1 /*
   2  * reserved comment block
   3  * DO NOT REMOVE OR ALTER!
   4  */
   5 /*
   6  * jchuff.c
   7  *
   8  * Copyright (C) 1991-1997, Thomas G. Lane.
   9  * This file is part of the Independent JPEG Group's software.
  10  * For conditions of distribution and use, see the accompanying README file.
  11  *
  12  * This file contains Huffman entropy encoding routines.
  13  *
  14  * Much of the complexity here has to do with supporting output suspension.
  15  * If the data destination module demands suspension, we want to be able to
  16  * back up to the start of the current MCU.  To do this, we copy state
  17  * variables into local working storage, and update them back to the
  18  * permanent JPEG objects only upon successful completion of an MCU.
  19  */
  20 
  21 #define JPEG_INTERNALS
  22 #include "jinclude.h"
  23 #include "jpeglib.h"
  24 #include "jchuff.h"             /* Declarations shared with jcphuff.c */
  25 
  26 
  27 /* Expanded entropy encoder object for Huffman encoding.
  28  *
  29  * The savable_state subrecord contains fields that change within an MCU,
  30  * but must not be updated permanently until we complete the MCU.
  31  */
  32 
  33 typedef struct {
  34   INT32 put_buffer;             /* current bit-accumulation buffer */
  35   int put_bits;                 /* # of bits now in it */
  36   int last_dc_val[MAX_COMPS_IN_SCAN]; /* last DC coef for each component */
  37 } savable_state;
  38 
  39 /* This macro is to work around compilers with missing or broken
  40  * structure assignment.  You'll need to fix this code if you have
  41  * such a compiler and you change MAX_COMPS_IN_SCAN.
  42  */
  43 
  44 #ifndef NO_STRUCT_ASSIGN
  45 #define ASSIGN_STATE(dest,src)  ((dest) = (src))
  46 #else
  47 #if MAX_COMPS_IN_SCAN == 4
  48 #define ASSIGN_STATE(dest,src)  \
  49         ((dest).put_buffer = (src).put_buffer, \
  50          (dest).put_bits = (src).put_bits, \
  51          (dest).last_dc_val[0] = (src).last_dc_val[0], \
  52          (dest).last_dc_val[1] = (src).last_dc_val[1], \
  53          (dest).last_dc_val[2] = (src).last_dc_val[2], \
  54          (dest).last_dc_val[3] = (src).last_dc_val[3])
  55 #endif
  56 #endif
  57 
  58 
  59 typedef struct {
  60   struct jpeg_entropy_encoder pub; /* public fields */
  61 
  62   savable_state saved;          /* Bit buffer & DC state at start of MCU */
  63 
  64   /* These fields are NOT loaded into local working state. */
  65   unsigned int restarts_to_go;  /* MCUs left in this restart interval */
  66   int next_restart_num;         /* next restart number to write (0-7) */
  67 
  68   /* Pointers to derived tables (these workspaces have image lifespan) */
  69   c_derived_tbl * dc_derived_tbls[NUM_HUFF_TBLS];
  70   c_derived_tbl * ac_derived_tbls[NUM_HUFF_TBLS];
  71 
  72 #ifdef ENTROPY_OPT_SUPPORTED    /* Statistics tables for optimization */
  73   long * dc_count_ptrs[NUM_HUFF_TBLS];
  74   long * ac_count_ptrs[NUM_HUFF_TBLS];
  75 #endif
  76 } huff_entropy_encoder;
  77 
  78 typedef huff_entropy_encoder * huff_entropy_ptr;
  79 
  80 /* Working state while writing an MCU.
  81  * This struct contains all the fields that are needed by subroutines.
  82  */
  83 
  84 typedef struct {
  85   JOCTET * next_output_byte;    /* => next byte to write in buffer */
  86   size_t free_in_buffer;        /* # of byte spaces remaining in buffer */
  87   savable_state cur;            /* Current bit buffer & DC state */
  88   j_compress_ptr cinfo;         /* dump_buffer needs access to this */
  89 } working_state;
  90 
  91 
  92 /* Forward declarations */
  93 METHODDEF(boolean) encode_mcu_huff JPP((j_compress_ptr cinfo,
  94                                         JBLOCKROW *MCU_data));
  95 METHODDEF(void) finish_pass_huff JPP((j_compress_ptr cinfo));
  96 #ifdef ENTROPY_OPT_SUPPORTED
  97 METHODDEF(boolean) encode_mcu_gather JPP((j_compress_ptr cinfo,
  98                                           JBLOCKROW *MCU_data));
  99 METHODDEF(void) finish_pass_gather JPP((j_compress_ptr cinfo));
 100 #endif
 101 
 102 
 103 /*
 104  * Initialize for a Huffman-compressed scan.
 105  * If gather_statistics is TRUE, we do not output anything during the scan,
 106  * just count the Huffman symbols used and generate Huffman code tables.
