jdarith.c

00001 /*
00002  * jdarith.c
00003  *
00004  * Developed 1997-2015 by Guido Vollbeding.
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 portable arithmetic entropy decoding routines for JPEG
00009  * (implementing the ISO/IEC IS 10918-1 and CCITT Recommendation ITU-T T.81).
00010  *
00011  * Both sequential and progressive modes are supported in this single module.
00012  *
00013  * Suspension is not currently supported in this module.
00014  */
00015 
00016 #define JPEG_INTERNALS
00017 #include "jinclude.h"
00018 #include "jpeglib.h"
00019 
00020 
00021 /* Expanded entropy decoder object for arithmetic decoding. */
00022 
00023 typedef struct {
00024   struct jpeg_entropy_decoder pub; /* public fields */
00025 
00026   INT32 c;       /* C register, base of coding interval + input bit buffer */
00027   INT32 a;               /* A register, normalized size of coding interval */
00028   int ct;     /* bit shift counter, # of bits left in bit buffer part of C */
00029                                                          /* init: ct = -16 */
00030                                                          /* run: ct = 0..7 */
00031                                                          /* error: ct = -1 */
00032   int last_dc_val[MAX_COMPS_IN_SCAN]; /* last DC coef for each component */
00033   int dc_context[MAX_COMPS_IN_SCAN]; /* context index for DC conditioning */
00034 
00035   unsigned int restarts_to_go;  /* MCUs left in this restart interval */
00036 
00037   /* Pointers to statistics areas (these workspaces have image lifespan) */
00038   unsigned char * dc_stats[NUM_ARITH_TBLS];
00039   unsigned char * ac_stats[NUM_ARITH_TBLS];
00040 
00041   /* Statistics bin for coding with fixed probability 0.5 */
00042   unsigned char fixed_bin[4];
00043 } arith_entropy_decoder;
00044 
00045 typedef arith_entropy_decoder * arith_entropy_ptr;
00046 
00047 /* The following two definitions specify the allocation chunk size
00048  * for the statistics area.
00049  * According to sections F.1.4.4.1.3 and F.1.4.4.2, we need at least
00050  * 49 statistics bins for DC, and 245 statistics bins for AC coding.
00051  *
00052  * We use a compact representation with 1 byte per statistics bin,
00053  * thus the numbers directly represent byte sizes.
00054  * This 1 byte per statistics bin contains the meaning of the MPS
00055  * (more probable symbol) in the highest bit (mask 0x80), and the
00056  * index into the probability estimation state machine table
00057  * in the lower bits (mask 0x7F).
00058  */
00059 
00060 #define DC_STAT_BINS 64
00061 #define AC_STAT_BINS 256
00062 
00063 
00064 LOCAL(int)
00065 get_byte (j_decompress_ptr cinfo)
00066 /* Read next input byte; we do not support suspension in this module. */
00067 {
00068   struct jpeg_source_mgr * src = cinfo->src;
00069 
00070   if (src->bytes_in_buffer == 0)
00071     if (! (*src->fill_input_buffer) (cinfo))
00072       ERREXIT(cinfo, JERR_CANT_SUSPEND);
00073   src->bytes_in_buffer--;
00074   return GETJOCTET(*src->next_input_byte++);
00075 }
00076 
00077 
00078 /*
00079  * The core arithmetic decoding routine (common in JPEG and JBIG).
00080  * This needs to go as fast as possible.
00081  * Machine-dependent optimization facilities
00082  * are not utilized in this portable implementation.
00083  * However, this code should be fairly efficient and
00084  * may be a good base for further optimizations anyway.
00085  *
00086  * Return value is 0 or 1 (binary decision).
00087  *
00088  * Note: I've changed the handling of the code base & bit
00089  * buffer register C compared to other implementations
00090  * based on the standards layout & procedures.
00091  * While it also contains both the actual base of the
00092  * coding interval (16 bits) and the next-bits buffer,
00093  * the cut-point between these two parts is floating
00094  * (instead of fixed) with the bit shift counter CT.
00095  * Thus, we also need only one (variable instead of
00096  * fixed size) shift for the LPS/MPS decision, and
00097  * we can do away with any renormalization update
00098  * of C (except for new data insertion, of course).
00099  *
00100  * I've also introduced a new scheme for accessing
00101  * the probability estimation state machine table,
00102  * derived from Markus Kuhn's JBIG implementation.
