jpeg/jchuff.c

   1 /*
   2  * jchuff.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 encoding routines.
   9  *
  10  * Much of the complexity here has to do with supporting output suspension.
  11  * If the data destination module demands suspension, we want to be able to
  12  * back up to the start of the current MCU.  To do this, we copy state
  13  * variables into local working storage, and update them back to the
  14  * permanent JPEG objects only upon successful completion of an MCU.
  15  */
  16
  17 #define JPEG_INTERNALS
  18 #include "jinclude.h"
  19 #include "jpeglib.h"
  20 #include "jchuff.h"             /* Declarations shared with jcphuff.c */
  21
  22
  23 /* Expanded entropy encoder object for Huffman encoding.
  24  *
  25  * The savable_state subrecord contains fields that change within an MCU,
  26  * but must not be updated permanently until we complete the MCU.
  27  */
  28
  29 typedef struct {
  30   INT32 put_buffer;             /* current bit-accumulation buffer */
  31   int put_bits;                 /* # of bits now in it */
  32   int last_dc_val[MAX_COMPS_IN_SCAN]; /* last DC coef for each component */
  33 } savable_state;
  34
  35 /* This macro is to work around compilers with missing or broken
  36  * structure assignment.  You'll need to fix this code if you have
  37  * such a compiler and you change MAX_COMPS_IN_SCAN.
  38  */
  39
  40 #ifndef NO_STRUCT_ASSIGN
  41 #define ASSIGN_STATE(dest,src)  ((dest) = (src))
  42 #else
  43 #if MAX_COMPS_IN_SCAN == 4
  44 #define ASSIGN_STATE(dest,src)  \
  45         ((dest).put_buffer = (src).put_buffer, \
  46          (dest).put_bits = (src).put_bits, \
  47          (dest).last_dc_val[0] = (src).last_dc_val[0], \
  48          (dest).last_dc_val[1] = (src).last_dc_val[1], \
  49          (dest).last_dc_val[2] = (src).last_dc_val[2], \
  50          (dest).last_dc_val[3] = (src).last_dc_val[3])
  51 #endif
  52 #endif
  53
  54
  55 typedef struct {
  56   struct jpeg_entropy_encoder pub; /* public fields */
  57
  58   savable_state saved;          /* Bit buffer & DC state at start of MCU */
  59
  60   /* These fields are NOT loaded into local working state. */
  61   unsigned int restarts_to_go;  /* MCUs left in this restart interval */
  62   int next_restart_num;         /* next restart number to write (0-7) */
  63
  64   /* Pointers to derived tables (these workspaces have image lifespan) */
  65   c_derived_tbl * dc_derived_tbls[NUM_HUFF_TBLS];
  66   c_derived_tbl * ac_derived_tbls[NUM_HUFF_TBLS];
  67
  68 #ifdef ENTROPY_OPT_SUPPORTED    /* Statistics tables for optimization */
  69   long * dc_count_ptrs[NUM_HUFF_TBLS];
  70   long * ac_count_ptrs[NUM_HUFF_TBLS];
  71 #endif
  72 } huff_entropy_encoder;
  73
  74 typedef huff_entropy_encoder * huff_entropy_ptr;
  75
  76 /* Working state while writing an MCU.
  77  * This struct contains all the fields that are needed by subroutines.
  78  */
  79
  80 typedef struct {
  81   JOCTET * next_output_byte;    /* => next byte to write in buffer */
  82   size_t free_in_buffer;        /* # of byte spaces remaining in buffer */
  83   savable_state cur;            /* Current bit buffer & DC state */
  84   j_compress_ptr cinfo;         /* dump_buffer needs access to this */
  85 } working_state;
  86
  87
  88 /* Forward declarations */
  89 METHODDEF(boolean) encode_mcu_huff JPP((j_compress_ptr cinfo,
  90                                         JBLOCKROW *MCU_data));
  91 METHODDEF(void) finish_pass_huff JPP((j_compress_ptr cinfo));
  92 #ifdef ENTROPY_OPT_SUPPORTED
  93 METHODDEF(boolean) encode_mcu_gather JPP((j_compress_ptr cinfo,
  94                                           JBLOCKROW *MCU_data));
  95 METHODDEF(void) finish_pass_gather JPP((j_compress_ptr cinfo));
  96 #endif
  97
  98
  99 /*
 100  * Initialize for a Huffman-compressed scan.
 101  * If gather_statistics is TRUE, we do not output anything during the scan,
 102  * just count the Huffman symbols used and generate Huffman code tables.
