1 /*

     2  * jdhuff.c

     3  *

     4  * Copyright (C) 1991-1997, Thomas G. Lane.

     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 Huffman entropy decoding routines.

     9  *

    10  * Much of the complexity here has to do with supporting input suspension.

    11  * If the data source module demands suspension, we want to be able to back

    12  * up to the start of the current MCU.  To do this, we copy state variables

    13  * into local working storage, and update them back to the permanent

    14  * storage only upon successful completion of an MCU.

    15  */

    16 

    17 #define JPEG_INTERNALS

    18 #include "jinclude.h"

    19 #include "jpeglib.h"

    20 #include "jdhuff.h"		/* Declarations shared with jdphuff.c */

    21 

    22 

    23 /*

    24  * Expanded entropy decoder object for Huffman decoding.

    25  *

    26  * The savable_state subrecord contains fields that change within an MCU,

    27  * but must not be updated permanently until we complete the MCU.

    28  */

    29 

    30 typedef struct {

    31   int last_dc_val[MAX_COMPS_IN_SCAN]; /* last DC coef for each component */

    32 } savable_state;

    33 

    34 /* This macro is to work around compilers with missing or broken

    35  * structure assignment.  You'll need to fix this code if you have

    36  * such a compiler and you change MAX_COMPS_IN_SCAN.

    37  */

    38 

    39 #ifndef NO_STRUCT_ASSIGN

    40 #define ASSIGN_STATE(dest,src)  ((dest) = (src))

    41 #else

    42 #if MAX_COMPS_IN_SCAN == 4

    43 #define ASSIGN_STATE(dest,src)  \

    44 	((dest).last_dc_val[0] = (src).last_dc_val[0], \

    45 	 (dest).last_dc_val[1] = (src).last_dc_val[1], \

    46 	 (dest).last_dc_val[2] = (src).last_dc_val[2], \

    47 	 (dest).last_dc_val[3] = (src).last_dc_val[3])

    48 #endif

    49 #endif

    50 

    51 

    52 typedef struct {

    53   struct jpeg_entropy_decoder pub; /* public fields */

    54 

    55   /* These fields are loaded into local variables at start of each MCU.

    56    * In case of suspension, we exit WITHOUT updating them.

    57    */

    58   bitread_perm_state bitstate;	/* Bit buffer at start of MCU */

    59   savable_state saved;		/* Other state at start of MCU */

    60 

    61   /* These fields are NOT loaded into local working state. */

    62   unsigned int restarts_to_go;	/* MCUs left in this restart interval */

    63 

    64   /* Pointers to derived tables (these workspaces have image lifespan) */

    65   d_derived_tbl * dc_derived_tbls[NUM_HUFF_TBLS];

    66   d_derived_tbl * ac_derived_tbls[NUM_HUFF_TBLS];

    67 

    68   /* Precalculated info set up by start_pass for use in decode_mcu: */

    69 

    70   /* Pointers to derived tables to be used for each block within an MCU */

    71   d_derived_tbl * dc_cur_tbls[D_MAX_BLOCKS_IN_MCU];

    72   d_derived_tbl * ac_cur_tbls[D_MAX_BLOCKS_IN_MCU];

    73   /* Whether we care about the DC and AC coefficient values for each block */

    74   boolean dc_needed[D_MAX_BLOCKS_IN_MCU];

    75   boolean ac_needed[D_MAX_BLOCKS_IN_MCU];

    76 } huff_entropy_decoder;

    77 

    78 typedef huff_entropy_decoder * huff_entropy_ptr;

    79 

    80 

    81 /*

    82  * Initialize for a Huffman-compressed scan.

    83  */

    84 

    85 METHODDEF(void)

    86 start_pass_huff_decoder (j_decompress_ptr cinfo)

    87 {

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

    89   int ci, blkn, dctbl, actbl;

    90   jpeg_component_info * compptr;

    91 

    92   /* Check that the scan parameters Ss, Se, Ah/Al are OK for sequential JPEG.

    93    * This ought to be an error condition, but we make it a warning because

    94    * there are some baseline files out there with all zeroes in these bytes.

