1 /*
2  * jdarith.c
3  *
4  * This file was part of the Independent JPEG Group's software:
5  * Developed 1997-2015 by Guido Vollbeding.
6  * libjpeg-turbo Modifications:
7  * Copyright (C) 2015-2020, D. R. Commander.
8  * For conditions of distribution and use, see the accompanying README.ijg
9  * file.
10  *
11  * This file contains portable arithmetic entropy encoding routines for JPEG
12  * (implementing Recommendation ITU-T T.81 | ISO/IEC 10918-1).
13  *
14  * Both sequential and progressive modes are supported in this single module.
15  *
16  * Suspension is not currently supported in this module.
17  *
18  * NOTE: All referenced figures are from
19  * Recommendation ITU-T T.81 (1992) | ISO/IEC 10918-1:1994.
20  */
21 
22 #define JPEG_INTERNALS
23 #include "jinclude.h"
24 #include "jpeglib.h"
25 
26 
27 #define NEG_1  ((unsigned int)-1)
28 
29 
30 /* Expanded entropy decoder object for arithmetic decoding. */
31 
32 typedef struct {
33   struct jpeg_entropy_decoder pub; /* public fields */
34 
35   JLONG c;       /* C register, base of coding interval + input bit buffer */
36   JLONG a;               /* A register, normalized size of coding interval */
37   int ct;     /* bit shift counter, # of bits left in bit buffer part of C */
38                                                          /* init: ct = -16 */
39                                                          /* run: ct = 0..7 */
40                                                          /* error: ct = -1 */
41   int last_dc_val[MAX_COMPS_IN_SCAN]; /* last DC coef for each component */
42   int dc_context[MAX_COMPS_IN_SCAN]; /* context index for DC conditioning */
43 
44   unsigned int restarts_to_go;  /* MCUs left in this restart interval */
45 
46   /* Pointers to statistics areas (these workspaces have image lifespan) */
47   unsigned char *dc_stats[NUM_ARITH_TBLS];
48   unsigned char *ac_stats[NUM_ARITH_TBLS];
49 
50   /* Statistics bin for coding with fixed probability 0.5 */
51   unsigned char fixed_bin[4];
52 } arith_entropy_decoder;
53 
54 typedef arith_entropy_decoder *arith_entropy_ptr;
55 
56 /* The following two definitions specify the allocation chunk size
57  * for the statistics area.
58  * According to sections F.1.4.4.1.3 and F.1.4.4.2, we need at least
59  * 49 statistics bins for DC, and 245 statistics bins for AC coding.
60  *
61  * We use a compact representation with 1 byte per statistics bin,
62  * thus the numbers directly represent byte sizes.
63  * This 1 byte per statistics bin contains the meaning of the MPS
64  * (more probable symbol) in the highest bit (mask 0x80), and the
65  * index into the probability estimation state machine table
66  * in the lower bits (mask 0x7F).
67  */
68 
69 #define DC_STAT_BINS  64
70 #define AC_STAT_BINS  256
71 
72 
73 LOCAL(int)
get_byte(j_decompress_ptr cinfo)74 get_byte(j_decompress_ptr cinfo)
75 /* Read next input byte; we do not support suspension in this module. */
76 {
77   struct jpeg_source_mgr *src = cinfo->src;
78 
79   if (src->bytes_in_buffer == 0)
80     if (!(*src->fill_input_buffer) (cinfo))
81       ERREXIT(cinfo, JERR_CANT_SUSPEND);
82   src->bytes_in_buffer--;
83   return *src->next_input_byte++;
84 }
85 
86 
87 /*
88  * The core arithmetic decoding routine (common in JPEG and JBIG).
89  * This needs to go as fast as possible.
90  * Machine-dependent optimization facilities
91  * are not utilized in this portable implementation.
92  * However, this code should be fairly efficient and
93  * may be a good base for further optimizations anyway.
94  *
95  * Return value is 0 or 1 (binary decision).
96  *
97  * Note: I've changed the handling of the code base & bit
98  * buffer register C compared to other implementations
99  * based on the standards layout & procedures.
100  * While it also contains both the actual base of the
101  * coding interval (16 bits) and the next-bits buffer,
102  * the cut-point between these two parts is floating
103  * (instead of fixed) with the bit shift counter CT.
