dll/3rdparty/libjpeg/jdarith.c

   1 /*
   2  * jdarith.c
   3  *
   4  * Developed 1997-2015 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, JBLOCKROW *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) == 0x8000) {
 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, JBLOCKROW *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) == 0x8000) {
 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, 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)
 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   const int * natural_order;
 445
 446   /* Process restart marker if needed */
 447   if (cinfo->restart_interval) {
 448     if (entropy->restarts_to_go == 0)
 449       process_restart(cinfo);
 450     entropy->restarts_to_go--;
 451   }
 452
 453   if (entropy->ct == -1) return TRUE;   /* if error do nothing */
 454
 455   natural_order = cinfo->natural_order;
 456
 457   /* There is always only one block per MCU */
 458   block = MCU_data[0];
 459   tbl = cinfo->cur_comp_info[0]->ac_tbl_no;
 460
 461   p1 = 1 << cinfo->Al;          /* 1 in the bit position being coded */
 462   m1 = (-1) << cinfo->Al;       /* -1 in the bit position being coded */
 463
 464   /* Establish EOBx (previous stage end-of-block) index */
 465   kex = cinfo->Se;
 466   do {
 467     if ((*block)[natural_order[kex]]) break;
 468   } while (--kex);
 469
 470   k = cinfo->Ss - 1;
 471   do {
 472     st = entropy->ac_stats[tbl] + 3 * k;
 473     if (k >= kex)
 474       if (arith_decode(cinfo, st)) break;       /* EOB flag */
 475     for (;;) {
 476       thiscoef = *block + natural_order[++k];
 477       if (*thiscoef) {                          /* previously nonzero coef */
 478         if (arith_decode(cinfo, st + 2)) {
 479           if (*thiscoef < 0)
 480             *thiscoef += m1;
 481           else
 482             *thiscoef += p1;
 483         }
 484         break;
 485       }
 486       if (arith_decode(cinfo, st + 1)) {        /* newly nonzero coef */
 487         if (arith_decode(cinfo, entropy->fixed_bin))
 488           *thiscoef = m1;
 489         else
 490           *thiscoef = p1;
 491         break;
 492       }
 493       st += 3;
 494       if (k >= cinfo->Se) {
 495         WARNMS(cinfo, JWRN_ARITH_BAD_CODE);
 496         entropy->ct = -1;                       /* spectral overflow */
 497         return TRUE;
 498       }
 499     }
 500   } while (k < cinfo->Se);
 501
 502   return TRUE;
 503 }
 504
 505
 506 /*
 507  * Decode one MCU's worth of arithmetic-compressed coefficients.
 508  */
 509
 510 METHODDEF(boolean)
 511 decode_mcu (j_decompress_ptr cinfo, JBLOCKROW *MCU_data)
 512 {
 513   arith_entropy_ptr entropy = (arith_entropy_ptr) cinfo->entropy;
 514   jpeg_component_info * compptr;
 515   JBLOCKROW block;
 516   unsigned char *st;
 517   int blkn, ci, tbl, sign, k;
 518   int v, m;
 519   const int * natural_order;
 520
 521   /* Process restart marker if needed */
 522   if (cinfo->restart_interval) {
 523     if (entropy->restarts_to_go == 0)
 524       process_restart(cinfo);
 525     entropy->restarts_to_go--;
 526   }
 527
 528   if (entropy->ct == -1) return TRUE;   /* if error do nothing */
 529
 530   natural_order = cinfo->natural_order;
 531
 532   /* Outer loop handles each block in the MCU */
 533
 534   for (blkn = 0; blkn < cinfo->blocks_in_MCU; blkn++) {
 535     block = MCU_data[blkn];
 536     ci = cinfo->MCU_membership[blkn];
 537     compptr = cinfo->cur_comp_info[ci];
 538
 539     /* Sections F.2.4.1 & F.1.4.4.1: Decoding of DC coefficients */
 540
 541     tbl = compptr->dc_tbl_no;
 542
 543     /* Table F.4: Point to statistics bin S0 for DC coefficient coding */
 544     st = entropy->dc_stats[tbl] + entropy->dc_context[ci];
 545
 546     /* Figure F.19: Decode_DC_DIFF */
 547     if (arith_decode(cinfo, st) == 0)
 548       entropy->dc_context[ci] = 0;
 549     else {
 550       /* Figure F.21: Decoding nonzero value v */
 551       /* Figure F.22: Decoding the sign of v */
 552       sign = arith_decode(cinfo, st + 1);
 553       st += 2; st += sign;
 554       /* Figure F.23: Decoding the magnitude category of v */
 555       if ((m = arith_decode(cinfo, st)) != 0) {
 556         st = entropy->dc_stats[tbl] + 20;       /* Table F.4: X1 = 20 */
 557         while (arith_decode(cinfo, st)) {