00103  */
00104 
00105 LOCAL(int)
00106 arith_decode (j_decompress_ptr cinfo, unsigned char *st)
00107 {
00108   register arith_entropy_ptr e = (arith_entropy_ptr) cinfo->entropy;
00109   register unsigned char nl, nm;
00110   register INT32 qe, temp;
00111   register int sv, data;
00112 
00113   /* Renormalization & data input per section D.2.6 */
00114   while (e->a < 0x8000L) {
00115     if (--e->ct < 0) {
00116       /* Need to fetch next data byte */
00117       if (cinfo->unread_marker)
00118     data = 0;       /* stuff zero data */
00119       else {
00120     data = get_byte(cinfo); /* read next input byte */
00121     if (data == 0xFF) { /* zero stuff or marker code */
00122       do data = get_byte(cinfo);
00123       while (data == 0xFF); /* swallow extra 0xFF bytes */
00124       if (data == 0)
00125         data = 0xFF;    /* discard stuffed zero byte */
00126       else {
00127         /* Note: Different from the Huffman decoder, hitting
00128          * a marker while processing the compressed data
00129          * segment is legal in arithmetic coding.
00130          * The convention is to supply zero data
00131          * then until decoding is complete.
00132          */
00133         cinfo->unread_marker = data;
00134         data = 0;
00135       }
00136     }
00137       }
00138       e->c = (e->c << 8) | data; /* insert data into C register */
00139       if ((e->ct += 8) < 0)  /* update bit shift counter */
00140     /* Need more initial bytes */
00141     if (++e->ct == 0)
00142       /* Got 2 initial bytes -> re-init A and exit loop */
00143       e->a = 0x8000L; /* => e->a = 0x10000L after loop exit */
00144     }
00145     e->a <<= 1;
00146   }
00147 
00148   /* Fetch values from our compact representation of Table D.3(D.2):
00149    * Qe values and probability estimation state machine
00150    */
00151   sv = *st;
00152   qe = jpeg_aritab[sv & 0x7F];  /* => Qe_Value */
00153   nl = qe & 0xFF; qe >>= 8; /* Next_Index_LPS + Switch_MPS */
00154   nm = qe & 0xFF; qe >>= 8; /* Next_Index_MPS */
00155 
00156   /* Decode & estimation procedures per sections D.2.4 & D.2.5 */
00157   temp = e->a - qe;
00158   e->a = temp;
00159   temp <<= e->ct;
00160   if (e->c >= temp) {
00161     e->c -= temp;
00162     /* Conditional LPS (less probable symbol) exchange */
00163     if (e->a < qe) {
00164       e->a = qe;
00165       *st = (sv & 0x80) ^ nm;   /* Estimate_after_MPS */
00166     } else {
00167       e->a = qe;
00168       *st = (sv & 0x80) ^ nl;   /* Estimate_after_LPS */
00169       sv ^= 0x80;       /* Exchange LPS/MPS */
00170     }
00171   } else if (e->a < 0x8000L) {
00172     /* Conditional MPS (more probable symbol) exchange */
00173     if (e->a < qe) {
00174       *st = (sv & 0x80) ^ nl;   /* Estimate_after_LPS */
00175       sv ^= 0x80;       /* Exchange LPS/MPS */
00176     } else {
00177       *st = (sv & 0x80) ^ nm;   /* Estimate_after_MPS */
00178     }
00179   }
00180 
00181   return sv >> 7;
00182 }
00183 
00184 
00185 /*
00186  * Check for a restart marker & resynchronize decoder.
