;
			} else {

				/* We have a repeated run, this byte indicates the count */

				bd->writeCopies=current;
				current=previous;
				bd->writeRunCountdown=5;

				/* Sometimes there are just 3 bytes (run length 0) */

				if(!bd->writeCopies) goto decode_next_byte;

				/* Subtract the 1 copy we'd output anyway to get extras */

				--bd->writeCopies;
			}
		}

		/* Decompression of this block completed successfully */

		bd->writeCRC=~bd->writeCRC;
		bd->totalCRC=((bd->totalCRC<<1) | (bd->totalCRC>>31)) ^ bd->writeCRC;

		/* If this block had a CRC error, force file level CRC error. */

		if(bd->writeCRC!=bd->headerCRC) {
			bd->totalCRC=bd->headerCRC+1;
			return RETVAL_LAST_BLOCK;
		}
	}

	/* Refill the intermediate buffer by Huffman-decoding next block of input */
	/* (previous is just a convenient unused temp variable here) */

	previous=get_next_block(bd);
	if(previous) {
		bd->writeCount=previous;
		return (previous!=RETVAL_LAST_BLOCK) ? previous : gotcount;
	}
	bd->writeCRC=0xffffffffUL;
	pos=bd->writePos;
	current=bd->writeCurrent;
	goto decode_next_byte;
}

/* Allocate the structure, read file header.  If in_fd==-1, inbuf must contain
   a complete bunzip file (len bytes long).  If in_fd!=-1, inbuf and len are
   ignored, and data is read from file handle into temporary buffer. */

static int start_bunzip(bunzip_data **bdp, int in_fd, unsigned char *inbuf,
						int len)
{
	bunzip_data *bd;
	unsigned int i,j,c;
	const unsigned int BZh0=(((unsigned int)'B')<<24)+(((unsigned int)'Z')<<16)
							+(((unsigned int)'h')<<8)+(unsigned int)'0';

	/* Figure out how much data to allocate */

	i=sizeof(bunzip_data);
	if(in_fd!=-1) i+=IOBUF_SIZE;

	/* Allocate bunzip_data.  Most fields initialize to zero. */

	bd=*bdp=xmalloc(i);
	memset(bd,0,sizeof(bunzip_data));

	/* Setup input buffer */

	if(-1==(bd->in_fd=in_fd)) {
		bd->inbuf=inbuf;
		bd->inbufCount=len;
	} else bd->inbuf=(unsigned char *)(bd+1);

	/* Init the CRC32 table (big endian) */

	for(i=0;i<256;i++) {
		c=i<<24;
		for(j=8;j;j--)
			c=c&0x80000000 ? (c<<1)^0x04c11db7 : (c<<1);
		bd->crc32Table[i]=c;
	}

	/* Setup for I/O error handling via longjmp */

	i=setjmp(bd->jmpbuf);
	if(i) return i;

	/* Ensure that file starts with "BZh['1'-'9']." */

	i = get_bits(bd,32);
	if (((unsigned int)(i-BZh0-1)) >= 9) return RETVAL_NOT_BZIP_DATA;

	/* Fourth byte (ascii '1'-'9'), indicates block size in units of 100k of
	   uncompressed data.  Allocate intermediate buffer for block. */

	bd->dbufSize=100000*(i-BZh0);

	bd->dbuf=xmalloc(bd->dbufSize * sizeof(int));
	return RETVAL_OK;
}

/* Example usage: decompress src_fd to dst_fd.  (Stops at end of bzip data,
   not end of file.) */

extern int uncompressStream(int src_fd, int dst_fd)
{
	char *outbuf;
	bunzip_data *bd;
	int i;

	outbuf=xmalloc(IOBUF_SIZE);
	if(!(i=start_bunzip(&bd,src_fd,0,0))) {
		for(;;) {
			if((i=read_bunzip(bd,outbuf,IOBUF_SIZE)) <= 0) break;
			if(i!=write(dst_fd,outbuf,i)) {
				i=RETVAL_UNEXPECTED_OUTPUT_EOF;
				break;
			}
		}
	}

	/* Check CRC and release memory */

	if(i==RETVAL_LAST_BLOCK) {
		if (bd->headerCRC!=bd->totalCRC) {
			bb_error_msg("Data integrity error when decompressing.");
		} else {
			i=RETVAL_OK;
		}
	} else if (i==RETVAL_UNEXPECTED_OUTPUT_EOF) {
		bb_error_msg("Compressed file ends unexpectedly");
	} else {
		bb_error_msg("Decompression failed");
	}
	if(bd->dbuf) free(bd->dbuf);
	free(bd);
	free(outbuf);

	return i;
}

#ifdef TESTING

static char * const bunzip_errors[]={NULL,"Bad file checksum","Not bzip data",
		"Unexpected input EOF","Unexpected output EOF","Data error",
		 "Out of memory","Obsolete (pre 0.9.5) bzip format not supported."};

/* Dumb little test thing, decompress stdin to stdout */
int main(int argc, char *argv[])
{
	int i=uncompressStream(0,1);
	char c;

	if(i) fprintf(stderr,"%s\n", bunzip_errors[-i]);
    else if(read(0,&c,1)) fprintf(stderr,"Trailing garbage ignored\n");
	return -i;
}
#endif