    95    */

    96   if (cinfo->Ss != 0 || cinfo->Se != DCTSIZE2-1 ||

    97       cinfo->Ah != 0 || cinfo->Al != 0)

    98     WARNMS(cinfo, JWRN_NOT_SEQUENTIAL);

    99 

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

   101     compptr = cinfo->cur_comp_info[ci];

   102     dctbl = compptr->dc_tbl_no;

   103     actbl = compptr->ac_tbl_no;

   104     /* Compute derived values for Huffman tables */

   105     /* We may do this more than once for a table, but it's not expensive */

   106     jpeg_make_d_derived_tbl(cinfo, TRUE, dctbl,

   107 			    & entropy->dc_derived_tbls[dctbl]);

   108     jpeg_make_d_derived_tbl(cinfo, FALSE, actbl,

   109 			    & entropy->ac_derived_tbls[actbl]);

   110     /* Initialize DC predictions to 0 */

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

   112   }

   113 

   114   /* Precalculate decoding info for each block in an MCU of this scan */

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

   116     ci = cinfo->MCU_membership[blkn];

   117     compptr = cinfo->cur_comp_info[ci];

   118     /* Precalculate which table to use for each block */

   119     entropy->dc_cur_tbls[blkn] = entropy->dc_derived_tbls[compptr->dc_tbl_no];

   120     entropy->ac_cur_tbls[blkn] = entropy->ac_derived_tbls[compptr->ac_tbl_no];

   121     /* Decide whether we really care about the coefficient values */

   122     if (compptr->component_needed) {

   123       entropy->dc_needed[blkn] = TRUE;

   124       /* we don't need the ACs if producing a 1/8th-size image */

   125       entropy->ac_needed[blkn] = (compptr->DCT_scaled_size > 1);

   126     } else {

   127       entropy->dc_needed[blkn] = entropy->ac_needed[blkn] = FALSE;

   128     }

   129   }

   130 

   131   /* Initialize bitread state variables */

   132   entropy->bitstate.bits_left = 0;

   133   entropy->bitstate.get_buffer = 0; /* unnecessary, but keeps Purify quiet */

   134   entropy->pub.insufficient_data = FALSE;

   135 

   136   /* Initialize restart counter */

   137   entropy->restarts_to_go = cinfo->restart_interval;

   138 }

   139 

   140 

   141 /*

   142  * Compute the derived values for a Huffman table.

   143  * This routine also performs some validation checks on the table.

   144  *

   145  * Note this is also used by jdphuff.c.

   146  */

   147 

   148 GLOBAL(void)

   149 jpeg_make_d_derived_tbl (j_decompress_ptr cinfo, boolean isDC, int tblno,

   150 			 d_derived_tbl ** pdtbl)

   151 {

   152   JHUFF_TBL *htbl;

   153   d_derived_tbl *dtbl;

   154   int p, i, l, si, numsymbols;

   155   int lookbits, ctr;

   156   char huffsize[257];

   157   unsigned int huffcode[257];

   158   unsigned int code;

   159 

   160   /* Note that huffsize[] and huffcode[] are filled in code-length order,

   161    * paralleling the order of the symbols themselves in htbl->huffval[].

   162    */

   163 

   164   /* Find the input Huffman table */

   165   if (tblno < 0 || tblno >= NUM_HUFF_TBLS)

   166     ERREXIT1(cinfo, JERR_NO_HUFF_TABLE, tblno);

   167   htbl =

   168     isDC ? cinfo->dc_huff_tbl_ptrs[tblno] : cinfo->ac_huff_tbl_ptrs[tblno];

   169   if (htbl == NULL)

   170     ERREXIT1(cinfo, JERR_NO_HUFF_TABLE, tblno);

   171 

   172   /* Allocate a workspace if we haven't already done so. */

   173   if (*pdtbl == NULL)

   174     *pdtbl = (d_derived_tbl *)

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

   176 				  SIZEOF(d_derived_tbl));

   177   dtbl = *pdtbl;

   178   dtbl->pub = htbl;		/* fill in back link */

   179   

   180   /* Figure C.1: make table of Huffman code length for each symbol */

   181 

   182   p = 0;

   183   for (l = 1; l <= 16; l++) {

   184     i = (int) htbl->bits[l];

