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,

   459     (int)(((unsigned)(~0)<<1)  + 1), (int)(((unsigned)(~0)<<2)  + 1),

   460     (int)(((unsigned)(~0)<<3)  + 1), (int)(((unsigned)(~0)<<4)  + 1),

   461     (int)(((unsigned)(~0)<<5)  + 1), (int)(((unsigned)(~0)<<6)  + 1),

   462     (int)(((unsigned)(~0)<<7)  + 1), (int)(((unsigned)(~0)<<8)  + 1),

   463     (int)(((unsigned)(~0)<<9)  + 1), (int)(((unsigned)(~0)<<10) + 1),

   464     (int)(((unsigned)(~0)<<11) + 1), (int)(((unsigned)(~0)<<12) + 1),

   465     (int)(((unsigned)(~0)<<13) + 1), (int)(((unsigned)(~0)<<14) + 1),

   466     (int)(((unsigned)(~0)<<15) + 1) };

   467 

   468 #endif /* AVOID_TABLES */

   469 

   470 

   471 /*

   472  * Check for a restart marker & resynchronize decoder.

   473  * Returns FALSE if must suspend.

   474  */

   475 

   476 LOCAL(boolean)

   477 process_restart (j_decompress_ptr cinfo)

   478 {

   479   huff_entropy_ptr entropy = (huff_entropy_ptr) cinfo->entropy;

   480   int ci;

   481 

   482   /* Throw away any unused bits remaining in bit buffer; */

   483   /* include any full bytes in next_marker's count of discarded bytes */

   484   cinfo->marker->discarded_bytes += entropy->bitstate.bits_left / 8;

   485   entropy->bitstate.bits_left = 0;

   486 

   487   /* Advance past the RSTn marker */

   488   if (! (*cinfo->marker->read_restart_marker) (cinfo))

   489     return FALSE;

   490 

   491   /* Re-initialize DC predictions to 0 */

   492   for (ci = 0; ci < cinfo->comps_in_scan; ci++)

   493     entropy->saved.last_dc_val[ci] = 0;

   494 

   495   /* Reset restart counter */

   496   entropy->restarts_to_go = cinfo->restart_interval;

   497 

   498   /* Reset out-of-data flag, unless read_restart_marker left us smack up

   499    * against a marker.  In that case we will end up treating the next data

   500    * segment as empty, and we can avoid producing bogus output pixels by

   501    * leaving the flag set.

   502    */

   503   if (cinfo->unread_marker == 0)

   504     entropy->pub.insufficient_data = FALSE;

   505 

   506   return TRUE;

   507 }

   508 

   509 

   510 /*

   511  * Decode and return one MCU's worth of Huffman-compressed coefficients.

   512  * The coefficients are reordered from zigzag order into natural array order,

   513  * but are not dequantized.

   514  *

   515  * The i'th block of the MCU is stored into the block pointed to by

   516  * MCU_data[i].  WE ASSUME THIS AREA HAS BEEN ZEROED BY THE CALLER.

   517  * (Wholesale zeroing is usually a little faster than retail...)

   518  *

   519  * Returns FALSE if data source requested suspension.  In that case no

   520  * changes have been made to permanent state.  (Exception: some output

   521  * coefficients may already have been assigned.  This is harmless for

   522  * this module, since we'll just re-assign them on the next call.)

   523  */

   524 

   525 METHODDEF(boolean)

   526 decode_mcu (j_decompress_ptr cinfo, JBLOCKROW *MCU_data)

   527 {

   528   huff_entropy_ptr entropy = (huff_entropy_ptr) cinfo->entropy;

   529   int blkn;

   530   BITREAD_STATE_VARS;

   531   savable_state state;

   532 

   533   /* Process restart marker if needed; may have to suspend */

   534   if (cinfo->restart_interval) {

   535     if (entropy->restarts_to_go == 0)

   536       if (! process_restart(cinfo))

   537         return FALSE;

   538   }

   539 

   540   /* If we've run out of data, just leave the MCU set to zeroes.

   541    * This way, we return uniform gray for the remainder of the segment.