104  * Thus, we also need only one (variable instead of
105  * fixed size) shift for the LPS/MPS decision, and
106  * we can do away with any renormalization update
107  * of C (except for new data insertion, of course).
108  *
109  * I've also introduced a new scheme for accessing
110  * the probability estimation state machine table,
111  * derived from Markus Kuhn's JBIG implementation.
112  */
113 
114 LOCAL(int)
arith_decode(j_decompress_ptr cinfo,unsigned char * st)115 arith_decode(j_decompress_ptr cinfo, unsigned char *st)
116 {
117   register arith_entropy_ptr e = (arith_entropy_ptr)cinfo->entropy;
118   register unsigned char nl, nm;
119   register JLONG qe, temp;
120   register int sv, data;
121 
122   /* Renormalization & data input per section D.2.6 */
123   while (e->a < 0x8000L) {
124     if (--e->ct < 0) {
125       /* Need to fetch next data byte */
126       if (cinfo->unread_marker)
127         data = 0;               /* stuff zero data */
128       else {
129         data = get_byte(cinfo); /* read next input byte */
130         if (data == 0xFF) {     /* zero stuff or marker code */
131           do data = get_byte(cinfo);
132           while (data == 0xFF); /* swallow extra 0xFF bytes */
133           if (data == 0)
134             data = 0xFF;        /* discard stuffed zero byte */
135           else {
136             /* Note: Different from the Huffman decoder, hitting
137              * a marker while processing the compressed data
138              * segment is legal in arithmetic coding.
139              * The convention is to supply zero data
140              * then until decoding is complete.
141              */
142             cinfo->unread_marker = data;
143             data = 0;
144           }
145         }
146       }
147       e->c = (e->c << 8) | data; /* insert data into C register */
148       if ((e->ct += 8) < 0)      /* update bit shift counter */
149         /* Need more initial bytes */
150         if (++e->ct == 0)
151           /* Got 2 initial bytes -> re-init A and exit loop */
152           e->a = 0x8000L; /* => e->a = 0x10000L after loop exit */
153     }
154     e->a <<= 1;
155   }
156 
157   /* Fetch values from our compact representation of Table D.2:
158    * Qe values and probability estimation state machine
159    */
160   sv = *st;
161   qe = jpeg_aritab[sv & 0x7F];  /* => Qe_Value */
162   nl = qe & 0xFF;  qe >>= 8;    /* Next_Index_LPS + Switch_MPS */
163   nm = qe & 0xFF;  qe >>= 8;    /* Next_Index_MPS */
164 
165   /* Decode & estimation procedures per sections D.2.4 & D.2.5 */
166   temp = e->a - qe;
167   e->a = temp;
168   temp <<= e->ct;
169   if (e->c >= temp) {
170     e->c -= temp;
171     /* Conditional LPS (less probable symbol) exchange */
172     if (e->a < qe) {
173       e->a = qe;
174       *st = (sv & 0x80) ^ nm;   /* Estimate_after_MPS */
175     } else {
176       e->a = qe;
177       *st = (sv & 0x80) ^ nl;   /* Estimate_after_LPS */
178       sv ^= 0x80;               /* Exchange LPS/MPS */
179     }
180   } else if (e->a < 0x8000L) {
181     /* Conditional MPS (more probable symbol) exchange */
182     if (e->a < qe) {
183       *st = (sv & 0x80) ^ nl;   /* Estimate_after_LPS */
184       sv ^= 0x80;               /* Exchange LPS/MPS */
185     } else {
186       *st = (sv & 0x80) ^ nm;   /* Estimate_after_MPS */
187     }
188   }
189 
190   return sv >> 7;
191 }
192 
193 
194 /*
195  * Check for a restart marker & resynchronize decoder.