00187  */
00188 
00189 LOCAL(void)
00190 process_restart (j_decompress_ptr cinfo)
00191 {
00192   arith_entropy_ptr entropy = (arith_entropy_ptr) cinfo->entropy;
00193   int ci;
00194   jpeg_component_info * compptr;
00195 
00196   /* Advance past the RSTn marker */
00197   if (! (*cinfo->marker->read_restart_marker) (cinfo))
00198     ERREXIT(cinfo, JERR_CANT_SUSPEND);
00199 
00200   /* Re-initialize statistics areas */
00201   for (ci = 0; ci < cinfo->comps_in_scan; ci++) {
00202     compptr = cinfo->cur_comp_info[ci];
00203     if (! cinfo->progressive_mode || (cinfo->Ss == 0 && cinfo->Ah == 0)) {
00204       MEMZERO(entropy->dc_stats[compptr->dc_tbl_no], DC_STAT_BINS);
00205       /* Reset DC predictions to 0 */
00206       entropy->last_dc_val[ci] = 0;
00207       entropy->dc_context[ci] = 0;
00208     }
00209     if ((! cinfo->progressive_mode && cinfo->lim_Se) ||
00210     (cinfo->progressive_mode && cinfo->Ss)) {
00211       MEMZERO(entropy->ac_stats[compptr->ac_tbl_no], AC_STAT_BINS);
00212     }
00213   }
00214 
00215   /* Reset arithmetic decoding variables */
00216   entropy->c = 0;
00217   entropy->a = 0;
00218   entropy->ct = -16;    /* force reading 2 initial bytes to fill C */
00219 
00220   /* Reset restart counter */
00221   entropy->restarts_to_go = cinfo->restart_interval;
00222 }
00223 
00224 
00225 /*
00226  * Arithmetic MCU decoding.
00227  * Each of these routines decodes and returns one MCU's worth of
00228  * arithmetic-compressed coefficients.
00229  * The coefficients are reordered from zigzag order into natural array order,
00230  * but are not dequantized.
00231  *
00232  * The i'th block of the MCU is stored into the block pointed to by
00233  * MCU_data[i].  WE ASSUME THIS AREA IS INITIALLY ZEROED BY THE CALLER.
00234  */
00235 
00236 /*
00237  * MCU decoding for DC initial scan (either spectral selection,
00238  * or first pass of successive approximation).
00239  */
00240 
00241 METHODDEF(boolean)
00242 decode_mcu_DC_first (j_decompress_ptr cinfo, JBLOCKROW *MCU_data)
00243 {
00244   arith_entropy_ptr entropy = (arith_entropy_ptr) cinfo->entropy;
00245   JBLOCKROW block;
00246   unsigned char *st;
00247   int blkn, ci, tbl, sign;
00248   int v, m;
00249 
00250   /* Process restart marker if needed */
00251   if (cinfo->restart_interval) {
00252     if (entropy->restarts_to_go == 0)
00253       process_restart(cinfo);
00254     entropy->restarts_to_go--;
00255   }
00256 
00257   if (entropy->ct == -1) return TRUE;   /* if error do nothing */
00258 
00259   /* Outer loop handles each block in the MCU */
00260 
00261   for (blkn = 0; blkn < cinfo->blocks_in_MCU; blkn++) {
00262     block = MCU_data[blkn];
00263     ci = cinfo->MCU_membership[blkn];
00264     tbl = cinfo->cur_comp_info[ci]->dc_tbl_no;
00265 
00266     /* Sections F.2.4.1 & F.1.4.4.1: Decoding of DC coefficients */
00267 
00268     /* Table F.4: Point to statistics bin S0 for DC coefficient coding */
00269     st = entropy->dc_stats[tbl] + entropy->dc_context[ci];
00270 
00271     /* Figure F.19: Decode_DC_DIFF */
00272     if (arith_decode(cinfo, st) == 0)
00273       entropy->dc_context[ci] = 0;
00274     else {
00275       /* Figure F.21: Decoding nonzero value v */
00276       /* Figure F.22: Decoding the sign of v */
00277       sign = arith_decode(cinfo, st + 1);
00278       st += 2; st += sign;
00279       /* Figure F.23: Decoding the magnitude category of v */
00280       if ((m = arith_decode(cinfo, st)) != 0) {
00281     st = entropy->dc_stats[tbl] + 20;   /* Table F.4: X1 = 20 */
00282     while (arith_decode(cinfo, st)) {
00283       if ((m <<= 1) == 0x8000) {
00284         WARNMS(cinfo, JWRN_ARITH_BAD_CODE);
00285         entropy->ct = -1;           /* magnitude overflow */
00286         return TRUE;
00287       }
00288       st += 1;
00289     }
00290       }
00291       /* Section F.1.4.4.1.2: Establish dc_context conditioning category */
00292       if (m < (int) ((1L << cinfo->arith_dc_L[tbl]) >> 1))
00293     entropy->dc_context[ci] = 0;           /* zero diff category */
00294       else if (m > (int) ((1L << cinfo->arith_dc_U[tbl]) >> 1))
00295     entropy->dc_context[ci] = 12 + (sign * 4); /* large diff category */
00296       else
00297     entropy->dc_context[ci] = 4 + (sign * 4);  /* small diff category */
00298       v = m;
00299       /* Figure F.24: Decoding the magnitude bit pattern of v */
00300       st += 14;
00301       while (m >>= 1)
00302     if (arith_decode(cinfo, st)) v |= m;
00303       v += 1; if (sign) v = -v;
00304       entropy->last_dc_val[ci] += v;
00305     }
00306 
00307     /* Scale and output the DC coefficient (assumes jpeg_natural_order[0]=0) */
00308     (*block)[0] = (JCOEF) (entropy->last_dc_val[ci] << cinfo->Al);
00309   }
00310 
00311   return TRUE;
00312 }
00313 
00314 
00315 /*
00316  * MCU decoding for AC initial scan (either spectral selection,
00317  * or first pass of successive approximation).