 107  */
 108 
 109 METHODDEF(void)
 110 start_pass_huff (j_compress_ptr cinfo, boolean gather_statistics)
 111 {
 112   huff_entropy_ptr entropy = (huff_entropy_ptr) cinfo->entropy;
 113   int ci, dctbl, actbl;
 114   jpeg_component_info * compptr;
 115 
 116   if (gather_statistics) {
 117 #ifdef ENTROPY_OPT_SUPPORTED
 118     entropy->pub.encode_mcu = encode_mcu_gather;
 119     entropy->pub.finish_pass = finish_pass_gather;
 120 #else
 121     ERREXIT(cinfo, JERR_NOT_COMPILED);
 122 #endif
 123   } else {
 124     entropy->pub.encode_mcu = encode_mcu_huff;
 125     entropy->pub.finish_pass = finish_pass_huff;
 126   }
 127 
 128   for (ci = 0; ci < cinfo->comps_in_scan; ci++) {
 129     compptr = cinfo->cur_comp_info[ci];
 130     dctbl = compptr->dc_tbl_no;
 131     actbl = compptr->ac_tbl_no;
 132     if (gather_statistics) {
 133 #ifdef ENTROPY_OPT_SUPPORTED
 134       /* Check for invalid table indexes */
 135       /* (make_c_derived_tbl does this in the other path) */
 136       if (dctbl < 0 || dctbl >= NUM_HUFF_TBLS)
 137         ERREXIT1(cinfo, JERR_NO_HUFF_TABLE, dctbl);
 138       if (actbl < 0 || actbl >= NUM_HUFF_TBLS)
 139         ERREXIT1(cinfo, JERR_NO_HUFF_TABLE, actbl);
 140       /* Allocate and zero the statistics tables */
 141       /* Note that jpeg_gen_optimal_table expects 257 entries in each table! */
 142       if (entropy->dc_count_ptrs[dctbl] == NULL)
 143         entropy->dc_count_ptrs[dctbl] = (long *)
 144           (*cinfo->mem->alloc_small) ((j_common_ptr) cinfo, JPOOL_IMAGE,
 145                                       257 * SIZEOF(long));
 146       MEMZERO(entropy->dc_count_ptrs[dctbl], 257 * SIZEOF(long));
 147       if (entropy->ac_count_ptrs[actbl] == NULL)
 148         entropy->ac_count_ptrs[actbl] = (long *)
 149           (*cinfo->mem->alloc_small) ((j_common_ptr) cinfo, JPOOL_IMAGE,
 150                                       257 * SIZEOF(long));
 151       MEMZERO(entropy->ac_count_ptrs[actbl], 257 * SIZEOF(long));
 152 #endif
 153     } else {
 154       /* Compute derived values for Huffman tables */
 155       /* We may do this more than once for a table, but it's not expensive */
 156       jpeg_make_c_derived_tbl(cinfo, TRUE, dctbl,
 157                               & entropy->dc_derived_tbls[dctbl]);
 158       jpeg_make_c_derived_tbl(cinfo, FALSE, actbl,
 159                               & entropy->ac_derived_tbls[actbl]);
 160     }
 161     /* Initialize DC predictions to 0 */
 162     entropy->saved.last_dc_val[ci] = 0;
 163   }
 164 
 165   /* Initialize bit buffer to empty */
 166   entropy->saved.put_buffer = 0;
 167   entropy->saved.put_bits = 0;
 168 
 169   /* Initialize restart stuff */
 170   entropy->restarts_to_go = cinfo->restart_interval;
 171   entropy->next_restart_num = 0;
 172 }
 173 
 174 
 175 /*
 176  * Compute the derived values for a Huffman table.
 177  * This routine also performs some validation checks on the table.
 178  *
 179  * Note this is also used by jcphuff.c.
 180  */
 181 
 182 GLOBAL(void)
 183 jpeg_make_c_derived_tbl (j_compress_ptr cinfo, boolean isDC, int tblno,
 184                          c_derived_tbl ** pdtbl)
 185 {
 186   JHUFF_TBL *htbl;
 187   c_derived_tbl *dtbl;
 188   int p, i, l, lastp, si, maxsymbol;
 189   char huffsize[257];
 190   unsigned int huffcode[257];
 191   unsigned int code;
 192 
 193   /* Note that huffsize[] and huffcode[] are filled in code-length order,
 194    * paralleling the order of the symbols themselves in htbl->huffval[].
