libs/jpeg/jdarith.c

   1 /*
   2  * jdarith.c
   3  *
   4  * Developed 1997-2020 by Guido Vollbeding.
   5  * This file is part of the Independent JPEG Group's software.
   6  * For conditions of distribution and use, see the accompanying README file.
   7  *
   8  * This file contains portable arithmetic entropy decoding routines for JPEG
   9  * (implementing the ISO/IEC IS 10918-1 and CCITT Recommendation ITU-T T.81).
  10  *
  11  * Both sequential and progressive modes are supported in this single module.
  12  *
  13  * Suspension is not currently supported in this module.
  14  */
  15
  16 #define JPEG_INTERNALS
  17 #include "jinclude.h"
  18 #include "jpeglib.h"
  19
  20
  21 /* Expanded entropy decoder object for arithmetic decoding. */
  22
  23 typedef struct {
  24   struct jpeg_entropy_decoder pub; /* public fields */
  25
  26   INT32 c;       /* C register, base of coding interval + input bit buffer */
  27   INT32 a;               /* A register, normalized size of coding interval */
  28   int ct;     /* bit shift counter, # of bits left in bit buffer part of C */
  29                                                          /* init: ct = -16 */
  30                                                          /* run: ct = 0..7 */
  31                                                          /* error: ct = -1 */
  32   int last_dc_val[MAX_COMPS_IN_SCAN]; /* last DC coef for each component */
  33   int dc_context[MAX_COMPS_IN_SCAN]; /* context index for DC conditioning */
  34
  35   unsigned int restarts_to_go;  /* MCUs left in this restart interval */
  36
  37   /* Pointers to statistics areas (these workspaces have image lifespan) */
  38   unsigned char * dc_stats[NUM_ARITH_TBLS];
  39   unsigned char * ac_stats[NUM_ARITH_TBLS];
  40
  41   /* Statistics bin for coding with fixed probability 0.5 */
  42   unsigned char fixed_bin[4];
  43 } arith_entropy_decoder;
  44
  45 typedef arith_entropy_decoder * arith_entropy_ptr;
  46
  47 /* The following two definitions specify the allocation chunk size
  48  * for the statistics area.
  49  * According to sections F.1.4.4.1.3 and F.1.4.4.2, we need at least
  50  * 49 statistics bins for DC, and 245 statistics bins for AC coding.
  51  *
  52  * We use a compact representation with 1 byte per statistics bin,
  53  * thus the numbers directly represent byte sizes.
  54  * This 1 byte per statistics bin contains the meaning of the MPS
  55  * (more probable symbol) in the highest bit (mask 0x80), and the
  56  * index into the probability estimation state machine table
  57  * in the lower bits (mask 0x7F).
  58  */
  59
  60 #define DC_STAT_BINS 64
  61 #define AC_STAT_BINS 256
  62
  63
  64 LOCAL(int)
  65 get_byte (j_decompress_ptr cinfo)
  66 /* Read next input byte; we do not support suspension in this module. */
  67 {
  68   struct jpeg_source_mgr * src = cinfo->src;
  69
  70   if (src->bytes_in_buffer == 0)
  71     if (! (*src->fill_input_buffer) (cinfo))
  72       ERREXIT(cinfo, JERR_CANT_SUSPEND);
  73   src->bytes_in_buffer--;
  74   return GETJOCTET(*src->next_input_byte++);
  75 }
  76
  77
  78 /*
  79  * The core arithmetic decoding routine (common in JPEG and JBIG).
  80  * This needs to go as fast as possible.
  81  * Machine-dependent optimization facilities
  82  * are not utilized in this portable implementation.
  83  * However, this code should be fairly efficient and
  84  * may be a good base for further optimizations anyway.
  85  *
  86  * Return value is 0 or 1 (binary decision).
  87  *
  88  * Note: I've changed the handling of the code base & bit
  89  * buffer register C compared to other implementations
  90  * based on the standards layout & procedures.
  91  * While it also contains both the actual base of the
  92  * coding interval (16 bits) and the next-bits buffer,
  93  * the cut-point between these two parts is floating
  94  * (instead of fixed) with the bit shift counter CT.
  95  * Thus, we also need only one (variable instead of
  96  * fixed size) shift for the LPS/MPS decision, and
  97  * we can do away with any renormalization update
  98  * of C (except for new data insertion, of course).