00318  */
00319 
00320 METHODDEF(boolean)
00321 decode_mcu_AC_first (j_decompress_ptr cinfo, JBLOCKROW *MCU_data)
00322 {
00323   arith_entropy_ptr entropy = (arith_entropy_ptr) cinfo->entropy;
00324   JBLOCKROW block;
00325   unsigned char *st;
00326   int tbl, sign, k;
00327   int v, m;
00328   const int * natural_order;
00329 
00330   /* Process restart marker if needed */
00331   if (cinfo->restart_interval) {
00332     if (entropy->restarts_to_go == 0)
00333       process_restart(cinfo);
00334     entropy->restarts_to_go--;
00335   }
00336 
00337   if (entropy->ct == -1) return TRUE;   /* if error do nothing */
00338 
00339   natural_order = cinfo->natural_order;
00340 
00341   /* There is always only one block per MCU */
00342   block = MCU_data[0];
00343   tbl = cinfo->cur_comp_info[0]->ac_tbl_no;
00344 
00345   /* Sections F.2.4.2 & F.1.4.4.2: Decoding of AC coefficients */
00346 
00347   /* Figure F.20: Decode_AC_coefficients */
00348   k = cinfo->Ss - 1;
00349   do {
00350     st = entropy->ac_stats[tbl] + 3 * k;
00351     if (arith_decode(cinfo, st)) break;     /* EOB flag */
00352     for (;;) {
00353       k++;
00354       if (arith_decode(cinfo, st + 1)) break;
00355       st += 3;
00356       if (k >= cinfo->Se) {
00357     WARNMS(cinfo, JWRN_ARITH_BAD_CODE);
00358     entropy->ct = -1;           /* spectral overflow */
00359     return TRUE;
00360       }
00361     }
00362     /* Figure F.21: Decoding nonzero value v */
00363     /* Figure F.22: Decoding the sign of v */
00364     sign = arith_decode(cinfo, entropy->fixed_bin);
00365     st += 2;
00366     /* Figure F.23: Decoding the magnitude category of v */
00367     if ((m = arith_decode(cinfo, st)) != 0) {
00368       if (arith_decode(cinfo, st)) {
00369     m <<= 1;
00370     st = entropy->ac_stats[tbl] +
00371          (k <= cinfo->arith_ac_K[tbl] ? 189 : 217);
00372     while (arith_decode(cinfo, st)) {
00373       if ((m <<= 1) == 0x8000) {
00374         WARNMS(cinfo, JWRN_ARITH_BAD_CODE);
00375         entropy->ct = -1;           /* magnitude overflow */
00376         return TRUE;
00377       }
00378       st += 1;
00379     }
00380       }
00381     }
00382     v = m;
00383     /* Figure F.24: Decoding the magnitude bit pattern of v */
00384     st += 14;
00385     while (m >>= 1)
00386       if (arith_decode(cinfo, st)) v |= m;
00387     v += 1; if (sign) v = -v;
00388     /* Scale and output coefficient in natural (dezigzagged) order */
00389     (*block)[natural_order[k]] = (JCOEF) (v << cinfo->Al);
00390   } while (k < cinfo->Se);
00391 
00392   return TRUE;
00393 }
00394 
00395 
00396 /*
00397  * MCU decoding for DC successive approximation refinement scan.
00398  * Note: we assume such scans can be multi-component,
00399  * although the spec is not very clear on the point.
