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plt-scheme  4.2.1
jdhuff.c
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00001 /*
00002  * jdhuff.c
00003  *
00004  * Copyright (C) 1991-1997, Thomas G. Lane.
00005  * This file is part of the Independent JPEG Group's software.
00006  * For conditions of distribution and use, see the accompanying README file.
00007  *
00008  * This file contains Huffman entropy decoding routines.
00009  *
00010  * Much of the complexity here has to do with supporting input suspension.
00011  * If the data source module demands suspension, we want to be able to back
00012  * up to the start of the current MCU.  To do this, we copy state variables
00013  * into local working storage, and update them back to the permanent
00014  * storage only upon successful completion of an MCU.
00015  */
00016 
00017 #define JPEG_INTERNALS
00018 #include "jinclude.h"
00019 #include "jpeglib.h"
00020 #include "jdhuff.h"         /* Declarations shared with jdphuff.c */
00021 
00022 
00023 /*
00024  * Expanded entropy decoder object for Huffman decoding.
00025  *
00026  * The savable_state subrecord contains fields that change within an MCU,
00027  * but must not be updated permanently until we complete the MCU.
00028  */
00029 
00030 typedef struct {
00031   int last_dc_val[MAX_COMPS_IN_SCAN]; /* last DC coef for each component */
00032 } savable_state;
00033 
00034 /* This macro is to work around compilers with missing or broken
00035  * structure assignment.  You'll need to fix this code if you have
00036  * such a compiler and you change MAX_COMPS_IN_SCAN.
00037  */
00038 
00039 #ifndef NO_STRUCT_ASSIGN
00040 #define ASSIGN_STATE(dest,src)  ((dest) = (src))
00041 #else
00042 #if MAX_COMPS_IN_SCAN == 4
00043 #define ASSIGN_STATE(dest,src)  \
00044        ((dest).last_dc_val[0] = (src).last_dc_val[0], \
00045         (dest).last_dc_val[1] = (src).last_dc_val[1], \
00046         (dest).last_dc_val[2] = (src).last_dc_val[2], \
00047         (dest).last_dc_val[3] = (src).last_dc_val[3])
00048 #endif
00049 #endif
00050 
00051 
00052 typedef struct {
00053   struct jpeg_entropy_decoder pub; /* public fields */
00054 
00055   /* These fields are loaded into local variables at start of each MCU.
00056    * In case of suspension, we exit WITHOUT updating them.
00057    */
00058   bitread_perm_state bitstate;     /* Bit buffer at start of MCU */
00059   savable_state saved;             /* Other state at start of MCU */
00060 
00061   /* These fields are NOT loaded into local working state. */
00062   unsigned int restarts_to_go;     /* MCUs left in this restart interval */
00063 
00064   /* Pointers to derived tables (these workspaces have image lifespan) */
00065   d_derived_tbl * dc_derived_tbls[NUM_HUFF_TBLS];
00066   d_derived_tbl * ac_derived_tbls[NUM_HUFF_TBLS];
00067 
00068   /* Precalculated info set up by start_pass for use in decode_mcu: */
00069 
00070   /* Pointers to derived tables to be used for each block within an MCU */
00071   d_derived_tbl * dc_cur_tbls[D_MAX_BLOCKS_IN_MCU];
00072   d_derived_tbl * ac_cur_tbls[D_MAX_BLOCKS_IN_MCU];
00073   /* Whether we care about the DC and AC coefficient values for each block */
00074   boolean dc_needed[D_MAX_BLOCKS_IN_MCU];
00075   boolean ac_needed[D_MAX_BLOCKS_IN_MCU];
00076 } huff_entropy_decoder;
00077 
00078 typedef huff_entropy_decoder * huff_entropy_ptr;
00079 
00080 
00081 /*
00082  * Initialize for a Huffman-compressed scan.
00083  */
00084 
00085 METHODDEF(void)
00086 start_pass_huff_decoder (j_decompress_ptr cinfo)
00087 {
00088   huff_entropy_ptr entropy = (huff_entropy_ptr) cinfo->entropy;
00089   int ci, blkn, dctbl, actbl;
00090   jpeg_component_info * compptr;
00091 
00092   /* Check that the scan parameters Ss, Se, Ah/Al are OK for sequential JPEG.