  99  *
 100  * I've also introduced a new scheme for accessing
 101  * the probability estimation state machine table,
 102  * derived from Markus Kuhn's JBIG implementation.
 103  */
 104
 105 LOCAL(int)
 106 arith_decode (j_decompress_ptr cinfo, unsigned char *st)
 107 {
 108   register arith_entropy_ptr e = (arith_entropy_ptr) cinfo->entropy;
 109   register unsigned char nl, nm;
 110   register INT32 qe, temp;
 111   register int sv, data;
 112
 113   /* Renormalization & data input per section D.2.6 */
 114   while (e->a < 0x8000L) {
 115     if (--e->ct < 0) {
 116       /* Need to fetch next data byte */
 117       if (cinfo->unread_marker)
 118         data = 0;               /* stuff zero data */
 119       else {
 120         data = get_byte(cinfo); /* read next input byte */
 121         if (data == 0xFF) {     /* zero stuff or marker code */
 122           do data = get_byte(cinfo);
 123           while (data == 0xFF); /* swallow extra 0xFF bytes */
 124           if (data == 0)
 125             data = 0xFF;        /* discard stuffed zero byte */
 126           else {
 127             /* Note: Different from the Huffman decoder, hitting
 128              * a marker while processing the compressed data
 129              * segment is legal in arithmetic coding.
 130              * The convention is to supply zero data
 131              * then until decoding is complete.
 132              */
 133             cinfo->unread_marker = data;
 134             data = 0;
 135           }
 136         }
 137       }
 138       e->c = (e->c << 8) | data; /* insert data into C register */
 139       if ((e->ct += 8) < 0)      /* update bit shift counter */
 140         /* Need more initial bytes */
 141         if (++e->ct == 0)
 142           /* Got 2 initial bytes -> re-init A and exit loop */
 143           e->a = 0x8000L; /* => e->a = 0x10000L after loop exit */
 144     }
 145     e->a <<= 1;
 146   }
 147
 148   /* Fetch values from our compact representation of Table D.3(D.2):
 149    * Qe values and probability estimation state machine
 150    */
 151   sv = *st;
 152   qe = jpeg_aritab[sv & 0x7F];  /* => Qe_Value */
 153   nl = qe & 0xFF; qe >>= 8;     /* Next_Index_LPS + Switch_MPS */
 154   nm = qe & 0xFF; qe >>= 8;     /* Next_Index_MPS */
 155
 156   /* Decode & estimation procedures per sections D.2.4 & D.2.5 */
 157   temp = e->a - qe;
 158   e->a = temp;
 159   temp <<= e->ct;
 160   if (e->c >= temp) {
 161     e->c -= temp;
 162     /* Conditional LPS (less probable symbol) exchange */
 163     if (e->a < qe) {
 164       e->a = qe;
 165       *st = (sv & 0x80) ^ nm;   /* Estimate_after_MPS */
 166     } else {
 167       e->a = qe;
 168       *st = (sv & 0x80) ^ nl;   /* Estimate_after_LPS */
 169       sv ^= 0x80;               /* Exchange LPS/MPS */
 170     }
 171   } else if (e->a < 0x8000L) {
 172     /* Conditional MPS (more probable symbol) exchange */
 173     if (e->a < qe) {
 174       *st = (sv & 0x80) ^ nl;   /* Estimate_after_LPS */
 175       sv ^= 0x80;               /* Exchange LPS/MPS */
 176     } else {
 177       *st = (sv & 0x80) ^ nm;   /* Estimate_after_MPS */
 178     }
 179   }
 180
 181   return sv >> 7;
 182 }
 183
 184
 185 /*
 186  * Check for a restart marker & resynchronize decoder.
