1 /*
   2  * jchuff.c
   3  *
   4  * Copyright (C) 1991-1997, Thomas G. Lane.
   5  * Modified 2006-2013 by Guido Vollbeding.
   6  * This file is part of the Independent JPEG Group's software.
   7  * For conditions of distribution and use, see the accompanying README file.
   8  *
   9  * This file contains Huffman entropy encoding routines.
  10  * Both sequential and progressive modes are supported in this single module.
  11  *
  12  * Much of the complexity here has to do with supporting output suspension.
  13  * If the data destination module demands suspension, we want to be able to
  14  * back up to the start of the current MCU.  To do this, we copy state
  15  * variables into local working storage, and update them back to the
  16  * permanent JPEG objects only upon successful completion of an MCU.
  17  *
  18  * We do not support output suspension for the progressive JPEG mode, since
  19  * the library currently does not allow multiple-scan files to be written
  20  * with output suspension.
  21  */
  22 
  23 #define JPEG_INTERNALS
  24 #include "jinclude.h"
  25 #include "jpeglib.h"
  26 
  27 
  28 /* The legal range of a DCT coefficient is
  29  *  -1024 .. +1023  for 8-bit data;
  30  * -16384 .. +16383 for 12-bit data.
  31  * Hence the magnitude should always fit in 10 or 14 bits respectively.
  32  */
  33 
  34 #if BITS_IN_JSAMPLE == 8
  35 #define MAX_COEF_BITS 10
  36 #else
  37 #define MAX_COEF_BITS 14
  38 #endif
  39 
  40 /* Derived data constructed for each Huffman table */
  41 
  42 typedef struct {
  43   unsigned int ehufco[256];     /* code for each symbol */
  44   char ehufsi[256];             /* length of code for each symbol */
  45   /* If no code has been allocated for a symbol S, ehufsi[S] contains 0 */
  46 } c_derived_tbl;
  47 
  48 
  49 /* Expanded entropy encoder object for Huffman encoding.
  50  *
  51  * The savable_state subrecord contains fields that change within an MCU,
  52  * but must not be updated permanently until we complete the MCU.
  53  */
  54 
  55 typedef struct {
  56   INT32 put_buffer;             /* current bit-accumulation buffer */
  57   int put_bits;                 /* # of bits now in it */
  58   int last_dc_val[MAX_COMPS_IN_SCAN]; /* last DC coef for each component */
  59 } savable_state;
  60 
  61 /* This macro is to work around compilers with missing or broken
  62  * structure assignment.  You'll need to fix this code if you have
  63  * such a compiler and you change MAX_COMPS_IN_SCAN.
  64  */
  65 
  66 #ifndef NO_STRUCT_ASSIGN
  67 #define ASSIGN_STATE(dest,src)  ((dest) = (src))
  68 #else
  69 #if MAX_COMPS_IN_SCAN == 4
  70 #define ASSIGN_STATE(dest,src)  \
  71         ((dest).put_buffer = (src).put_buffer, \
  72          (dest).put_bits = (src).put_bits, \
  73          (dest).last_dc_val[0] = (src).last_dc_val[0], \
  74          (dest).last_dc_val[1] = (src).last_dc_val[1], \
  75          (dest).last_dc_val[2] = (src).last_dc_val[2], \
  76          (dest).last_dc_val[3] = (src).last_dc_val[3])
  77 #endif
  78 #endif
  79 
  80 
  81 typedef struct {
  82   struct jpeg_entropy_encoder pub; /* public fields */
  83 
  84   savable_state saved;          /* Bit buffer & DC state at start of MCU */
  85 
  86   /* These fields are NOT loaded into local working state. */
  87   unsigned int restarts_to_go;  /* MCUs left in this restart interval */
  88   int next_restart_num;         /* next restart number to write (0-7) */
  89 
  90   /* Pointers to derived tables (these workspaces have image lifespan) */
  91   c_derived_tbl * dc_derived_tbls[NUM_HUFF_TBLS];
  92   c_derived_tbl * ac_derived_tbls[NUM_HUFF_TBLS];
  93 
  94   /* Statistics tables for optimization */
  95   long * dc_count_ptrs[NUM_HUFF_TBLS];
  96   long * ac_count_ptrs[NUM_HUFF_TBLS];
  97 
  98   /* Following fields used only in progressive mode */
  99 
 100   /* Mode flag: TRUE for optimization, FALSE for actual data output */
 101   boolean gather_statistics;
 102 
 103   /* next_output_byte/free_in_buffer are local copies of cinfo->dest fields.
 104    */
 105   JOCTET * next_output_byte;    /* => next byte to write in buffer */
 106   size_t free_in_buffer;        /* # of byte spaces remaining in buffer */
 107   j_compress_ptr cinfo;         /* link to cinfo (needed for dump_buffer) */
 108 
 109   /* Coding status for AC components */
 110   int ac_tbl_no;                /* the table number of the single component */
 111   unsigned int EOBRUN;          /* run length of EOBs */
 112   unsigned int BE;              /* # of buffered correction bits before MCU */
 113   char * bit_buffer;            /* buffer for correction bits (1 per char) */
 114   /* packing correction bits tightly would save some space but cost time... */
 115 } huff_entropy_encoder;
 116 
 117 typedef huff_entropy_encoder * huff_entropy_ptr;
 118 
 119 /* Working state while writing an MCU (sequential mode).
 120  * This struct contains all the fields that are needed by subroutines.
 121  */
 122 
 123 typedef struct {
 124   JOCTET * next_output_byte;    /* => next byte to write in buffer */
 125   size_t free_in_buffer;        /* # of byte spaces remaining in buffer */
 126   savable_state cur;            /* Current bit buffer & DC state */
 127   j_compress_ptr cinfo;         /* dump_buffer needs access to this */
 128 } working_state;
 129 
 130 /* MAX_CORR_BITS is the number of bits the AC refinement correction-bit
 131  * buffer can hold.  Larger sizes may slightly improve compression, but
 132  * 1000 is already well into the realm of overkill.
 133  * The minimum safe size is 64 bits.
 134  */
 135 
 136 #define MAX_CORR_BITS  1000     /* Max # of correction bits I can buffer */
 137 
 138 /* IRIGHT_SHIFT is like RIGHT_SHIFT, but works on int rather than INT32.
 139  * We assume that int right shift is unsigned if INT32 right shift is,
 140  * which should be safe.
 141  */
 142 
 143 #ifdef RIGHT_SHIFT_IS_UNSIGNED
 144 #define ISHIFT_TEMPS    int ishift_temp;
 145 #define IRIGHT_SHIFT(x,shft)  \
 146         ((ishift_temp = (x)) < 0 ? \
 147          (ishift_temp >> (shft)) | ((~0) << (16-(shft))) : \
 148          (ishift_temp >> (shft)))
 149 #else
 150 #define ISHIFT_TEMPS
 151 #define IRIGHT_SHIFT(x,shft)    ((x) >> (shft))
 152 #endif
 153 
 154 
 155 /*
 156  * Compute the derived values for a Huffman table.
 157  * This routine also performs some validation checks on the table.
 158  */
 159 
 160 LOCAL(void)
 161 jpeg_make_c_derived_tbl (j_compress_ptr cinfo, boolean isDC, int tblno,
 162                          c_derived_tbl ** pdtbl)
 163 {
 164   JHUFF_TBL *htbl;
 165   c_derived_tbl *dtbl;
 166   int p, i, l, lastp, si, maxsymbol;
 167   char huffsize[257];
 168   unsigned int huffcode[257];
 169   unsigned int code;
 170 
 171   /* Note that huffsize[] and huffcode[] are filled in code-length order,
 172    * paralleling the order of the symbols themselves in htbl->huffval[].