   185     if (i < 0 || p + i > 256)	/* protect against table overrun */

   186       ERREXIT(cinfo, JERR_BAD_HUFF_TABLE);

   187     while (i--)

   188       huffsize[p++] = (char) l;

   189   }

   190   huffsize[p] = 0;

   191   numsymbols = p;

   192   

   193   /* Figure C.2: generate the codes themselves */

   194   /* We also validate that the counts represent a legal Huffman code tree. */

   195   

   196   code = 0;

   197   si = huffsize[0];

   198   p = 0;

   199   while (huffsize[p]) {

   200     while (((int) huffsize[p]) == si) {

   201       huffcode[p++] = code;

   202       code++;

   203     }

   204     /* code is now 1 more than the last code used for codelength si; but

   205      * it must still fit in si bits, since no code is allowed to be all ones.

   206      */

   207     if (((INT32) code) >= (((INT32) 1) << si))

   208       ERREXIT(cinfo, JERR_BAD_HUFF_TABLE);

   209     code <<= 1;

   210     si++;

   211   }

   212 

   213   /* Figure F.15: generate decoding tables for bit-sequential decoding */

   214 

   215   p = 0;

   216   for (l = 1; l <= 16; l++) {

   217     if (htbl->bits[l]) {

   218       /* valoffset[l] = huffval[] index of 1st symbol of code length l,

   219        * minus the minimum code of length l

   220        */

   221       dtbl->valoffset[l] = (INT32) p - (INT32) huffcode[p];

   222       p += htbl->bits[l];

   223       dtbl->maxcode[l] = huffcode[p-1]; /* maximum code of length l */

   224     } else {

   225       dtbl->maxcode[l] = -1;	/* -1 if no codes of this length */

   226     }

   227   }

   228   dtbl->maxcode[17] = 0xFFFFFL; /* ensures jpeg_huff_decode terminates */

   229 

   230   /* Compute lookahead tables to speed up decoding.

   231    * First we set all the table entries to 0, indicating "too long";

   232    * then we iterate through the Huffman codes that are short enough and

   233    * fill in all the entries that correspond to bit sequences starting

   234    * with that code.

   235    */

   236 

   237   MEMZERO(dtbl->look_nbits, SIZEOF(dtbl->look_nbits));

   238 

   239   p = 0;

   240   for (l = 1; l <= HUFF_LOOKAHEAD; l++) {

   241     for (i = 1; i <= (int) htbl->bits[l]; i++, p++) {

   242       /* l = current code's length, p = its index in huffcode[] & huffval[]. */

   243       /* Generate left-justified code followed by all possible bit sequences */

   244       lookbits = huffcode[p] << (HUFF_LOOKAHEAD-l);

   245       for (ctr = 1 << (HUFF_LOOKAHEAD-l); ctr > 0; ctr--) {

   246 	dtbl->look_nbits[lookbits] = l;

   247 	dtbl->look_sym[lookbits] = htbl->huffval[p];

   248 	lookbits++;

   249       }

   250     }

   251   }

   252 

   253   /* Validate symbols as being reasonable.

   254    * For AC tables, we make no check, but accept all byte values 0..255.

   255    * For DC tables, we require the symbols to be in range 0..15.

   256    * (Tighter bounds could be applied depending on the data depth and mode,

   257    * but this is sufficient to ensure safe decoding.)

   258    */

   259   if (isDC) {

   260     for (i = 0; i < numsymbols; i++) {

   261       int sym = htbl->huffval[i];

   262       if (sym < 0 || sym > 15)

   263 	ERREXIT(cinfo, JERR_BAD_HUFF_TABLE);

   264     }

   265   }

   266 }

   267 

   268 

   269 /*

   270  * Out-of-line code for bit fetching (shared with jdphuff.c).

   271  * See jdhuff.h for info about usage.

   272  * Note: current values of get_buffer and bits_left are passed as parameters,

   273  * but are returned in the corresponding fields of the state struct.

   274  *

   275  * On most machines MIN_GET_BITS should be 25 to allow the full 32-bit width

   276  * of get_buffer to be used.  (On machines with wider words, an even larger

   277  * buffer could be used.)  However, on some machines 32-bit shifts are

   278  * quite slow and take time proportional to the number of places shifted.