 187  */
 188
 189 LOCAL(void)
 190 process_restart (j_decompress_ptr cinfo)
 191 {
 192   arith_entropy_ptr entropy = (arith_entropy_ptr) cinfo->entropy;
 193   int ci;
 194   jpeg_component_info * compptr;
 195
 196   /* Advance past the RSTn marker */
 197   if (! (*cinfo->marker->read_restart_marker) (cinfo))
 198     ERREXIT(cinfo, JERR_CANT_SUSPEND);
 199
 200   /* Re-initialize statistics areas */
 201   for (ci = 0; ci < cinfo->comps_in_scan; ci++) {
 202     compptr = cinfo->cur_comp_info[ci];
 203     if (! cinfo->progressive_mode || (cinfo->Ss == 0 && cinfo->Ah == 0)) {
 204       MEMZERO(entropy->dc_stats[compptr->dc_tbl_no], DC_STAT_BINS);
 205       /* Reset DC predictions to 0 */
 206       entropy->last_dc_val[ci] = 0;
 207       entropy->dc_context[ci] = 0;
 208     }
 209     if ((! cinfo->progressive_mode && cinfo->lim_Se) ||
 210         (cinfo->progressive_mode && cinfo->Ss)) {
 211       MEMZERO(entropy->ac_stats[compptr->ac_tbl_no], AC_STAT_BINS);
 212     }
 213   }
 214
 215   /* Reset arithmetic decoding variables */
 216   entropy->c = 0;
 217   entropy->a = 0;
 218   entropy->ct = -16;    /* force reading 2 initial bytes to fill C */
 219
 220   /* Reset restart counter */
 221   entropy->restarts_to_go = cinfo->restart_interval;
 222 }
 223
 224
 225 /*
 226  * Arithmetic MCU decoding.
 227  * Each of these routines decodes and returns one MCU's worth of
 228  * arithmetic-compressed coefficients.
 229  * The coefficients are reordered from zigzag order into natural array order,
 230  * but are not dequantized.
 231  *
 232  * The i'th block of the MCU is stored into the block pointed to by
 233  * MCU_data[i].  WE ASSUME THIS AREA IS INITIALLY ZEROED BY THE CALLER.
 234  */
 235
 236 /*
 237  * MCU decoding for DC initial scan (either spectral selection,
 238  * or first pass of successive approximation).
 239  */
 240
 241 METHODDEF(boolean)
 242 decode_mcu_DC_first (j_decompress_ptr cinfo, JBLOCKARRAY MCU_data)
 243 {
 244   arith_entropy_ptr entropy = (arith_entropy_ptr) cinfo->entropy;
 245   JBLOCKROW block;
 246   unsigned char *st;
 247   int blkn, ci, tbl, sign;
 248   int v, m;
 249
 250   /* Process restart marker if needed */
 251   if (cinfo->restart_interval) {
 252     if (entropy->restarts_to_go == 0)
 253       process_restart(cinfo);
 254     entropy->restarts_to_go--;
 255   }
 256
 257   if (entropy->ct == -1) return TRUE;   /* if error do nothing */
 258
 259   /* Outer loop handles each block in the MCU */
 260
 261   for (blkn = 0; blkn < cinfo->blocks_in_MCU; blkn++) {
 262     block = MCU_data[blkn];
 263     ci = cinfo->MCU_membership[blkn];
 264     tbl = cinfo->cur_comp_info[ci]->dc_tbl_no;
 265
 266     /* Sections F.2.4.1 & F.1.4.4.1: Decoding of DC coefficients */
 267
 268     /* Table F.4: Point to statistics bin S0 for DC coefficient coding */
 269     st = entropy->dc_stats[tbl] + entropy->dc_context[ci];
 270
 271     /* Figure F.19: Decode_DC_DIFF */
 272     if (arith_decode(cinfo, st) == 0)
 273       entropy->dc_context[ci] = 0;
 274     else {
 275       /* Figure F.21: Decoding nonzero value v */
 276       /* Figure F.22: Decoding the sign of v */
 277       sign = arith_decode(cinfo, st + 1);
 278       st += 2; st += sign;
 279       /* Figure F.23: Decoding the magnitude category of v */
 280       if ((m = arith_decode(cinfo, st)) != 0) {
 281         st = entropy->dc_stats[tbl] + 20;       /* Table F.4: X1 = 20 */
 282         while (arith_decode(cinfo, st)) {
 283           if ((m <<= 1) == (int) 0x8000U) {
 284             WARNMS(cinfo, JWRN_ARITH_BAD_CODE);
 285             entropy->ct = -1;                   /* magnitude overflow */
 286             return TRUE;
 287           }
 288           st += 1;
 289         }
 290       }
 291       /* Section F.1.4.4.1.2: Establish dc_context conditioning category */
 292       if (m < (int) ((1L << cinfo->arith_dc_L[tbl]) >> 1))
 293         entropy->dc_context[ci] = 0;               /* zero diff category */
 294       else if (m > (int) ((1L << cinfo->arith_dc_U[tbl]) >> 1))
 295         entropy->dc_context[ci] = 12 + (sign * 4); /* large diff category */
 296       else
 297         entropy->dc_context[ci] = 4 + (sign * 4);  /* small diff category */
 298       v = m;
 299       /* Figure F.24: Decoding the magnitude bit pattern of v */
 300       st += 14;
 301       while (m >>= 1)
 302         if (arith_decode(cinfo, st)) v |= m;
 303       v += 1; if (sign) v = -v;
 304       entropy->last_dc_val[ci] += v;
 305     }
 306
 307     /* Scale and output the DC coefficient (assumes jpeg_natural_order[0]=0) */
 308     (*block)[0] = (JCOEF) (entropy->last_dc_val[ci] << cinfo->Al);
 309   }
 310
 311   return TRUE;
 312 }
 313
 314
 315 /*
 316  * MCU decoding for AC initial scan (either spectral selection,
 317  * or first pass of successive approximation).