00093    * This ought to be an error condition, but we make it a warning because
00094    * there are some baseline files out there with all zeroes in these bytes.
00095    */
00096   if (cinfo->Ss != 0 || cinfo->Se != DCTSIZE2-1 ||
00097       cinfo->Ah != 0 || cinfo->Al != 0)
00098     WARNMS(cinfo, JWRN_NOT_SEQUENTIAL);
00099 
00100   for (ci = 0; ci < cinfo->comps_in_scan; ci++) {
00101     compptr = cinfo->cur_comp_info[ci];
00102     dctbl = compptr->dc_tbl_no;
00103     actbl = compptr->ac_tbl_no;
00104     /* Compute derived values for Huffman tables */
00105     /* We may do this more than once for a table, but it's not expensive */
00106     jpeg_make_d_derived_tbl(cinfo, TRUE, dctbl,
00107                          & entropy->dc_derived_tbls[dctbl]);
00108     jpeg_make_d_derived_tbl(cinfo, FALSE, actbl,
00109                          & entropy->ac_derived_tbls[actbl]);
00110     /* Initialize DC predictions to 0 */
00111     entropy->saved.last_dc_val[ci] = 0;
00112   }
00113 
00114   /* Precalculate decoding info for each block in an MCU of this scan */
00115   for (blkn = 0; blkn < cinfo->blocks_in_MCU; blkn++) {
00116     ci = cinfo->MCU_membership[blkn];
00117     compptr = cinfo->cur_comp_info[ci];
00118     /* Precalculate which table to use for each block */
00119     entropy->dc_cur_tbls[blkn] = entropy->dc_derived_tbls[compptr->dc_tbl_no];
00120     entropy->ac_cur_tbls[blkn] = entropy->ac_derived_tbls[compptr->ac_tbl_no];
00121     /* Decide whether we really care about the coefficient values */
00122     if (compptr->component_needed) {
00123       entropy->dc_needed[blkn] = TRUE;
00124       /* we don't need the ACs if producing a 1/8th-size image */
00125       entropy->ac_needed[blkn] = (compptr->DCT_scaled_size > 1);
00126     } else {
00127       entropy->dc_needed[blkn] = entropy->ac_needed[blkn] = FALSE;
00128     }
00129   }
00130 
00131   /* Initialize bitread state variables */
00132   entropy->bitstate.bits_left = 0;
00133   entropy->bitstate.get_buffer = 0; /* unnecessary, but keeps Purify quiet */
00134   entropy->pub.insufficient_data = FALSE;
00135 
00136   /* Initialize restart counter */
00137   entropy->restarts_to_go = cinfo->restart_interval;
00138 }
00139 
00140 
00141 /*
00142  * Compute the derived values for a Huffman table.
00143  * This routine also performs some validation checks on the table.
00144  *
00145  * Note this is also used by jdphuff.c.
00146  */
00147 
00148 GLOBAL(void)
00149 jpeg_make_d_derived_tbl (j_decompress_ptr cinfo, boolean isDC, int tblno,
00150                       d_derived_tbl ** pdtbl)
00151 {
00152   JHUFF_TBL *htbl;
00153   d_derived_tbl *dtbl;
00154   int p, i, l, si, numsymbols;
00155   int lookbits, ctr;
00156   char huffsize[257];
00157   unsigned int huffcode[257];
00158   unsigned int code;
00159 
00160   /* Note that huffsize[] and huffcode[] are filled in code-length order,
00161    * paralleling the order of the symbols themselves in htbl->huffval[].