 173    */
 174 
 175   /* Find the input Huffman table */
 176   if (tblno < 0 || tblno >= NUM_HUFF_TBLS)
 177     ERREXIT1(cinfo, JERR_NO_HUFF_TABLE, tblno);
 178   htbl =
 179     isDC ? cinfo->dc_huff_tbl_ptrs[tblno] : cinfo->ac_huff_tbl_ptrs[tblno];
 180   if (htbl == NULL)
 181     ERREXIT1(cinfo, JERR_NO_HUFF_TABLE, tblno);
 182 
 183   /* Allocate a workspace if we haven't already done so. */
 184   if (*pdtbl == NULL)
 185     *pdtbl = (c_derived_tbl *)
 186       (*cinfo->mem->alloc_small) ((j_common_ptr) cinfo, JPOOL_IMAGE,
 187                                   SIZEOF(c_derived_tbl));
 188   dtbl = *pdtbl;
 189   
 190   /* Figure C.1: make table of Huffman code length for each symbol */
 191 
 192   p = 0;
 193   for (l = 1; l <= 16; l++) {
 194     i = (int) htbl->bits[l];
 195     if (i < 0 || p + i > 256)     /* protect against table overrun */
 196       ERREXIT(cinfo, JERR_BAD_HUFF_TABLE);
 197     while (i--)
 198       huffsize[p++] = (char) l;
 199   }
 200   huffsize[p] = 0;
 201   lastp = p;
 202   
 203   /* Figure C.2: generate the codes themselves */
 204   /* We also validate that the counts represent a legal Huffman code tree. */
 205 
 206   code = 0;
 207   si = huffsize[0];
 208   p = 0;
 209   while (huffsize[p]) {
 210     while (((int) huffsize[p]) == si) {
 211       huffcode[p++] = code;
 212       code++;
 213     }
 214     /* code is now 1 more than the last code used for codelength si; but
 215      * it must still fit in si bits, since no code is allowed to be all ones.
 216      */
 217     if (((INT32) code) >= (((INT32) 1) << si))
 218       ERREXIT(cinfo, JERR_BAD_HUFF_TABLE);
 219     code <<= 1;
 220     si++;
 221   }
 222   
 223   /* Figure C.3: generate encoding tables */
 224   /* These are code and size indexed by symbol value */
 225 
 226   /* Set all codeless symbols to have code length 0;
 227    * this lets us detect duplicate VAL entries here, and later
 228    * allows emit_bits to detect any attempt to emit such symbols.
 229    */
 230   MEMZERO(dtbl->ehufsi, SIZEOF(dtbl->ehufsi));
 231 
 232   /* This is also a convenient place to check for out-of-range
 233    * and duplicated VAL entries.  We allow 0..255 for AC symbols
 234    * but only 0..15 for DC.  (We could constrain them further
 235    * based on data depth and mode, but this seems enough.)
 236    */
 237   maxsymbol = isDC ? 15 : 255;
 238 
 239   for (p = 0; p < lastp; p++) {
 240     i = htbl->huffval[p];
 241     if (i < 0 || i > maxsymbol || dtbl->ehufsi[i])
 242       ERREXIT(cinfo, JERR_BAD_HUFF_TABLE);
 243     dtbl->ehufco[i] = huffcode[p];
 244     dtbl->ehufsi[i] = huffsize[p];
 245   }
 246 }
 247 
 248 
 249 /* Outputting bytes to the file.
 250  * NB: these must be called only when actually outputting,
 251  * that is, entropy->gather_statistics == FALSE.
 252  */
 253 
 254 /* Emit a byte, taking 'action' if must suspend. */
 255 #define emit_byte_s(state,val,action)  \
 256         { *(state)->next_output_byte++ = (JOCTET) (val);  \
 257           if (--(state)->free_in_buffer == 0)  \
 258             if (! dump_buffer_s(state))  \
 259               { action; } }
 260 
 261 /* Emit a byte */
 262 #define emit_byte_e(entropy,val)  \
 263         { *(entropy)->next_output_byte++ = (JOCTET) (val);  \
 264           if (--(entropy)->free_in_buffer == 0)  \
 265             dump_buffer_e(entropy); }
 266 
 267 
 268 LOCAL(boolean)
 269 dump_buffer_s (working_state * state)
 270 /* Empty the output buffer; return TRUE if successful, FALSE if must suspend */
 271 {
 272   struct jpeg_destination_mgr * dest = state->cinfo->dest;
 273 
 274   if (! (*dest->empty_output_buffer) (state->cinfo))
 275     return FALSE;
 276   /* After a successful buffer dump, must reset buffer pointers */
 277   state->next_output_byte = dest->next_output_byte;
 278   state->free_in_buffer = dest->free_in_buffer;
 279   return TRUE;
 280 }
 281 
 282 
 283 LOCAL(void)
 284 dump_buffer_e (huff_entropy_ptr entropy)
 285 /* Empty the output buffer; we do not support suspension in this case. */
 286 {
 287   struct jpeg_destination_mgr * dest = entropy->cinfo->dest;
 288 
 289   if (! (*dest->empty_output_buffer) (entropy->cinfo))
 290     ERREXIT(entropy->cinfo, JERR_CANT_SUSPEND);
 291   /* After a successful buffer dump, must reset buffer pointers */
 292   entropy->next_output_byte = dest->next_output_byte;
 293   entropy->free_in_buffer = dest->free_in_buffer;
 294 }
 295 
 296 
 297 /* Outputting bits to the file */
 298 
 299 /* Only the right 24 bits of put_buffer are used; the valid bits are
 300  * left-justified in this part.  At most 16 bits can be passed to emit_bits
 301  * in one call, and we never retain more than 7 bits in put_buffer
 302  * between calls, so 24 bits are sufficient.
 303  */
 304 
 305 INLINE
 306 LOCAL(boolean)
 307 emit_bits_s (working_state * state, unsigned int code, int size)
 308 /* Emit some bits; return TRUE if successful, FALSE if must suspend */
 309 {
 310   /* This routine is heavily used, so it's worth coding tightly. */
 311   register INT32 put_buffer;
 312   register int put_bits;
 313 
 314   /* if size is 0, caller used an invalid Huffman table entry */
 315   if (size == 0)
 316     ERREXIT(state->cinfo, JERR_HUFF_MISSING_CODE);
 317 
 318   /* mask off any extra bits in code */
 319   put_buffer = ((INT32) code) & ((((INT32) 1) << size) - 1);
 320 
 321   /* new number of bits in buffer */
 322   put_bits = size + state->cur.put_bits;
 323 
 324   put_buffer <<= 24 - put_bits; /* align incoming bits */
 325 
 326   /* and merge with old buffer contents */
 327   put_buffer |= state->cur.put_buffer;
 328 
 329   while (put_bits >= 8) {
 330     int c = (int) ((put_buffer >> 16) & 0xFF);
 331 
 332     emit_byte_s(state, c, return FALSE);
 333     if (c == 0xFF) {            /* need to stuff a zero byte? */
 334       emit_byte_s(state, 0, return FALSE);
 335     }
 336     put_buffer <<= 8;
 337     put_bits -= 8;
 338   }
 339 
 340   state->cur.put_buffer = put_buffer; /* update state variables */
 341   state->cur.put_bits = put_bits;
 342 
 343   return TRUE;
 344 }
 345 
 346 
 347 INLINE
 348 LOCAL(void)
 349 emit_bits_e (huff_entropy_ptr entropy, unsigned int code, int size)
 350 /* Emit some bits, unless we are in gather mode */
 351 {
 352   /* This routine is heavily used, so it's worth coding tightly. */
 353   register INT32 put_buffer;
 354   register int put_bits;
 355 
 356   /* if size is 0, caller used an invalid Huffman table entry */
 357   if (size == 0)
 358     ERREXIT(entropy->cinfo, JERR_HUFF_MISSING_CODE);
 359 
 360   if (entropy->gather_statistics)
 361     return;                     /* do nothing if we're only getting stats */
 362 
 363   /* mask off any extra bits in code */
 364   put_buffer = ((INT32) code) & ((((INT32) 1) << size) - 1);
 365 
 366   /* new number of bits in buffer */
 367   put_bits = size + entropy->saved.put_bits;
 368 
 369   put_buffer <<= 24 - put_bits; /* align incoming bits */
 370 
 371   /* and merge with old buffer contents */
 372   put_buffer |= entropy->saved.put_buffer;
 373 
 374   while (put_bits >= 8) {
 375     int c = (int) ((put_buffer >> 16) & 0xFF);
 376 
 377     emit_byte_e(entropy, c);
 378     if (c == 0xFF) {            /* need to stuff a zero byte? */
 379       emit_byte_e(entropy, 0);
 380     }
 381     put_buffer <<= 8;
 382     put_bits -= 8;
 383   }
 384 
 385   entropy->saved.put_buffer = put_buffer; /* update variables */
 386   entropy->saved.put_bits = put_bits;
 387 }
 388 
 389 
 390 LOCAL(boolean)
 391 flush_bits_s (working_state * state)
 392 {
 393   if (! emit_bits_s(state, 0x7F, 7)) /* fill any partial byte with ones */
 394     return FALSE;
 395   state->cur.put_buffer = 0;      /* and reset bit-buffer to empty */
 396   state->cur.put_bits = 0;
 397   return TRUE;
 398 }
 399 
 400 
 401 LOCAL(void)
 402 flush_bits_e (huff_entropy_ptr entropy)
 403 {
 404   emit_bits_e(entropy, 0x7F, 7); /* fill any partial byte with ones */
 405   entropy->saved.put_buffer = 0; /* and reset bit-buffer to empty */
 406   entropy->saved.put_bits = 0;
 407 }
 408 
 409 
 410 /*
 411  * Emit (or just count) a Huffman symbol.
