media/libjpeg/jdarith.c

   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, 2022, 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)
  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)
 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)
 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       memset(entropy->dc_stats[compptr->dc_tbl_no], 0, 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       memset(entropy->ac_stats[compptr->ac_tbl_no], 0, 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)
 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)
 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)
 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
 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 += (JCOEF)m1;
 475           else
 476             *thiscoef += (JCOEF)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 = (JCOEF)m1;
 483         else
 484           *thiscoef = (JCOEF)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)
 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)
 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->Se != DCTSIZE2 - 1 ||
 702         cinfo->Ah != 0 || cinfo->Al != 0)
 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       memset(entropy->dc_stats[tbl], 0, 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       memset(entropy->ac_stats[tbl], 0, 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)
 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 }