1 /*
   2  * reserved comment block
   3  * DO NOT REMOVE OR ALTER!
   4  */
   5 /*
   6  * jdhuff.c
   7  *
   8  * Copyright (C) 1991-1997, Thomas G. Lane.
   9  * This file is part of the Independent JPEG Group's software.
  10  * For conditions of distribution and use, see the accompanying README file.
  11  *
  12  * This file contains Huffman entropy decoding routines.
  13  *
  14  * Much of the complexity here has to do with supporting input suspension.
  15  * If the data source module demands suspension, we want to be able to back
  16  * up to the start of the current MCU.  To do this, we copy state variables
  17  * into local working storage, and update them back to the permanent
  18  * storage only upon successful completion of an MCU.
  19  */
  20 
  21 #define JPEG_INTERNALS
  22 #include "jinclude.h"
  23 #include "jpeglib.h"
  24 #include "jdhuff.h"             /* Declarations shared with jdphuff.c */
  25 
  26 
  27 /*
  28  * Expanded entropy decoder object for Huffman decoding.
  29  *
  30  * The savable_state subrecord contains fields that change within an MCU,
  31  * but must not be updated permanently until we complete the MCU.
  32  */
  33 
  34 typedef struct {
  35   int last_dc_val[MAX_COMPS_IN_SCAN]; /* last DC coef for each component */
  36 } savable_state;
  37 
  38 /* This macro is to work around compilers with missing or broken
  39  * structure assignment.  You'll need to fix this code if you have
  40  * such a compiler and you change MAX_COMPS_IN_SCAN.
  41  */
  42 
  43 #ifndef NO_STRUCT_ASSIGN
  44 #define ASSIGN_STATE(dest,src)  ((dest) = (src))
  45 #else
  46 #if MAX_COMPS_IN_SCAN == 4
  47 #define ASSIGN_STATE(dest,src)  \
  48         ((dest).last_dc_val[0] = (src).last_dc_val[0], \
  49          (dest).last_dc_val[1] = (src).last_dc_val[1], \
  50          (dest).last_dc_val[2] = (src).last_dc_val[2], \
  51          (dest).last_dc_val[3] = (src).last_dc_val[3])
  52 #endif
  53 #endif
  54 
  55 
  56 typedef struct {
  57   struct jpeg_entropy_decoder pub; /* public fields */
  58 
  59   /* These fields are loaded into local variables at start of each MCU.
  60    * In case of suspension, we exit WITHOUT updating them.
  61    */
  62   bitread_perm_state bitstate;  /* Bit buffer at start of MCU */
  63   savable_state saved;          /* Other state at start of MCU */
  64 
  65   /* These fields are NOT loaded into local working state. */
  66   unsigned int restarts_to_go;  /* MCUs left in this restart interval */
  67 
  68   /* Pointers to derived tables (these workspaces have image lifespan) */
  69   d_derived_tbl * dc_derived_tbls[NUM_HUFF_TBLS];
  70   d_derived_tbl * ac_derived_tbls[NUM_HUFF_TBLS];
  71 
  72   /* Precalculated info set up by start_pass for use in decode_mcu: */
  73 
  74   /* Pointers to derived tables to be used for each block within an MCU */
  75   d_derived_tbl * dc_cur_tbls[D_MAX_BLOCKS_IN_MCU];
  76   d_derived_tbl * ac_cur_tbls[D_MAX_BLOCKS_IN_MCU];
  77   /* Whether we care about the DC and AC coefficient values for each block */
  78   boolean dc_needed[D_MAX_BLOCKS_IN_MCU];
  79   boolean ac_needed[D_MAX_BLOCKS_IN_MCU];
  80 } huff_entropy_decoder;
  81 
  82 typedef huff_entropy_decoder * huff_entropy_ptr;
  83 
  84 
  85 /*
  86  * Initialize for a Huffman-compressed scan.
  87  */
  88 
  89 METHODDEF(void)
  90 start_pass_huff_decoder (j_decompress_ptr cinfo)
  91 {
  92   huff_entropy_ptr entropy = (huff_entropy_ptr) cinfo->entropy;
  93   int ci, blkn, dctbl, actbl;
  94   jpeg_component_info * compptr;
  95 
  96   /* Check that the scan parameters Ss, Se, Ah/Al are OK for sequential JPEG.