196  */
197 
198 LOCAL(void)
process_restart(j_decompress_ptr cinfo)199 process_restart(j_decompress_ptr cinfo)
200 {
201   arith_entropy_ptr entropy = (arith_entropy_ptr)cinfo->entropy;
202   int ci;
203   jpeg_component_info *compptr;
204 
205   /* Advance past the RSTn marker */
206   if (!(*cinfo->marker->read_restart_marker) (cinfo))
207     ERREXIT(cinfo, JERR_CANT_SUSPEND);
208 
209   /* Re-initialize statistics areas */
210   for (ci = 0; ci < cinfo->comps_in_scan; ci++) {
211     compptr = cinfo->cur_comp_info[ci];
212     if (!cinfo->progressive_mode || (cinfo->Ss == 0 && cinfo->Ah == 0)) {
213       MEMZERO(entropy->dc_stats[compptr->dc_tbl_no], DC_STAT_BINS);
214       /* Reset DC predictions to 0 */
215       entropy->last_dc_val[ci] = 0;
216       entropy->dc_context[ci] = 0;
217     }
218     if (!cinfo->progressive_mode || cinfo->Ss) {
219       MEMZERO(entropy->ac_stats[compptr->ac_tbl_no], AC_STAT_BINS);
220     }
221   }
222 
223   /* Reset arithmetic decoding variables */
224   entropy->c = 0;
225   entropy->a = 0;
226   entropy->ct = -16;    /* force reading 2 initial bytes to fill C */
227 
228   /* Reset restart counter */
229   entropy->restarts_to_go = cinfo->restart_interval;
230 }
231 
232 
233 /*
234  * Arithmetic MCU decoding.
235  * Each of these routines decodes and returns one MCU's worth of
236  * arithmetic-compressed coefficients.
237  * The coefficients are reordered from zigzag order into natural array order,
238  * but are not dequantized.
239  *
240  * The i'th block of the MCU is stored into the block pointed to by
241  * MCU_data[i].  WE ASSUME THIS AREA IS INITIALLY ZEROED BY THE CALLER.
242  */
243 
244 /*
245  * MCU decoding for DC initial scan (either spectral selection,
246  * or first pass of successive approximation).
247  */
248 
249 METHODDEF(boolean)
decode_mcu_DC_first(j_decompress_ptr cinfo,JBLOCKROW * MCU_data)250 decode_mcu_DC_first(j_decompress_ptr cinfo, JBLOCKROW *MCU_data)
251 {
252   arith_entropy_ptr entropy = (arith_entropy_ptr)cinfo->entropy;
253   JBLOCKROW block;
254   unsigned char *st;
255   int blkn, ci, tbl, sign;
256   int v, m;
257 
258   /* Process restart marker if needed */
259   if (cinfo->restart_interval) {
260     if (entropy->restarts_to_go == 0)
261       process_restart(cinfo);
262     entropy->restarts_to_go--;
263   }
264 
265   if (entropy->ct == -1) return TRUE;   /* if error do nothing */
266 
267   /* Outer loop handles each block in the MCU */
268 
269   for (blkn = 0; blkn < cinfo->blocks_in_MCU; blkn++) {
270     block = MCU_data[blkn];
271     ci = cinfo->MCU_membership[blkn];
272     tbl = cinfo->cur_comp_info[ci]->dc_tbl_no;
273 
274     /* Sections F.2.4.1 & F.1.4.4.1: Decoding of DC coefficients */
275 
276     /* Table F.4: Point to statistics bin S0 for DC coefficient coding */
277     st = entropy->dc_stats[tbl] + entropy->dc_context[ci];
278 
279     /* Figure F.19: Decode_DC_DIFF */
280     if (arith_decode(cinfo, st) == 0)
281       entropy->dc_context[ci] = 0;
282     else {
283       /* Figure F.21: Decoding nonzero value v */
284       /* Figure F.22: Decoding the sign of v */
285       sign = arith_decode(cinfo, st + 1);
286       st += 2;  st += sign;
287       /* Figure F.23: Decoding the magnitude category of v */
288       if ((m = arith_decode(cinfo, st)) != 0) {
289         st = entropy->dc_stats[tbl] + 20;       /* Table F.4: X1 = 20 */
290         while (arith_decode(cinfo, st)) {
291           if ((m <<= 1) == 0x8000) {
292             WARNMS(cinfo, JWRN_ARITH_BAD_CODE);
293             entropy->ct = -1;                   /* magnitude overflow */
294             return TRUE;
295           }
296           st += 1;
297         }
298       }
299       /* Section F.1.4.4.1.2: Establish dc_context conditioning category */
300       if (m < (int)((1L << cinfo->arith_dc_L[tbl]) >> 1))
301         entropy->dc_context[ci] = 0;               /* zero diff category */
302       else if (m > (int)((1L << cinfo->arith_dc_U[tbl]) >> 1))
303         entropy->dc_context[ci] = 12 + (sign * 4); /* large diff category */
304       else
305         entropy->dc_context[ci] = 4 + (sign * 4);  /* small diff category */
306       v = m;
307       /* Figure F.24: Decoding the magnitude bit pattern of v */
308       st += 14;
309       while (m >>= 1)
310         if (arith_decode(cinfo, st)) v |= m;
311       v += 1;  if (sign) v = -v;
312       entropy->last_dc_val[ci] = (entropy->last_dc_val[ci] + v) & 0xffff;
313     }
314 
315     /* Scale and output the DC coefficient (assumes jpeg_natural_order[0]=0) */
316     (*block)[0] = (JCOEF)LEFT_SHIFT(entropy->last_dc_val[ci], cinfo->Al);
317   }
318 
319   return TRUE;
320 }
321 
322 
323 /*
324  * MCU decoding for AC initial scan (either spectral selection,
325  * or first pass of successive approximation).