00162    */
00163 
00164   /* Find the input Huffman table */
00165   if (tblno < 0 || tblno >= NUM_HUFF_TBLS)
00166     ERREXIT1(cinfo, JERR_NO_HUFF_TABLE, tblno);
00167   htbl =
00168     isDC ? cinfo->dc_huff_tbl_ptrs[tblno] : cinfo->ac_huff_tbl_ptrs[tblno];
00169   if (htbl == NULL)
00170     ERREXIT1(cinfo, JERR_NO_HUFF_TABLE, tblno);
00171 
00172   /* Allocate a workspace if we haven't already done so. */
00173   if (*pdtbl == NULL)
00174     *pdtbl = (d_derived_tbl *)
00175       (*cinfo->mem->alloc_small) ((j_common_ptr) cinfo, JPOOL_IMAGE,
00176                               SIZEOF(d_derived_tbl));
00177   dtbl = *pdtbl;
00178   dtbl->pub = htbl;         /* fill in back link */
00179   
00180   /* Figure C.1: make table of Huffman code length for each symbol */
00181 
00182   p = 0;
00183   for (l = 1; l <= 16; l++) {
00184     i = (int) htbl->bits[l];
00185     if (i < 0 || p + i > 256)      /* protect against table overrun */
00186       ERREXIT(cinfo, JERR_BAD_HUFF_TABLE);
00187     while (i--)
00188       huffsize[p++] = (char) l;
00189   }
00190   huffsize[p] = 0;
00191   numsymbols = p;
00192   
00193   /* Figure C.2: generate the codes themselves */
00194   /* We also validate that the counts represent a legal Huffman code tree. */
00195   
00196   code = 0;
00197   si = huffsize[0];
00198   p = 0;
00199   while (huffsize[p]) {
00200     while (((int) huffsize[p]) == si) {
00201       huffcode[p++] = code;
00202       code++;
00203     }
00204     /* code is now 1 more than the last code used for codelength si; but
00205      * it must still fit in si bits, since no code is allowed to be all ones.
00206      */
00207     if (((INT32) code) >= (((INT32) 1) << si))
00208       ERREXIT(cinfo, JERR_BAD_HUFF_TABLE);
00209     code <<= 1;
00210     si++;
00211   }
00212 
00213   /* Figure F.15: generate decoding tables for bit-sequential decoding */
00214 
00215   p = 0;
00216   for (l = 1; l <= 16; l++) {
00217     if (htbl->bits[l]) {
00218       /* valoffset[l] = huffval[] index of 1st symbol of code length l,
00219        * minus the minimum code of length l
00220        */
00221       dtbl->valoffset[l] = (INT32) p - (INT32) huffcode[p];
00222       p += htbl->bits[l];
00223       dtbl->maxcode[l] = huffcode[p-1]; /* maximum code of length l */
00224     } else {
00225       dtbl->maxcode[l] = -1;       /* -1 if no codes of this length */
00226     }
00227   }
00228   dtbl->maxcode[17] = 0xFFFFFL; /* ensures jpeg_huff_decode terminates */
00229 
00230   /* Compute lookahead tables to speed up decoding.
00231    * First we set all the table entries to 0, indicating "too long";
00232    * then we iterate through the Huffman codes that are short enough and
00233    * fill in all the entries that correspond to bit sequences starting
00234    * with that code.
00235    */
00236 
00237   MEMZERO(dtbl->look_nbits, SIZEOF(dtbl->look_nbits));
00238 
00239   p = 0;
00240   for (l = 1; l <= HUFF_LOOKAHEAD; l++) {
00241     for (i = 1; i <= (int) htbl->bits[l]; i++, p++) {
00242       /* l = current code's length, p = its index in huffcode[] & huffval[]. */
00243       /* Generate left-justified code followed by all possible bit sequences */
00244       lookbits = huffcode[p] << (HUFF_LOOKAHEAD-l);
00245       for (ctr = 1 << (HUFF_LOOKAHEAD-l); ctr > 0; ctr--) {
00246        dtbl->look_nbits[lookbits] = l;
00247        dtbl->look_sym[lookbits] = htbl->huffval[p];
00248        lookbits++;
00249       }
00250     }
00251   }
00252 
00253   /* Validate symbols as being reasonable.
00254    * For AC tables, we make no check, but accept all byte values 0..255.
00255    * For DC tables, we require the symbols to be in range 0..15.
00256    * (Tighter bounds could be applied depending on the data depth and mode,
00257    * but this is sufficient to ensure safe decoding.)