 318  */
 319
 320 METHODDEF(boolean)
 321 decode_mcu_AC_first (j_decompress_ptr cinfo, JBLOCKARRAY MCU_data)
 322 {
 323   arith_entropy_ptr entropy = (arith_entropy_ptr) cinfo->entropy;
 324   JBLOCKROW block;
 325   unsigned char *st;
 326   int tbl, sign, k;
 327   int v, m;
 328   const int * natural_order;
 329
 330   /* Process restart marker if needed */
 331   if (cinfo->restart_interval) {
 332     if (entropy->restarts_to_go == 0)
 333       process_restart(cinfo);
 334     entropy->restarts_to_go--;
 335   }
 336
 337   if (entropy->ct == -1) return TRUE;   /* if error do nothing */
 338
 339   natural_order = cinfo->natural_order;
 340
 341   /* There is always only one block per MCU */
 342   block = MCU_data[0];
 343   tbl = cinfo->cur_comp_info[0]->ac_tbl_no;
 344
 345   /* Sections F.2.4.2 & F.1.4.4.2: Decoding of AC coefficients */
 346
 347   /* Figure F.20: Decode_AC_coefficients */
 348   k = cinfo->Ss - 1;
 349   do {
 350     st = entropy->ac_stats[tbl] + 3 * k;
 351     if (arith_decode(cinfo, st)) break;         /* EOB flag */
 352     for (;;) {
 353       k++;
 354       if (arith_decode(cinfo, st + 1)) break;
 355       st += 3;
 356       if (k >= cinfo->Se) {
 357         WARNMS(cinfo, JWRN_ARITH_BAD_CODE);
 358         entropy->ct = -1;                       /* spectral overflow */
 359         return TRUE;
 360       }
 361     }
 362     /* Figure F.21: Decoding nonzero value v */
 363     /* Figure F.22: Decoding the sign of v */
 364     sign = arith_decode(cinfo, entropy->fixed_bin);
 365     st += 2;
 366     /* Figure F.23: Decoding the magnitude category of v */
 367     if ((m = arith_decode(cinfo, st)) != 0) {
 368       if (arith_decode(cinfo, st)) {
 369         m <<= 1;
 370         st = entropy->ac_stats[tbl] +
 371              (k <= cinfo->arith_ac_K[tbl] ? 189 : 217);
 372         while (arith_decode(cinfo, st)) {
 373           if ((m <<= 1) == (int) 0x8000U) {
 374             WARNMS(cinfo, JWRN_ARITH_BAD_CODE);
 375             entropy->ct = -1;                   /* magnitude overflow */
 376             return TRUE;
 377           }
 378           st += 1;
 379         }
 380       }
 381     }
 382     v = m;
 383     /* Figure F.24: Decoding the magnitude bit pattern of v */
 384     st += 14;
 385     while (m >>= 1)
 386       if (arith_decode(cinfo, st)) v |= m;
 387     v += 1; if (sign) v = -v;
 388     /* Scale and output coefficient in natural (dezigzagged) order */
 389     (*block)[natural_order[k]] = (JCOEF) (v << cinfo->Al);
 390   } while (k < cinfo->Se);
 391
 392   return TRUE;
 393 }
 394
 395
 396 /*
 397  * MCU decoding for DC successive approximation refinement scan.
 398  * Note: we assume such scans can be multi-component,
 399  * although the spec is not very clear on the point.