 195    */
 196 
 197   /* Find the input Huffman table */
 198   if (tblno < 0 || tblno >= NUM_HUFF_TBLS)
 199     ERREXIT1(cinfo, JERR_NO_HUFF_TABLE, tblno);
 200   htbl =
 201     isDC ? cinfo->dc_huff_tbl_ptrs[tblno] : cinfo->ac_huff_tbl_ptrs[tblno];
 202   if (htbl == NULL)
 203     ERREXIT1(cinfo, JERR_NO_HUFF_TABLE, tblno);
 204 
 205   /* Allocate a workspace if we haven't already done so. */
 206   if (*pdtbl == NULL)
 207     *pdtbl = (c_derived_tbl *)
 208       (*cinfo->mem->alloc_small) ((j_common_ptr) cinfo, JPOOL_IMAGE,
 209                                   SIZEOF(c_derived_tbl));
 210   dtbl = *pdtbl;
 211 
 212   /* Figure C.1: make table of Huffman code length for each symbol */
 213 
 214   p = 0;
 215   for (l = 1; l <= 16; l++) {
 216     i = (int) htbl->bits[l];
 217     if (i < 0 || p + i > 256)   /* protect against table overrun */
 218       ERREXIT(cinfo, JERR_BAD_HUFF_TABLE);
 219     while (i--)
 220       huffsize[p++] = (char) l;
 221   }
 222   huffsize[p] = 0;
 223   lastp = p;
 224 
 225   /* Figure C.2: generate the codes themselves */
 226   /* We also validate that the counts represent a legal Huffman code tree. */
 227 
 228   code = 0;
 229   si = huffsize[0];
 230   p = 0;
 231   while (huffsize[p]) {
 232     while (((int) huffsize[p]) == si) {
 233       huffcode[p++] = code;
 234       code++;
 235     }
 236     /* code is now 1 more than the last code used for codelength si; but
 237      * it must still fit in si bits, since no code is allowed to be all ones.
 238      */
 239     if (((INT32) code) >= (((INT32) 1) << si))
 240       ERREXIT(cinfo, JERR_BAD_HUFF_TABLE);
 241     code <<= 1;
 242     si++;
 243   }
 244 
 245   /* Figure C.3: generate encoding tables */
 246   /* These are code and size indexed by symbol value */
 247 
 248   /* Set all codeless symbols to have code length 0;
 249    * this lets us detect duplicate VAL entries here, and later
 250    * allows emit_bits to detect any attempt to emit such symbols.
 251    */
 252   MEMZERO(dtbl->ehufsi, SIZEOF(dtbl->ehufsi));
 253 
 254   /* This is also a convenient place to check for out-of-range
 255    * and duplicated VAL entries.  We allow 0..255 for AC symbols
 256    * but only 0..15 for DC.  (We could constrain them further
 257    * based on data depth and mode, but this seems enough.)
 258    */
 259   maxsymbol = isDC ? 15 : 255;
 260 
 261   for (p = 0; p < lastp; p++) {
 262     i = htbl->huffval[p];
 263     if (i < 0 || i > maxsymbol || dtbl->ehufsi[i])
 264       ERREXIT(cinfo, JERR_BAD_HUFF_TABLE);
 265     dtbl->ehufco[i] = huffcode[p];
 266     dtbl->ehufsi[i] = huffsize[p];
 267   }
 268 }
 269 
 270 
 271 /* Outputting bytes to the file */
 272 
 273 /* Emit a byte, taking 'action' if must suspend. */
 274 #define emit_byte(state,val,action)  \
 275         { *(state)->next_output_byte++ = (JOCTET) (val);  \
 276           if (--(state)->free_in_buffer == 0)  \
 277             if (! dump_buffer(state))  \
 278               { action; } }
 279 
 280 
 281 LOCAL(boolean)
 282 dump_buffer (working_state * state)
 283 /* Empty the output buffer; return TRUE if successful, FALSE if must suspend */
 284 {
 285   struct jpeg_destination_mgr * dest = state->cinfo->dest;
 286 
 287   if (! (*dest->empty_output_buffer) (state->cinfo))
 288     return FALSE;
 289   /* After a successful buffer dump, must reset buffer pointers */
 290   state->next_output_byte = dest->next_output_byte;
 291   state->free_in_buffer = dest->free_in_buffer;
 292   return TRUE;
 293 }
 294 
 295 
 296 /* Outputting bits to the file */
 297 
 298 /* Only the right 24 bits of put_buffer are used; the valid bits are
 299  * left-justified in this part.  At most 16 bits can be passed to emit_bits
 300  * in one call, and we never retain more than 7 bits in put_buffer
 301  * between calls, so 24 bits are sufficient.