   279  * (This is true with most PC compilers, for instance.)  In this case it may

   280  * be a win to set MIN_GET_BITS to the minimum value of 15.  This reduces the

   281  * average shift distance at the cost of more calls to jpeg_fill_bit_buffer.

   282  */

   283 

   284 #ifdef SLOW_SHIFT_32

   285 #define MIN_GET_BITS  15	/* minimum allowable value */

   286 #else

   287 #define MIN_GET_BITS  (BIT_BUF_SIZE-7)

   288 #endif

   289 

   290 

   291 GLOBAL(boolean)

   292 jpeg_fill_bit_buffer (bitread_working_state * state,

   293 		      register bit_buf_type get_buffer, register int bits_left,

   294 		      int nbits)

   295 /* Load up the bit buffer to a depth of at least nbits */

   296 {

   297   /* Copy heavily used state fields into locals (hopefully registers) */

   298   register const JOCTET * next_input_byte = state->next_input_byte;

   299   register size_t bytes_in_buffer = state->bytes_in_buffer;

   300   j_decompress_ptr cinfo = state->cinfo;

   301 

   302   /* Attempt to load at least MIN_GET_BITS bits into get_buffer. */

   303   /* (It is assumed that no request will be for more than that many bits.) */

   304   /* We fail to do so only if we hit a marker or are forced to suspend. */

   305 

   306   if (cinfo->unread_marker == 0) {	/* cannot advance past a marker */

   307     while (bits_left < MIN_GET_BITS) {

   308       register int c;

   309 

   310       /* Attempt to read a byte */

   311       if (bytes_in_buffer == 0) {

   312 	if (! (*cinfo->src->fill_input_buffer) (cinfo))

   313 	  return FALSE;

   314 	next_input_byte = cinfo->src->next_input_byte;

   315 	bytes_in_buffer = cinfo->src->bytes_in_buffer;

   316       }

   317       bytes_in_buffer--;

   318       c = GETJOCTET(*next_input_byte++);

   319 

   320       /* If it's 0xFF, check and discard stuffed zero byte */

   321       if (c == 0xFF) {

   322 	/* Loop here to discard any padding FF's on terminating marker,

   323 	 * so that we can save a valid unread_marker value.  NOTE: we will

   324 	 * accept multiple FF's followed by a 0 as meaning a single FF data

   325 	 * byte.  This data pattern is not valid according to the standard.

   326 	 */

   327 	do {

   328 	  if (bytes_in_buffer == 0) {

   329 	    if (! (*cinfo->src->fill_input_buffer) (cinfo))

   330 	      return FALSE;

   331 	    next_input_byte = cinfo->src->next_input_byte;

   332 	    bytes_in_buffer = cinfo->src->bytes_in_buffer;

   333 	  }

   334 	  bytes_in_buffer--;

   335 	  c = GETJOCTET(*next_input_byte++);

   336 	} while (c == 0xFF);

   337 

   338 	if (c == 0) {

   339 	  /* Found FF/00, which represents an FF data byte */

   340 	  c = 0xFF;

   341 	} else {

   342 	  /* Oops, it's actually a marker indicating end of compressed data.

   343 	   * Save the marker code for later use.

   344 	   * Fine point: it might appear that we should save the marker into

   345 	   * bitread working state, not straight into permanent state.  But

   346 	   * once we have hit a marker, we cannot need to suspend within the

   347 	   * current MCU, because we will read no more bytes from the data

   348 	   * source.  So it is OK to update permanent state right away.

   349 	   */

   350 	  cinfo->unread_marker = c;

   351 	  /* See if we need to insert some fake zero bits. */

   352 	  goto no_more_bytes;

   353 	}

   354       }

   355 

   356       /* OK, load c into get_buffer */

   357       get_buffer = (get_buffer << 8) | c;

   358       bits_left += 8;

   359     } /* end while */

   360   } else {

   361   no_more_bytes:

   362     /* We get here if we've read the marker that terminates the compressed

   363      * data segment.  There should be enough bits in the buffer register

   364      * to satisfy the request; if so, no problem.