 412  */
 413 
 414 INLINE
 415 LOCAL(void)
 416 emit_dc_symbol (huff_entropy_ptr entropy, int tbl_no, int symbol)
 417 {
 418   if (entropy->gather_statistics)
 419     entropy->dc_count_ptrs[tbl_no][symbol]++;
 420   else {
 421     c_derived_tbl * tbl = entropy->dc_derived_tbls[tbl_no];
 422     emit_bits_e(entropy, tbl->ehufco[symbol], tbl->ehufsi[symbol]);
 423   }
 424 }
 425 
 426 
 427 INLINE
 428 LOCAL(void)
 429 emit_ac_symbol (huff_entropy_ptr entropy, int tbl_no, int symbol)
 430 {
 431   if (entropy->gather_statistics)
 432     entropy->ac_count_ptrs[tbl_no][symbol]++;
 433   else {
 434     c_derived_tbl * tbl = entropy->ac_derived_tbls[tbl_no];
 435     emit_bits_e(entropy, tbl->ehufco[symbol], tbl->ehufsi[symbol]);
 436   }
 437 }
 438 
 439 
 440 /*
 441  * Emit bits from a correction bit buffer.
 442  */
 443 
 444 LOCAL(void)
 445 emit_buffered_bits (huff_entropy_ptr entropy, char * bufstart,
 446                     unsigned int nbits)
 447 {
 448   if (entropy->gather_statistics)
 449     return;                     /* no real work */
 450 
 451   while (nbits > 0) {
 452     emit_bits_e(entropy, (unsigned int) (*bufstart), 1);
 453     bufstart++;
 454     nbits--;
 455   }
 456 }
 457 
 458 
 459 /*
 460  * Emit any pending EOBRUN symbol.
 461  */
 462 
 463 LOCAL(void)
 464 emit_eobrun (huff_entropy_ptr entropy)
 465 {
 466   register int temp, nbits;
 467 
 468   if (entropy->EOBRUN > 0) {      /* if there is any pending EOBRUN */
 469     temp = entropy->EOBRUN;
 470     nbits = 0;
 471     while ((temp >>= 1))
 472       nbits++;
 473     /* safety check: shouldn't happen given limited correction-bit buffer */
 474     if (nbits > 14)
 475       ERREXIT(entropy->cinfo, JERR_HUFF_MISSING_CODE);
 476 
 477     emit_ac_symbol(entropy, entropy->ac_tbl_no, nbits << 4);
 478     if (nbits)
 479       emit_bits_e(entropy, entropy->EOBRUN, nbits);
 480 
 481     entropy->EOBRUN = 0;
 482 
 483     /* Emit any buffered correction bits */
 484     emit_buffered_bits(entropy, entropy->bit_buffer, entropy->BE);
 485     entropy->BE = 0;
 486   }
 487 }
 488 
 489 
 490 /*
 491  * Emit a restart marker & resynchronize predictions.
 492  */
 493 
 494 LOCAL(boolean)
 495 emit_restart_s (working_state * state, int restart_num)
 496 {
 497   int ci;
 498 
 499   if (! flush_bits_s(state))
 500     return FALSE;
 501 
 502   emit_byte_s(state, 0xFF, return FALSE);
 503   emit_byte_s(state, JPEG_RST0 + restart_num, return FALSE);
 504 
 505   /* Re-initialize DC predictions to 0 */
 506   for (ci = 0; ci < state->cinfo->comps_in_scan; ci++)
 507     state->cur.last_dc_val[ci] = 0;
 508 
 509   /* The restart counter is not updated until we successfully write the MCU. */
 510 
 511   return TRUE;
 512 }
 513 
 514 
 515 LOCAL(void)
 516 emit_restart_e (huff_entropy_ptr entropy, int restart_num)
 517 {
 518   int ci;
 519 
 520   emit_eobrun(entropy);
 521 
 522   if (! entropy->gather_statistics) {
 523     flush_bits_e(entropy);
 524     emit_byte_e(entropy, 0xFF);
 525     emit_byte_e(entropy, JPEG_RST0 + restart_num);
 526   }
 527 
 528   if (entropy->cinfo->Ss == 0) {
 529     /* Re-initialize DC predictions to 0 */
 530     for (ci = 0; ci < entropy->cinfo->comps_in_scan; ci++)
 531       entropy->saved.last_dc_val[ci] = 0;
 532   } else {
 533     /* Re-initialize all AC-related fields to 0 */
 534     entropy->EOBRUN = 0;
 535     entropy->BE = 0;
 536   }
 537 }
 538 
 539 
 540 /*
 541  * MCU encoding for DC initial scan (either spectral selection,
 542  * or first pass of successive approximation).
 543  */
 544 
 545 METHODDEF(boolean)
 546 encode_mcu_DC_first (j_compress_ptr cinfo, JBLOCKROW *MCU_data)
 547 {
 548   huff_entropy_ptr entropy = (huff_entropy_ptr) cinfo->entropy;
 549   register int temp, temp2;
 550   register int nbits;
 551   int blkn, ci, tbl;
 552   ISHIFT_TEMPS
 553 
 554   entropy->next_output_byte = cinfo->dest->next_output_byte;
 555   entropy->free_in_buffer = cinfo->dest->free_in_buffer;
 556 
 557   /* Emit restart marker if needed */
 558   if (cinfo->restart_interval)
 559     if (entropy->restarts_to_go == 0)
 560       emit_restart_e(entropy, entropy->next_restart_num);
 561 
 562   /* Encode the MCU data blocks */
 563   for (blkn = 0; blkn < cinfo->blocks_in_MCU; blkn++) {
 564     ci = cinfo->MCU_membership[blkn];
 565     tbl = cinfo->cur_comp_info[ci]->dc_tbl_no;
 566 
 567     /* Compute the DC value after the required point transform by Al.
 568      * This is simply an arithmetic right shift.
 569      */
 570     temp = IRIGHT_SHIFT((int) (MCU_data[blkn][0][0]), cinfo->Al);
 571 
 572     /* DC differences are figured on the point-transformed values. */
 573     temp2 = temp - entropy->saved.last_dc_val[ci];
 574     entropy->saved.last_dc_val[ci] = temp;
 575 
 576     /* Encode the DC coefficient difference per section G.1.2.1 */
 577     temp = temp2;
 578     if (temp < 0) {
 579       temp = -temp;             /* temp is abs value of input */
 580       /* For a negative input, want temp2 = bitwise complement of abs(input) */
 581       /* This code assumes we are on a two's complement machine */
 582       temp2--;
 583     }
 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/emit the Huffman-coded symbol for the number of bits */
 598     emit_dc_symbol(entropy, tbl, nbits);
 599 
 600     /* Emit that number of bits of the value, if positive, */
 601     /* or the complement of its magnitude, if negative. */
 602     if (nbits)                  /* emit_bits rejects calls with size 0 */
 603       emit_bits_e(entropy, (unsigned int) temp2, nbits);
 604   }
 605 
 606   cinfo->dest->next_output_byte = entropy->next_output_byte;
 607   cinfo->dest->free_in_buffer = entropy->free_in_buffer;
 608 
 609   /* Update restart-interval state too */
 610   if (cinfo->restart_interval) {
 611     if (entropy->restarts_to_go == 0) {
 612       entropy->restarts_to_go = cinfo->restart_interval;
 613       entropy->next_restart_num++;
 614       entropy->next_restart_num &= 7;
 615     }
 616     entropy->restarts_to_go--;
 617   }
 618 
 619   return TRUE;
 620 }
 621 
 622 
 623 /*
 624  * MCU encoding for AC initial scan (either spectral selection,
 625  * or first pass of successive approximation).