 558           if ((m <<= 1) == 0x8000) {
 559             WARNMS(cinfo, JWRN_ARITH_BAD_CODE);
 560             entropy->ct = -1;                   /* magnitude overflow */
 561             return TRUE;
 562           }
 563           st += 1;
 564         }
 565       }
 566       /* Section F.1.4.4.1.2: Establish dc_context conditioning category */
 567       if (m < (int) ((1L << cinfo->arith_dc_L[tbl]) >> 1))
 568         entropy->dc_context[ci] = 0;               /* zero diff category */
 569       else if (m > (int) ((1L << cinfo->arith_dc_U[tbl]) >> 1))
 570         entropy->dc_context[ci] = 12 + (sign * 4); /* large diff category */
 571       else
 572         entropy->dc_context[ci] = 4 + (sign * 4);  /* small diff category */
 573       v = m;
 574       /* Figure F.24: Decoding the magnitude bit pattern of v */
 575       st += 14;
 576       while (m >>= 1)
 577         if (arith_decode(cinfo, st)) v |= m;
 578       v += 1; if (sign) v = -v;
 579       entropy->last_dc_val[ci] += v;
 580     }
 581
 582     (*block)[0] = (JCOEF) entropy->last_dc_val[ci];
 583
 584     /* Sections F.2.4.2 & F.1.4.4.2: Decoding of AC coefficients */
 585
 586     if (cinfo->lim_Se == 0) continue;
 587     tbl = compptr->ac_tbl_no;
 588     k = 0;
 589
 590     /* Figure F.20: Decode_AC_coefficients */
 591     do {
 592       st = entropy->ac_stats[tbl] + 3 * k;
 593       if (arith_decode(cinfo, st)) break;       /* EOB flag */
 594       for (;;) {
 595         k++;
 596         if (arith_decode(cinfo, st + 1)) break;
 597         st += 3;
 598         if (k >= cinfo->lim_Se) {
 599           WARNMS(cinfo, JWRN_ARITH_BAD_CODE);
 600           entropy->ct = -1;                     /* spectral overflow */
 601           return TRUE;
 602         }
 603       }
 604       /* Figure F.21: Decoding nonzero value v */
 605       /* Figure F.22: Decoding the sign of v */
 606       sign = arith_decode(cinfo, entropy->fixed_bin);
 607       st += 2;
 608       /* Figure F.23: Decoding the magnitude category of v */
 609       if ((m = arith_decode(cinfo, st)) != 0) {
 610         if (arith_decode(cinfo, st)) {
 611           m <<= 1;
 612           st = entropy->ac_stats[tbl] +
 613                (k <= cinfo->arith_ac_K[tbl] ? 189 : 217);
 614           while (arith_decode(cinfo, st)) {
 615             if ((m <<= 1) == 0x8000) {
 616               WARNMS(cinfo, JWRN_ARITH_BAD_CODE);
 617               entropy->ct = -1;                 /* magnitude overflow */
 618               return TRUE;
 619             }
 620             st += 1;
 621           }
 622         }
 623       }
 624       v = m;
 625       /* Figure F.24: Decoding the magnitude bit pattern of v */
 626       st += 14;
 627       while (m >>= 1)
 628         if (arith_decode(cinfo, st)) v |= m;
 629       v += 1; if (sign) v = -v;
 630       (*block)[natural_order[k]] = (JCOEF) v;
 631     } while (k < cinfo->lim_Se);
 632   }
 633
 634   return TRUE;
 635 }
 636
 637
 638 /*
 639  * Initialize for an arithmetic-compressed scan.
 640  */
 641
 642 METHODDEF(void)
 643 start_pass (j_decompress_ptr cinfo)
 644 {
 645   arith_entropy_ptr entropy = (arith_entropy_ptr) cinfo->entropy;
 646   int ci, tbl;
 647   jpeg_component_info * compptr;
 648
 649   if (cinfo->progressive_mode) {
 650     /* Validate progressive scan parameters */
 651     if (cinfo->Ss == 0) {
 652       if (cinfo->Se != 0)
 653         goto bad;
 654     } else {
 655       /* need not check Ss/Se < 0 since they came from unsigned bytes */
 656       if (cinfo->Se < cinfo->Ss || cinfo->Se > cinfo->lim_Se)
 657         goto bad;
 658       /* AC scans may have only one component */
 659       if (cinfo->comps_in_scan != 1)
 660         goto bad;
 661     }
 662     if (cinfo->Ah != 0) {
 663       /* Successive approximation refinement scan: must have Al = Ah-1. */
 664       if (cinfo->Ah-1 != cinfo->Al)
 665         goto bad;
 666     }
 667     if (cinfo->Al > 13) {       /* need not check for < 0 */
 668       bad:
 669       ERREXIT4(cinfo, JERR_BAD_PROGRESSION,
 670                cinfo->Ss, cinfo->Se, cinfo->Ah, cinfo->Al);
 671     }
 672     /* Update progression status, and verify that scan order is legal.
 673      * Note that inter-scan inconsistencies are treated as warnings
 674      * not fatal errors ... not clear if this is right way to behave.