 302  */
 303 
 304 INLINE
 305 LOCAL(boolean)
 306 emit_bits (working_state * state, unsigned int code, int size)
 307 /* Emit some bits; return TRUE if successful, FALSE if must suspend */
 308 {
 309   /* This routine is heavily used, so it's worth coding tightly. */
 310   register INT32 put_buffer = (INT32) code;
 311   register int put_bits = state->cur.put_bits;
 312 
 313   /* if size is 0, caller used an invalid Huffman table entry */
 314   if (size == 0)
 315     ERREXIT(state->cinfo, JERR_HUFF_MISSING_CODE);
 316 
 317   put_buffer &= (((INT32) 1)<<size) - 1; /* mask off any extra bits in code */
 318 
 319   put_bits += size;             /* new number of bits in buffer */
 320 
 321   put_buffer <<= 24 - put_bits; /* align incoming bits */
 322 
 323   put_buffer |= state->cur.put_buffer; /* and merge with old buffer contents */
 324 
 325   while (put_bits >= 8) {
 326     int c = (int) ((put_buffer >> 16) & 0xFF);
 327 
 328     emit_byte(state, c, return FALSE);
 329     if (c == 0xFF) {            /* need to stuff a zero byte? */
 330       emit_byte(state, 0, return FALSE);
 331     }
 332     put_buffer <<= 8;
 333     put_bits -= 8;
 334   }
 335 
 336   state->cur.put_buffer = put_buffer; /* update state variables */
 337   state->cur.put_bits = put_bits;
 338 
 339   return TRUE;
 340 }
 341 
 342 
 343 LOCAL(boolean)
 344 flush_bits (working_state * state)
 345 {
 346   if (! emit_bits(state, 0x7F, 7)) /* fill any partial byte with ones */
 347     return FALSE;
 348   state->cur.put_buffer = 0;    /* and reset bit-buffer to empty */
 349   state->cur.put_bits = 0;
 350   return TRUE;
 351 }
 352 
 353 
 354 /* Encode a single block's worth of coefficients */
 355 
 356 LOCAL(boolean)
 357 encode_one_block (working_state * state, JCOEFPTR block, int last_dc_val,
 358                   c_derived_tbl *dctbl, c_derived_tbl *actbl)
 359 {
 360   register int temp, temp2;
 361   register int nbits;
 362   register int k, r, i;
 363 
 364   /* Encode the DC coefficient difference per section F.1.2.1 */
 365 
 366   temp = temp2 = block[0] - last_dc_val;
 367 
 368   if (temp < 0) {
 369     temp = -temp;               /* temp is abs value of input */
 370     /* For a negative input, want temp2 = bitwise complement of abs(input) */
 371     /* This code assumes we are on a two's complement machine */
 372     temp2--;
 373   }
 374 
 375   /* Find the number of bits needed for the magnitude of the coefficient */
 376   nbits = 0;
 377   while (temp) {
 378     nbits++;
 379     temp >>= 1;
 380   }
 381   /* Check for out-of-range coefficient values.
 382    * Since we're encoding a difference, the range limit is twice as much.
 383    */
 384   if (nbits > MAX_COEF_BITS+1)
 385     ERREXIT(state->cinfo, JERR_BAD_DCT_COEF);
 386 
 387   /* Emit the Huffman-coded symbol for the number of bits */
 388   if (! emit_bits(state, dctbl->ehufco[nbits], dctbl->ehufsi[nbits]))
 389     return FALSE;
 390 
 391   /* Emit that number of bits of the value, if positive, */
 392   /* or the complement of its magnitude, if negative. */
 393   if (nbits)                    /* emit_bits rejects calls with size 0 */
 394     if (! emit_bits(state, (unsigned int) temp2, nbits))
 395       return FALSE;
 396 
 397   /* Encode the AC coefficients per section F.1.2.2 */
 398 
 399   r = 0;                        /* r = run length of zeros */
 400 
 401   for (k = 1; k < DCTSIZE2; k++) {
 402     if ((temp = block[jpeg_natural_order[k]]) == 0) {
 403       r++;
 404     } else {
 405       /* if run length > 15, must emit special run-length-16 codes (0xF0) */
 406       while (r > 15) {
 407         if (! emit_bits(state, actbl->ehufco[0xF0], actbl->ehufsi[0xF0]))
 408           return FALSE;
 409         r -= 16;
 410       }
 411 
 412       temp2 = temp;
 413       if (temp < 0) {
 414         temp = -temp;           /* temp is abs value of input */
 415         /* This code assumes we are on a two's complement machine */
 416         temp2--;
 417       }
 418 
 419       /* Find the number of bits needed for the magnitude of the coefficient */
 420       nbits = 1;                /* there must be at least one 1 bit */
 421       while ((temp >>= 1))
 422         nbits++;
 423       /* Check for out-of-range coefficient values */
 424       if (nbits > MAX_COEF_BITS)
 425         ERREXIT(state->cinfo, JERR_BAD_DCT_COEF);
 426 
 427       /* Emit Huffman symbol for run length / number of bits */
 428       i = (r << 4) + nbits;
 429       if (! emit_bits(state, actbl->ehufco[i], actbl->ehufsi[i]))
 430         return FALSE;
 431 
 432       /* Emit that number of bits of the value, if positive, */
 433       /* or the complement of its magnitude, if negative. */
 434       if (! emit_bits(state, (unsigned int) temp2, nbits))
 435         return FALSE;
 436 
 437       r = 0;
 438     }
 439   }
 440 
 441   /* If the last coef(s) were zero, emit an end-of-block code */
 442   if (r > 0)
 443     if (! emit_bits(state, actbl->ehufco[0], actbl->ehufsi[0]))
 444       return FALSE;
 445 
 446   return TRUE;
 447 }
 448 
 449 
 450 /*
 451  * Emit a restart marker & resynchronize predictions.