 400  */
 401
 402 METHODDEF(boolean)
 403 decode_mcu_DC_refine (j_decompress_ptr cinfo, JBLOCKARRAY MCU_data)
 404 {
 405   arith_entropy_ptr entropy = (arith_entropy_ptr) cinfo->entropy;
 406   unsigned char *st;
 407   JCOEF p1;
 408   int blkn;
 409
 410   /* Process restart marker if needed */
 411   if (cinfo->restart_interval) {
 412     if (entropy->restarts_to_go == 0)
 413       process_restart(cinfo);
 414     entropy->restarts_to_go--;
 415   }
 416
 417   st = entropy->fixed_bin;      /* use fixed probability estimation */
 418   p1 = 1 << cinfo->Al;          /* 1 in the bit position being coded */
 419
 420   /* Outer loop handles each block in the MCU */
 421
 422   for (blkn = 0; blkn < cinfo->blocks_in_MCU; blkn++) {
 423     /* Encoded data is simply the next bit of the two's-complement DC value */
 424     if (arith_decode(cinfo, st))
 425       MCU_data[blkn][0][0] |= p1;
 426   }
 427
 428   return TRUE;
 429 }
 430
 431
 432 /*
 433  * MCU decoding for AC successive approximation refinement scan.
 434  */
 435
 436 METHODDEF(boolean)
 437 decode_mcu_AC_refine (j_decompress_ptr cinfo, JBLOCKARRAY MCU_data)
 438 {
 439   arith_entropy_ptr entropy = (arith_entropy_ptr) cinfo->entropy;
 440   JBLOCKROW block;
 441   JCOEFPTR thiscoef;
 442   unsigned char *st;
 443   int tbl, k, kex;
 444   JCOEF p1, m1;
 445   const int * natural_order;
 446
 447   /* Process restart marker if needed */
 448   if (cinfo->restart_interval) {
 449     if (entropy->restarts_to_go == 0)
 450       process_restart(cinfo);
 451     entropy->restarts_to_go--;
 452   }
 453
 454   if (entropy->ct == -1) return TRUE;   /* if error do nothing */
 455
 456   natural_order = cinfo->natural_order;
 457
 458   /* There is always only one block per MCU */
 459   block = MCU_data[0];
 460   tbl = cinfo->cur_comp_info[0]->ac_tbl_no;
 461
 462   p1 = 1 << cinfo->Al;          /* 1 in the bit position being coded */
 463   m1 = -p1;                     /* -1 in the bit position being coded */
 464
 465   /* Establish EOBx (previous stage end-of-block) index */
 466   kex = cinfo->Se;
 467   do {
 468     if ((*block)[natural_order[kex]]) break;
 469   } while (--kex);
 470
 471   k = cinfo->Ss - 1;
 472   do {
 473     st = entropy->ac_stats[tbl] + 3 * k;
 474     if (k >= kex)
 475       if (arith_decode(cinfo, st)) break;       /* EOB flag */
 476     for (;;) {
 477       thiscoef = *block + natural_order[++k];
 478       if (*thiscoef) {                          /* previously nonzero coef */
 479         if (arith_decode(cinfo, st + 2)) {
 480           if (*thiscoef < 0)
 481             *thiscoef += m1;
 482           else
 483             *thiscoef += p1;
 484         }
 485         break;
 486       }
 487       if (arith_decode(cinfo, st + 1)) {        /* newly nonzero coef */
 488         if (arith_decode(cinfo, entropy->fixed_bin))
 489           *thiscoef = m1;
 490         else
 491           *thiscoef = p1;
 492         break;
 493       }
 494       st += 3;
 495       if (k >= cinfo->Se) {
 496         WARNMS(cinfo, JWRN_ARITH_BAD_CODE);
 497         entropy->ct = -1;                       /* spectral overflow */
 498         return TRUE;
 499       }
 500     }
 501   } while (k < cinfo->Se);
 502
 503   return TRUE;
 504 }
 505
 506
 507 /*
 508  * Decode one MCU's worth of arithmetic-compressed coefficients.