326  */
327 
328 METHODDEF(boolean)
decode_mcu_AC_first(j_decompress_ptr cinfo,JBLOCKROW * MCU_data)329 decode_mcu_AC_first(j_decompress_ptr cinfo, JBLOCKROW *MCU_data)
330 {
331   arith_entropy_ptr entropy = (arith_entropy_ptr)cinfo->entropy;
332   JBLOCKROW block;
333   unsigned char *st;
334   int tbl, sign, k;
335   int v, m;
336 
337   /* Process restart marker if needed */
338   if (cinfo->restart_interval) {
339     if (entropy->restarts_to_go == 0)
340       process_restart(cinfo);
341     entropy->restarts_to_go--;
342   }
343 
344   if (entropy->ct == -1) return TRUE;   /* if error do nothing */
345 
346   /* There is always only one block per MCU */
347   block = MCU_data[0];
348   tbl = cinfo->cur_comp_info[0]->ac_tbl_no;
349 
350   /* Sections F.2.4.2 & F.1.4.4.2: Decoding of AC coefficients */
351 
352   /* Figure F.20: Decode_AC_coefficients */
353   for (k = cinfo->Ss; k <= cinfo->Se; k++) {
354     st = entropy->ac_stats[tbl] + 3 * (k - 1);
355     if (arith_decode(cinfo, st)) break;         /* EOB flag */
356     while (arith_decode(cinfo, st + 1) == 0) {
357       st += 3;  k++;
358       if (k > cinfo->Se) {
359         WARNMS(cinfo, JWRN_ARITH_BAD_CODE);
360         entropy->ct = -1;                       /* spectral overflow */
361         return TRUE;
362       }
363     }
364     /* Figure F.21: Decoding nonzero value v */
365     /* Figure F.22: Decoding the sign of v */
366     sign = arith_decode(cinfo, entropy->fixed_bin);
367     st += 2;
368     /* Figure F.23: Decoding the magnitude category of v */
369     if ((m = arith_decode(cinfo, st)) != 0) {
370       if (arith_decode(cinfo, st)) {
371         m <<= 1;
372         st = entropy->ac_stats[tbl] +
373              (k <= cinfo->arith_ac_K[tbl] ? 189 : 217);
374         while (arith_decode(cinfo, st)) {
375           if ((m <<= 1) == 0x8000) {
376             WARNMS(cinfo, JWRN_ARITH_BAD_CODE);
377             entropy->ct = -1;                   /* magnitude overflow */
378             return TRUE;
379           }
380           st += 1;
381         }
382       }
383     }
384     v = m;
385     /* Figure F.24: Decoding the magnitude bit pattern of v */
386     st += 14;
387     while (m >>= 1)
388       if (arith_decode(cinfo, st)) v |= m;
389     v += 1;  if (sign) v = -v;
390     /* Scale and output coefficient in natural (dezigzagged) order */
391     (*block)[jpeg_natural_order[k]] = (JCOEF)((unsigned)v << cinfo->Al);
392   }
393 
394   return TRUE;
395 }
396 
397 
398 /*
399  * MCU decoding for DC successive approximation refinement scan.