   365      */

   366     if (nbits > bits_left) {

   367       /* Uh-oh.  Report corrupted data to user and stuff zeroes into

   368        * the data stream, so that we can produce some kind of image.

   369        * We use a nonvolatile flag to ensure that only one warning message

   370        * appears per data segment.

   371        */

   372       if (! cinfo->entropy->insufficient_data) {

   373 	WARNMS(cinfo, JWRN_HIT_MARKER);

   374 	cinfo->entropy->insufficient_data = TRUE;

   375       }

   376       /* Fill the buffer with zero bits */

   377       get_buffer <<= MIN_GET_BITS - bits_left;

   378       bits_left = MIN_GET_BITS;

   379     }

   380   }

   381 

   382   /* Unload the local registers */

   383   state->next_input_byte = next_input_byte;

   384   state->bytes_in_buffer = bytes_in_buffer;

   385   state->get_buffer = get_buffer;

   386   state->bits_left = bits_left;

   387 

   388   return TRUE;

   389 }

   390 

   391 

   392 /*

   393  * Out-of-line code for Huffman code decoding.

   394  * See jdhuff.h for info about usage.

   395  */

   396 

   397 GLOBAL(int)

   398 jpeg_huff_decode (bitread_working_state * state,

   399 		  register bit_buf_type get_buffer, register int bits_left,

   400 		  d_derived_tbl * htbl, int min_bits)

   401 {

   402   register int l = min_bits;

   403   register INT32 code;

   404 

   405   /* HUFF_DECODE has determined that the code is at least min_bits */

   406   /* bits long, so fetch that many bits in one swoop. */

   407 

   408   CHECK_BIT_BUFFER(*state, l, return -1);

   409   code = GET_BITS(l);

   410 

   411   /* Collect the rest of the Huffman code one bit at a time. */

   412   /* This is per Figure F.16 in the JPEG spec. */

   413 

   414   while (code > htbl->maxcode[l]) {

   415     code <<= 1;

   416     CHECK_BIT_BUFFER(*state, 1, return -1);

   417     code |= GET_BITS(1);

   418     l++;

   419   }

   420 

   421   /* Unload the local registers */

   422   state->get_buffer = get_buffer;

   423   state->bits_left = bits_left;

   424 

   425   /* With garbage input we may reach the sentinel value l = 17. */

   426 

   427   if (l > 16) {

   428     WARNMS(state->cinfo, JWRN_HUFF_BAD_CODE);

   429     return 0;			/* fake a zero as the safest result */

   430   }

   431 

   432   return htbl->pub->huffval[ (int) (code + htbl->valoffset[l]) ];

   433 }

   434 

   435 

   436 /*

   437  * Figure F.12: extend sign bit.

   438  * On some machines, a shift and add will be faster than a table lookup.

   439  */

   440 

   441 #ifdef AVOID_TABLES

   442 

   443 #define HUFF_EXTEND(x,s)  ((x) < (1<<((s)-1)) ? (x) + (((-1)<<(s)) + 1) : (x))

   444 

   445 #else

   446 

   447 #define HUFF_EXTEND(x,s)  ((x) < extend_test[s] ? (x) + extend_offset[s] : (x))

   448 

   449 static const int extend_test[16] =   /* entry n is 2**(n-1) */

   450   { 0, 0x0001, 0x0002, 0x0004, 0x0008, 0x0010, 0x0020, 0x0040, 0x0080,

   451     0x0100, 0x0200, 0x0400, 0x0800, 0x1000, 0x2000, 0x4000 };

   452 

   453 static const int extend_offset[16] = /* entry n is (-1 << n) + 1 */

   454   { 0, ((-1)<<1) + 1, ((-1)<<2) + 1, ((-1)<<3) + 1, ((-1)<<4) + 1,

   455     ((-1)<<5) + 1, ((-1)<<6) + 1, ((-1)<<7) + 1, ((-1)<<8) + 1,

   456     ((-1)<<9) + 1, ((-1)<<10) + 1, ((-1)<<11) + 1, ((-1)<<12) + 1,

   457     ((-1)<<13) + 1, ((-1)<<14) + 1, ((-1)<<15) + 1 };

   458 

   459 #endif /* AVOID_TABLES */

   460 

   461 

   462 /*

   463  * Check for a restart marker & resynchronize decoder.