 626  */
 627 
 628 METHODDEF(boolean)
 629 encode_mcu_AC_first (j_compress_ptr cinfo, JBLOCKROW *MCU_data)
 630 {
 631   huff_entropy_ptr entropy = (huff_entropy_ptr) cinfo->entropy;
 632   const int * natural_order;
 633   JBLOCKROW block;
 634   register int temp, temp2;
 635   register int nbits;
 636   register int r, k;
 637   int Se, Al;
 638 
 639   entropy->next_output_byte = cinfo->dest->next_output_byte;
 640   entropy->free_in_buffer = cinfo->dest->free_in_buffer;
 641 
 642   /* Emit restart marker if needed */
 643   if (cinfo->restart_interval)
 644     if (entropy->restarts_to_go == 0)
 645       emit_restart_e(entropy, entropy->next_restart_num);
 646 
 647   Se = cinfo->Se;
 648   Al = cinfo->Al;
 649   natural_order = cinfo->natural_order;
 650 
 651   /* Encode the MCU data block */
 652   block = MCU_data[0];
 653 
 654   /* Encode the AC coefficients per section G.1.2.2, fig. G.3 */
 655   
 656   r = 0;                        /* r = run length of zeros */
 657    
 658   for (k = cinfo->Ss; k <= Se; k++) {
 659     if ((temp = (*block)[natural_order[k]]) == 0) {
 660       r++;
 661       continue;
 662     }
 663     /* We must apply the point transform by Al.  For AC coefficients this
 664      * is an integer division with rounding towards 0.  To do this portably
 665      * in C, we shift after obtaining the absolute value; so the code is
 666      * interwoven with finding the abs value (temp) and output bits (temp2).
 667      */
 668     if (temp < 0) {
 669       temp = -temp;             /* temp is abs value of input */
 670       temp >>= Al;                /* apply the point transform */
 671       /* For a negative coef, want temp2 = bitwise complement of abs(coef) */
 672       temp2 = ~temp;
 673     } else {
 674       temp >>= Al;                /* apply the point transform */
 675       temp2 = temp;
 676     }
 677     /* Watch out for case that nonzero coef is zero after point transform */
 678     if (temp == 0) {
 679       r++;
 680       continue;
 681     }
 682 
 683     /* Emit any pending EOBRUN */
 684     if (entropy->EOBRUN > 0)
 685       emit_eobrun(entropy);
 686     /* if run length > 15, must emit special run-length-16 codes (0xF0) */
 687     while (r > 15) {
 688       emit_ac_symbol(entropy, entropy->ac_tbl_no, 0xF0);
 689       r -= 16;
 690     }
 691 
 692     /* Find the number of bits needed for the magnitude of the coefficient */
 693     nbits = 1;                  /* there must be at least one 1 bit */
 694     while ((temp >>= 1))
 695       nbits++;
 696     /* Check for out-of-range coefficient values */
 697     if (nbits > MAX_COEF_BITS)
 698       ERREXIT(cinfo, JERR_BAD_DCT_COEF);
 699 
 700     /* Count/emit Huffman symbol for run length / number of bits */
 701     emit_ac_symbol(entropy, entropy->ac_tbl_no, (r << 4) + nbits);
 702 
 703     /* Emit that number of bits of the value, if positive, */
 704     /* or the complement of its magnitude, if negative. */
 705     emit_bits_e(entropy, (unsigned int) temp2, nbits);
 706 
 707     r = 0;                      /* reset zero run length */
 708   }
 709 
 710   if (r > 0) {                       /* If there are trailing zeroes, */
 711     entropy->EOBRUN++;               /* count an EOB */
 712     if (entropy->EOBRUN == 0x7FFF)
 713       emit_eobrun(entropy);     /* force it out to avoid overflow */
 714   }
 715 
 716   cinfo->dest->next_output_byte = entropy->next_output_byte;
 717   cinfo->dest->free_in_buffer = entropy->free_in_buffer;
 718 
 719   /* Update restart-interval state too */
 720   if (cinfo->restart_interval) {
 721     if (entropy->restarts_to_go == 0) {
 722       entropy->restarts_to_go = cinfo->restart_interval;
 723       entropy->next_restart_num++;
 724       entropy->next_restart_num &= 7;
 725     }
 726     entropy->restarts_to_go--;
 727   }
 728 
 729   return TRUE;
 730 }
 731 
 732 
 733 /*
 734  * MCU encoding for DC successive approximation refinement scan.
 735  * Note: we assume such scans can be multi-component,
 736  * although the spec is not very clear on the point.
 737  */
 738 
 739 METHODDEF(boolean)
 740 encode_mcu_DC_refine (j_compress_ptr cinfo, JBLOCKROW *MCU_data)
 741 {
 742   huff_entropy_ptr entropy = (huff_entropy_ptr) cinfo->entropy;
 743   int Al, blkn;
 744 
 745   entropy->next_output_byte = cinfo->dest->next_output_byte;
 746   entropy->free_in_buffer = cinfo->dest->free_in_buffer;
 747 
 748   /* Emit restart marker if needed */
 749   if (cinfo->restart_interval)
 750     if (entropy->restarts_to_go == 0)
 751       emit_restart_e(entropy, entropy->next_restart_num);
 752 
 753   Al = cinfo->Al;
 754 
 755   /* Encode the MCU data blocks */
 756   for (blkn = 0; blkn < cinfo->blocks_in_MCU; blkn++) {
 757     /* We simply emit the Al'th bit of the DC coefficient value. */
 758     emit_bits_e(entropy, (unsigned int) (MCU_data[blkn][0][0] >> Al), 1);
 759   }
 760 
 761   cinfo->dest->next_output_byte = entropy->next_output_byte;
 762   cinfo->dest->free_in_buffer = entropy->free_in_buffer;
 763 
 764   /* Update restart-interval state too */
 765   if (cinfo->restart_interval) {
 766     if (entropy->restarts_to_go == 0) {
 767       entropy->restarts_to_go = cinfo->restart_interval;
 768       entropy->next_restart_num++;
 769       entropy->next_restart_num &= 7;
 770     }
 771     entropy->restarts_to_go--;
 772   }
 773 
 774   return TRUE;
 775 }
 776 
 777 
 778 /*
 779  * MCU encoding for AC successive approximation refinement scan.
 780  */
 781 
 782 METHODDEF(boolean)
 783 encode_mcu_AC_refine (j_compress_ptr cinfo, JBLOCKROW *MCU_data)
 784 {
 785   huff_entropy_ptr entropy = (huff_entropy_ptr) cinfo->entropy;
 786   const int * natural_order;
 787   JBLOCKROW block;
 788   register int temp;
 789   register int r, k;
 790   int Se, Al;
 791   int EOB;
 792   char *BR_buffer;
 793   unsigned int BR;
 794   int absvalues[DCTSIZE2];
 795 
 796   entropy->next_output_byte = cinfo->dest->next_output_byte;
 797   entropy->free_in_buffer = cinfo->dest->free_in_buffer;
 798 
 799   /* Emit restart marker if needed */
 800   if (cinfo->restart_interval)
 801     if (entropy->restarts_to_go == 0)
 802       emit_restart_e(entropy, entropy->next_restart_num);
 803 
 804   Se = cinfo->Se;
 805   Al = cinfo->Al;
 806   natural_order = cinfo->natural_order;
 807 
 808   /* Encode the MCU data block */
 809   block = MCU_data[0];
 810 
 811   /* It is convenient to make a pre-pass to determine the transformed
 812    * coefficients' absolute values and the EOB position.
