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)
start_pass_huff(j_compress_ptr cinfo,boolean gather_statistics)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)
jpeg_make_c_derived_tbl(j_compress_ptr cinfo,boolean isDC,int tblno,c_derived_tbl ** pdtbl)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)
dump_buffer(working_state * state)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
LOCAL(boolean)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)
flush_bits(working_state * state)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)
encode_one_block(working_state * state,JCOEFPTR block,int last_dc_val,c_derived_tbl * dctbl,c_derived_tbl * actbl)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)
emit_restart(working_state * state,int restart_num)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)
encode_mcu_huff(j_compress_ptr cinfo,JBLOCKROW * MCU_data)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)
finish_pass_huff(j_compress_ptr cinfo)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)
htest_one_block(j_compress_ptr cinfo,JCOEFPTR block,int last_dc_val,long dc_counts[],long ac_counts[])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)
encode_mcu_gather(j_compress_ptr cinfo,JBLOCKROW * MCU_data)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)
jpeg_gen_optimal_table(j_compress_ptr cinfo,JHUFF_TBL * htbl,long freq[])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)
finish_pass_gather(j_compress_ptr cinfo)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)
jinit_huff_encoder(j_compress_ptr cinfo)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 }
910