00400  */
00401 
00402 METHODDEF(boolean)
00403 decode_mcu_DC_refine (j_decompress_ptr cinfo, JBLOCKROW *MCU_data)
00404 {
00405   arith_entropy_ptr entropy = (arith_entropy_ptr) cinfo->entropy;
00406   unsigned char *st;
00407   int p1, blkn;
00408 
00409   /* Process restart marker if needed */
00410   if (cinfo->restart_interval) {
00411     if (entropy->restarts_to_go == 0)
00412       process_restart(cinfo);
00413     entropy->restarts_to_go--;
00414   }
00415 
00416   st = entropy->fixed_bin;  /* use fixed probability estimation */
00417   p1 = 1 << cinfo->Al;      /* 1 in the bit position being coded */
00418 
00419   /* Outer loop handles each block in the MCU */
00420 
00421   for (blkn = 0; blkn < cinfo->blocks_in_MCU; blkn++) {
00422     /* Encoded data is simply the next bit of the two's-complement DC value */
00423     if (arith_decode(cinfo, st))
00424       MCU_data[blkn][0][0] |= p1;
00425   }
00426 
00427   return TRUE;
00428 }
00429 
00430 
00431 /*
00432  * MCU decoding for AC successive approximation refinement scan.
00433  */
00434 
00435 METHODDEF(boolean)
00436 decode_mcu_AC_refine (j_decompress_ptr cinfo, JBLOCKROW *MCU_data)
00437 {
00438   arith_entropy_ptr entropy = (arith_entropy_ptr) cinfo->entropy;
00439   JBLOCKROW block;
00440   JCOEFPTR thiscoef;
00441   unsigned char *st;
00442   int tbl, k, kex;
00443   int p1, m1;
00444   const int * natural_order;
00445 
00446   /* Process restart marker if needed */
00447   if (cinfo->restart_interval) {
00448     if (entropy->restarts_to_go == 0)
00449       process_restart(cinfo);
00450     entropy->restarts_to_go--;
00451   }
00452 
00453   if (entropy->ct == -1) return TRUE;   /* if error do nothing */
00454 
00455   natural_order = cinfo->natural_order;
00456 
00457   /* There is always only one block per MCU */
00458   block = MCU_data[0];
00459   tbl = cinfo->cur_comp_info[0]->ac_tbl_no;
00460 
00461   p1 = 1 << cinfo->Al;      /* 1 in the bit position being coded */
00462   m1 = (-1) << cinfo->Al;   /* -1 in the bit position being coded */
00463 
00464   /* Establish EOBx (previous stage end-of-block) index */
00465   kex = cinfo->Se;
00466   do {
00467     if ((*block)[natural_order[kex]]) break;
00468   } while (--kex);
00469 
00470   k = cinfo->Ss - 1;
00471   do {
00472     st = entropy->ac_stats[tbl] + 3 * k;
00473     if (k >= kex)
00474       if (arith_decode(cinfo, st)) break;   /* EOB flag */
00475     for (;;) {
00476       thiscoef = *block + natural_order[++k];
00477       if (*thiscoef) {              /* previously nonzero coef */
00478     if (arith_decode(cinfo, st + 2)) {
00479       if (*thiscoef < 0)
00480         *thiscoef += m1;
00481       else
00482         *thiscoef += p1;
00483     }
00484     break;
00485       }
00486       if (arith_decode(cinfo, st + 1)) {    /* newly nonzero coef */
00487     if (arith_decode(cinfo, entropy->fixed_bin))
00488       *thiscoef = m1;
00489     else
00490       *thiscoef = p1;
00491     break;
00492       }
00493       st += 3;
00494       if (k >= cinfo->Se) {
00495     WARNMS(cinfo, JWRN_ARITH_BAD_CODE);
00496     entropy->ct = -1;           /* spectral overflow */
00497     return TRUE;
00498       }
00499     }
00500   } while (k < cinfo->Se);
00501 
00502   return TRUE;
00503 }
00504 
00505 
00506 /*
00507  * Decode one MCU's worth of arithmetic-compressed coefficients.