400  */
401 
402 METHODDEF(boolean)
decode_mcu_DC_refine(j_decompress_ptr cinfo,JBLOCKROW * MCU_data)403 decode_mcu_DC_refine(j_decompress_ptr cinfo, JBLOCKROW *MCU_data)
404 {
405   arith_entropy_ptr entropy = (arith_entropy_ptr)cinfo->entropy;
406   unsigned char *st;
407   int p1, blkn;
408 
409   /* Process restart marker if needed */
410   if (cinfo->restart_interval) {
411     if (entropy->restarts_to_go == 0)
412       process_restart(cinfo);
413     entropy->restarts_to_go--;
414   }
415 
416   st = entropy->fixed_bin;      /* use fixed probability estimation */
417   p1 = 1 << cinfo->Al;          /* 1 in the bit position being coded */
418 
419   /* Outer loop handles each block in the MCU */
420 
421   for (blkn = 0; blkn < cinfo->blocks_in_MCU; blkn++) {
422     /* Encoded data is simply the next bit of the two's-complement DC value */
423     if (arith_decode(cinfo, st))
424       MCU_data[blkn][0][0] |= p1;
425   }
426 
427   return TRUE;
428 }
429 
430 
431 /*
432  * MCU decoding for AC successive approximation refinement scan.
433  */
434 
435 METHODDEF(boolean)
decode_mcu_AC_refine(j_decompress_ptr cinfo,JBLOCKROW * MCU_data)436 decode_mcu_AC_refine(j_decompress_ptr cinfo, JBLOCKROW *MCU_data)
437 {
438   arith_entropy_ptr entropy = (arith_entropy_ptr)cinfo->entropy;
439   JBLOCKROW block;
440   JCOEFPTR thiscoef;
441   unsigned char *st;
442   int tbl, k, kex;
443   int p1, m1;
444 
445   /* Process restart marker if needed */
446   if (cinfo->restart_interval) {
447     if (entropy->restarts_to_go == 0)
448       process_restart(cinfo);
449     entropy->restarts_to_go--;
450   }
451 
452   if (entropy->ct == -1) return TRUE;   /* if error do nothing */
453 
454   /* There is always only one block per MCU */
455   block = MCU_data[0];
456   tbl = cinfo->cur_comp_info[0]->ac_tbl_no;
457 
458   p1 = 1 << cinfo->Al;          /* 1 in the bit position being coded */
459   m1 = (NEG_1) << cinfo->Al;    /* -1 in the bit position being coded */
460 
461   /* Establish EOBx (previous stage end-of-block) index */
462   for (kex = cinfo->Se; kex > 0; kex--)
463     if ((*block)[jpeg_natural_order[kex]]) break;
464 
465   for (k = cinfo->Ss; k <= cinfo->Se; k++) {
466     st = entropy->ac_stats[tbl] + 3 * (k - 1);
467     if (k > kex)
468       if (arith_decode(cinfo, st)) break;       /* EOB flag */
469     for (;;) {
470       thiscoef = *block + jpeg_natural_order[k];
471       if (*thiscoef) {                          /* previously nonzero coef */
472         if (arith_decode(cinfo, st + 2)) {
473           if (*thiscoef < 0)
474             *thiscoef += m1;
475           else
476             *thiscoef += p1;
477         }
478         break;
479       }
480       if (arith_decode(cinfo, st + 1)) {        /* newly nonzero coef */
481         if (arith_decode(cinfo, entropy->fixed_bin))
482           *thiscoef = m1;
483         else
484           *thiscoef = p1;
485         break;
486       }
487       st += 3;  k++;
488       if (k > cinfo->Se) {
489         WARNMS(cinfo, JWRN_ARITH_BAD_CODE);
490         entropy->ct = -1;                       /* spectral overflow */
491         return TRUE;
492       }
493     }
494   }
495 
496   return TRUE;
497 }
498 
499 
500 /*
501  * Decode one MCU's worth of arithmetic-compressed coefficients.