00258    */
00259   if (isDC) {
00260     for (i = 0; i < numsymbols; i++) {
00261       int sym = htbl->huffval[i];
00262       if (sym < 0 || sym > 15)
00263        ERREXIT(cinfo, JERR_BAD_HUFF_TABLE);
00264     }
00265   }
00266 }
00267 
00268 
00269 /*
00270  * Out-of-line code for bit fetching (shared with jdphuff.c).
00271  * See jdhuff.h for info about usage.
00272  * Note: current values of get_buffer and bits_left are passed as parameters,
00273  * but are returned in the corresponding fields of the state struct.
00274  *
00275  * On most machines MIN_GET_BITS should be 25 to allow the full 32-bit width
00276  * of get_buffer to be used.  (On machines with wider words, an even larger
00277  * buffer could be used.)  However, on some machines 32-bit shifts are
00278  * quite slow and take time proportional to the number of places shifted.
00279  * (This is true with most PC compilers, for instance.)  In this case it may
00280  * be a win to set MIN_GET_BITS to the minimum value of 15.  This reduces the
00281  * average shift distance at the cost of more calls to jpeg_fill_bit_buffer.
00282  */
00283 
00284 #ifdef SLOW_SHIFT_32
00285 #define MIN_GET_BITS  15    /* minimum allowable value */
00286 #else
00287 #define MIN_GET_BITS  (BIT_BUF_SIZE-7)
00288 #endif
00289 
00290 
00291 GLOBAL(boolean)
00292 jpeg_fill_bit_buffer (bitread_working_state * state,
00293                     register bit_buf_type get_buffer, register int bits_left,
00294                     int nbits)
00295 /* Load up the bit buffer to a depth of at least nbits */
00296 {
00297   /* Copy heavily used state fields into locals (hopefully registers) */
00298   register const JOCTET * next_input_byte = state->next_input_byte;
00299   register size_t bytes_in_buffer = state->bytes_in_buffer;
00300   j_decompress_ptr cinfo = state->cinfo;
00301 
00302   /* Attempt to load at least MIN_GET_BITS bits into get_buffer. */
00303   /* (It is assumed that no request will be for more than that many bits.) */
00304   /* We fail to do so only if we hit a marker or are forced to suspend. */
00305 
00306   if (cinfo->unread_marker == 0) { /* cannot advance past a marker */
00307     while (bits_left < MIN_GET_BITS) {
00308       register int c;
00309 
00310       /* Attempt to read a byte */
00311       if (bytes_in_buffer == 0) {
00312        if (! (*cinfo->src->fill_input_buffer) (cinfo))
00313          return FALSE;
00314        next_input_byte = cinfo->src->next_input_byte;
00315        bytes_in_buffer = cinfo->src->bytes_in_buffer;
00316       }
00317       bytes_in_buffer--;
00318       c = GETJOCTET(*next_input_byte++);
00319 
00320       /* If it's 0xFF, check and discard stuffed zero byte */
00321       if (c == 0xFF) {
00322        /* Loop here to discard any padding FF's on terminating marker,
00323         * so that we can save a valid unread_marker value.  NOTE: we will
00324         * accept multiple FF's followed by a 0 as meaning a single FF data
00325         * byte.  This data pattern is not valid according to the standard.
00326         */
00327        do {
00328          if (bytes_in_buffer == 0) {
00329            if (! (*cinfo->src->fill_input_buffer) (cinfo))
00330              return FALSE;
00331            next_input_byte = cinfo->src->next_input_byte;
00332            bytes_in_buffer = cinfo->src->bytes_in_buffer;
00333          }
00334          bytes_in_buffer--;
00335          c = GETJOCTET(*next_input_byte++);
00336        } while (c == 0xFF);
00337 
00338        if (c == 0) {
00339          /* Found FF/00, which represents an FF data byte */
00340          c = 0xFF;
00341        } else {
00342          /* Oops, it's actually a marker indicating end of compressed data.
00343           * Save the marker code for later use.
00344           * Fine point: it might appear that we should save the marker into
00345           * bitread working state, not straight into permanent state.  But
00346           * once we have hit a marker, we cannot need to suspend within the
00347           * current MCU, because we will read no more bytes from the data
00348           * source.  So it is OK to update permanent state right away.