  97    * This ought to be an error condition, but we make it a warning because
  98    * there are some baseline files out there with all zeroes in these bytes.
  99    */
 100   if (cinfo->Ss != 0 || cinfo->Se != DCTSIZE2-1 ||
 101       cinfo->Ah != 0 || cinfo->Al != 0)
 102     WARNMS(cinfo, JWRN_NOT_SEQUENTIAL);
 103 
 104   for (ci = 0; ci < cinfo->comps_in_scan; ci++) {
 105     compptr = cinfo->cur_comp_info[ci];
 106     dctbl = compptr->dc_tbl_no;
 107     actbl = compptr->ac_tbl_no;
 108     /* Compute derived values for Huffman tables */
 109     /* We may do this more than once for a table, but it's not expensive */
 110     jpeg_make_d_derived_tbl(cinfo, TRUE, dctbl,
 111                             & entropy->dc_derived_tbls[dctbl]);
 112     jpeg_make_d_derived_tbl(cinfo, FALSE, actbl,
 113                             & entropy->ac_derived_tbls[actbl]);
 114     /* Initialize DC predictions to 0 */
 115     entropy->saved.last_dc_val[ci] = 0;
 116   }
 117 
 118   /* Precalculate decoding info for each block in an MCU of this scan */
 119   for (blkn = 0; blkn < cinfo->blocks_in_MCU; blkn++) {
 120     ci = cinfo->MCU_membership[blkn];
 121     compptr = cinfo->cur_comp_info[ci];
 122     /* Precalculate which table to use for each block */
 123     entropy->dc_cur_tbls[blkn] = entropy->dc_derived_tbls[compptr->dc_tbl_no];
 124     entropy->ac_cur_tbls[blkn] = entropy->ac_derived_tbls[compptr->ac_tbl_no];
 125     /* Decide whether we really care about the coefficient values */
 126     if (compptr->component_needed) {
 127       entropy->dc_needed[blkn] = TRUE;
 128       /* we don't need the ACs if producing a 1/8th-size image */
 129       entropy->ac_needed[blkn] = (compptr->DCT_scaled_size > 1);
 130     } else {
 131       entropy->dc_needed[blkn] = entropy->ac_needed[blkn] = FALSE;
 132     }
 133   }
 134 
 135   /* Initialize bitread state variables */
 136   entropy->bitstate.bits_left = 0;
 137   entropy->bitstate.get_buffer = 0; /* unnecessary, but keeps Purify quiet */
 138   entropy->pub.insufficient_data = FALSE;
 139 
 140   /* Initialize restart counter */
 141   entropy->restarts_to_go = cinfo->restart_interval;
 142 }
 143 
 144 
 145 /*
 146  * Compute the derived values for a Huffman table.
 147  * This routine also performs some validation checks on the table.
 148  *
 149  * Note this is also used by jdphuff.c.
 150  */
 151 
 152 GLOBAL(void)
 153 jpeg_make_d_derived_tbl (j_decompress_ptr cinfo, boolean isDC, int tblno,
 154                          d_derived_tbl ** pdtbl)
 155 {
 156   JHUFF_TBL *htbl;
 157   d_derived_tbl *dtbl;
 158   int p, i, l, si, numsymbols;
 159   int lookbits, ctr;
 160   char huffsize[257];
 161   unsigned int huffcode[257];
 162   unsigned int code;
 163 
 164   /* Note that huffsize[] and huffcode[] are filled in code-length order,
 165    * paralleling the order of the symbols themselves in htbl->huffval[].