 452  */
 453 
 454 LOCAL(boolean)
 455 emit_restart (working_state * state, int restart_num)
 456 {
 457   int ci;
 458 
 459   if (! flush_bits(state))
 460     return FALSE;
 461 
 462   emit_byte(state, 0xFF, return FALSE);
 463   emit_byte(state, JPEG_RST0 + restart_num, return FALSE);
 464 
 465   /* Re-initialize DC predictions to 0 */
 466   for (ci = 0; ci < state->cinfo->comps_in_scan; ci++)
 467     state->cur.last_dc_val[ci] = 0;
 468 
 469   /* The restart counter is not updated until we successfully write the MCU. */
 470 
 471   return TRUE;
 472 }
 473 
 474 
 475 /*
 476  * Encode and output one MCU's worth of Huffman-compressed coefficients.
 477  */
 478 
 479 METHODDEF(boolean)
 480 encode_mcu_huff (j_compress_ptr cinfo, JBLOCKROW *MCU_data)
 481 {
 482   huff_entropy_ptr entropy = (huff_entropy_ptr) cinfo->entropy;
 483   working_state state;
 484   int blkn, ci;
 485   jpeg_component_info * compptr;
 486 
 487   /* Load up working state */
 488   state.next_output_byte = cinfo->dest->next_output_byte;
 489   state.free_in_buffer = cinfo->dest->free_in_buffer;
 490   ASSIGN_STATE(state.cur, entropy->saved);
 491   state.cinfo = cinfo;
 492 
 493   /* Emit restart marker if needed */
 494   if (cinfo->restart_interval) {
 495     if (entropy->restarts_to_go == 0)
 496       if (! emit_restart(&state, entropy->next_restart_num))
 497         return FALSE;
 498   }
 499 
 500   /* Encode the MCU data blocks */
 501   for (blkn = 0; blkn < cinfo->blocks_in_MCU; blkn++) {
 502     ci = cinfo->MCU_membership[blkn];
 503     compptr = cinfo->cur_comp_info[ci];
 504     if (! encode_one_block(&state,
 505                            MCU_data[blkn][0], state.cur.last_dc_val[ci],
 506                            entropy->dc_derived_tbls[compptr->dc_tbl_no],
 507                            entropy->ac_derived_tbls[compptr->ac_tbl_no]))
 508       return FALSE;
 509     /* Update last_dc_val */
 510     state.cur.last_dc_val[ci] = MCU_data[blkn][0][0];
 511   }
 512 
 513   /* Completed MCU, so update state */
 514   cinfo->dest->next_output_byte = state.next_output_byte;
 515   cinfo->dest->free_in_buffer = state.free_in_buffer;
 516   ASSIGN_STATE(entropy->saved, state.cur);
 517 
 518   /* Update restart-interval state too */
 519   if (cinfo->restart_interval) {
 520     if (entropy->restarts_to_go == 0) {
 521       entropy->restarts_to_go = cinfo->restart_interval;
 522       entropy->next_restart_num++;
 523       entropy->next_restart_num &= 7;
 524     }
 525     entropy->restarts_to_go--;
 526   }
 527 
 528   return TRUE;
 529 }
 530 
 531 
 532 /*
 533  * Finish up at the end of a Huffman-compressed scan.
 534  */
 535 
 536 METHODDEF(void)
 537 finish_pass_huff (j_compress_ptr cinfo)
 538 {
 539   huff_entropy_ptr entropy = (huff_entropy_ptr) cinfo->entropy;
 540   working_state state;
 541 
 542   /* Load up working state ... flush_bits needs it */
 543   state.next_output_byte = cinfo->dest->next_output_byte;
 544   state.free_in_buffer = cinfo->dest->free_in_buffer;
 545   ASSIGN_STATE(state.cur, entropy->saved);
 546   state.cinfo = cinfo;
 547 
 548   /* Flush out the last data */
 549   if (! flush_bits(&state))
 550     ERREXIT(cinfo, JERR_CANT_SUSPEND);
 551 
 552   /* Update state */
 553   cinfo->dest->next_output_byte = state.next_output_byte;
 554   cinfo->dest->free_in_buffer = state.free_in_buffer;
 555   ASSIGN_STATE(entropy->saved, state.cur);
 556 }
 557 
 558 
 559 /*
 560  * Huffman coding optimization.
 561  *
 562  * We first scan the supplied data and count the number of uses of each symbol
 563  * that is to be Huffman-coded. (This process MUST agree with the code above.)
 564  * Then we build a Huffman coding tree for the observed counts.
 565  * Symbols which are not needed at all for the particular image are not
 566  * assigned any code, which saves space in the DHT marker as well as in
 567  * the compressed data.