   464  * Returns FALSE if must suspend.

   465  */

   466 

   467 LOCAL(boolean)

   468 process_restart (j_decompress_ptr cinfo)

   469 {

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

   471   int ci;

   472 

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

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

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

   476   entropy->bitstate.bits_left = 0;

   477 

   478   /* Advance past the RSTn marker */

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

   480     return FALSE;

   481 

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

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

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

   485 

   486   /* Reset restart counter */

   487   entropy->restarts_to_go = cinfo->restart_interval;

   488 

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

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

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

   492    * leaving the flag set.

   493    */

   494   if (cinfo->unread_marker == 0)

   495     entropy->pub.insufficient_data = FALSE;

   496 

   497   return TRUE;

   498 }

   499 

   500 

   501 /*

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

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

   504  * but are not dequantized.

   505  *

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

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

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

   509  *

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

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

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

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

   514  */

   515 

   516 METHODDEF(boolean)

   517 decode_mcu (j_decompress_ptr cinfo, JBLOCKROW *MCU_data)

   518 {

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

   520   int blkn;

   521   BITREAD_STATE_VARS;

   522   savable_state state;

   523 

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

   525   if (cinfo->restart_interval) {

   526     if (entropy->restarts_to_go == 0)

   527       if (! process_restart(cinfo))

   528 	return FALSE;

   529   }

   530 

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

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

   533    */

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

   535 

   536     /* Load up working state */

   537     BITREAD_LOAD_STATE(cinfo,entropy->bitstate);

   538     ASSIGN_STATE(state, entropy->saved);

   539 

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

   541 

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

   543       JBLOCKROW block = MCU_data[blkn];

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

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

   546       register int s, k, r;

   547 

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

   549 

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

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

   552       if (s) {

   553 	CHECK_BIT_BUFFER(br_state, s, return FALSE);

   554 	r = GET_BITS(s);

   555 	s = HUFF_EXTEND(r, s);

   556       }

   557 

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

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

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

   561 	s += state.last_dc_val[ci];

   562 	state.last_dc_val[ci] = s;

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

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

   565       }

   566 

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

   568 

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

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

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

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

   573       

   574 	  r = s >> 4;

   575 	  s &= 15;

   576       

   577 	  if (s) {

   578 	    k += r;

   579 	    CHECK_BIT_BUFFER(br_state, s, return FALSE);

   580 	    r = GET_BITS(s);

   581 	    s = HUFF_EXTEND(r, s);

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

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

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

   585 	     */

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

   587 	  } else {

   588 	    if (r != 15)

   589 	      break;

   590 	    k += 15;

   591 	  }

   592 	}

   593 

   594       } else {

   595 

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

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

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

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

   600       

   601 	  r = s >> 4;

   602 	  s &= 15;

   603       

   604 	  if (s) {

   605 	    k += r;

   606 	    CHECK_BIT_BUFFER(br_state, s, return FALSE);

   607 	    DROP_BITS(s);

   608 	  } else {

   609 	    if (r != 15)

   610 	      break;

   611 	    k += 15;

   612 	  }

   613 	}

   614 

   615       }

   616     }

   617 

   618     /* Completed MCU, so update state */

   619     BITREAD_SAVE_STATE(cinfo,entropy->bitstate);

   620     ASSIGN_STATE(entropy->saved, state);

   621   }

   622 

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

   624   entropy->restarts_to_go--;

   625 

   626   return TRUE;

   627 }

   628 

   629 

   630 /*

   631  * Module initialization routine for Huffman entropy decoding.

   632  */

   633 

   634 GLOBAL(void)

   635 jinit_huff_decoder (j_decompress_ptr cinfo)

   636 {

   637   huff_entropy_ptr entropy;

   638   int i;

   639 

   640   entropy = (huff_entropy_ptr)

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

   642 				SIZEOF(huff_entropy_decoder));

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

   644   entropy->pub.start_pass = start_pass_huff_decoder;

   645   entropy->pub.decode_mcu = decode_mcu;

   646 

   647   /* Mark tables unallocated */

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

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

   650   }

   651 }