502  */
503 
504 METHODDEF(boolean)
decode_mcu(j_decompress_ptr cinfo,JBLOCKROW * MCU_data)505 decode_mcu(j_decompress_ptr cinfo, JBLOCKROW *MCU_data)
506 {
507   arith_entropy_ptr entropy = (arith_entropy_ptr)cinfo->entropy;
508   jpeg_component_info *compptr;
509   JBLOCKROW block;
510   unsigned char *st;
511   int blkn, ci, tbl, sign, k;
512   int v, m;
513 
514   /* Process restart marker if needed */
515   if (cinfo->restart_interval) {
516     if (entropy->restarts_to_go == 0)
517       process_restart(cinfo);
518     entropy->restarts_to_go--;
519   }
520 
521   if (entropy->ct == -1) return TRUE;   /* if error do nothing */
522 
523   /* Outer loop handles each block in the MCU */
524 
525   for (blkn = 0; blkn < cinfo->blocks_in_MCU; blkn++) {
526     block = MCU_data ? MCU_data[blkn] : NULL;
527     ci = cinfo->MCU_membership[blkn];
528     compptr = cinfo->cur_comp_info[ci];
529 
530     /* Sections F.2.4.1 & F.1.4.4.1: Decoding of DC coefficients */
531 
532     tbl = compptr->dc_tbl_no;
533 
534     /* Table F.4: Point to statistics bin S0 for DC coefficient coding */
535     st = entropy->dc_stats[tbl] + entropy->dc_context[ci];
536 
537     /* Figure F.19: Decode_DC_DIFF */
538     if (arith_decode(cinfo, st) == 0)
539       entropy->dc_context[ci] = 0;
540     else {
541       /* Figure F.21: Decoding nonzero value v */
542       /* Figure F.22: Decoding the sign of v */
543       sign = arith_decode(cinfo, st + 1);
544       st += 2;  st += sign;
545       /* Figure F.23: Decoding the magnitude category of v */
546       if ((m = arith_decode(cinfo, st)) != 0) {
547         st = entropy->dc_stats[tbl] + 20;       /* Table F.4: X1 = 20 */
548         while (arith_decode(cinfo, st)) {
549           if ((m <<= 1) == 0x8000) {
550             WARNMS(cinfo, JWRN_ARITH_BAD_CODE);
551             entropy->ct = -1;                   /* magnitude overflow */
552             return TRUE;
553           }
554           st += 1;
555         }
556       }
557       /* Section F.1.4.4.1.2: Establish dc_context conditioning category */
558       if (m < (int)((1L << cinfo->arith_dc_L[tbl]) >> 1))
559         entropy->dc_context[ci] = 0;               /* zero diff category */
560       else if (m > (int)((1L << cinfo->arith_dc_U[tbl]) >> 1))
561         entropy->dc_context[ci] = 12 + (sign * 4); /* large diff category */
562       else
563         entropy->dc_context[ci] = 4 + (sign * 4);  /* small diff category */
564       v = m;
565       /* Figure F.24: Decoding the magnitude bit pattern of v */
566       st += 14;
567       while (m >>= 1)
568         if (arith_decode(cinfo, st)) v |= m;
569       v += 1;  if (sign) v = -v;
570       entropy->last_dc_val[ci] = (entropy->last_dc_val[ci] + v) & 0xffff;
571     }
572 
573     if (block)
574       (*block)[0] = (JCOEF)entropy->last_dc_val[ci];
575 
576     /* Sections F.2.4.2 & F.1.4.4.2: Decoding of AC coefficients */
577 
578     tbl = compptr->ac_tbl_no;
579 
580     /* Figure F.20: Decode_AC_coefficients */
581     for (k = 1; k <= DCTSIZE2 - 1; k++) {
582       st = entropy->ac_stats[tbl] + 3 * (k - 1);
583       if (arith_decode(cinfo, st)) break;       /* EOB flag */
584       while (arith_decode(cinfo, st + 1) == 0) {
585         st += 3;  k++;
586         if (k > DCTSIZE2 - 1) {
587           WARNMS(cinfo, JWRN_ARITH_BAD_CODE);
588           entropy->ct = -1;                     /* spectral overflow */
589           return TRUE;
590         }
591       }
592       /* Figure F.21: Decoding nonzero value v */
593       /* Figure F.22: Decoding the sign of v */
594       sign = arith_decode(cinfo, entropy->fixed_bin);
595       st += 2;
596       /* Figure F.23: Decoding the magnitude category of v */
597       if ((m = arith_decode(cinfo, st)) != 0) {
598         if (arith_decode(cinfo, st)) {
599           m <<= 1;
600           st = entropy->ac_stats[tbl] +
601                (k <= cinfo->arith_ac_K[tbl] ? 189 : 217);
602           while (arith_decode(cinfo, st)) {
603             if ((m <<= 1) == 0x8000) {
604               WARNMS(cinfo, JWRN_ARITH_BAD_CODE);
605               entropy->ct = -1;                 /* magnitude overflow */
606               return TRUE;
607             }
608             st += 1;
609           }
610         }
611       }
612       v = m;
613       /* Figure F.24: Decoding the magnitude bit pattern of v */
614       st += 14;
615       while (m >>= 1)
616         if (arith_decode(cinfo, st)) v |= m;
617       v += 1;  if (sign) v = -v;
618       if (block)
619         (*block)[jpeg_natural_order[k]] = (JCOEF)v;
620     }
621   }
622 
623   return TRUE;
624 }
625 
626 
627 /*
628  * Initialize for an arithmetic-compressed scan.