 103  */
 104
 105 METHODDEF(void)
 106 start_pass_huff (j_compress_ptr cinfo, boolean gather_statistics)
 107 {
 108   huff_entropy_ptr entropy = (huff_entropy_ptr) cinfo->entropy;
 109   int ci, dctbl, actbl;
 110   jpeg_component_info * compptr;
 111
 112   if (gather_statistics) {
 113 #ifdef ENTROPY_OPT_SUPPORTED
 114     entropy->pub.encode_mcu = encode_mcu_gather;
 115     entropy->pub.finish_pass = finish_pass_gather;
 116 #else
 117     ERREXIT(cinfo, JERR_NOT_COMPILED);
 118 #endif
 119   } else {
 120     entropy->pub.encode_mcu = encode_mcu_huff;
 121     entropy->pub.finish_pass = finish_pass_huff;
 122   }
 123
 124   for (ci = 0; ci < cinfo->comps_in_scan; ci++) {
 125     compptr = cinfo->cur_comp_info[ci];
 126     dctbl = compptr->dc_tbl_no;
 127     actbl = compptr->ac_tbl_no;
 128     if (gather_statistics) {
 129 #ifdef ENTROPY_OPT_SUPPORTED
 130       /* Check for invalid table indexes */
 131       /* (make_c_derived_tbl does this in the other path) */
 132       if (dctbl < 0 || dctbl >= NUM_HUFF_TBLS)
 133         ERREXIT1(cinfo, JERR_NO_HUFF_TABLE, dctbl);
 134       if (actbl < 0 || actbl >= NUM_HUFF_TBLS)
 135         ERREXIT1(cinfo, JERR_NO_HUFF_TABLE, actbl);
 136       /* Allocate and zero the statistics tables */
 137       /* Note that jpeg_gen_optimal_table expects 257 entries in each table! */
 138       if (entropy->dc_count_ptrs[dctbl] == NULL)
 139         entropy->dc_count_ptrs[dctbl] = (long *)
 140           (*cinfo->mem->alloc_small) ((j_common_ptr) cinfo, JPOOL_IMAGE,
 141                                       257 * SIZEOF(long));
 142       MEMZERO(entropy->dc_count_ptrs[dctbl], 257 * SIZEOF(long));
 143       if (entropy->ac_count_ptrs[actbl] == NULL)
 144         entropy->ac_count_ptrs[actbl] = (long *)
 145           (*cinfo->mem->alloc_small) ((j_common_ptr) cinfo, JPOOL_IMAGE,
 146                                       257 * SIZEOF(long));
 147       MEMZERO(entropy->ac_count_ptrs[actbl], 257 * SIZEOF(long));
 148 #endif
 149     } else {
 150       /* Compute derived values for Huffman tables */
 151       /* We may do this more than once for a table, but it's not expensive */
 152       jpeg_make_c_derived_tbl(cinfo, TRUE, dctbl,
 153                               & entropy->dc_derived_tbls[dctbl]);
 154       jpeg_make_c_derived_tbl(cinfo, FALSE, actbl,
 155                               & entropy->ac_derived_tbls[actbl]);
 156     }
 157     /* Initialize DC predictions to 0 */
 158     entropy->saved.last_dc_val[ci] = 0;
 159   }
 160
 161   /* Initialize bit buffer to empty */
 162   entropy->saved.put_buffer = 0;
 163   entropy->saved.put_bits = 0;
 164
 165   /* Initialize restart stuff */
 166   entropy->restarts_to_go = cinfo->restart_interval;
 167   entropy->next_restart_num = 0;
 168 }
 169
 170
 171 /*
 172  * Compute the derived values for a Huffman table.
 173  * This routine also performs some validation checks on the table.
 174  *
 175  * Note this is also used by jcphuff.c.
 176  */
 177
 178 GLOBAL(void)
 179 jpeg_make_c_derived_tbl (j_compress_ptr cinfo, boolean isDC, int tblno,
 180                          c_derived_tbl ** pdtbl)
 181 {
 182   JHUFF_TBL *htbl;
 183   c_derived_tbl *dtbl;
 184   int p, i, l, lastp, si, maxsymbol;
 185   char huffsize[257];
 186   unsigned int huffcode[257];
 187   unsigned int code;
 188
 189   /* Note that huffsize[] and huffcode[] are filled in code-length order,
 190    * paralleling the order of the symbols themselves in htbl->huffval[].