 166    */
 167 
 168   /* Find the input Huffman table */
 169   if (tblno < 0 || tblno >= NUM_HUFF_TBLS)
 170     ERREXIT1(cinfo, JERR_NO_HUFF_TABLE, tblno);
 171   htbl =
 172     isDC ? cinfo->dc_huff_tbl_ptrs[tblno] : cinfo->ac_huff_tbl_ptrs[tblno];
 173   if (htbl == NULL)
 174     ERREXIT1(cinfo, JERR_NO_HUFF_TABLE, tblno);
 175 
 176   /* Allocate a workspace if we haven't already done so. */
 177   if (*pdtbl == NULL)
 178     *pdtbl = (d_derived_tbl *)
 179       (*cinfo->mem->alloc_small) ((j_common_ptr) cinfo, JPOOL_IMAGE,
 180                                   SIZEOF(d_derived_tbl));
 181   dtbl = *pdtbl;
 182   dtbl->pub = htbl;             /* fill in back link */
 183 
 184   /* Figure C.1: make table of Huffman code length for each symbol */
 185 
 186   p = 0;
 187   for (l = 1; l <= 16; l++) {
 188     i = (int) htbl->bits[l];
 189     if (i < 0 || p + i > 256)   /* protect against table overrun */
 190       ERREXIT(cinfo, JERR_BAD_HUFF_TABLE);
 191     while (i--)
 192       huffsize[p++] = (char) l;
 193   }
 194   huffsize[p] = 0;
 195   numsymbols = p;
 196 
 197   /* Figure C.2: generate the codes themselves */
 198   /* We also validate that the counts represent a legal Huffman code tree. */
 199 
 200   code = 0;
 201   si = huffsize[0];
 202   p = 0;
 203   while (huffsize[p]) {
 204     while (((int) huffsize[p]) == si) {
 205       huffcode[p++] = code;
 206       code++;
 207     }
 208     /* code is now 1 more than the last code used for codelength si; but
 209      * it must still fit in si bits, since no code is allowed to be all ones.
 210      */
 211     if (((INT32) code) >= (((INT32) 1) << si))
 212       ERREXIT(cinfo, JERR_BAD_HUFF_TABLE);
 213     code <<= 1;
 214     si++;
 215   }
 216 
 217   /* Figure F.15: generate decoding tables for bit-sequential decoding */
 218 
 219   p = 0;
 220   for (l = 1; l <= 16; l++) {
 221     if (htbl->bits[l]) {
 222       /* valoffset[l] = huffval[] index of 1st symbol of code length l,
 223        * minus the minimum code of length l
 224        */
 225       dtbl->valoffset[l] = (INT32) p - (INT32) huffcode[p];
 226       p += htbl->bits[l];
 227       dtbl->maxcode[l] = huffcode[p-1]; /* maximum code of length l */
 228     } else {
 229       dtbl->maxcode[l] = -1;    /* -1 if no codes of this length */
 230     }
 231   }
 232   dtbl->maxcode[17] = 0xFFFFFL; /* ensures jpeg_huff_decode terminates */
 233 
 234   /* Compute lookahead tables to speed up decoding.
 235    * First we set all the table entries to 0, indicating "too long";
 236    * then we iterate through the Huffman codes that are short enough and
 237    * fill in all the entries that correspond to bit sequences starting
 238    * with that code.
 239    */
 240 
 241   MEMZERO(dtbl->look_nbits, SIZEOF(dtbl->look_nbits));
 242 
 243   p = 0;
 244   for (l = 1; l <= HUFF_LOOKAHEAD; l++) {
 245     for (i = 1; i <= (int) htbl->bits[l]; i++, p++) {
 246       /* l = current code's length, p = its index in huffcode[] & huffval[]. */
 247       /* Generate left-justified code followed by all possible bit sequences */
 248       lookbits = huffcode[p] << (HUFF_LOOKAHEAD-l);
 249       for (ctr = 1 << (HUFF_LOOKAHEAD-l); ctr > 0; ctr--) {
 250         dtbl->look_nbits[lookbits] = l;
 251         dtbl->look_sym[lookbits] = htbl->huffval[p];
 252         lookbits++;
 253       }
 254     }
 255   }
 256 
 257   /* Validate symbols as being reasonable.
 258    * For AC tables, we make no check, but accept all byte values 0..255.
 259    * For DC tables, we require the symbols to be in range 0..15.
 260    * (Tighter bounds could be applied depending on the data depth and mode,
 261    * but this is sufficient to ensure safe decoding.)