629  */
630 
631 METHODDEF(void)
start_pass(j_decompress_ptr cinfo)632 start_pass(j_decompress_ptr cinfo)
633 {
634   arith_entropy_ptr entropy = (arith_entropy_ptr)cinfo->entropy;
635   int ci, tbl;
636   jpeg_component_info *compptr;
637 
638   if (cinfo->progressive_mode) {
639     /* Validate progressive scan parameters */
640     if (cinfo->Ss == 0) {
641       if (cinfo->Se != 0)
642         goto bad;
643     } else {
644       /* need not check Ss/Se < 0 since they came from unsigned bytes */
645       if (cinfo->Se < cinfo->Ss || cinfo->Se > DCTSIZE2 - 1)
646         goto bad;
647       /* AC scans may have only one component */
648       if (cinfo->comps_in_scan != 1)
649         goto bad;
650     }
651     if (cinfo->Ah != 0) {
652       /* Successive approximation refinement scan: must have Al = Ah-1. */
653       if (cinfo->Ah - 1 != cinfo->Al)
654         goto bad;
655     }
656     if (cinfo->Al > 13) {       /* need not check for < 0 */
657 bad:
658       ERREXIT4(cinfo, JERR_BAD_PROGRESSION,
659                cinfo->Ss, cinfo->Se, cinfo->Ah, cinfo->Al);
660     }
661     /* Update progression status, and verify that scan order is legal.
662      * Note that inter-scan inconsistencies are treated as warnings
663      * not fatal errors ... not clear if this is right way to behave.
664      */
665     for (ci = 0; ci < cinfo->comps_in_scan; ci++) {
666       int coefi, cindex = cinfo->cur_comp_info[ci]->component_index;
667       int *coef_bit_ptr = &cinfo->coef_bits[cindex][0];
668       int *prev_coef_bit_ptr =
669         &cinfo->coef_bits[cindex + cinfo->num_components][0];
670       if (cinfo->Ss && coef_bit_ptr[0] < 0) /* AC without prior DC scan */
671         WARNMS2(cinfo, JWRN_BOGUS_PROGRESSION, cindex, 0);
672       for (coefi = MIN(cinfo->Ss, 1); coefi <= MAX(cinfo->Se, 9); coefi++) {
673         if (cinfo->input_scan_number > 1)
674           prev_coef_bit_ptr[coefi] = coef_bit_ptr[coefi];
675         else
676           prev_coef_bit_ptr[coefi] = 0;
677       }
678       for (coefi = cinfo->Ss; coefi <= cinfo->Se; coefi++) {
679         int expected = (coef_bit_ptr[coefi] < 0) ? 0 : coef_bit_ptr[coefi];
680         if (cinfo->Ah != expected)
681           WARNMS2(cinfo, JWRN_BOGUS_PROGRESSION, cindex, coefi);
682         coef_bit_ptr[coefi] = cinfo->Al;
683       }
684     }
685     /* Select MCU decoding routine */
686     if (cinfo->Ah == 0) {
687       if (cinfo->Ss == 0)
688         entropy->pub.decode_mcu = decode_mcu_DC_first;
689       else
690         entropy->pub.decode_mcu = decode_mcu_AC_first;
691     } else {
692       if (cinfo->Ss == 0)
693         entropy->pub.decode_mcu = decode_mcu_DC_refine;
694       else
695         entropy->pub.decode_mcu = decode_mcu_AC_refine;
696     }
697   } else {
698     /* Check that the scan parameters Ss, Se, Ah/Al are OK for sequential JPEG.