 191    */
 192
 193   /* Find the input Huffman table */
 194   if (tblno < 0 || tblno >= NUM_HUFF_TBLS)
 195     ERREXIT1(cinfo, JERR_NO_HUFF_TABLE, tblno);
 196   htbl =
 197     isDC ? cinfo->dc_huff_tbl_ptrs[tblno] : cinfo->ac_huff_tbl_ptrs[tblno];
 198   if (htbl == NULL)
 199     ERREXIT1(cinfo, JERR_NO_HUFF_TABLE, tblno);
 200
 201   /* Allocate a workspace if we haven't already done so. */
 202   if (*pdtbl == NULL)
 203     *pdtbl = (c_derived_tbl *)
 204       (*cinfo->mem->alloc_small) ((j_common_ptr) cinfo, JPOOL_IMAGE,
 205                                   SIZEOF(c_derived_tbl));
 206   dtbl = *pdtbl;
 207
 208   /* Figure C.1: make table of Huffman code length for each symbol */
 209
 210   p = 0;
 211   for (l = 1; l <= 16; l++) {
 212     i = (int) htbl->bits[l];
 213     if (i < 0 || p + i > 256)   /* protect against table overrun */
 214       ERREXIT(cinfo, JERR_BAD_HUFF_TABLE);
 215     while (i--)
 216       huffsize[p++] = (char) l;
 217   }
 218   huffsize[p] = 0;
 219   lastp = p;
 220
 221   /* Figure C.2: generate the codes themselves */
 222   /* We also validate that the counts represent a legal Huffman code tree. */
 223
 224   code = 0;
 225   si = huffsize[0];
 226   p = 0;
 227   while (huffsize[p]) {
 228     while (((int) huffsize[p]) == si) {
 229       huffcode[p++] = code;
 230       code++;
 231     }
 232     /* code is now 1 more than the last code used for codelength si; but
 233      * it must still fit in si bits, since no code is allowed to be all ones.
 234      */
 235     if (((INT32) code) >= (((INT32) 1) << si))
 236       ERREXIT(cinfo, JERR_BAD_HUFF_TABLE);
 237     code <<= 1;
 238     si++;
 239   }
 240
 241   /* Figure C.3: generate encoding tables */
 242   /* These are code and size indexed by symbol value */
 243
 244   /* Set all codeless symbols to have code length 0;
 245    * this lets us detect duplicate VAL entries here, and later
 246    * allows emit_bits to detect any attempt to emit such symbols.
 247    */
 248   MEMZERO(dtbl->ehufsi, SIZEOF(dtbl->ehufsi));
 249
 250   /* This is also a convenient place to check for out-of-range
 251    * and duplicated VAL entries.  We allow 0..255 for AC symbols
 252    * but only 0..15 for DC.  (We could constrain them further
 253    * based on data depth and mode, but this seems enough.)
 254    */
 255   maxsymbol = isDC ? 15 : 255;
 256
 257   for (p = 0; p < lastp; p++) {
 258     i = htbl->huffval[p];
 259     if (i < 0 || i > maxsymbol || dtbl->ehufsi[i])
 260       ERREXIT(cinfo, JERR_BAD_HUFF_TABLE);
 261     dtbl->ehufco[i] = huffcode[p];
 262     dtbl->ehufsi[i] = huffsize[p];
 263   }
 264 }
 265
 266
 267 /* Outputting bytes to the file */
 268
 269 /* Emit a byte, taking 'action' if must suspend. */
 270 #define emit_byte(state,val,action)  \
 271         { *(state)->next_output_byte++ = (JOCTET) (val);  \
 272           if (--(state)->free_in_buffer == 0)  \
 273             if (! dump_buffer(state))  \
 274               { action; } }
 275
 276
 277 LOCAL(boolean)
 278 dump_buffer (working_state * state)
 279 /* Empty the output buffer; return TRUE if successful, FALSE if must suspend */
 280 {
 281   struct jpeg_destination_mgr * dest = state->cinfo->dest;
 282
 283   if (! (*dest->empty_output_buffer) (state->cinfo))
 284     return FALSE;
 285   /* After a successful buffer dump, must reset buffer pointers */
 286   state->next_output_byte = dest->next_output_byte;
 287   state->free_in_buffer = dest->free_in_buffer;
 288   return TRUE;
 289 }
 290
 291
 292 /* Outputting bits to the file */
 293
 294 /* Only the right 24 bits of put_buffer are used; the valid bits are
 295  * left-justified in this part.  At most 16 bits can be passed to emit_bits
 296  * in one call, and we never retain more than 7 bits in put_buffer
 297  * between calls, so 24 bits are sufficient.