 262    */
 263   if (isDC) {
 264     for (i = 0; i < numsymbols; i++) {
 265       int sym = htbl->huffval[i];
 266       if (sym < 0 || sym > 15)
 267         ERREXIT(cinfo, JERR_BAD_HUFF_TABLE);
 268     }
 269   }
 270 }
 271 
 272 
 273 /*
 274  * Out-of-line code for bit fetching (shared with jdphuff.c).
 275  * See jdhuff.h for info about usage.
 276  * Note: current values of get_buffer and bits_left are passed as parameters,
 277  * but are returned in the corresponding fields of the state struct.
 278  *
 279  * On most machines MIN_GET_BITS should be 25 to allow the full 32-bit width
 280  * of get_buffer to be used.  (On machines with wider words, an even larger
 281  * buffer could be used.)  However, on some machines 32-bit shifts are
 282  * quite slow and take time proportional to the number of places shifted.
 283  * (This is true with most PC compilers, for instance.)  In this case it may
 284  * be a win to set MIN_GET_BITS to the minimum value of 15.  This reduces the
 285  * average shift distance at the cost of more calls to jpeg_fill_bit_buffer.
 286  */
 287 
 288 #ifdef SLOW_SHIFT_32
 289 #define MIN_GET_BITS  15        /* minimum allowable value */
 290 #else
 291 #define MIN_GET_BITS  (BIT_BUF_SIZE-7)
 292 #endif
 293 
 294 
 295 GLOBAL(boolean)
 296 jpeg_fill_bit_buffer (bitread_working_state * state,
 297                       register bit_buf_type get_buffer, register int bits_left,
 298                       int nbits)
 299 /* Load up the bit buffer to a depth of at least nbits */
 300 {
 301   /* Copy heavily used state fields into locals (hopefully registers) */
 302   register const JOCTET * next_input_byte = state->next_input_byte;
 303   register size_t bytes_in_buffer = state->bytes_in_buffer;
 304   j_decompress_ptr cinfo = state->cinfo;
 305 
 306   /* Attempt to load at least MIN_GET_BITS bits into get_buffer. */
 307   /* (It is assumed that no request will be for more than that many bits.) */
 308   /* We fail to do so only if we hit a marker or are forced to suspend. */
 309 
 310   if (cinfo->unread_marker == 0) {      /* cannot advance past a marker */
 311     while (bits_left < MIN_GET_BITS) {
 312       register int c;
 313 
 314       /* Attempt to read a byte */
 315       if (bytes_in_buffer == 0) {
 316         if (! (*cinfo->src->fill_input_buffer) (cinfo))
 317           return FALSE;
 318         next_input_byte = cinfo->src->next_input_byte;
 319         bytes_in_buffer = cinfo->src->bytes_in_buffer;
 320       }
 321       bytes_in_buffer--;
 322       c = GETJOCTET(*next_input_byte++);
 323 
 324       /* If it's 0xFF, check and discard stuffed zero byte */
 325       if (c == 0xFF) {
 326         /* Loop here to discard any padding FF's on terminating marker,
 327          * so that we can save a valid unread_marker value.  NOTE: we will
 328          * accept multiple FF's followed by a 0 as meaning a single FF data
 329          * byte.  This data pattern is not valid according to the standard.
 330          */
 331         do {
 332           if (bytes_in_buffer == 0) {
 333             if (! (*cinfo->src->fill_input_buffer) (cinfo))
 334               return FALSE;
 335             next_input_byte = cinfo->src->next_input_byte;
 336             bytes_in_buffer = cinfo->src->bytes_in_buffer;
 337           }
 338           bytes_in_buffer--;
 339           c = GETJOCTET(*next_input_byte++);
 340         } while (c == 0xFF);
 341 
 342         if (c == 0) {
 343           /* Found FF/00, which represents an FF data byte */
 344           c = 0xFF;
 345         } else {
 346           /* Oops, it's actually a marker indicating end of compressed data.
 347            * Save the marker code for later use.
 348            * Fine point: it might appear that we should save the marker into
 349            * bitread working state, not straight into permanent state.  But
 350            * once we have hit a marker, we cannot need to suspend within the
 351            * current MCU, because we will read no more bytes from the data
 352            * source.  So it is OK to update permanent state right away.