 509  */
 510
 511 METHODDEF(boolean)
 512 decode_mcu (j_decompress_ptr cinfo, JBLOCKARRAY MCU_data)
 513 {
 514   arith_entropy_ptr entropy = (arith_entropy_ptr) cinfo->entropy;
 515   jpeg_component_info * compptr;
 516   JBLOCKROW block;
 517   unsigned char *st;
 518   int blkn, ci, tbl, sign, k;
 519   int v, m;
 520   const int * natural_order;
 521
 522   /* Process restart marker if needed */
 523   if (cinfo->restart_interval) {
 524     if (entropy->restarts_to_go == 0)
 525       process_restart(cinfo);
 526     entropy->restarts_to_go--;
 527   }
 528
 529   if (entropy->ct == -1) return TRUE;   /* if error do nothing */
 530
 531   natural_order = cinfo->natural_order;
 532
 533   /* Outer loop handles each block in the MCU */
 534
 535   for (blkn = 0; blkn < cinfo->blocks_in_MCU; blkn++) {
 536     block = MCU_data[blkn];
 537     ci = cinfo->MCU_membership[blkn];
 538     compptr = cinfo->cur_comp_info[ci];
 539
 540     /* Sections F.2.4.1 & F.1.4.4.1: Decoding of DC coefficients */
 541
 542     tbl = compptr->dc_tbl_no;
 543
 544     /* Table F.4: Point to statistics bin S0 for DC coefficient coding */
 545     st = entropy->dc_stats[tbl] + entropy->dc_context[ci];
 546
 547     /* Figure F.19: Decode_DC_DIFF */
 548     if (arith_decode(cinfo, st) == 0)
 549       entropy->dc_context[ci] = 0;
 550     else {
 551       /* Figure F.21: Decoding nonzero value v */
 552       /* Figure F.22: Decoding the sign of v */
 553       sign = arith_decode(cinfo, st + 1);
 554       st += 2; st += sign;
 555       /* Figure F.23: Decoding the magnitude category of v */
 556       if ((m = arith_decode(cinfo, st)) != 0) {
 557         st = entropy->dc_stats[tbl] + 20;       /* Table F.4: X1 = 20 */
 558         while (arith_decode(cinfo, st)) {
 559           if ((m <<= 1) == (int) 0x8000U) {
 560             WARNMS(cinfo, JWRN_ARITH_BAD_CODE);
 561             entropy->ct = -1;                   /* magnitude overflow */
 562             return TRUE;
 563           }
 564           st += 1;
 565         }
 566       }
 567       /* Section F.1.4.4.1.2: Establish dc_context conditioning category */
 568       if (m < (int) ((1L << cinfo->arith_dc_L[tbl]) >> 1))
 569         entropy->dc_context[ci] = 0;               /* zero diff category */
 570       else if (m > (int) ((1L << cinfo->arith_dc_U[tbl]) >> 1))
 571         entropy->dc_context[ci] = 12 + (sign * 4); /* large diff category */
 572       else
 573         entropy->dc_context[ci] = 4 + (sign * 4);  /* small diff category */
 574       v = m;
 575       /* Figure F.24: Decoding the magnitude bit pattern of v */
 576       st += 14;
 577       while (m >>= 1)
 578         if (arith_decode(cinfo, st)) v |= m;
 579       v += 1; if (sign) v = -v;
 580       entropy->last_dc_val[ci] += v;
 581     }
 582
 583     (*block)[0] = (JCOEF) entropy->last_dc_val[ci];
 584
 585     /* Sections F.2.4.2 & F.1.4.4.2: Decoding of AC coefficients */
 586
 587     if (cinfo->lim_Se == 0) continue;
 588     tbl = compptr->ac_tbl_no;
 589     k = 0;
 590
 591     /* Figure F.20: Decode_AC_coefficients */
 592     do {
 593       st = entropy->ac_stats[tbl] + 3 * k;
 594       if (arith_decode(cinfo, st)) break;       /* EOB flag */
 595       for (;;) {
 596         k++;
 597         if (arith_decode(cinfo, st + 1)) break;
 598         st += 3;
 599         if (k >= cinfo->lim_Se) {
 600           WARNMS(cinfo, JWRN_ARITH_BAD_CODE);
 601           entropy->ct = -1;                     /* spectral overflow */
 602           return TRUE;
 603         }
 604       }
 605       /* Figure F.21: Decoding nonzero value v */
 606       /* Figure F.22: Decoding the sign of v */
 607       sign = arith_decode(cinfo, entropy->fixed_bin);
 608       st += 2;
 609       /* Figure F.23: Decoding the magnitude category of v */
 610       if ((m = arith_decode(cinfo, st)) != 0) {
 611         if (arith_decode(cinfo, st)) {
 612           m <<= 1;
 613           st = entropy->ac_stats[tbl] +
 614                (k <= cinfo->arith_ac_K[tbl] ? 189 : 217);
 615           while (arith_decode(cinfo, st)) {
 616             if ((m <<= 1) == (int) 0x8000U) {
 617               WARNMS(cinfo, JWRN_ARITH_BAD_CODE);
 618               entropy->ct = -1;                 /* magnitude overflow */
 619               return TRUE;
 620             }
 621             st += 1;
 622           }
 623         }
 624       }
 625       v = m;
 626       /* Figure F.24: Decoding the magnitude bit pattern of v */
 627       st += 14;
 628       while (m >>= 1)
 629         if (arith_decode(cinfo, st)) v |= m;
 630       v += 1; if (sign) v = -v;
 631       (*block)[natural_order[k]] = (JCOEF) v;
 632     } while (k < cinfo->lim_Se);
 633   }
 634
 635   return TRUE;
 636 }
 637
 638
 639 /*
 640  * Initialize for an arithmetic-compressed scan.