 298  */
 299
 300 INLINE
 301 LOCAL(boolean)
 302 emit_bits (working_state * state, unsigned int code, int size)
 303 /* Emit some bits; return TRUE if successful, FALSE if must suspend */
 304 {
 305   /* This routine is heavily used, so it's worth coding tightly. */
 306   register INT32 put_buffer = (INT32) code;
 307   register int put_bits = state->cur.put_bits;
 308
 309   /* if size is 0, caller used an invalid Huffman table entry */
 310   if (size == 0)
 311     ERREXIT(state->cinfo, JERR_HUFF_MISSING_CODE);
 312
 313   put_buffer &= (((INT32) 1)<<size) - 1; /* mask off any extra bits in code */
 314
 315   put_bits += size;             /* new number of bits in buffer */
 316
 317   put_buffer <<= 24 - put_bits; /* align incoming bits */
 318
 319   put_buffer |= state->cur.put_buffer; /* and merge with old buffer contents */
 320
 321   while (put_bits >= 8) {
 322     int c = (int) ((put_buffer >> 16) & 0xFF);
 323
 324     emit_byte(state, c, return FALSE);
 325     if (c == 0xFF) {            /* need to stuff a zero byte? */
 326       emit_byte(state, 0, return FALSE);
 327     }
 328     put_buffer <<= 8;
 329     put_bits -= 8;
 330   }
 331
 332   state->cur.put_buffer = put_buffer; /* update state variables */
 333   state->cur.put_bits = put_bits;
 334
 335   return TRUE;
 336 }
 337
 338
 339 LOCAL(boolean)
 340 flush_bits (working_state * state)
 341 {
 342   if (! emit_bits(state, 0x7F, 7)) /* fill any partial byte with ones */
 343     return FALSE;
 344   state->cur.put_buffer = 0;    /* and reset bit-buffer to empty */
 345   state->cur.put_bits = 0;
 346   return TRUE;
 347 }
 348
 349
 350 /* Encode a single block's worth of coefficients */
 351
 352 LOCAL(boolean)
 353 encode_one_block (working_state * state, JCOEFPTR block, int last_dc_val,
 354                   c_derived_tbl *dctbl, c_derived_tbl *actbl)
 355 {
 356   register int temp, temp2;
 357   register int nbits;
 358   register int k, r, i;
 359
 360   /* Encode the DC coefficient difference per section F.1.2.1 */
 361
 362   temp = temp2 = block[0] - last_dc_val;
 363
 364   if (temp < 0) {
 365     temp = -temp;               /* temp is abs value of input */
 366     /* For a negative input, want temp2 = bitwise complement of abs(input) */
 367     /* This code assumes we are on a two's complement machine */
 368     temp2--;
 369   }
 370
 371   /* Find the number of bits needed for the magnitude of the coefficient */
 372   nbits = 0;
 373   while (temp) {
 374     nbits++;
 375     temp >>= 1;
 376   }
 377   /* Check for out-of-range coefficient values.
 378    * Since we're encoding a difference, the range limit is twice as much.
 379    */
 380   if (nbits > MAX_COEF_BITS+1)
 381     ERREXIT(state->cinfo, JERR_BAD_DCT_COEF);
 382
 383   /* Emit the Huffman-coded symbol for the number of bits */
 384   if (! emit_bits(state, dctbl->ehufco[nbits], dctbl->ehufsi[nbits]))
 385     return FALSE;
 386
 387   /* Emit that number of bits of the value, if positive, */
 388   /* or the complement of its magnitude, if negative. */
 389   if (nbits)                    /* emit_bits rejects calls with size 0 */
 390     if (! emit_bits(state, (unsigned int) temp2, nbits))
 391       return FALSE;
 392
 393   /* Encode the AC coefficients per section F.1.2.2 */
 394
 395   r = 0;                        /* r = run length of zeros */
 396
 397   for (k = 1; k < DCTSIZE2; k++) {
 398     if ((temp = block[jpeg_natural_order[k]]) == 0) {
 399       r++;
 400     } else {
 401       /* if run length > 15, must emit special run-length-16 codes (0xF0) */
 402       while (r > 15) {
 403         if (! emit_bits(state, actbl->ehufco[0xF0], actbl->ehufsi[0xF0]))
 404           return FALSE;
 405         r -= 16;
 406       }
 407
 408       temp2 = temp;
 409       if (temp < 0) {
 410         temp = -temp;           /* temp is abs value of input */
 411         /* This code assumes we are on a two's complement machine */
 412         temp2--;
 413       }
 414
 415       /* Find the number of bits needed for the magnitude of the coefficient */
 416       nbits = 1;                /* there must be at least one 1 bit */
 417       while ((temp >>= 1))
 418         nbits++;
 419       /* Check for out-of-range coefficient values */
 420       if (nbits > MAX_COEF_BITS)
 421         ERREXIT(state->cinfo, JERR_BAD_DCT_COEF);
 422
 423       /* Emit Huffman symbol for run length / number of bits */
 424       i = (r << 4) + nbits;
 425       if (! emit_bits(state, actbl->ehufco[i], actbl->ehufsi[i]))
 426         return FALSE;
 427
 428       /* Emit that number of bits of the value, if positive, */
 429       /* or the complement of its magnitude, if negative. */
 430       if (! emit_bits(state, (unsigned int) temp2, nbits))
 431         return FALSE;
 432
 433       r = 0;
 434     }
 435   }
 436
 437   /* If the last coef(s) were zero, emit an end-of-block code */
 438   if (r > 0)
 439     if (! emit_bits(state, actbl->ehufco[0], actbl->ehufsi[0]))
 440       return FALSE;
 441
 442   return TRUE;
 443 }
 444
 445
 446 /*
 447  * Emit a restart marker & resynchronize predictions.