   542    */

   543   if (! entropy->pub.insufficient_data) {

   544 

   545     /* Load up working state */

   546     BITREAD_LOAD_STATE(cinfo,entropy->bitstate);

   547     ASSIGN_STATE(state, entropy->saved);

   548 

   549     /* Outer loop handles each block in the MCU */

   550 

   551     for (blkn = 0; blkn < cinfo->blocks_in_MCU; blkn++) {

   552       JBLOCKROW block = MCU_data[blkn];

   553       d_derived_tbl * dctbl = entropy->dc_cur_tbls[blkn];

   554       d_derived_tbl * actbl = entropy->ac_cur_tbls[blkn];

   555       register int s, k, r;

   556 

   557       /* Decode a single block's worth of coefficients */

   558 

   559       /* Section F.2.2.1: decode the DC coefficient difference */

   560       HUFF_DECODE(s, br_state, dctbl, return FALSE, label1);

   561       if (s) {

   562         CHECK_BIT_BUFFER(br_state, s, return FALSE);

   563         r = GET_BITS(s);

   564         s = HUFF_EXTEND(r, s);

   565       }

   566 

   567       if (entropy->dc_needed[blkn]) {

   568         /* Convert DC difference to actual value, update last_dc_val */

   569         int ci = cinfo->MCU_membership[blkn];

   570         s += state.last_dc_val[ci];

   571         state.last_dc_val[ci] = s;

   572         /* Output the DC coefficient (assumes jpeg_natural_order[0] = 0) */

   573         (*block)[0] = (JCOEF) s;

   574       }

   575 

   576       if (entropy->ac_needed[blkn]) {

   577 

   578         /* Section F.2.2.2: decode the AC coefficients */

   579         /* Since zeroes are skipped, output area must be cleared beforehand */

   580         for (k = 1; k < DCTSIZE2; k++) {

   581           HUFF_DECODE(s, br_state, actbl, return FALSE, label2);

   582 

   583           r = s >> 4;

   584           s &= 15;

   585 

   586           if (s) {

   587             k += r;

   588             CHECK_BIT_BUFFER(br_state, s, return FALSE);

   589             r = GET_BITS(s);

   590             s = HUFF_EXTEND(r, s);

   591             /* Output coefficient in natural (dezigzagged) order.

   592              * Note: the extra entries in jpeg_natural_order[] will save us

   593              * if k >= DCTSIZE2, which could happen if the data is corrupted.

   594              */

   595             (*block)[jpeg_natural_order[k]] = (JCOEF) s;

   596           } else {

   597             if (r != 15)

   598               break;

   599             k += 15;

   600           }

   601         }

   602 

   603       } else {

   604 

   605         /* Section F.2.2.2: decode the AC coefficients */

   606         /* In this path we just discard the values */

   607         for (k = 1; k < DCTSIZE2; k++) {

   608           HUFF_DECODE(s, br_state, actbl, return FALSE, label3);

   609 

   610           r = s >> 4;

   611           s &= 15;

   612 

   613           if (s) {

   614             k += r;

   615             CHECK_BIT_BUFFER(br_state, s, return FALSE);

   616             DROP_BITS(s);

   617           } else {

   618             if (r != 15)

   619               break;

   620             k += 15;

   621           }

   622         }

   623 

   624       }

   625     }

   626 

   627     /* Completed MCU, so update state */

   628     BITREAD_SAVE_STATE(cinfo,entropy->bitstate);

   629     ASSIGN_STATE(entropy->saved, state);

   630   }

   631 

   632   /* Account for restart interval (no-op if not using restarts) */

   633   entropy->restarts_to_go--;

   634 

   635   return TRUE;

   636 }

   637 

   638 

   639 /*

   640  * Module initialization routine for Huffman entropy decoding.

   641  */

   642 

   643 GLOBAL(void)

   644 jinit_huff_decoder (j_decompress_ptr cinfo)

   645 {

   646   huff_entropy_ptr entropy;

   647   int i;

   648 

   649   entropy = (huff_entropy_ptr)

   650     (*cinfo->mem->alloc_small) ((j_common_ptr) cinfo, JPOOL_IMAGE,

   651                                 SIZEOF(huff_entropy_decoder));

   652   cinfo->entropy = (struct jpeg_entropy_decoder *) entropy;

   653   entropy->pub.start_pass = start_pass_huff_decoder;

   654   entropy->pub.decode_mcu = decode_mcu;

   655 

   656   /* Mark tables unallocated */

   657   for (i = 0; i < NUM_HUFF_TBLS; i++) {

   658     entropy->dc_derived_tbls[i] = entropy->ac_derived_tbls[i] = NULL;

   659   }

   660 }