 568  */
 569 
 570 #ifdef ENTROPY_OPT_SUPPORTED
 571 
 572 
 573 /* Process a single block's worth of coefficients */
 574 
 575 LOCAL(void)
 576 htest_one_block (j_compress_ptr cinfo, JCOEFPTR block, int last_dc_val,
 577                  long dc_counts[], long ac_counts[])
 578 {
 579   register int temp;
 580   register int nbits;
 581   register int k, r;
 582 
 583   /* Encode the DC coefficient difference per section F.1.2.1 */
 584 
 585   temp = block[0] - last_dc_val;
 586   if (temp < 0)
 587     temp = -temp;
 588 
 589   /* Find the number of bits needed for the magnitude of the coefficient */
 590   nbits = 0;
 591   while (temp) {
 592     nbits++;
 593     temp >>= 1;
 594   }
 595   /* Check for out-of-range coefficient values.
 596    * Since we're encoding a difference, the range limit is twice as much.
 597    */
 598   if (nbits > MAX_COEF_BITS+1)
 599     ERREXIT(cinfo, JERR_BAD_DCT_COEF);
 600 
 601   /* Count the Huffman symbol for the number of bits */
 602   dc_counts[nbits]++;
 603 
 604   /* Encode the AC coefficients per section F.1.2.2 */
 605 
 606   r = 0;                        /* r = run length of zeros */
 607 
 608   for (k = 1; k < DCTSIZE2; k++) {
 609     if ((temp = block[jpeg_natural_order[k]]) == 0) {
 610       r++;
 611     } else {
 612       /* if run length > 15, must emit special run-length-16 codes (0xF0) */
 613       while (r > 15) {
 614         ac_counts[0xF0]++;
 615         r -= 16;
 616       }
 617 
 618       /* Find the number of bits needed for the magnitude of the coefficient */
 619       if (temp < 0)
 620         temp = -temp;
 621 
 622       /* Find the number of bits needed for the magnitude of the coefficient */
 623       nbits = 1;                /* there must be at least one 1 bit */
 624       while ((temp >>= 1))
 625         nbits++;
 626       /* Check for out-of-range coefficient values */
 627       if (nbits > MAX_COEF_BITS)
 628         ERREXIT(cinfo, JERR_BAD_DCT_COEF);
 629 
 630       /* Count Huffman symbol for run length / number of bits */
 631       ac_counts[(r << 4) + nbits]++;
 632 
 633       r = 0;
 634     }
 635   }
 636 
 637   /* If the last coef(s) were zero, emit an end-of-block code */
 638   if (r > 0)
 639     ac_counts[0]++;
 640 }
 641 
 642 
 643 /*
 644  * Trial-encode one MCU's worth of Huffman-compressed coefficients.
 645  * No data is actually output, so no suspension return is possible.
 646  */
 647 
 648 METHODDEF(boolean)
 649 encode_mcu_gather (j_compress_ptr cinfo, JBLOCKROW *MCU_data)
 650 {
 651   huff_entropy_ptr entropy = (huff_entropy_ptr) cinfo->entropy;
 652   int blkn, ci;
 653   jpeg_component_info * compptr;
 654 
 655   /* Take care of restart intervals if needed */
 656   if (cinfo->restart_interval) {
 657     if (entropy->restarts_to_go == 0) {
 658       /* Re-initialize DC predictions to 0 */
 659       for (ci = 0; ci < cinfo->comps_in_scan; ci++)
 660         entropy->saved.last_dc_val[ci] = 0;
 661       /* Update restart state */
 662       entropy->restarts_to_go = cinfo->restart_interval;
 663     }
 664     entropy->restarts_to_go--;
 665   }
 666 
 667   for (blkn = 0; blkn < cinfo->blocks_in_MCU; blkn++) {
 668     ci = cinfo->MCU_membership[blkn];
 669     compptr = cinfo->cur_comp_info[ci];
 670     htest_one_block(cinfo, MCU_data[blkn][0], entropy->saved.last_dc_val[ci],
 671                     entropy->dc_count_ptrs[compptr->dc_tbl_no],
 672                     entropy->ac_count_ptrs[compptr->ac_tbl_no]);
 673     entropy->saved.last_dc_val[ci] = MCU_data[blkn][0][0];
 674   }
 675 
 676   return TRUE;
 677 }
 678 
 679 
 680 /*
 681  * Generate the best Huffman code table for the given counts, fill htbl.
 682  * Note this is also used by jcphuff.c.
 683  *
 684  * The JPEG standard requires that no symbol be assigned a codeword of all
 685  * one bits (so that padding bits added at the end of a compressed segment
 686  * can't look like a valid code).  Because of the canonical ordering of
 687  * codewords, this just means that there must be an unused slot in the
 688  * longest codeword length category.  Section K.2 of the JPEG spec suggests
 689  * reserving such a slot by pretending that symbol 256 is a valid symbol
 690  * with count 1.  In theory that's not optimal; giving it count zero but
 691  * including it in the symbol set anyway should give a better Huffman code.