 813    */
 814   EOB = 0;
 815   for (k = cinfo->Ss; k <= Se; k++) {
 816     temp = (*block)[natural_order[k]];
 817     /* We must apply the point transform by Al.  For AC coefficients this
 818      * is an integer division with rounding towards 0.  To do this portably
 819      * in C, we shift after obtaining the absolute value.
 820      */
 821     if (temp < 0)
 822       temp = -temp;             /* temp is abs value of input */
 823     temp >>= Al;          /* apply the point transform */
 824     absvalues[k] = temp;        /* save abs value for main pass */
 825     if (temp == 1)
 826       EOB = k;                  /* EOB = index of last newly-nonzero coef */
 827   }
 828 
 829   /* Encode the AC coefficients per section G.1.2.3, fig. G.7 */
 830   
 831   r = 0;                        /* r = run length of zeros */
 832   BR = 0;                       /* BR = count of buffered bits added now */
 833   BR_buffer = entropy->bit_buffer + entropy->BE; /* Append bits to buffer */
 834 
 835   for (k = cinfo->Ss; k <= Se; k++) {
 836     if ((temp = absvalues[k]) == 0) {
 837       r++;
 838       continue;
 839     }
 840 
 841     /* Emit any required ZRLs, but not if they can be folded into EOB */
 842     while (r > 15 && k <= EOB) {
 843       /* emit any pending EOBRUN and the BE correction bits */
 844       emit_eobrun(entropy);
 845       /* Emit ZRL */
 846       emit_ac_symbol(entropy, entropy->ac_tbl_no, 0xF0);
 847       r -= 16;
 848       /* Emit buffered correction bits that must be associated with ZRL */
 849       emit_buffered_bits(entropy, BR_buffer, BR);
 850       BR_buffer = entropy->bit_buffer; /* BE bits are gone now */
 851       BR = 0;
 852     }
 853 
 854     /* If the coef was previously nonzero, it only needs a correction bit.
 855      * NOTE: a straight translation of the spec's figure G.7 would suggest
 856      * that we also need to test r > 15.  But if r > 15, we can only get here
 857      * if k > EOB, which implies that this coefficient is not 1.
 858      */
 859     if (temp > 1) {
 860       /* The correction bit is the next bit of the absolute value. */
 861       BR_buffer[BR++] = (char) (temp & 1);
 862       continue;
 863     }
 864 
 865     /* Emit any pending EOBRUN and the BE correction bits */
 866     emit_eobrun(entropy);
 867 
 868     /* Count/emit Huffman symbol for run length / number of bits */
 869     emit_ac_symbol(entropy, entropy->ac_tbl_no, (r << 4) + 1);
 870 
 871     /* Emit output bit for newly-nonzero coef */
 872     temp = ((*block)[natural_order[k]] < 0) ? 0 : 1;
 873     emit_bits_e(entropy, (unsigned int) temp, 1);
 874 
 875     /* Emit buffered correction bits that must be associated with this code */
 876     emit_buffered_bits(entropy, BR_buffer, BR);
 877     BR_buffer = entropy->bit_buffer; /* BE bits are gone now */
 878     BR = 0;
 879     r = 0;                      /* reset zero run length */
 880   }
 881 
 882   if (r > 0 || BR > 0) {  /* If there are trailing zeroes, */
 883     entropy->EOBRUN++;               /* count an EOB */
 884     entropy->BE += BR;               /* concat my correction bits to older ones */
 885     /* We force out the EOB if we risk either:
 886      * 1. overflow of the EOB counter;
 887      * 2. overflow of the correction bit buffer during the next MCU.
 888      */
 889     if (entropy->EOBRUN == 0x7FFF || entropy->BE > (MAX_CORR_BITS-DCTSIZE2+1))
 890       emit_eobrun(entropy);
 891   }
 892 
 893   cinfo->dest->next_output_byte = entropy->next_output_byte;
 894   cinfo->dest->free_in_buffer = entropy->free_in_buffer;
 895 
 896   /* Update restart-interval state too */
 897   if (cinfo->restart_interval) {
 898     if (entropy->restarts_to_go == 0) {
 899       entropy->restarts_to_go = cinfo->restart_interval;
 900       entropy->next_restart_num++;
 901       entropy->next_restart_num &= 7;
 902     }
 903     entropy->restarts_to_go--;
 904   }
 905 
 906   return TRUE;
 907 }
 908 
 909 
 910 /* Encode a single block's worth of coefficients */
 911 
 912 LOCAL(boolean)
 913 encode_one_block (working_state * state, JCOEFPTR block, int last_dc_val,
 914                   c_derived_tbl *dctbl, c_derived_tbl *actbl)
 915 {
 916   register int temp, temp2;
 917   register int nbits;
 918   register int r, k;
 919   int Se = state->cinfo->lim_Se;
 920   const int * natural_order = state->cinfo->natural_order;
 921 
 922   /* Encode the DC coefficient difference per section F.1.2.1 */
 923 
 924   temp = temp2 = block[0] - last_dc_val;
 925 
 926   if (temp < 0) {
 927     temp = -temp;               /* temp is abs value of input */
 928     /* For a negative input, want temp2 = bitwise complement of abs(input) */
 929     /* This code assumes we are on a two's complement machine */
 930     temp2--;
 931   }
 932 
 933   /* Find the number of bits needed for the magnitude of the coefficient */
 934   nbits = 0;
 935   while (temp) {
 936     nbits++;
 937     temp >>= 1;
 938   }
 939   /* Check for out-of-range coefficient values.
 940    * Since we're encoding a difference, the range limit is twice as much.
 941    */
 942   if (nbits > MAX_COEF_BITS+1)
 943     ERREXIT(state->cinfo, JERR_BAD_DCT_COEF);
 944 
 945   /* Emit the Huffman-coded symbol for the number of bits */
 946   if (! emit_bits_s(state, dctbl->ehufco[nbits], dctbl->ehufsi[nbits]))
 947     return FALSE;
 948 
 949   /* Emit that number of bits of the value, if positive, */
 950   /* or the complement of its magnitude, if negative. */
 951   if (nbits)                    /* emit_bits rejects calls with size 0 */
 952     if (! emit_bits_s(state, (unsigned int) temp2, nbits))
 953       return FALSE;
 954 
 955   /* Encode the AC coefficients per section F.1.2.2 */
 956 
 957   r = 0;                        /* r = run length of zeros */
 958 
 959   for (k = 1; k <= Se; k++) {
 960     if ((temp2 = block[natural_order[k]]) == 0) {
 961       r++;
 962     } else {
 963       /* if run length > 15, must emit special run-length-16 codes (0xF0) */
 964       while (r > 15) {
 965         if (! emit_bits_s(state, actbl->ehufco[0xF0], actbl->ehufsi[0xF0]))
 966           return FALSE;
 967         r -= 16;
 968       }
 969 
 970       temp = temp2;
 971       if (temp < 0) {
 972         temp = -temp;           /* temp is abs value of input */
 973         /* This code assumes we are on a two's complement machine */
 974         temp2--;
 975       }
 976 
 977       /* Find the number of bits needed for the magnitude of the coefficient */
 978       nbits = 1;                /* there must be at least one 1 bit */
 979       while ((temp >>= 1))
 980         nbits++;
 981       /* Check for out-of-range coefficient values */
 982       if (nbits > MAX_COEF_BITS)
 983         ERREXIT(state->cinfo, JERR_BAD_DCT_COEF);
 984 
 985       /* Emit Huffman symbol for run length / number of bits */
 986       temp = (r << 4) + nbits;
 987       if (! emit_bits_s(state, actbl->ehufco[temp], actbl->ehufsi[temp]))
 988         return FALSE;
 989 
 990       /* Emit that number of bits of the value, if positive, */
 991       /* or the complement of its magnitude, if negative. */
 992       if (! emit_bits_s(state, (unsigned int) temp2, nbits))
 993         return FALSE;
 994 
 995       r = 0;
 996     }
 997   }
 998 
 999   /* If the last coef(s) were zero, emit an end-of-block code */
1000   if (r > 0)
1001     if (! emit_bits_s(state, actbl->ehufco[0], actbl->ehufsi[0]))
1002       return FALSE;
1003 
1004   return TRUE;
1005 }
1006 
1007 
1008 /*
1009  * Encode and output one MCU's worth of Huffman-compressed coefficients.