 448  */
 449
 450 LOCAL(boolean)
 451 emit_restart (working_state * state, int restart_num)
 452 {
 453   int ci;
 454
 455   if (! flush_bits(state))
 456     return FALSE;
 457
 458   emit_byte(state, 0xFF, return FALSE);
 459   emit_byte(state, JPEG_RST0 + restart_num, return FALSE);
 460
 461   /* Re-initialize DC predictions to 0 */
 462   for (ci = 0; ci < state->cinfo->comps_in_scan; ci++)
 463     state->cur.last_dc_val[ci] = 0;
 464
 465   /* The restart counter is not updated until we successfully write the MCU. */
 466
 467   return TRUE;
 468 }
 469
 470
 471 /*
 472  * Encode and output one MCU's worth of Huffman-compressed coefficients.
 473  */
 474
 475 METHODDEF(boolean)
 476 encode_mcu_huff (j_compress_ptr cinfo, JBLOCKROW *MCU_data)
 477 {
 478   huff_entropy_ptr entropy = (huff_entropy_ptr) cinfo->entropy;
 479   working_state state;
 480   int blkn, ci;
 481   jpeg_component_info * compptr;
 482
 483   /* Load up working state */
 484   state.next_output_byte = cinfo->dest->next_output_byte;
 485   state.free_in_buffer = cinfo->dest->free_in_buffer;
 486   ASSIGN_STATE(state.cur, entropy->saved);
 487   state.cinfo = cinfo;
 488
 489   /* Emit restart marker if needed */
 490   if (cinfo->restart_interval) {
 491     if (entropy->restarts_to_go == 0)
 492       if (! emit_restart(&state, entropy->next_restart_num))
 493         return FALSE;
 494   }
 495
 496   /* Encode the MCU data blocks */
 497   for (blkn = 0; blkn < cinfo->blocks_in_MCU; blkn++) {
 498     ci = cinfo->MCU_membership[blkn];
 499     compptr = cinfo->cur_comp_info[ci];
 500     if (! encode_one_block(&state,
 501                            MCU_data[blkn][0], state.cur.last_dc_val[ci],
 502                            entropy->dc_derived_tbls[compptr->dc_tbl_no],
 503                            entropy->ac_derived_tbls[compptr->ac_tbl_no]))
 504       return FALSE;
 505     /* Update last_dc_val */
 506     state.cur.last_dc_val[ci] = MCU_data[blkn][0][0];
 507   }
 508
 509   /* Completed MCU, so update state */
 510   cinfo->dest->next_output_byte = state.next_output_byte;
 511   cinfo->dest->free_in_buffer = state.free_in_buffer;
 512   ASSIGN_STATE(entropy->saved, state.cur);
 513
 514   /* Update restart-interval state too */
 515   if (cinfo->restart_interval) {
 516     if (entropy->restarts_to_go == 0) {
 517       entropy->restarts_to_go = cinfo->restart_interval;
 518       entropy->next_restart_num++;
 519       entropy->next_restart_num &= 7;
 520     }
 521     entropy->restarts_to_go--;
 522   }
 523
 524   return TRUE;
 525 }
 526
 527
 528 /*
 529  * Finish up at the end of a Huffman-compressed scan.
 530  */
 531
 532 METHODDEF(void)
 533 finish_pass_huff (j_compress_ptr cinfo)
 534 {
 535   huff_entropy_ptr entropy = (huff_entropy_ptr) cinfo->entropy;
 536   working_state state;
 537
 538   /* Load up working state ... flush_bits needs it */
 539   state.next_output_byte = cinfo->dest->next_output_byte;
 540   state.free_in_buffer = cinfo->dest->free_in_buffer;
 541   ASSIGN_STATE(state.cur, entropy->saved);
 542   state.cinfo = cinfo;
 543
 544   /* Flush out the last data */
 545   if (! flush_bits(&state))
 546     ERREXIT(cinfo, JERR_CANT_SUSPEND);
 547
 548   /* Update state */
 549   cinfo->dest->next_output_byte = state.next_output_byte;
 550   cinfo->dest->free_in_buffer = state.free_in_buffer;
 551   ASSIGN_STATE(entropy->saved, state.cur);
 552 }
 553
 554
 555 /*
 556  * Huffman coding optimization.
 557  *
 558  * We first scan the supplied data and count the number of uses of each symbol
 559  * that is to be Huffman-coded. (This process MUST agree with the code above.)
 560  * Then we build a Huffman coding tree for the observed counts.
 561  * Symbols which are not needed at all for the particular image are not
 562  * assigned any code, which saves space in the DHT marker as well as in
 563  * the compressed data.