 692  * But the theoretically better code actually seems to come out worse in
 693  * practice, because it produces more all-ones bytes (which incur stuffed
 694  * zero bytes in the final file).  In any case the difference is tiny.
 695  *
 696  * The JPEG standard requires Huffman codes to be no more than 16 bits long.
 697  * If some symbols have a very small but nonzero probability, the Huffman tree
 698  * must be adjusted to meet the code length restriction.  We currently use
 699  * the adjustment method suggested in JPEG section K.2.  This method is *not*
 700  * optimal; it may not choose the best possible limited-length code.  But
 701  * typically only very-low-frequency symbols will be given less-than-optimal
 702  * lengths, so the code is almost optimal.  Experimental comparisons against
 703  * an optimal limited-length-code algorithm indicate that the difference is
 704  * microscopic --- usually less than a hundredth of a percent of total size.
 705  * So the extra complexity of an optimal algorithm doesn't seem worthwhile.
 706  */
 707 
 708 GLOBAL(void)
 709 jpeg_gen_optimal_table (j_compress_ptr cinfo, JHUFF_TBL * htbl, long freq[])
 710 {
 711 #define MAX_CLEN 32             /* assumed maximum initial code length */
 712   UINT8 bits[MAX_CLEN+1];       /* bits[k] = # of symbols with code length k */
 713   int codesize[257];            /* codesize[k] = code length of symbol k */
 714   int others[257];              /* next symbol in current branch of tree */
 715   int c1, c2;
 716   int p, i, j;
 717   long v;
 718 
 719   /* This algorithm is explained in section K.2 of the JPEG standard */
 720 
 721   MEMZERO(bits, SIZEOF(bits));
 722   MEMZERO(codesize, SIZEOF(codesize));
 723   for (i = 0; i < 257; i++)
 724     others[i] = -1;             /* init links to empty */
 725 
 726   freq[256] = 1;                /* make sure 256 has a nonzero count */
 727   /* Including the pseudo-symbol 256 in the Huffman procedure guarantees
 728    * that no real symbol is given code-value of all ones, because 256
 729    * will be placed last in the largest codeword category.
 730    */
 731 
 732   /* Huffman's basic algorithm to assign optimal code lengths to symbols */
 733 
 734   for (;;) {
 735     /* Find the smallest nonzero frequency, set c1 = its symbol */
 736     /* In case of ties, take the larger symbol number */
 737     c1 = -1;
 738     v = 1000000000L;
 739     for (i = 0; i <= 256; i++) {
 740       if (freq[i] && freq[i] <= v) {
 741         v = freq[i];
 742         c1 = i;
 743       }
 744     }
 745 
 746     /* Find the next smallest nonzero frequency, set c2 = its symbol */
 747     /* In case of ties, take the larger symbol number */
 748     c2 = -1;
 749     v = 1000000000L;
 750     for (i = 0; i <= 256; i++) {
 751       if (freq[i] && freq[i] <= v && i != c1) {
 752         v = freq[i];
 753         c2 = i;
 754       }
 755     }
 756 
 757     /* Done if we've merged everything into one frequency */
 758     if (c2 < 0)
 759       break;
 760 
 761     /* Else merge the two counts/trees */
 762     freq[c1] += freq[c2];
 763     freq[c2] = 0;
 764 
 765     /* Increment the codesize of everything in c1's tree branch */
 766     codesize[c1]++;
 767     while (others[c1] >= 0) {
 768       c1 = others[c1];
 769       codesize[c1]++;
 770     }
 771 
 772     others[c1] = c2;            /* chain c2 onto c1's tree branch */
 773 
 774     /* Increment the codesize of everything in c2's tree branch */
 775     codesize[c2]++;
 776     while (others[c2] >= 0) {
 777       c2 = others[c2];
 778       codesize[c2]++;
 779     }
 780   }
 781 
 782   /* Now count the number of symbols of each code length */
 783   for (i = 0; i <= 256; i++) {
 784     if (codesize[i]) {
 785       /* The JPEG standard seems to think that this can't happen, */
 786       /* but I'm paranoid... */
 787       if (codesize[i] > MAX_CLEN)
 788         ERREXIT(cinfo, JERR_HUFF_CLEN_OVERFLOW);
 789 
 790       bits[codesize[i]]++;
 791     }
 792   }
 793 
 794   /* JPEG doesn't allow symbols with code lengths over 16 bits, so if the pure
 795    * Huffman procedure assigned any such lengths, we must adjust the coding.
 796    * Here is what the JPEG spec says about how this next bit works:
 797    * Since symbols are paired for the longest Huffman code, the symbols are
 798    * removed from this length category two at a time.  The prefix for the pair
 799    * (which is one bit shorter) is allocated to one of the pair; then,
 800    * skipping the BITS entry for that prefix length, a code word from the next
 801    * shortest nonzero BITS entry is converted into a prefix for two code words
 802    * one bit longer.