1010  */
1011 
1012 METHODDEF(boolean)
1013 encode_mcu_huff (j_compress_ptr cinfo, JBLOCKROW *MCU_data)
1014 {
1015   huff_entropy_ptr entropy = (huff_entropy_ptr) cinfo->entropy;
1016   working_state state;
1017   int blkn, ci;
1018   jpeg_component_info * compptr;
1019 
1020   /* Load up working state */
1021   state.next_output_byte = cinfo->dest->next_output_byte;
1022   state.free_in_buffer = cinfo->dest->free_in_buffer;
1023   ASSIGN_STATE(state.cur, entropy->saved);
1024   state.cinfo = cinfo;
1025 
1026   /* Emit restart marker if needed */
1027   if (cinfo->restart_interval) {
1028     if (entropy->restarts_to_go == 0)
1029       if (! emit_restart_s(&state, entropy->next_restart_num))
1030         return FALSE;
1031   }
1032 
1033   /* Encode the MCU data blocks */
1034   for (blkn = 0; blkn < cinfo->blocks_in_MCU; blkn++) {
1035     ci = cinfo->MCU_membership[blkn];
1036     compptr = cinfo->cur_comp_info[ci];
1037     if (! encode_one_block(&state,
1038                            MCU_data[blkn][0], state.cur.last_dc_val[ci],
1039                            entropy->dc_derived_tbls[compptr->dc_tbl_no],
1040                            entropy->ac_derived_tbls[compptr->ac_tbl_no]))
1041       return FALSE;
1042     /* Update last_dc_val */
1043     state.cur.last_dc_val[ci] = MCU_data[blkn][0][0];
1044   }
1045 
1046   /* Completed MCU, so update state */
1047   cinfo->dest->next_output_byte = state.next_output_byte;
1048   cinfo->dest->free_in_buffer = state.free_in_buffer;
1049   ASSIGN_STATE(entropy->saved, state.cur);
1050 
1051   /* Update restart-interval state too */
1052   if (cinfo->restart_interval) {
1053     if (entropy->restarts_to_go == 0) {
1054       entropy->restarts_to_go = cinfo->restart_interval;
1055       entropy->next_restart_num++;
1056       entropy->next_restart_num &= 7;
1057     }
1058     entropy->restarts_to_go--;
1059   }
1060 
1061   return TRUE;
1062 }
1063 
1064 
1065 /*
1066  * Finish up at the end of a Huffman-compressed scan.
1067  */
1068 
1069 METHODDEF(void)
1070 finish_pass_huff (j_compress_ptr cinfo)
1071 {
1072   huff_entropy_ptr entropy = (huff_entropy_ptr) cinfo->entropy;
1073   working_state state;
1074 
1075   if (cinfo->progressive_mode) {
1076     entropy->next_output_byte = cinfo->dest->next_output_byte;
1077     entropy->free_in_buffer = cinfo->dest->free_in_buffer;
1078 
1079     /* Flush out any buffered data */
1080     emit_eobrun(entropy);
1081     flush_bits_e(entropy);
1082 
1083     cinfo->dest->next_output_byte = entropy->next_output_byte;
1084     cinfo->dest->free_in_buffer = entropy->free_in_buffer;
1085   } else {
1086     /* Load up working state ... flush_bits needs it */
1087     state.next_output_byte = cinfo->dest->next_output_byte;
1088     state.free_in_buffer = cinfo->dest->free_in_buffer;
1089     ASSIGN_STATE(state.cur, entropy->saved);
1090     state.cinfo = cinfo;
1091 
1092     /* Flush out the last data */
1093     if (! flush_bits_s(&state))
1094       ERREXIT(cinfo, JERR_CANT_SUSPEND);
1095 
1096     /* Update state */
1097     cinfo->dest->next_output_byte = state.next_output_byte;
1098     cinfo->dest->free_in_buffer = state.free_in_buffer;
1099     ASSIGN_STATE(entropy->saved, state.cur);
1100   }
1101 }
1102 
1103 
1104 /*
1105  * Huffman coding optimization.
1106  *
1107  * We first scan the supplied data and count the number of uses of each symbol
1108  * that is to be Huffman-coded. (This process MUST agree with the code above.)
1109  * Then we build a Huffman coding tree for the observed counts.
1110  * Symbols which are not needed at all for the particular image are not
1111  * assigned any code, which saves space in the DHT marker as well as in
1112  * the compressed data.
1113  */
1114 
1115 
1116 /* Process a single block's worth of coefficients */
1117 
1118 LOCAL(void)
1119 htest_one_block (j_compress_ptr cinfo, JCOEFPTR block, int last_dc_val,
1120                  long dc_counts[], long ac_counts[])
1121 {
1122   register int temp;
1123   register int nbits;
1124   register int r, k;
1125   int Se = cinfo->lim_Se;
1126   const int * natural_order = cinfo->natural_order;
1127 
1128   /* Encode the DC coefficient difference per section F.1.2.1 */
1129 
1130   temp = block[0] - last_dc_val;
1131   if (temp < 0)
1132     temp = -temp;
1133 
1134   /* Find the number of bits needed for the magnitude of the coefficient */
1135   nbits = 0;
1136   while (temp) {
1137     nbits++;
1138     temp >>= 1;
1139   }
1140   /* Check for out-of-range coefficient values.
1141    * Since we're encoding a difference, the range limit is twice as much.
1142    */
1143   if (nbits > MAX_COEF_BITS+1)
1144     ERREXIT(cinfo, JERR_BAD_DCT_COEF);
1145 
1146   /* Count the Huffman symbol for the number of bits */
1147   dc_counts[nbits]++;
1148 
1149   /* Encode the AC coefficients per section F.1.2.2 */
1150 
1151   r = 0;                        /* r = run length of zeros */
1152 
1153   for (k = 1; k <= Se; k++) {
1154     if ((temp = block[natural_order[k]]) == 0) {
1155       r++;
1156     } else {
1157       /* if run length > 15, must emit special run-length-16 codes (0xF0) */
1158       while (r > 15) {
1159         ac_counts[0xF0]++;
1160         r -= 16;
1161       }
1162 
1163       /* Find the number of bits needed for the magnitude of the coefficient */
1164       if (temp < 0)
1165         temp = -temp;
1166 
1167       /* Find the number of bits needed for the magnitude of the coefficient */
1168       nbits = 1;                /* there must be at least one 1 bit */
1169       while ((temp >>= 1))
1170         nbits++;
1171       /* Check for out-of-range coefficient values */
1172       if (nbits > MAX_COEF_BITS)
1173         ERREXIT(cinfo, JERR_BAD_DCT_COEF);
1174 
1175       /* Count Huffman symbol for run length / number of bits */
1176       ac_counts[(r << 4) + nbits]++;
1177 
1178       r = 0;
1179     }
1180   }
1181 
1182   /* If the last coef(s) were zero, emit an end-of-block code */
1183   if (r > 0)
1184     ac_counts[0]++;
1185 }
1186 
1187 
1188 /*
1189  * Trial-encode one MCU's worth of Huffman-compressed coefficients.
1190  * No data is actually output, so no suspension return is possible.
1191  */
1192 
1193 METHODDEF(boolean)
1194 encode_mcu_gather (j_compress_ptr cinfo, JBLOCKROW *MCU_data)
1195 {
1196   huff_entropy_ptr entropy = (huff_entropy_ptr) cinfo->entropy;
1197   int blkn, ci;
1198   jpeg_component_info * compptr;
1199 
1200   /* Take care of restart intervals if needed */
1201   if (cinfo->restart_interval) {
1202     if (entropy->restarts_to_go == 0) {
1203       /* Re-initialize DC predictions to 0 */
1204       for (ci = 0; ci < cinfo->comps_in_scan; ci++)
1205         entropy->saved.last_dc_val[ci] = 0;
1206       /* Update restart state */
1207       entropy->restarts_to_go = cinfo->restart_interval;
1208     }
1209     entropy->restarts_to_go--;
1210   }
1211 
1212   for (blkn = 0; blkn < cinfo->blocks_in_MCU; blkn++) {
1213     ci = cinfo->MCU_membership[blkn];
1214     compptr = cinfo->cur_comp_info[ci];
1215     htest_one_block(cinfo, MCU_data[blkn][0], entropy->saved.last_dc_val[ci],
1216                     entropy->dc_count_ptrs[compptr->dc_tbl_no],
1217                     entropy->ac_count_ptrs[compptr->ac_tbl_no]);
1218     entropy->saved.last_dc_val[ci] = MCU_data[blkn][0][0];
1219   }
1220 
1221   return TRUE;
1222 }
1223 
1224 
1225 /*
1226  * Generate the best Huffman code table for the given counts, fill htbl.