 353            */
 354           cinfo->unread_marker = c;
 355           /* See if we need to insert some fake zero bits. */
 356           goto no_more_bytes;
 357         }
 358       }
 359 
 360       /* OK, load c into get_buffer */
 361       get_buffer = (get_buffer << 8) | c;
 362       bits_left += 8;
 363     } /* end while */
 364   } else {
 365   no_more_bytes:
 366     /* We get here if we've read the marker that terminates the compressed
 367      * data segment.  There should be enough bits in the buffer register
 368      * to satisfy the request; if so, no problem.
 369      */
 370     if (nbits > bits_left) {
 371       /* Uh-oh.  Report corrupted data to user and stuff zeroes into
 372        * the data stream, so that we can produce some kind of image.
 373        * We use a nonvolatile flag to ensure that only one warning message
 374        * appears per data segment.
 375        */
 376       if (! cinfo->entropy->insufficient_data) {
 377         WARNMS(cinfo, JWRN_HIT_MARKER);
 378         cinfo->entropy->insufficient_data = TRUE;
 379       }
 380       /* Fill the buffer with zero bits */
 381       get_buffer <<= MIN_GET_BITS - bits_left;
 382       bits_left = MIN_GET_BITS;
 383     }
 384   }
 385 
 386   /* Unload the local registers */
 387   state->next_input_byte = next_input_byte;
 388   state->bytes_in_buffer = bytes_in_buffer;
 389   state->get_buffer = get_buffer;
 390   state->bits_left = bits_left;
 391 
 392   return TRUE;
 393 }
 394 
 395 
 396 /*
 397  * Out-of-line code for Huffman code decoding.
 398  * See jdhuff.h for info about usage.
 399  */
 400 
 401 GLOBAL(int)
 402 jpeg_huff_decode (bitread_working_state * state,
 403                   register bit_buf_type get_buffer, register int bits_left,
 404                   d_derived_tbl * htbl, int min_bits)
 405 {
 406   register int l = min_bits;
 407   register INT32 code;
 408 
 409   /* HUFF_DECODE has determined that the code is at least min_bits */
 410   /* bits long, so fetch that many bits in one swoop. */
 411 
 412   CHECK_BIT_BUFFER(*state, l, return -1);
 413   code = GET_BITS(l);
 414 
 415   /* Collect the rest of the Huffman code one bit at a time. */
 416   /* This is per Figure F.16 in the JPEG spec. */
 417 
 418   while (code > htbl->maxcode[l]) {
 419     code <<= 1;
 420     CHECK_BIT_BUFFER(*state, 1, return -1);
 421     code |= GET_BITS(1);
 422     l++;
 423   }
 424 
 425   /* Unload the local registers */
 426   state->get_buffer = get_buffer;
 427   state->bits_left = bits_left;
 428 
 429   /* With garbage input we may reach the sentinel value l = 17. */
 430 
 431   if (l > 16) {
 432     WARNMS(state->cinfo, JWRN_HUFF_BAD_CODE);
 433     return 0;                   /* fake a zero as the safest result */
 434   }
 435 
 436   return htbl->pub->huffval[ (int) (code + htbl->valoffset[l]) ];
 437 }
 438 
 439 
 440 /*
 441  * Figure F.12: extend sign bit.
 442  * On some machines, a shift and add will be faster than a table lookup.
 443  */
 444 
 445 #ifdef AVOID_TABLES
 446 
 447 #define HUFF_EXTEND(x,s)  ((x) < (1<<((s)-1)) ? (x) + (((-1)<<(s)) + 1) : (x))
 448 
 449 #else
 450 
 451 #define HUFF_EXTEND(x,s)  ((x) < extend_test[s] ? (x) + extend_offset[s] : (x))
 452 
 453 static const int extend_test[16] =   /* entry n is 2**(n-1) */
 454   { 0, 0x0001, 0x0002, 0x0004, 0x0008, 0x0010, 0x0020, 0x0040, 0x0080,
 455     0x0100, 0x0200, 0x0400, 0x0800, 0x1000, 0x2000, 0x4000 };
 456 
 457 static const int extend_offset[16] = /* entry n is (-1 << n) + 1 */
 458   { 0, (0xffffffff<<1) + 1, (0xffffffff<<2) + 1, (0xffffffff<<3) + 1, (0xffffffff<<4) + 1,
 459     (0xffffffff<<5) + 1, (0xffffffff<<6) + 1, (0xffffffff<<7) + 1, (0xffffffff<<8) + 1,
 460     (0xffffffff<<9) + 1, (0xffffffff<<10) + 1, (0xffffffff<<11) + 1, (0xffffffff<<12) + 1,
 461     (0xffffffff<<13) + 1, (0xffffffff<<14) + 1, (0xffffffff<<15) + 1 };
 462 
 463 #endif /* AVOID_TABLES */
 464 
 465 
 466 /*
 467  * Check for a restart marker & resynchronize decoder.