 641  */
 642
 643 METHODDEF(void)
 644 start_pass (j_decompress_ptr cinfo)
 645 {
 646   arith_entropy_ptr entropy = (arith_entropy_ptr) cinfo->entropy;
 647   int ci, tbl;
 648   jpeg_component_info * compptr;
 649
 650   if (cinfo->progressive_mode) {
 651     /* Validate progressive scan parameters */
 652     if (cinfo->Ss == 0) {
 653       if (cinfo->Se != 0)
 654         goto bad;
 655     } else {
 656       /* need not check Ss/Se < 0 since they came from unsigned bytes */
 657       if (cinfo->Se < cinfo->Ss || cinfo->Se > cinfo->lim_Se)
 658         goto bad;
 659       /* AC scans may have only one component */
 660       if (cinfo->comps_in_scan != 1)
 661         goto bad;
 662     }
 663     if (cinfo->Ah != 0) {
 664       /* Successive approximation refinement scan: must have Al = Ah-1. */
 665       if (cinfo->Ah-1 != cinfo->Al)
 666         goto bad;
 667     }
 668     if (cinfo->Al > 13) {       /* need not check for < 0 */
 669       bad:
 670       ERREXIT4(cinfo, JERR_BAD_PROGRESSION,
 671                cinfo->Ss, cinfo->Se, cinfo->Ah, cinfo->Al);
 672     }
 673     /* Update progression status, and verify that scan order is legal.
 674      * Note that inter-scan inconsistencies are treated as warnings
 675      * not fatal errors ... not clear if this is right way to behave.
 676      */
 677     for (ci = 0; ci < cinfo->comps_in_scan; ci++) {
 678       int coefi, cindex = cinfo->cur_comp_info[ci]->component_index;
 679       int *coef_bit_ptr = & cinfo->coef_bits[cindex][0];
 680       if (cinfo->Ss && coef_bit_ptr[0] < 0) /* AC without prior DC scan */
 681         WARNMS2(cinfo, JWRN_BOGUS_PROGRESSION, cindex, 0);
 682       for (coefi = cinfo->Ss; coefi <= cinfo->Se; coefi++) {
 683         int expected = (coef_bit_ptr[coefi] < 0) ? 0 : coef_bit_ptr[coefi];
 684         if (cinfo->Ah != expected)
 685           WARNMS2(cinfo, JWRN_BOGUS_PROGRESSION, cindex, coefi);
 686         coef_bit_ptr[coefi] = cinfo->Al;
 687       }
 688     }
 689     /* Select MCU decoding routine */
 690     if (cinfo->Ah == 0) {
 691       if (cinfo->Ss == 0)
 692         entropy->pub.decode_mcu = decode_mcu_DC_first;
 693       else
 694         entropy->pub.decode_mcu = decode_mcu_AC_first;
 695     } else {
 696       if (cinfo->Ss == 0)
 697         entropy->pub.decode_mcu = decode_mcu_DC_refine;
 698       else
 699         entropy->pub.decode_mcu = decode_mcu_AC_refine;
 700     }
 701   } else {
 702     /* Check that the scan parameters Ss, Se, Ah/Al are OK for sequential JPEG.
 703      * This ought to be an error condition, but we make it a warning.