1227  *
1228  * The JPEG standard requires that no symbol be assigned a codeword of all
1229  * one bits (so that padding bits added at the end of a compressed segment
1230  * can't look like a valid code).  Because of the canonical ordering of
1231  * codewords, this just means that there must be an unused slot in the
1232  * longest codeword length category.  Section K.2 of the JPEG spec suggests
1233  * reserving such a slot by pretending that symbol 256 is a valid symbol
1234  * with count 1.  In theory that's not optimal; giving it count zero but
1235  * including it in the symbol set anyway should give a better Huffman code.
1236  * But the theoretically better code actually seems to come out worse in
1237  * practice, because it produces more all-ones bytes (which incur stuffed
1238  * zero bytes in the final file).  In any case the difference is tiny.
1239  *
1240  * The JPEG standard requires Huffman codes to be no more than 16 bits long.
1241  * If some symbols have a very small but nonzero probability, the Huffman tree
1242  * must be adjusted to meet the code length restriction.  We currently use
1243  * the adjustment method suggested in JPEG section K.2.  This method is *not*
1244  * optimal; it may not choose the best possible limited-length code.  But
1245  * typically only very-low-frequency symbols will be given less-than-optimal
1246  * lengths, so the code is almost optimal.  Experimental comparisons against
1247  * an optimal limited-length-code algorithm indicate that the difference is
1248  * microscopic --- usually less than a hundredth of a percent of total size.
1249  * So the extra complexity of an optimal algorithm doesn't seem worthwhile.
1250  */
1251 
1252 LOCAL(void)
1253 jpeg_gen_optimal_table (j_compress_ptr cinfo, JHUFF_TBL * htbl, long freq[])
1254 {
1255 #define MAX_CLEN 32             /* assumed maximum initial code length */
1256   UINT8 bits[MAX_CLEN+1];       /* bits[k] = # of symbols with code length k */
1257   int codesize[257];            /* codesize[k] = code length of symbol k */
1258   int others[257];              /* next symbol in current branch of tree */
1259   int c1, c2;
1260   int p, i, j;
1261   long v;
1262 
1263   /* This algorithm is explained in section K.2 of the JPEG standard */
1264 
1265   MEMZERO(bits, SIZEOF(bits));
1266   MEMZERO(codesize, SIZEOF(codesize));
1267   for (i = 0; i < 257; i++)
1268     others[i] = -1;             /* init links to empty */
1269   
1270   freq[256] = 1;                /* make sure 256 has a nonzero count */
1271   /* Including the pseudo-symbol 256 in the Huffman procedure guarantees
1272    * that no real symbol is given code-value of all ones, because 256
1273    * will be placed last in the largest codeword category.
1274    */
1275 
1276   /* Huffman's basic algorithm to assign optimal code lengths to symbols */
1277 
1278   for (;;) {
1279     /* Find the smallest nonzero frequency, set c1 = its symbol */
1280     /* In case of ties, take the larger symbol number */
1281     c1 = -1;
1282     v = 1000000000L;
1283     for (i = 0; i <= 256; i++) {
1284       if (freq[i] && freq[i] <= v) {
1285         v = freq[i];
1286         c1 = i;
1287       }
1288     }
1289 
1290     /* Find the next smallest nonzero frequency, set c2 = its symbol */
1291     /* In case of ties, take the larger symbol number */
1292     c2 = -1;
1293     v = 1000000000L;
1294     for (i = 0; i <= 256; i++) {
1295       if (freq[i] && freq[i] <= v && i != c1) {
1296         v = freq[i];
1297         c2 = i;
1298       }
1299     }
1300 
1301     /* Done if we've merged everything into one frequency */
1302     if (c2 < 0)
1303       break;
1304     
1305     /* Else merge the two counts/trees */
1306     freq[c1] += freq[c2];
1307     freq[c2] = 0;
1308 
1309     /* Increment the codesize of everything in c1's tree branch */
1310     codesize[c1]++;
1311     while (others[c1] >= 0) {
1312       c1 = others[c1];
1313       codesize[c1]++;
1314     }
1315     
1316     others[c1] = c2;            /* chain c2 onto c1's tree branch */
1317     
1318     /* Increment the codesize of everything in c2's tree branch */
1319     codesize[c2]++;
1320     while (others[c2] >= 0) {
1321       c2 = others[c2];
1322       codesize[c2]++;
1323     }
1324   }
1325 
1326   /* Now count the number of symbols of each code length */
1327   for (i = 0; i <= 256; i++) {
1328     if (codesize[i]) {
1329       /* The JPEG standard seems to think that this can't happen, */
1330       /* but I'm paranoid... */
1331       if (codesize[i] > MAX_CLEN)
1332         ERREXIT(cinfo, JERR_HUFF_CLEN_OVERFLOW);
1333 
1334       bits[codesize[i]]++;
1335     }
1336   }
1337 
1338   /* JPEG doesn't allow symbols with code lengths over 16 bits, so if the pure
1339    * Huffman procedure assigned any such lengths, we must adjust the coding.
1340    * Here is what the JPEG spec says about how this next bit works:
1341    * Since symbols are paired for the longest Huffman code, the symbols are
1342    * removed from this length category two at a time.  The prefix for the pair
1343    * (which is one bit shorter) is allocated to one of the pair; then,
1344    * skipping the BITS entry for that prefix length, a code word from the next
1345    * shortest nonzero BITS entry is converted into a prefix for two code words
1346    * one bit longer.
1347    */
1348   
1349   for (i = MAX_CLEN; i > 16; i--) {
1350     while (bits[i] > 0) {
1351       j = i - 2;                /* find length of new prefix to be used */
1352       while (bits[j] == 0)
1353         j--;
1354       
1355       bits[i] -= 2;             /* remove two symbols */
1356       bits[i-1]++;              /* one goes in this length */
1357       bits[j+1] += 2;           /* two new symbols in this length */
1358       bits[j]--;                /* symbol of this length is now a prefix */
1359     }
1360   }
1361 
1362   /* Remove the count for the pseudo-symbol 256 from the largest codelength */
1363   while (bits[i] == 0)          /* find largest codelength still in use */
1364     i--;
1365   bits[i]--;
1366   
1367   /* Return final symbol counts (only for lengths 0..16) */
1368   MEMCOPY(htbl->bits, bits, SIZEOF(htbl->bits));
1369   
1370   /* Return a list of the symbols sorted by code length */
1371   /* It's not real clear to me why we don't need to consider the codelength
1372    * changes made above, but the JPEG spec seems to think this works.
1373    */
1374   p = 0;
1375   for (i = 1; i <= MAX_CLEN; i++) {
1376     for (j = 0; j <= 255; j++) {
1377       if (codesize[j] == i) {
1378         htbl->huffval[p] = (UINT8) j;
1379         p++;
1380       }
1381     }
1382   }
1383 
1384   /* Set sent_table FALSE so updated table will be written to JPEG file. */
1385   htbl->sent_table = FALSE;
1386 }
1387 
1388 
1389 /*
1390  * Finish up a statistics-gathering pass and create the new Huffman tables.
1391  */
1392 
1393 METHODDEF(void)
1394 finish_pass_gather (j_compress_ptr cinfo)
1395 {
1396   huff_entropy_ptr entropy = (huff_entropy_ptr) cinfo->entropy;
1397   int ci, tbl;
1398   jpeg_component_info * compptr;
1399   JHUFF_TBL **htblptr;
1400   boolean did_dc[NUM_HUFF_TBLS];
1401   boolean did_ac[NUM_HUFF_TBLS];
1402 
1403   /* It's important not to apply jpeg_gen_optimal_table more than once
1404    * per table, because it clobbers the input frequency counts!