 468  * Returns FALSE if must suspend.
 469  */
 470 
 471 LOCAL(boolean)
 472 process_restart (j_decompress_ptr cinfo)
 473 {
 474   huff_entropy_ptr entropy = (huff_entropy_ptr) cinfo->entropy;
 475   int ci;
 476 
 477   /* Throw away any unused bits remaining in bit buffer; */
 478   /* include any full bytes in next_marker's count of discarded bytes */
 479   cinfo->marker->discarded_bytes += entropy->bitstate.bits_left / 8;
 480   entropy->bitstate.bits_left = 0;
 481 
 482   /* Advance past the RSTn marker */
 483   if (! (*cinfo->marker->read_restart_marker) (cinfo))
 484     return FALSE;
 485 
 486   /* Re-initialize DC predictions to 0 */
 487   for (ci = 0; ci < cinfo->comps_in_scan; ci++)
 488     entropy->saved.last_dc_val[ci] = 0;
 489 
 490   /* Reset restart counter */
 491   entropy->restarts_to_go = cinfo->restart_interval;
 492 
 493   /* Reset out-of-data flag, unless read_restart_marker left us smack up
 494    * against a marker.  In that case we will end up treating the next data
 495    * segment as empty, and we can avoid producing bogus output pixels by
 496    * leaving the flag set.
 497    */
 498   if (cinfo->unread_marker == 0)
 499     entropy->pub.insufficient_data = FALSE;
 500 
 501   return TRUE;
 502 }
 503 
 504 
 505 /*
 506  * Decode and return one MCU's worth of Huffman-compressed coefficients.
 507  * The coefficients are reordered from zigzag order into natural array order,
 508  * but are not dequantized.
 509  *
 510  * The i'th block of the MCU is stored into the block pointed to by
 511  * MCU_data[i].  WE ASSUME THIS AREA HAS BEEN ZEROED BY THE CALLER.
 512  * (Wholesale zeroing is usually a little faster than retail...)
 513  *
 514  * Returns FALSE if data source requested suspension.  In that case no
 515  * changes have been made to permanent state.  (Exception: some output
 516  * coefficients may already have been assigned.  This is harmless for
 517  * this module, since we'll just re-assign them on the next call.)
 518  */
 519 
 520 METHODDEF(boolean)
 521 decode_mcu (j_decompress_ptr cinfo, JBLOCKROW *MCU_data)
 522 {
 523   huff_entropy_ptr entropy = (huff_entropy_ptr) cinfo->entropy;
 524   int blkn;
 525   BITREAD_STATE_VARS;
 526   savable_state state;
 527 
 528   /* Process restart marker if needed; may have to suspend */
 529   if (cinfo->restart_interval) {
 530     if (entropy->restarts_to_go == 0)
 531       if (! process_restart(cinfo))
 532         return FALSE;
 533   }
 534 
 535   /* If we've run out of data, just leave the MCU set to zeroes.
 536    * This way, we return uniform gray for the remainder of the segment.