00349           */
00350          cinfo->unread_marker = c;
00351          /* See if we need to insert some fake zero bits. */
00352          goto no_more_bytes;
00353        }
00354       }
00355 
00356       /* OK, load c into get_buffer */
00357       get_buffer = (get_buffer << 8) | c;
00358       bits_left += 8;
00359     } /* end while */
00360   } else {
00361   no_more_bytes:
00362     /* We get here if we've read the marker that terminates the compressed
00363      * data segment.  There should be enough bits in the buffer register
00364      * to satisfy the request; if so, no problem.
00365      */
00366     if (nbits > bits_left) {
00367       /* Uh-oh.  Report corrupted data to user and stuff zeroes into
00368        * the data stream, so that we can produce some kind of image.
00369        * We use a nonvolatile flag to ensure that only one warning message
00370        * appears per data segment.
00371        */
00372       if (! cinfo->entropy->insufficient_data) {
00373        WARNMS(cinfo, JWRN_HIT_MARKER);
00374        cinfo->entropy->insufficient_data = TRUE;
00375       }
00376       /* Fill the buffer with zero bits */
00377       get_buffer <<= MIN_GET_BITS - bits_left;
00378       bits_left = MIN_GET_BITS;
00379     }
00380   }
00381 
00382   /* Unload the local registers */
00383   state->next_input_byte = next_input_byte;
00384   state->bytes_in_buffer = bytes_in_buffer;
00385   state->get_buffer = get_buffer;
00386   state->bits_left = bits_left;
00387 
00388   return TRUE;
00389 }
00390 
00391 
00392 /*
00393  * Out-of-line code for Huffman code decoding.
00394  * See jdhuff.h for info about usage.
00395  */
00396 
00397 GLOBAL(int)
00398 jpeg_huff_decode (bitread_working_state * state,
00399                 register bit_buf_type get_buffer, register int bits_left,
00400                 d_derived_tbl * htbl, int min_bits)
00401 {
00402   register int l = min_bits;
00403   register INT32 code;
00404 
00405   /* HUFF_DECODE has determined that the code is at least min_bits */
00406   /* bits long, so fetch that many bits in one swoop. */
00407 
00408   CHECK_BIT_BUFFER(*state, l, return -1);
00409   code = GET_BITS(l);
00410 
00411   /* Collect the rest of the Huffman code one bit at a time. */
00412   /* This is per Figure F.16 in the JPEG spec. */
00413 
00414   while (code > htbl->maxcode[l]) {
00415     code <<= 1;
00416     CHECK_BIT_BUFFER(*state, 1, return -1);
00417     code |= GET_BITS(1);
00418     l++;
00419   }
00420 
00421   /* Unload the local registers */
00422   state->get_buffer = get_buffer;
00423   state->bits_left = bits_left;
00424 
00425   /* With garbage input we may reach the sentinel value l = 17. */
00426 
00427   if (l > 16) {
00428     WARNMS(state->cinfo, JWRN_HUFF_BAD_CODE);
00429     return 0;               /* fake a zero as the safest result */
00430   }
00431 
00432   return htbl->pub->huffval[ (int) (code + htbl->valoffset[l]) ];
00433 }
00434 
00435 
00436 /*
00437  * Figure F.12: extend sign bit.
00438  * On some machines, a shift and add will be faster than a table lookup.
00439  */
00440 
00441 #ifdef AVOID_TABLES
00442 
00443 #define HUFF_EXTEND(x,s)  ((x) < (1<<((s)-1)) ? (x) + (((-1)<<(s)) + 1) : (x))
00444 
00445 #else
00446 
00447 #define HUFF_EXTEND(x,s)  ((x) < extend_test[s] ? (x) + extend_offset[s] : (x))
00448 
00449 static const int extend_test[16] =   /* entry n is 2**(n-1) */
00450   { 0, 0x0001, 0x0002, 0x0004, 0x0008, 0x0010, 0x0020, 0x0040, 0x0080,
00451     0x0100, 0x0200, 0x0400, 0x0800, 0x1000, 0x2000, 0x4000 };
00452 
00453 static const int extend_offset[16] = /* entry n is (-1 << n) + 1 */
00454   { 0, ((-1)<<1) + 1, ((-1)<<2) + 1, ((-1)<<3) + 1, ((-1)<<4) + 1,
00455     ((-1)<<5) + 1, ((-1)<<6) + 1, ((-1)<<7) + 1, ((-1)<<8) + 1,
00456     ((-1)<<9) + 1, ((-1)<<10) + 1, ((-1)<<11) + 1, ((-1)<<12) + 1,
00457     ((-1)<<13) + 1, ((-1)<<14) + 1, ((-1)<<15) + 1 };
00458 
00459 #endif /* AVOID_TABLES */
00460 
00461 
00462 /*
00463  * Check for a restart marker & resynchronize decoder.