1405    */
1406   if (cinfo->progressive_mode)
1407     /* Flush out buffered data (all we care about is counting the EOB symbol) */
1408     emit_eobrun(entropy);
1409 
1410   MEMZERO(did_dc, SIZEOF(did_dc));
1411   MEMZERO(did_ac, SIZEOF(did_ac));
1412 
1413   for (ci = 0; ci < cinfo->comps_in_scan; ci++) {
1414     compptr = cinfo->cur_comp_info[ci];
1415     /* DC needs no table for refinement scan */
1416     if (cinfo->Ss == 0 && cinfo->Ah == 0) {
1417       tbl = compptr->dc_tbl_no;
1418       if (! did_dc[tbl]) {
1419         htblptr = & cinfo->dc_huff_tbl_ptrs[tbl];
1420         if (*htblptr == NULL)
1421           *htblptr = jpeg_alloc_huff_table((j_common_ptr) cinfo);
1422         jpeg_gen_optimal_table(cinfo, *htblptr, entropy->dc_count_ptrs[tbl]);
1423         did_dc[tbl] = TRUE;
1424       }
1425     }
1426     /* AC needs no table when not present */
1427     if (cinfo->Se) {
1428       tbl = compptr->ac_tbl_no;
1429       if (! did_ac[tbl]) {
1430         htblptr = & cinfo->ac_huff_tbl_ptrs[tbl];
1431         if (*htblptr == NULL)
1432           *htblptr = jpeg_alloc_huff_table((j_common_ptr) cinfo);
1433         jpeg_gen_optimal_table(cinfo, *htblptr, entropy->ac_count_ptrs[tbl]);
1434         did_ac[tbl] = TRUE;
1435       }
1436     }
1437   }
1438 }
1439 
1440 
1441 /*
1442  * Initialize for a Huffman-compressed scan.
1443  * If gather_statistics is TRUE, we do not output anything during the scan,
1444  * just count the Huffman symbols used and generate Huffman code tables.
1445  */
1446 
1447 METHODDEF(void)
1448 start_pass_huff (j_compress_ptr cinfo, boolean gather_statistics)
1449 {
1450   huff_entropy_ptr entropy = (huff_entropy_ptr) cinfo->entropy;
1451   int ci, tbl;
1452   jpeg_component_info * compptr;
1453 
1454   if (gather_statistics)
1455     entropy->pub.finish_pass = finish_pass_gather;
1456   else
1457     entropy->pub.finish_pass = finish_pass_huff;
1458 
1459   if (cinfo->progressive_mode) {
1460     entropy->cinfo = cinfo;
1461     entropy->gather_statistics = gather_statistics;
1462 
1463     /* We assume jcmaster.c already validated the scan parameters. */
1464 
1465     /* Select execution routine */
1466     if (cinfo->Ah == 0) {
1467       if (cinfo->Ss == 0)
1468         entropy->pub.encode_mcu = encode_mcu_DC_first;
1469       else
1470         entropy->pub.encode_mcu = encode_mcu_AC_first;
1471     } else {
1472       if (cinfo->Ss == 0)
1473         entropy->pub.encode_mcu = encode_mcu_DC_refine;
1474       else {
1475         entropy->pub.encode_mcu = encode_mcu_AC_refine;
1476         /* AC refinement needs a correction bit buffer */
1477         if (entropy->bit_buffer == NULL)
1478           entropy->bit_buffer = (char *)
1479             (*cinfo->mem->alloc_small) ((j_common_ptr) cinfo, JPOOL_IMAGE,
1480                                         MAX_CORR_BITS * SIZEOF(char));
1481       }
1482     }
1483 
1484     /* Initialize AC stuff */
1485     entropy->ac_tbl_no = cinfo->cur_comp_info[0]->ac_tbl_no;
1486     entropy->EOBRUN = 0;
1487     entropy->BE = 0;
1488   } else {
1489     if (gather_statistics)
1490       entropy->pub.encode_mcu = encode_mcu_gather;
1491     else
1492       entropy->pub.encode_mcu = encode_mcu_huff;
1493   }
1494 
1495   for (ci = 0; ci < cinfo->comps_in_scan; ci++) {
1496     compptr = cinfo->cur_comp_info[ci];
1497     /* DC needs no table for refinement scan */
1498     if (cinfo->Ss == 0 && cinfo->Ah == 0) {
1499       tbl = compptr->dc_tbl_no;
1500       if (gather_statistics) {
1501         /* Check for invalid table index */
1502         /* (make_c_derived_tbl does this in the other path) */
1503         if (tbl < 0 || tbl >= NUM_HUFF_TBLS)
1504           ERREXIT1(cinfo, JERR_NO_HUFF_TABLE, tbl);
1505         /* Allocate and zero the statistics tables */
1506         /* Note that jpeg_gen_optimal_table expects 257 entries in each table! */
1507         if (entropy->dc_count_ptrs[tbl] == NULL)
1508           entropy->dc_count_ptrs[tbl] = (long *)
1509             (*cinfo->mem->alloc_small) ((j_common_ptr) cinfo, JPOOL_IMAGE,
1510                                         257 * SIZEOF(long));
1511         MEMZERO(entropy->dc_count_ptrs[tbl], 257 * SIZEOF(long));
1512       } else {
1513         /* Compute derived values for Huffman tables */
1514         /* We may do this more than once for a table, but it's not expensive */
1515         jpeg_make_c_derived_tbl(cinfo, TRUE, tbl,
1516                                 & entropy->dc_derived_tbls[tbl]);
1517       }
1518       /* Initialize DC predictions to 0 */
1519       entropy->saved.last_dc_val[ci] = 0;
1520     }
1521     /* AC needs no table when not present */
1522     if (cinfo->Se) {
1523       tbl = compptr->ac_tbl_no;
1524       if (gather_statistics) {
1525         if (tbl < 0 || tbl >= NUM_HUFF_TBLS)
1526           ERREXIT1(cinfo, JERR_NO_HUFF_TABLE, tbl);
1527         if (entropy->ac_count_ptrs[tbl] == NULL)
1528           entropy->ac_count_ptrs[tbl] = (long *)
1529             (*cinfo->mem->alloc_small) ((j_common_ptr) cinfo, JPOOL_IMAGE,
1530                                         257 * SIZEOF(long));
1531         MEMZERO(entropy->ac_count_ptrs[tbl], 257 * SIZEOF(long));
1532       } else {
1533         jpeg_make_c_derived_tbl(cinfo, FALSE, tbl,
1534                                 & entropy->ac_derived_tbls[tbl]);
1535       }
1536     }
1537   }
1538 
1539   /* Initialize bit buffer to empty */
1540   entropy->saved.put_buffer = 0;
1541   entropy->saved.put_bits = 0;
1542 
1543   /* Initialize restart stuff */
1544   entropy->restarts_to_go = cinfo->restart_interval;
1545   entropy->next_restart_num = 0;
1546 }
1547 
1548 
1549 /*
1550  * Module initialization routine for Huffman entropy encoding.
1551  */
1552 
1553 GLOBAL(void)
1554 jinit_huff_encoder (j_compress_ptr cinfo)
1555 {
1556   huff_entropy_ptr entropy;
1557   int i;
1558 
1559   entropy = (huff_entropy_ptr)
1560     (*cinfo->mem->alloc_small) ((j_common_ptr) cinfo, JPOOL_IMAGE,
1561                                 SIZEOF(huff_entropy_encoder));
1562   cinfo->entropy = &entropy->pub;
1563   entropy->pub.start_pass = start_pass_huff;
1564 
1565   /* Mark tables unallocated */
1566   for (i = 0; i < NUM_HUFF_TBLS; i++) {
1567     entropy->dc_derived_tbls[i] = entropy->ac_derived_tbls[i] = NULL;
1568     entropy->dc_count_ptrs[i] = entropy->ac_count_ptrs[i] = NULL;
1569   }
1570 
1571   if (cinfo->progressive_mode)
1572     entropy->bit_buffer = NULL;      /* needed only in AC refinement scan */
1573 }