 564  */
 565
 566 #ifdef ENTROPY_OPT_SUPPORTED
 567
 568
 569 /* Process a single block's worth of coefficients */
 570
 571 LOCAL(void)
 572 htest_one_block (j_compress_ptr cinfo, JCOEFPTR block, int last_dc_val,
 573                  long dc_counts[], long ac_counts[])
 574 {
 575   register int temp;
 576   register int nbits;
 577   register int k, r;
 578
 579   /* Encode the DC coefficient difference per section F.1.2.1 */
 580
 581   temp = block[0] - last_dc_val;
 582   if (temp < 0)
 583     temp = -temp;
 584
 585   /* Find the number of bits needed for the magnitude of the coefficient */
 586   nbits = 0;
 587   while (temp) {
 588     nbits++;
 589     temp >>= 1;
 590   }
 591   /* Check for out-of-range coefficient values.
 592    * Since we're encoding a difference, the range limit is twice as much.
 593    */
 594   if (nbits > MAX_COEF_BITS+1)
 595     ERREXIT(cinfo, JERR_BAD_DCT_COEF);
 596
 597   /* Count the Huffman symbol for the number of bits */
 598   dc_counts[nbits]++;
 599
 600   /* Encode the AC coefficients per section F.1.2.2 */
 601
 602   r = 0;                        /* r = run length of zeros */
 603
 604   for (k = 1; k < DCTSIZE2; k++) {
 605     if ((temp = block[jpeg_natural_order[k]]) == 0) {
 606       r++;
 607     } else {
 608       /* if run length > 15, must emit special run-length-16 codes (0xF0) */
 609       while (r > 15) {
 610         ac_counts[0xF0]++;
 611         r -= 16;
 612       }
 613
 614       /* Find the number of bits needed for the magnitude of the coefficient */
 615       if (temp < 0)
 616         temp = -temp;
 617
 618       /* Find the number of bits needed for the magnitude of the coefficient */
 619       nbits = 1;                /* there must be at least one 1 bit */
 620       while ((temp >>= 1))
 621         nbits++;
 622       /* Check for out-of-range coefficient values */
 623       if (nbits > MAX_COEF_BITS)
 624         ERREXIT(cinfo, JERR_BAD_DCT_COEF);
 625
 626       /* Count Huffman symbol for run length / number of bits */
 627       ac_counts[(r << 4) + nbits]++;
 628
 629       r = 0;
 630     }
 631   }
 632
 633   /* If the last coef(s) were zero, emit an end-of-block code */
 634   if (r > 0)
 635     ac_counts[0]++;
 636 }
 637
 638
 639 /*
 640  * Trial-encode one MCU's worth of Huffman-compressed coefficients.
 641  * No data is actually output, so no suspension return is possible.
 642  */
 643
 644 METHODDEF(boolean)
 645 encode_mcu_gather (j_compress_ptr cinfo, JBLOCKROW *MCU_data)
 646 {
 647   huff_entropy_ptr entropy = (huff_entropy_ptr) cinfo->entropy;
 648   int blkn, ci;
 649   jpeg_component_info * compptr;
 650
 651   /* Take care of restart intervals if needed */
 652   if (cinfo->restart_interval) {
 653     if (entropy->restarts_to_go == 0) {
 654       /* Re-initialize DC predictions to 0 */
 655       for (ci = 0; ci < cinfo->comps_in_scan; ci++)
 656         entropy->saved.last_dc_val[ci] = 0;
 657       /* Update restart state */
 658       entropy->restarts_to_go = cinfo->restart_interval;
 659     }
 660     entropy->restarts_to_go--;
 661   }
 662
 663   for (blkn = 0; blkn < cinfo->blocks_in_MCU; blkn++) {
 664     ci = cinfo->MCU_membership[blkn];
 665     compptr = cinfo->cur_comp_info[ci];
 666     htest_one_block(cinfo, MCU_data[blkn][0], entropy->saved.last_dc_val[ci],
 667                     entropy->dc_count_ptrs[compptr->dc_tbl_no],
 668                     entropy->ac_count_ptrs[compptr->ac_tbl_no]);
 669     entropy->saved.last_dc_val[ci] = MCU_data[blkn][0][0];
 670   }
 671
 672   return TRUE;
 673 }
 674
 675
 676 /*
 677  * Generate the best Huffman code table for the given counts, fill htbl.
 678  * Note this is also used by jcphuff.c.
 679  *
 680  * The JPEG standard requires that no symbol be assigned a codeword of all
 681  * one bits (so that padding bits added at the end of a compressed segment
 682  * can't look like a valid code).  Because of the canonical ordering of
 683  * codewords, this just means that there must be an unused slot in the
 684  * longest codeword length category.  Section K.2 of the JPEG spec suggests
 685  * reserving such a slot by pretending that symbol 256 is a valid symbol
 686  * with count 1.  In theory that's not optimal; giving it count zero but
 687  * including it in the symbol set anyway should give a better Huffman code.