00464  * Returns FALSE if must suspend.
00465  */
00466 
00467 LOCAL(boolean)
00468 process_restart (j_decompress_ptr cinfo)
00469 {
00470   huff_entropy_ptr entropy = (huff_entropy_ptr) cinfo->entropy;
00471   int ci;
00472 
00473   /* Throw away any unused bits remaining in bit buffer; */
00474   /* include any full bytes in next_marker's count of discarded bytes */
00475   cinfo->marker->discarded_bytes += entropy->bitstate.bits_left / 8;
00476   entropy->bitstate.bits_left = 0;
00477 
00478   /* Advance past the RSTn marker */
00479   if (! (*cinfo->marker->read_restart_marker) (cinfo))
00480     return FALSE;
00481 
00482   /* Re-initialize DC predictions to 0 */
00483   for (ci = 0; ci < cinfo->comps_in_scan; ci++)
00484     entropy->saved.last_dc_val[ci] = 0;
00485 
00486   /* Reset restart counter */
00487   entropy->restarts_to_go = cinfo->restart_interval;
00488 
00489   /* Reset out-of-data flag, unless read_restart_marker left us smack up
00490    * against a marker.  In that case we will end up treating the next data
00491    * segment as empty, and we can avoid producing bogus output pixels by
00492    * leaving the flag set.
00493    */
00494   if (cinfo->unread_marker == 0)
00495     entropy->pub.insufficient_data = FALSE;
00496 
00497   return TRUE;
00498 }
00499 
00500 
00501 /*
00502  * Decode and return one MCU's worth of Huffman-compressed coefficients.
00503  * The coefficients are reordered from zigzag order into natural array order,
00504  * but are not dequantized.
00505  *
00506  * The i'th block of the MCU is stored into the block pointed to by
00507  * MCU_data[i].  WE ASSUME THIS AREA HAS BEEN ZEROED BY THE CALLER.
00508  * (Wholesale zeroing is usually a little faster than retail...)
00509  *
00510  * Returns FALSE if data source requested suspension.  In that case no
00511  * changes have been made to permanent state.  (Exception: some output
00512  * coefficients may already have been assigned.  This is harmless for
00513  * this module, since we'll just re-assign them on the next call.)
00514  */
00515 
00516 METHODDEF(boolean)
00517 decode_mcu (j_decompress_ptr cinfo, JBLOCKROW *MCU_data)
00518 {
00519   huff_entropy_ptr entropy = (huff_entropy_ptr) cinfo->entropy;
00520   int blkn;
00521   BITREAD_STATE_VARS;
00522   savable_state state;
00523 
00524   /* Process restart marker if needed; may have to suspend */
00525   if (cinfo->restart_interval) {
00526     if (entropy->restarts_to_go == 0)
00527       if (! process_restart(cinfo))
00528        return FALSE;
00529   }
00530 
00531   /* If we've run out of data, just leave the MCU set to zeroes.
00532    * This way, we return uniform gray for the remainder of the segment.