 803    */
 804 
 805   for (i = MAX_CLEN; i > 16; i--) {
 806     while (bits[i] > 0) {
 807       j = i - 2;                /* find length of new prefix to be used */
 808       while ((bits[j] == 0) && (j > 0))
 809         j--;
 810 
 811       bits[i] -= 2;             /* remove two symbols */
 812       bits[i-1]++;              /* one goes in this length */
 813       bits[j+1] += 2;           /* two new symbols in this length */
 814       bits[j]--;                /* symbol of this length is now a prefix */
 815     }
 816   }
 817 
 818   /* Remove the count for the pseudo-symbol 256 from the largest codelength */
 819   while (bits[i] == 0)          /* find largest codelength still in use */
 820     i--;
 821   bits[i]--;
 822 
 823   /* Return final symbol counts (only for lengths 0..16) */
 824   MEMCOPY(htbl->bits, bits, SIZEOF(htbl->bits));
 825 
 826   /* Return a list of the symbols sorted by code length */
 827   /* It's not real clear to me why we don't need to consider the codelength
 828    * changes made above, but the JPEG spec seems to think this works.
 829    */
 830   p = 0;
 831   for (i = 1; i <= MAX_CLEN; i++) {
 832     for (j = 0; j <= 255; j++) {
 833       if (codesize[j] == i) {
 834         htbl->huffval[p] = (UINT8) j;
 835         p++;
 836       }
 837     }
 838   }
 839 
 840   /* Set sent_table FALSE so updated table will be written to JPEG file. */
 841   htbl->sent_table = FALSE;
 842 }
 843 
 844 
 845 /*
 846  * Finish up a statistics-gathering pass and create the new Huffman tables.
 847  */
 848 
 849 METHODDEF(void)
 850 finish_pass_gather (j_compress_ptr cinfo)
 851 {
 852   huff_entropy_ptr entropy = (huff_entropy_ptr) cinfo->entropy;
 853   int ci, dctbl, actbl;
 854   jpeg_component_info * compptr;
 855   JHUFF_TBL **htblptr;
 856   boolean did_dc[NUM_HUFF_TBLS];
 857   boolean did_ac[NUM_HUFF_TBLS];
 858 
 859   /* It's important not to apply jpeg_gen_optimal_table more than once
 860    * per table, because it clobbers the input frequency counts!
 861    */
 862   MEMZERO(did_dc, SIZEOF(did_dc));
 863   MEMZERO(did_ac, SIZEOF(did_ac));
 864 
 865   for (ci = 0; ci < cinfo->comps_in_scan; ci++) {
 866     compptr = cinfo->cur_comp_info[ci];
 867     dctbl = compptr->dc_tbl_no;
 868     actbl = compptr->ac_tbl_no;
 869     if (! did_dc[dctbl]) {
 870       htblptr = & cinfo->dc_huff_tbl_ptrs[dctbl];
 871       if (*htblptr == NULL)
 872         *htblptr = jpeg_alloc_huff_table((j_common_ptr) cinfo);
 873       jpeg_gen_optimal_table(cinfo, *htblptr, entropy->dc_count_ptrs[dctbl]);
 874       did_dc[dctbl] = TRUE;
 875     }
 876     if (! did_ac[actbl]) {
 877       htblptr = & cinfo->ac_huff_tbl_ptrs[actbl];
 878       if (*htblptr == NULL)
 879         *htblptr = jpeg_alloc_huff_table((j_common_ptr) cinfo);
 880       jpeg_gen_optimal_table(cinfo, *htblptr, entropy->ac_count_ptrs[actbl]);
 881       did_ac[actbl] = TRUE;
 882     }
 883   }
 884 }
 885 
 886 
 887 #endif /* ENTROPY_OPT_SUPPORTED */
 888 
 889 
 890 /*
 891  * Module initialization routine for Huffman entropy encoding.
 892  */
 893 
 894 GLOBAL(void)
 895 jinit_huff_encoder (j_compress_ptr cinfo)
 896 {
 897   huff_entropy_ptr entropy;
 898   int i;
 899 
 900   entropy = (huff_entropy_ptr)
 901     (*cinfo->mem->alloc_small) ((j_common_ptr) cinfo, JPOOL_IMAGE,
 902                                 SIZEOF(huff_entropy_encoder));
 903   cinfo->entropy = (struct jpeg_entropy_encoder *) entropy;
 904   entropy->pub.start_pass = start_pass_huff;
 905 
 906   /* Mark tables unallocated */
 907   for (i = 0; i < NUM_HUFF_TBLS; i++) {
 908     entropy->dc_derived_tbls[i] = entropy->ac_derived_tbls[i] = NULL;
 909 #ifdef ENTROPY_OPT_SUPPORTED
 910     entropy->dc_count_ptrs[i] = entropy->ac_count_ptrs[i] = NULL;
 911 #endif
 912   }
 913 }