 704      */
 705     if (cinfo->Ss != 0 || cinfo->Ah != 0 || cinfo->Al != 0 ||
 706         (cinfo->Se < DCTSIZE2 && cinfo->Se != cinfo->lim_Se))
 707       WARNMS(cinfo, JWRN_NOT_SEQUENTIAL);
 708     /* Select MCU decoding routine */
 709     entropy->pub.decode_mcu = decode_mcu;
 710   }
 711
 712   /* Allocate & initialize requested statistics areas */
 713   for (ci = 0; ci < cinfo->comps_in_scan; ci++) {
 714     compptr = cinfo->cur_comp_info[ci];
 715     if (! cinfo->progressive_mode || (cinfo->Ss == 0 && cinfo->Ah == 0)) {
 716       tbl = compptr->dc_tbl_no;
 717       if (tbl < 0 || tbl >= NUM_ARITH_TBLS)
 718         ERREXIT1(cinfo, JERR_NO_ARITH_TABLE, tbl);
 719       if (entropy->dc_stats[tbl] == NULL)
 720         entropy->dc_stats[tbl] = (unsigned char *) (*cinfo->mem->alloc_small)
 721           ((j_common_ptr) cinfo, JPOOL_IMAGE, DC_STAT_BINS);
 722       MEMZERO(entropy->dc_stats[tbl], DC_STAT_BINS);
 723       /* Initialize DC predictions to 0 */
 724       entropy->last_dc_val[ci] = 0;
 725       entropy->dc_context[ci] = 0;
 726     }
 727     if ((! cinfo->progressive_mode && cinfo->lim_Se) ||
 728         (cinfo->progressive_mode && cinfo->Ss)) {
 729       tbl = compptr->ac_tbl_no;
 730       if (tbl < 0 || tbl >= NUM_ARITH_TBLS)
 731         ERREXIT1(cinfo, JERR_NO_ARITH_TABLE, tbl);
 732       if (entropy->ac_stats[tbl] == NULL)
 733         entropy->ac_stats[tbl] = (unsigned char *) (*cinfo->mem->alloc_small)
 734           ((j_common_ptr) cinfo, JPOOL_IMAGE, AC_STAT_BINS);
 735       MEMZERO(entropy->ac_stats[tbl], AC_STAT_BINS);
 736     }
 737   }
 738
 739   /* Initialize arithmetic decoding variables */
 740   entropy->c = 0;
 741   entropy->a = 0;
 742   entropy->ct = -16;    /* force reading 2 initial bytes to fill C */
 743
 744   /* Initialize restart counter */
 745   entropy->restarts_to_go = cinfo->restart_interval;
 746 }
 747
 748
 749 /*
 750  * Finish up at the end of an arithmetic-compressed scan.
 751  */
 752
 753 METHODDEF(void)
 754 finish_pass (j_decompress_ptr cinfo)
 755 {
 756   /* no work necessary here */
 757 }
 758
 759
 760 /*
 761  * Module initialization routine for arithmetic entropy decoding.
 762  */
 763
 764 GLOBAL(void)
 765 jinit_arith_decoder (j_decompress_ptr cinfo)
 766 {
 767   arith_entropy_ptr entropy;
 768   int i;
 769
 770   entropy = (arith_entropy_ptr) (*cinfo->mem->alloc_small)
 771     ((j_common_ptr) cinfo, JPOOL_IMAGE, SIZEOF(arith_entropy_decoder));
 772   cinfo->entropy = &entropy->pub;
 773   entropy->pub.start_pass = start_pass;
 774   entropy->pub.finish_pass = finish_pass;
 775
 776   /* Mark tables unallocated */
 777   for (i = 0; i < NUM_ARITH_TBLS; i++) {
 778     entropy->dc_stats[i] = NULL;
 779     entropy->ac_stats[i] = NULL;
 780   }
 781
 782   /* Initialize index for fixed probability estimation */
 783   entropy->fixed_bin[0] = 113;
 784
 785   if (cinfo->progressive_mode) {
 786     /* Create progression status table */
 787     int *coef_bit_ptr, ci;
 788     cinfo->coef_bits = (int (*)[DCTSIZE2]) (*cinfo->mem->alloc_small)
 789       ((j_common_ptr) cinfo, JPOOL_IMAGE,
 790        cinfo->num_components * DCTSIZE2 * SIZEOF(int));
 791     coef_bit_ptr = & cinfo->coef_bits[0][0];
 792     for (ci = 0; ci < cinfo->num_components; ci++)
 793       for (i = 0; i < DCTSIZE2; i++)
 794         *coef_bit_ptr++ = -1;
 795   }
 796 }