00533    */
00534   if (! entropy->pub.insufficient_data) {
00535 
00536     /* Load up working state */
00537     BITREAD_LOAD_STATE(cinfo,entropy->bitstate);
00538     ASSIGN_STATE(state, entropy->saved);
00539 
00540     /* Outer loop handles each block in the MCU */
00541 
00542     for (blkn = 0; blkn < cinfo->blocks_in_MCU; blkn++) {
00543       JBLOCKROW block = MCU_data[blkn];
00544       d_derived_tbl * dctbl = entropy->dc_cur_tbls[blkn];
00545       d_derived_tbl * actbl = entropy->ac_cur_tbls[blkn];
00546       register int s, k, r;
00547 
00548       /* Decode a single block's worth of coefficients */
00549 
00550       /* Section F.2.2.1: decode the DC coefficient difference */
00551       HUFF_DECODE(s, br_state, dctbl, return FALSE, label1);
00552       if (s) {
00553        CHECK_BIT_BUFFER(br_state, s, return FALSE);
00554        r = GET_BITS(s);
00555        s = HUFF_EXTEND(r, s);
00556       }
00557 
00558       if (entropy->dc_needed[blkn]) {
00559        /* Convert DC difference to actual value, update last_dc_val */
00560        int ci = cinfo->MCU_membership[blkn];
00561        s += state.last_dc_val[ci];
00562        state.last_dc_val[ci] = s;
00563        /* Output the DC coefficient (assumes jpeg_natural_order[0] = 0) */
00564        (*block)[0] = (JCOEF) s;
00565       }
00566 
00567       if (entropy->ac_needed[blkn]) {
00568 
00569        /* Section F.2.2.2: decode the AC coefficients */
00570        /* Since zeroes are skipped, output area must be cleared beforehand */
00571        for (k = 1; k < DCTSIZE2; k++) {
00572          HUFF_DECODE(s, br_state, actbl, return FALSE, label2);
00573       
00574          r = s >> 4;
00575          s &= 15;
00576       
00577          if (s) {
00578            k += r;
00579            CHECK_BIT_BUFFER(br_state, s, return FALSE);
00580            r = GET_BITS(s);
00581            s = HUFF_EXTEND(r, s);
00582            /* Output coefficient in natural (dezigzagged) order.
00583             * Note: the extra entries in jpeg_natural_order[] will save us
00584             * if k >= DCTSIZE2, which could happen if the data is corrupted.
00585             */
00586            (*block)[jpeg_natural_order[k]] = (JCOEF) s;
00587          } else {
00588            if (r != 15)
00589              break;
00590            k += 15;
00591          }
00592        }
00593 
00594       } else {
00595 
00596        /* Section F.2.2.2: decode the AC coefficients */
00597        /* In this path we just discard the values */
00598        for (k = 1; k < DCTSIZE2; k++) {
00599          HUFF_DECODE(s, br_state, actbl, return FALSE, label3);
00600       
00601          r = s >> 4;
00602          s &= 15;
00603       
00604          if (s) {
00605            k += r;
00606            CHECK_BIT_BUFFER(br_state, s, return FALSE);
00607            DROP_BITS(s);
00608          } else {
00609            if (r != 15)
00610              break;
00611            k += 15;
00612          }
00613        }
00614 
00615       }
00616     }
00617 
00618     /* Completed MCU, so update state */
00619     BITREAD_SAVE_STATE(cinfo,entropy->bitstate);
00620     ASSIGN_STATE(entropy->saved, state);
00621   }
00622 
00623   /* Account for restart interval (no-op if not using restarts) */
00624   entropy->restarts_to_go--;
00625 
00626   return TRUE;
00627 }
00628 
00629 
00630 /*
00631  * Module initialization routine for Huffman entropy decoding.
00632  */
00633 
00634 GLOBAL(void)
00635 jinit_huff_decoder (j_decompress_ptr cinfo)
00636 {
00637   huff_entropy_ptr entropy;
00638   int i;
00639 
00640   entropy = (huff_entropy_ptr)
00641     (*cinfo->mem->alloc_small) ((j_common_ptr) cinfo, JPOOL_IMAGE,
00642                             SIZEOF(huff_entropy_decoder));
00643   cinfo->entropy = (struct jpeg_entropy_decoder *) entropy;
00644   entropy->pub.start_pass = start_pass_huff_decoder;
00645   entropy->pub.decode_mcu = decode_mcu;
00646 
00647   /* Mark tables unallocated */
00648   for (i = 0; i < NUM_HUFF_TBLS; i++) {
00649     entropy->dc_derived_tbls[i] = entropy->ac_derived_tbls[i] = NULL;
00650   }
00651 }