00508  */
00509 
00510 METHODDEF(boolean)
00511 decode_mcu (j_decompress_ptr cinfo, JBLOCKROW *MCU_data)
00512 {
00513   arith_entropy_ptr entropy = (arith_entropy_ptr) cinfo->entropy;
00514   jpeg_component_info * compptr;
00515   JBLOCKROW block;
00516   unsigned char *st;
00517   int blkn, ci, tbl, sign, k;
00518   int v, m;
00519   const int * natural_order;
00520 
00521   /* Process restart marker if needed */
00522   if (cinfo->restart_interval) {
00523     if (entropy->restarts_to_go == 0)
00524       process_restart(cinfo);
00525     entropy->restarts_to_go--;
00526   }
00527 
00528   if (entropy->ct == -1) return TRUE;   /* if error do nothing */
00529 
00530   natural_order = cinfo->natural_order;
00531 
00532   /* Outer loop handles each block in the MCU */
00533 
00534   for (blkn = 0; blkn < cinfo->blocks_in_MCU; blkn++) {
00535     block = MCU_data[blkn];
00536     ci = cinfo->MCU_membership[blkn];
00537     compptr = cinfo->cur_comp_info[ci];
00538 
00539     /* Sections F.2.4.1 & F.1.4.4.1: Decoding of DC coefficients */
00540 
00541     tbl = compptr->dc_tbl_no;
00542 
00543     /* Table F.4: Point to statistics bin S0 for DC coefficient coding */
00544     st = entropy->dc_stats[tbl] + entropy->dc_context[ci];
00545 
00546     /* Figure F.19: Decode_DC_DIFF */
00547     if (arith_decode(cinfo, st) == 0)
00548       entropy->dc_context[ci] = 0;
00549     else {
00550       /* Figure F.21: Decoding nonzero value v */
00551       /* Figure F.22: Decoding the sign of v */
00552       sign = arith_decode(cinfo, st + 1);
00553       st += 2; st += sign;
00554       /* Figure F.23: Decoding the magnitude category of v */
00555       if ((m = arith_decode(cinfo, st)) != 0) {
00556     st = entropy->dc_stats[tbl] + 20;   /* Table F.4: X1 = 20 */
00557     while (arith_decode(cinfo, st)) {
00558       if ((m <<= 1) == 0x8000) {
00559         WARNMS(cinfo, JWRN_ARITH_BAD_CODE);
00560         entropy->ct = -1;           /* magnitude overflow */
00561         return TRUE;
00562       }
00563       st += 1;
00564     }
00565       }
00566       /* Section F.1.4.4.1.2: Establish dc_context conditioning category */
00567       if (m < (int) ((1L << cinfo->arith_dc_L[tbl]) >> 1))
00568     entropy->dc_context[ci] = 0;           /* zero diff category */
00569       else if (m > (int) ((1L << cinfo->arith_dc_U[tbl]) >> 1))
00570     entropy->dc_context[ci] = 12 + (sign * 4); /* large diff category */
00571       else
00572     entropy->dc_context[ci] = 4 + (sign * 4);  /* small diff category */
00573       v = m;
00574       /* Figure F.24: Decoding the magnitude bit pattern of v */
00575       st += 14;
00576       while (m >>= 1)
00577     if (arith_decode(cinfo, st)) v |= m;
00578       v += 1; if (sign) v = -v;
00579       entropy->last_dc_val[ci] += v;
00580     }
00581 
00582     (*block)[0] = (JCOEF) entropy->last_dc_val[ci];
00583 
00584     /* Sections F.2.4.2 & F.1.4.4.2: Decoding of AC coefficients */
00585 
00586     if (cinfo->lim_Se == 0) continue;
00587     tbl = compptr->ac_tbl_no;
00588     k = 0;
00589 
00590     /* Figure F.20: Decode_AC_coefficients */
00591     do {
00592       st = entropy->ac_stats[tbl] + 3 * k;
00593       if (arith_decode(cinfo, st)) break;   /* EOB flag */
00594       for (;;) {
00595     k++;
00596     if (arith_decode(cinfo, st + 1)) break;
00597     st += 3;
00598     if (k >= cinfo->lim_Se) {
00599       WARNMS(cinfo, JWRN_ARITH_BAD_CODE);
00600       entropy->ct = -1;         /* spectral overflow */
00601       return TRUE;
00602     }
00603       }
00604       /* Figure F.21: Decoding nonzero value v */
00605       /* Figure F.22: Decoding the sign of v */
00606       sign = arith_decode(cinfo, entropy->fixed_bin);
00607       st += 2;
00608       /* Figure F.23: Decoding the magnitude category of v */
00609       if ((m = arith_decode(cinfo, st)) != 0) {
00610     if (arith_decode(cinfo, st)) {
00611       m <<= 1;
00612       st = entropy->ac_stats[tbl] +
00613            (k <= cinfo->arith_ac_K[tbl] ? 189 : 217);
00614       while (arith_decode(cinfo, st)) {
00615         if ((m <<= 1) == 0x8000) {
00616           WARNMS(cinfo, JWRN_ARITH_BAD_CODE);
00617           entropy->ct = -1;         /* magnitude overflow */
00618           return TRUE;
00619         }
00620         st += 1;
00621       }
00622     }
00623       }
00624       v = m;
00625       /* Figure F.24: Decoding the magnitude bit pattern of v */
00626       st += 14;
00627       while (m >>= 1)
00628     if (arith_decode(cinfo, st)) v |= m;
00629       v += 1; if (sign) v = -v;
00630       (*block)[natural_order[k]] = (JCOEF) v;
00631     } while (k < cinfo->lim_Se);
00632   }
00633 
00634   return TRUE;
00635 }
00636 
00637 
00638 /*
00639  * Initialize for an arithmetic-compressed scan.