699      * This ought to be an error condition, but we make it a warning.
700      */
701     if (cinfo->Ss != 0 || cinfo->Ah != 0 || cinfo->Al != 0 ||
702         (cinfo->Se < DCTSIZE2 && cinfo->Se != DCTSIZE2 - 1))
703       WARNMS(cinfo, JWRN_NOT_SEQUENTIAL);
704     /* Select MCU decoding routine */
705     entropy->pub.decode_mcu = decode_mcu;
706   }
707 
708   /* Allocate & initialize requested statistics areas */
709   for (ci = 0; ci < cinfo->comps_in_scan; ci++) {
710     compptr = cinfo->cur_comp_info[ci];
711     if (!cinfo->progressive_mode || (cinfo->Ss == 0 && cinfo->Ah == 0)) {
712       tbl = compptr->dc_tbl_no;
713       if (tbl < 0 || tbl >= NUM_ARITH_TBLS)
714         ERREXIT1(cinfo, JERR_NO_ARITH_TABLE, tbl);
715       if (entropy->dc_stats[tbl] == NULL)
716         entropy->dc_stats[tbl] = (unsigned char *)(*cinfo->mem->alloc_small)
717           ((j_common_ptr)cinfo, JPOOL_IMAGE, DC_STAT_BINS);
718       MEMZERO(entropy->dc_stats[tbl], DC_STAT_BINS);
719       /* Initialize DC predictions to 0 */
720       entropy->last_dc_val[ci] = 0;
721       entropy->dc_context[ci] = 0;
722     }
723     if (!cinfo->progressive_mode || cinfo->Ss) {
724       tbl = compptr->ac_tbl_no;
725       if (tbl < 0 || tbl >= NUM_ARITH_TBLS)
726         ERREXIT1(cinfo, JERR_NO_ARITH_TABLE, tbl);
727       if (entropy->ac_stats[tbl] == NULL)
728         entropy->ac_stats[tbl] = (unsigned char *)(*cinfo->mem->alloc_small)
729           ((j_common_ptr)cinfo, JPOOL_IMAGE, AC_STAT_BINS);
730       MEMZERO(entropy->ac_stats[tbl], AC_STAT_BINS);
731     }
732   }
733 
734   /* Initialize arithmetic decoding variables */
735   entropy->c = 0;
736   entropy->a = 0;
737   entropy->ct = -16;    /* force reading 2 initial bytes to fill C */
738   entropy->pub.insufficient_data = FALSE;
739 
740   /* Initialize restart counter */
741   entropy->restarts_to_go = cinfo->restart_interval;
742 }
743 
744 
745 /*
746  * Module initialization routine for arithmetic entropy decoding.
747  */
748 
749 GLOBAL(void)
jinit_arith_decoder(j_decompress_ptr cinfo)750 jinit_arith_decoder(j_decompress_ptr cinfo)
751 {
752   arith_entropy_ptr entropy;
753   int i;
754 
755   entropy = (arith_entropy_ptr)
756     (*cinfo->mem->alloc_small) ((j_common_ptr)cinfo, JPOOL_IMAGE,
757                                 sizeof(arith_entropy_decoder));
758   cinfo->entropy = (struct jpeg_entropy_decoder *)entropy;
759   entropy->pub.start_pass = start_pass;
760 
761   /* Mark tables unallocated */
762   for (i = 0; i < NUM_ARITH_TBLS; i++) {
763     entropy->dc_stats[i] = NULL;
764     entropy->ac_stats[i] = NULL;
765   }
766 
767   /* Initialize index for fixed probability estimation */
768   entropy->fixed_bin[0] = 113;
769 
770   if (cinfo->progressive_mode) {
771     /* Create progression status table */
772     int *coef_bit_ptr, ci;
773     cinfo->coef_bits = (int (*)[DCTSIZE2])
774       (*cinfo->mem->alloc_small) ((j_common_ptr)cinfo, JPOOL_IMAGE,
775                                   cinfo->num_components * 2 * DCTSIZE2 *
776                                   sizeof(int));
777     coef_bit_ptr = &cinfo->coef_bits[0][0];
778     for (ci = 0; ci < cinfo->num_components; ci++)
779       for (i = 0; i < DCTSIZE2; i++)
780         *coef_bit_ptr++ = -1;
781   }
782 }
783