 688  * But the theoretically better code actually seems to come out worse in
 689  * practice, because it produces more all-ones bytes (which incur stuffed
 690  * zero bytes in the final file).  In any case the difference is tiny.
 691  *
 692  * The JPEG standard requires Huffman codes to be no more than 16 bits long.
 693  * If some symbols have a very small but nonzero probability, the Huffman tree
 694  * must be adjusted to meet the code length restriction.  We currently use
 695  * the adjustment method suggested in JPEG section K.2.  This method is *not*
 696  * optimal; it may not choose the best possible limited-length code.  But
 697  * typically only very-low-frequency symbols will be given less-than-optimal
 698  * lengths, so the code is almost optimal.  Experimental comparisons against
 699  * an optimal limited-length-code algorithm indicate that the difference is
 700  * microscopic --- usually less than a hundredth of a percent of total size.
 701  * So the extra complexity of an optimal algorithm doesn't seem worthwhile.
 702  */
 703
 704 GLOBAL(void)
 705 jpeg_gen_optimal_table (j_compress_ptr cinfo, JHUFF_TBL * htbl, long freq[])
 706 {
 707 #define MAX_CLEN 32             /* assumed maximum initial code length */
 708   UINT8 bits[MAX_CLEN+1];       /* bits[k] = # of symbols with code length k */
 709   int codesize[257];            /* codesize[k] = code length of symbol k */
 710   int others[257];              /* next symbol in current branch of tree */
 711   int c1, c2;
 712   int p, i, j;
 713   long v;
 714
 715   /* This algorithm is explained in section K.2 of the JPEG standard */
 716
 717   MEMZERO(bits, SIZEOF(bits));
 718   MEMZERO(codesize, SIZEOF(codesize));
 719   for (i = 0; i < 257; i++)
 720     others[i] = -1;             /* init links to empty */
 721
 722   freq[256] = 1;                /* make sure 256 has a nonzero count */
 723   /* Including the pseudo-symbol 256 in the Huffman procedure guarantees
 724    * that no real symbol is given code-value of all ones, because 256
 725    * will be placed last in the largest codeword category.
 726    */
 727
 728   /* Huffman's basic algorithm to assign optimal code lengths to symbols */
 729
 730   for (;;) {
 731     /* Find the smallest nonzero frequency, set c1 = its symbol */
 732     /* In case of ties, take the larger symbol number */
 733     c1 = -1;
 734     v = 1000000000L;
 735     for (i = 0; i <= 256; i++) {
 736       if (freq[i] && freq[i] <= v) {
 737         v = freq[i];
 738         c1 = i;
 739       }
 740     }
 741
 742     /* Find the next smallest nonzero frequency, set c2 = its symbol */
 743     /* In case of ties, take the larger symbol number */
 744     c2 = -1;
 745     v = 1000000000L;
 746     for (i = 0; i <= 256; i++) {
 747       if (freq[i] && freq[i] <= v && i != c1) {
 748         v = freq[i];
 749         c2 = i;
 750       }
 751     }
 752
 753     /* Done if we've merged everything into one frequency */
 754     if (c2 < 0)
 755       break;
 756
 757     /* Else merge the two counts/trees */
 758     freq[c1] += freq[c2];
 759     freq[c2] = 0;
 760
 761     /* Increment the codesize of everything in c1's tree branch */
 762     codesize[c1]++;
 763     while (others[c1] >= 0) {
 764       c1 = others[c1];
 765       codesize[c1]++;
 766     }
 767
 768     others[c1] = c2;            /* chain c2 onto c1's tree branch */
 769
 770     /* Increment the codesize of everything in c2's tree branch */
 771     codesize[c2]++;
 772     while (others[c2] >= 0) {
 773       c2 = others[c2];
 774       codesize[c2]++;
 775     }
 776   }
 777
 778   /* Now count the number of symbols of each code length */
 779   for (i = 0; i <= 256; i++) {
 780     if (codesize[i]) {
 781       /* The JPEG standard seems to think that this can't happen, */
 782       /* but I'm paranoid... */
 783       if (codesize[i] > MAX_CLEN)
 784         ERREXIT(cinfo, JERR_HUFF_CLEN_OVERFLOW);
 785
 786       bits[codesize[i]]++;
 787     }
 788   }
 789
 790   /* JPEG doesn't allow symbols with code lengths over 16 bits, so if the pure
 791    * Huffman procedure assigned any such lengths, we must adjust the coding.
 792    * Here is what the JPEG spec says about how this next bit works:
 793    * Since symbols are paired for the longest Huffman code, the symbols are
 794    * removed from this length category two at a time.  The prefix for the pair
 795    * (which is one bit shorter) is allocated to one of the pair; then,
 796    * skipping the BITS entry for that prefix length, a code word from the next
 797    * shortest nonzero BITS entry is converted into a prefix for two code words
 798    * one bit longer.