 675      */
 676     for (ci = 0; ci < cinfo->comps_in_scan; ci++) {
 677       int coefi, cindex = cinfo->cur_comp_info[ci]->component_index;
 678       int *coef_bit_ptr = & cinfo->coef_bits[cindex][0];
 679       if (cinfo->Ss && coef_bit_ptr[0] < 0) /* AC without prior DC scan */
 680         WARNMS2(cinfo, JWRN_BOGUS_PROGRESSION, cindex, 0);
 681       for (coefi = cinfo->Ss; coefi <= cinfo->Se; coefi++) {
 682         int expected = (coef_bit_ptr[coefi] < 0) ? 0 : coef_bit_ptr[coefi];
 683         if (cinfo->Ah != expected)
 684           WARNMS2(cinfo, JWRN_BOGUS_PROGRESSION, cindex, coefi);
 685         coef_bit_ptr[coefi] = cinfo->Al;
 686       }
 687     }
 688     /* Select MCU decoding routine */
 689     if (cinfo->Ah == 0) {
 690       if (cinfo->Ss == 0)
 691         entropy->pub.decode_mcu = decode_mcu_DC_first;
 692       else
 693         entropy->pub.decode_mcu = decode_mcu_AC_first;
 694     } else {
 695       if (cinfo->Ss == 0)
 696         entropy->pub.decode_mcu = decode_mcu_DC_refine;
 697       else
 698         entropy->pub.decode_mcu = decode_mcu_AC_refine;
 699     }
 700   } else {
 701     /* Check that the scan parameters Ss, Se, Ah/Al are OK for sequential JPEG.
 702      * This ought to be an error condition, but we make it a warning.
 703      */
 704     if (cinfo->Ss != 0 || cinfo->Ah != 0 || cinfo->Al != 0 ||
 705         (cinfo->Se < DCTSIZE2 && cinfo->Se != cinfo->lim_Se))
 706       WARNMS(cinfo, JWRN_NOT_SEQUENTIAL);
 707     /* Select MCU decoding routine */
 708     entropy->pub.decode_mcu = decode_mcu;
 709   }
 710
 711   /* Allocate & initialize requested statistics areas */
 712   for (ci = 0; ci < cinfo->comps_in_scan; ci++) {
 713     compptr = cinfo->cur_comp_info[ci];
 714     if (! cinfo->progressive_mode || (cinfo->Ss == 0 && cinfo->Ah == 0)) {
 715       tbl = compptr->dc_tbl_no;
 716       if (tbl < 0 || tbl >= NUM_ARITH_TBLS)
 717         ERREXIT1(cinfo, JERR_NO_ARITH_TABLE, tbl);
 718       if (entropy->dc_stats[tbl] == NULL)
 719         entropy->dc_stats[tbl] = (unsigned char *) (*cinfo->mem->alloc_small)
 720           ((j_common_ptr) cinfo, JPOOL_IMAGE, DC_STAT_BINS);
 721       MEMZERO(entropy->dc_stats[tbl], DC_STAT_BINS);
 722       /* Initialize DC predictions to 0 */
 723       entropy->last_dc_val[ci] = 0;
 724       entropy->dc_context[ci] = 0;
 725     }
 726     if ((! cinfo->progressive_mode && cinfo->lim_Se) ||
 727         (cinfo->progressive_mode && cinfo->Ss)) {
 728       tbl = compptr->ac_tbl_no;
 729       if (tbl < 0 || tbl >= NUM_ARITH_TBLS)
 730         ERREXIT1(cinfo, JERR_NO_ARITH_TABLE, tbl);
 731       if (entropy->ac_stats[tbl] == NULL)
 732         entropy->ac_stats[tbl] = (unsigned char *) (*cinfo->mem->alloc_small)
 733           ((j_common_ptr) cinfo, JPOOL_IMAGE, AC_STAT_BINS);
 734       MEMZERO(entropy->ac_stats[tbl], AC_STAT_BINS);
 735     }
 736   }
 737
 738   /* Initialize arithmetic decoding variables */
 739   entropy->c = 0;
 740   entropy->a = 0;
 741   entropy->ct = -16;    /* force reading 2 initial bytes to fill C */
 742
 743   /* Initialize restart counter */
 744   entropy->restarts_to_go = cinfo->restart_interval;
 745 }
 746
 747
 748 /*
 749  * Finish up at the end of an arithmetic-compressed scan.
 750  */
 751
 752 METHODDEF(void)
 753 finish_pass (j_decompress_ptr cinfo)
 754 {
 755   /* no work necessary here */
 756 }
 757
 758
 759 /*
 760  * Module initialization routine for arithmetic entropy decoding.
 761  */
 762
 763 GLOBAL(void)
 764 jinit_arith_decoder (j_decompress_ptr cinfo)
 765 {
 766   arith_entropy_ptr entropy;
 767   int i;
 768
 769   entropy = (arith_entropy_ptr)
 770     (*cinfo->mem->alloc_small) ((j_common_ptr) cinfo, JPOOL_IMAGE,
 771                                 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])
 789       (*cinfo->mem->alloc_small) ((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 }