00640  */
00641 
00642 METHODDEF(void)
00643 start_pass (j_decompress_ptr cinfo)
00644 {
00645   arith_entropy_ptr entropy = (arith_entropy_ptr) cinfo->entropy;
00646   int ci, tbl;
00647   jpeg_component_info * compptr;
00648 
00649   if (cinfo->progressive_mode) {
00650     /* Validate progressive scan parameters */
00651     if (cinfo->Ss == 0) {
00652       if (cinfo->Se != 0)
00653     goto bad;
00654     } else {
00655       /* need not check Ss/Se < 0 since they came from unsigned bytes */
00656       if (cinfo->Se < cinfo->Ss || cinfo->Se > cinfo->lim_Se)
00657     goto bad;
00658       /* AC scans may have only one component */
00659       if (cinfo->comps_in_scan != 1)
00660     goto bad;
00661     }
00662     if (cinfo->Ah != 0) {
00663       /* Successive approximation refinement scan: must have Al = Ah-1. */
00664       if (cinfo->Ah-1 != cinfo->Al)
00665     goto bad;
00666     }
00667     if (cinfo->Al > 13) {   /* need not check for < 0 */
00668       bad:
00669       ERREXIT4(cinfo, JERR_BAD_PROGRESSION,
00670            cinfo->Ss, cinfo->Se, cinfo->Ah, cinfo->Al);
00671     }
00672     /* Update progression status, and verify that scan order is legal.
00673      * Note that inter-scan inconsistencies are treated as warnings
00674      * not fatal errors ... not clear if this is right way to behave.
00675      */
00676     for (ci = 0; ci < cinfo->comps_in_scan; ci++) {
00677       int coefi, cindex = cinfo->cur_comp_info[ci]->component_index;
00678       int *coef_bit_ptr = & cinfo->coef_bits[cindex][0];
00679       if (cinfo->Ss && coef_bit_ptr[0] < 0) /* AC without prior DC scan */
00680     WARNMS2(cinfo, JWRN_BOGUS_PROGRESSION, cindex, 0);
00681       for (coefi = cinfo->Ss; coefi <= cinfo->Se; coefi++) {
00682     int expected = (coef_bit_ptr[coefi] < 0) ? 0 : coef_bit_ptr[coefi];
00683     if (cinfo->Ah != expected)
00684       WARNMS2(cinfo, JWRN_BOGUS_PROGRESSION, cindex, coefi);
00685     coef_bit_ptr[coefi] = cinfo->Al;
00686       }
00687     }
00688     /* Select MCU decoding routine */
00689     if (cinfo->Ah == 0) {
00690       if (cinfo->Ss == 0)
00691     entropy->pub.decode_mcu = decode_mcu_DC_first;
00692       else
00693     entropy->pub.decode_mcu = decode_mcu_AC_first;
00694     } else {
00695       if (cinfo->Ss == 0)
00696     entropy->pub.decode_mcu = decode_mcu_DC_refine;
00697       else
00698     entropy->pub.decode_mcu = decode_mcu_AC_refine;
00699     }
00700   } else {
00701     /* Check that the scan parameters Ss, Se, Ah/Al are OK for sequential JPEG.
00702      * This ought to be an error condition, but we make it a warning.