 537    */
 538   if (! entropy->pub.insufficient_data) {
 539 
 540     /* Load up working state */
 541     BITREAD_LOAD_STATE(cinfo,entropy->bitstate);
 542     ASSIGN_STATE(state, entropy->saved);
 543 
 544     /* Outer loop handles each block in the MCU */
 545 
 546     for (blkn = 0; blkn < cinfo->blocks_in_MCU; blkn++) {
 547       JBLOCKROW block = MCU_data[blkn];
 548       d_derived_tbl * dctbl = entropy->dc_cur_tbls[blkn];
 549       d_derived_tbl * actbl = entropy->ac_cur_tbls[blkn];
 550       register int s, k, r;
 551 
 552       /* Decode a single block's worth of coefficients */
 553 
 554       /* Section F.2.2.1: decode the DC coefficient difference */
 555       HUFF_DECODE(s, br_state, dctbl, return FALSE, label1);
 556       if (s) {
 557         CHECK_BIT_BUFFER(br_state, s, return FALSE);
 558         r = GET_BITS(s);
 559         s = HUFF_EXTEND(r, s);
 560       }
 561 
 562       if (entropy->dc_needed[blkn]) {
 563         /* Convert DC difference to actual value, update last_dc_val */
 564         int ci = cinfo->MCU_membership[blkn];
 565         s += state.last_dc_val[ci];
 566         state.last_dc_val[ci] = s;
 567         /* Output the DC coefficient (assumes jpeg_natural_order[0] = 0) */
 568         (*block)[0] = (JCOEF) s;
 569       }
 570 
 571       if (entropy->ac_needed[blkn]) {
 572 
 573         /* Section F.2.2.2: decode the AC coefficients */
 574         /* Since zeroes are skipped, output area must be cleared beforehand */
 575         for (k = 1; k < DCTSIZE2; k++) {
 576           HUFF_DECODE(s, br_state, actbl, return FALSE, label2);
 577 
 578           r = s >> 4;
 579           s &= 15;
 580 
 581           if (s) {
 582             k += r;
 583             CHECK_BIT_BUFFER(br_state, s, return FALSE);
 584             r = GET_BITS(s);
 585             s = HUFF_EXTEND(r, s);
 586             /* Output coefficient in natural (dezigzagged) order.
 587              * Note: the extra entries in jpeg_natural_order[] will save us
 588              * if k >= DCTSIZE2, which could happen if the data is corrupted.
 589              */
 590             (*block)[jpeg_natural_order[k]] = (JCOEF) s;
 591           } else {
 592             if (r != 15)
 593               break;
 594             k += 15;
 595           }
 596         }
 597 
 598       } else {
 599 
 600         /* Section F.2.2.2: decode the AC coefficients */
 601         /* In this path we just discard the values */
 602         for (k = 1; k < DCTSIZE2; k++) {
 603           HUFF_DECODE(s, br_state, actbl, return FALSE, label3);
 604 
 605           r = s >> 4;
 606           s &= 15;
 607 
 608           if (s) {
 609             k += r;
 610             CHECK_BIT_BUFFER(br_state, s, return FALSE);
 611             DROP_BITS(s);
 612           } else {
 613             if (r != 15)
 614               break;
 615             k += 15;
 616           }
 617         }
 618 
 619       }
 620     }
 621 
 622     /* Completed MCU, so update state */
 623     BITREAD_SAVE_STATE(cinfo,entropy->bitstate);
 624     ASSIGN_STATE(entropy->saved, state);
 625   }
 626 
 627   /* Account for restart interval (no-op if not using restarts) */
 628   entropy->restarts_to_go--;
 629 
 630   return TRUE;
 631 }
 632 
 633 
 634 /*
 635  * Module initialization routine for Huffman entropy decoding.
 636  */
 637 
 638 GLOBAL(void)
 639 jinit_huff_decoder (j_decompress_ptr cinfo)
 640 {
 641   huff_entropy_ptr entropy;
 642   int i;
 643 
 644   entropy = (huff_entropy_ptr)
 645     (*cinfo->mem->alloc_small) ((j_common_ptr) cinfo, JPOOL_IMAGE,
 646                                 SIZEOF(huff_entropy_decoder));
 647   cinfo->entropy = (struct jpeg_entropy_decoder *) entropy;
 648   entropy->pub.start_pass = start_pass_huff_decoder;
 649   entropy->pub.decode_mcu = decode_mcu;
 650 
 651   /* Mark tables unallocated */
 652   for (i = 0; i < NUM_HUFF_TBLS; i++) {
 653     entropy->dc_derived_tbls[i] = entropy->ac_derived_tbls[i] = NULL;
 654   }
 655 }
--- EOF ---