 799    */
 800
 801   for (i = MAX_CLEN; i > 16; i--) {
 802     while (bits[i] > 0) {
 803       j = i - 2;                /* find length of new prefix to be used */
 804       while (bits[j] == 0)
 805         j--;
 806
 807       bits[i] -= 2;             /* remove two symbols */
 808       bits[i-1]++;              /* one goes in this length */
 809       bits[j+1] += 2;           /* two new symbols in this length */
 810       bits[j]--;                /* symbol of this length is now a prefix */
 811     }
 812   }
 813
 814   /* Remove the count for the pseudo-symbol 256 from the largest codelength */
 815   while (bits[i] == 0)          /* find largest codelength still in use */
 816     i--;
 817   bits[i]--;
 818
 819   /* Return final symbol counts (only for lengths 0..16) */
 820   MEMCOPY(htbl->bits, bits, SIZEOF(htbl->bits));
 821
 822   /* Return a list of the symbols sorted by code length */
 823   /* It's not real clear to me why we don't need to consider the codelength
 824    * changes made above, but the JPEG spec seems to think this works.
 825    */
 826   p = 0;
 827   for (i = 1; i <= MAX_CLEN; i++) {
 828     for (j = 0; j <= 255; j++) {
 829       if (codesize[j] == i) {
 830         htbl->huffval[p] = (UINT8) j;
 831         p++;
 832       }
 833     }
 834   }
 835
 836   /* Set sent_table FALSE so updated table will be written to JPEG file. */
 837   htbl->sent_table = FALSE;
 838 }
 839
 840
 841 /*
 842  * Finish up a statistics-gathering pass and create the new Huffman tables.
 843  */
 844
 845 METHODDEF(void)
 846 finish_pass_gather (j_compress_ptr cinfo)
 847 {
 848   huff_entropy_ptr entropy = (huff_entropy_ptr) cinfo->entropy;
 849   int ci, dctbl, actbl;
 850   jpeg_component_info * compptr;
 851   JHUFF_TBL **htblptr;
 852   boolean did_dc[NUM_HUFF_TBLS];
 853   boolean did_ac[NUM_HUFF_TBLS];
 854
 855   /* It's important not to apply jpeg_gen_optimal_table more than once
 856    * per table, because it clobbers the input frequency counts!
 857    */
 858   MEMZERO(did_dc, SIZEOF(did_dc));
 859   MEMZERO(did_ac, SIZEOF(did_ac));
 860
 861   for (ci = 0; ci < cinfo->comps_in_scan; ci++) {
 862     compptr = cinfo->cur_comp_info[ci];
 863     dctbl = compptr->dc_tbl_no;
 864     actbl = compptr->ac_tbl_no;
 865     if (! did_dc[dctbl]) {
 866       htblptr = & cinfo->dc_huff_tbl_ptrs[dctbl];
 867       if (*htblptr == NULL)
 868         *htblptr = jpeg_alloc_huff_table((j_common_ptr) cinfo);
 869       jpeg_gen_optimal_table(cinfo, *htblptr, entropy->dc_count_ptrs[dctbl]);
 870       did_dc[dctbl] = TRUE;
 871     }
 872     if (! did_ac[actbl]) {
 873       htblptr = & cinfo->ac_huff_tbl_ptrs[actbl];
 874       if (*htblptr == NULL)
 875         *htblptr = jpeg_alloc_huff_table((j_common_ptr) cinfo);
 876       jpeg_gen_optimal_table(cinfo, *htblptr, entropy->ac_count_ptrs[actbl]);
 877       did_ac[actbl] = TRUE;
 878     }
 879   }
 880 }
 881
 882
 883 #endif /* ENTROPY_OPT_SUPPORTED */
 884
 885
 886 /*
 887  * Module initialization routine for Huffman entropy encoding.
 888  */
 889
 890 GLOBAL(void)
 891 jinit_huff_encoder (j_compress_ptr cinfo)
 892 {
 893   huff_entropy_ptr entropy;
 894   int i;
 895
 896   entropy = (huff_entropy_ptr)
 897     (*cinfo->mem->alloc_small) ((j_common_ptr) cinfo, JPOOL_IMAGE,
 898                                 SIZEOF(huff_entropy_encoder));
 899   cinfo->entropy = (struct jpeg_entropy_encoder *) entropy;
 900   entropy->pub.start_pass = start_pass_huff;
 901
 902   /* Mark tables unallocated */
 903   for (i = 0; i < NUM_HUFF_TBLS; i++) {
 904     entropy->dc_derived_tbls[i] = entropy->ac_derived_tbls[i] = NULL;
 905 #ifdef ENTROPY_OPT_SUPPORTED
 906     entropy->dc_count_ptrs[i] = entropy->ac_count_ptrs[i] = NULL;
 907 #endif
 908   }
 909 }