00703      */
00704     if (cinfo->Ss != 0 || cinfo->Ah != 0 || cinfo->Al != 0 ||
00705     (cinfo->Se < DCTSIZE2 && cinfo->Se != cinfo->lim_Se))
00706       WARNMS(cinfo, JWRN_NOT_SEQUENTIAL);
00707     /* Select MCU decoding routine */
00708     entropy->pub.decode_mcu = decode_mcu;
00709   }
00710 
00711   /* Allocate & initialize requested statistics areas */
00712   for (ci = 0; ci < cinfo->comps_in_scan; ci++) {
00713     compptr = cinfo->cur_comp_info[ci];
00714     if (! cinfo->progressive_mode || (cinfo->Ss == 0 && cinfo->Ah == 0)) {
00715       tbl = compptr->dc_tbl_no;
00716       if (tbl < 0 || tbl >= NUM_ARITH_TBLS)
00717     ERREXIT1(cinfo, JERR_NO_ARITH_TABLE, tbl);
00718       if (entropy->dc_stats[tbl] == NULL)
00719     entropy->dc_stats[tbl] = (unsigned char *) (*cinfo->mem->alloc_small)
00720       ((j_common_ptr) cinfo, JPOOL_IMAGE, DC_STAT_BINS);
00721       MEMZERO(entropy->dc_stats[tbl], DC_STAT_BINS);
00722       /* Initialize DC predictions to 0 */
00723       entropy->last_dc_val[ci] = 0;
00724       entropy->dc_context[ci] = 0;
00725     }
00726     if ((! cinfo->progressive_mode && cinfo->lim_Se) ||
00727     (cinfo->progressive_mode && cinfo->Ss)) {
00728       tbl = compptr->ac_tbl_no;
00729       if (tbl < 0 || tbl >= NUM_ARITH_TBLS)
00730     ERREXIT1(cinfo, JERR_NO_ARITH_TABLE, tbl);
00731       if (entropy->ac_stats[tbl] == NULL)
00732     entropy->ac_stats[tbl] = (unsigned char *) (*cinfo->mem->alloc_small)
00733       ((j_common_ptr) cinfo, JPOOL_IMAGE, AC_STAT_BINS);
00734       MEMZERO(entropy->ac_stats[tbl], AC_STAT_BINS);
00735     }
00736   }
00737 
00738   /* Initialize arithmetic decoding variables */
00739   entropy->c = 0;
00740   entropy->a = 0;
00741   entropy->ct = -16;    /* force reading 2 initial bytes to fill C */
00742 
00743   /* Initialize restart counter */
00744   entropy->restarts_to_go = cinfo->restart_interval;
00745 }
00746 
00747 
00748 /*
00749  * Finish up at the end of an arithmetic-compressed scan.
00750  */
00751 
00752 METHODDEF(void)
00753 finish_pass (j_decompress_ptr cinfo)
00754 {
00755   /* no work necessary here */
00756 }
00757 
00758 
00759 /*
00760  * Module initialization routine for arithmetic entropy decoding.
00761  */
00762 
00763 GLOBAL(void)
00764 jinit_arith_decoder (j_decompress_ptr cinfo)
00765 {
00766   arith_entropy_ptr entropy;
00767   int i;
00768 
00769   entropy = (arith_entropy_ptr)
00770     (*cinfo->mem->alloc_small) ((j_common_ptr) cinfo, JPOOL_IMAGE,
00771                 SIZEOF(arith_entropy_decoder));
00772   cinfo->entropy = &entropy->pub;
00773   entropy->pub.start_pass = start_pass;
00774   entropy->pub.finish_pass = finish_pass;
00775 
00776   /* Mark tables unallocated */
00777   for (i = 0; i < NUM_ARITH_TBLS; i++) {
00778     entropy->dc_stats[i] = NULL;
00779     entropy->ac_stats[i] = NULL;
00780   }
00781 
00782   /* Initialize index for fixed probability estimation */
00783   entropy->fixed_bin[0] = 113;
00784 
00785   if (cinfo->progressive_mode) {
00786     /* Create progression status table */
00787     int *coef_bit_ptr, ci;
00788     cinfo->coef_bits = (int (*)[DCTSIZE2])
00789       (*cinfo->mem->alloc_small) ((j_common_ptr) cinfo, JPOOL_IMAGE,
00790                   cinfo->num_components*DCTSIZE2*SIZEOF(int));
00791     coef_bit_ptr = & cinfo->coef_bits[0][0];
00792     for (ci = 0; ci < cinfo->num_components; ci++) 
00793       for (i = 0; i < DCTSIZE2; i++)
00794     *coef_bit_ptr++ = -1;
00795   }
00796 }