summaryrefslogtreecommitdiff
path: root/src/support/huffman.c
blob: 17342c53ced3c12429fcd18c0bf80cc589968dd0 (plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
750
751
752
753
754
755
756
757
758
759
760
761
762
763
764
765
766
767
768
769
770
771
772
773
774
775
776
777
778
779
780
781
782
783
784
785
786
787
788
789
790
791
792
793
794
795
796
797
798
799
800
801
802
803
804
805
806
807
808
809
810
811
812
813
814
815
816
817
818
819
820
821
822
823
824
825
826
827
828
829
830
831
832
833
834
835
836
837
838
839
840
841
842
843
844
845
846
847
848
849
850
851
852
853
854
855
856
857
858
859
860
861
862
863
864
865
866
867
868
869
870
871
872
873
874
875
876
877
878
879
880
881
882
883
884
885
886
887
888
889
890
891
892
893
894
895
896
897
898
899
900
901
902
903
904
905
906
907
908
909
910
911
912
913
914
915
916
917
918
919
920
921
922
923
924
925
926
927
928
929
930
931
/*-
 * Copyright (c) 2014-2017 MongoDB, Inc.
 * Copyright (c) 2008-2014 WiredTiger, Inc.
 *	All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 * 1. Redistributions of source code must retain the above copyright
 *    notice, this list of conditions and the following disclaimer.
 * 2. Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in the
 *    documentation and/or other materials provided with the distribution.
 * 4. Neither the name MongoDB or the name WiredTiger
 *    may be used to endorse or promote products derived from this software
 *    without specific prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY MONGODB INC. ``AS IS'' AND
 * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
 * ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
 * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
 * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
 * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
 * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
 * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
 * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 * SUCH DAMAGE.
 */

#include "wt_internal.h"

#define	__HUFFMAN_DETAIL	0	/* Set to 1 for debugging output. */

/* Length of header in compressed message, in bits. */
#define	WT_HUFFMAN_HEADER 	3

/*
 * Maximum allowed length of Huffman code words, which otherwise can range up
 * to (#symbols - 1) bits long.  Lower value to use less memory for tables,
 * higher value for better compression.  Max value = 16 (or 32-7=25 or 64-7=57
 * if adjust data types).  FYI, JPEG uses 16.  A side effect of limiting max
 * code length is that the worst case compression (a message of the least
 * frequent symbols) is shorter.
 */
#define	MAX_CODE_LENGTH		16

typedef struct __wt_freqtree_node {
	/*
	 * Data structure representing a node of the huffman tree. It holds a
	 * 64-bit weight and pointers to the left and right child nodes.  The
	 * node either has two child nodes or none.
	 */
	uint8_t  symbol;			/* only used in leaf nodes */
	uint64_t weight;
	struct __wt_freqtree_node *left;	/* bit 0 */
	struct __wt_freqtree_node *right;	/* bit 1 */
} WT_FREQTREE_NODE;

typedef struct __wt_huffman_code {
	uint16_t pattern;		/* requirement: length of field's type
					 * in bits >= MAX_CODE_LENGTH.
					 */
	uint8_t length;
} WT_HUFFMAN_CODE;

typedef struct __wt_huffman_obj {
	/*
	 * Data structure here defines specific instance of the encoder/decoder.
	 */
	u_int	numSymbols;		/* Symbols: UINT16_MAX or UINT8_MAX */

	uint16_t max_depth, min_depth;	/* Tree max/min depths */

	/*
	 * use: codes[symbol] = struct with pattern and length.
	 * Used in encoding and decoding.
	 * memory: codes[0-to-(number of symbols - 1)]
	 */
	WT_HUFFMAN_CODE *codes;

	/*
	 * use: code2symbol[Huffman_code] = symbol.
	 * Used in decoding.
	 * memory: code2symbol[1 << max_code_length]
	 */
	uint8_t *code2symbol;
} WT_HUFFMAN_OBJ;

/*
 * Queue element data structure.
 *
 * Consists of a pointer to a huffman tree node, and a pointer to the next
 * element in the queue.
 */
typedef struct node_queue_elem {
	WT_FREQTREE_NODE *node;
	struct node_queue_elem *next;
} NODE_QUEUE_ELEM;

/*
 * Queue of huffman tree nodes.
 *
 * Contains a pointer to the beginning and the end of the queue, which is
 * implemented as a linked list.
 */
typedef struct node_queue {
	NODE_QUEUE_ELEM *first;
	NODE_QUEUE_ELEM *last;
} NODE_QUEUE;

/*
 * Internal data structure used to preserve the symbol when rearranging the
 * frequency array.
 */
typedef struct __indexed_byte {
	uint32_t symbol;	/* not uint8_t: match external data structure */
	uint32_t frequency;
} INDEXED_SYMBOL;

static int WT_CDECL indexed_freq_compare(const void *, const void *);
static int WT_CDECL indexed_symbol_compare(const void *, const void *);
static void make_table(
	WT_SESSION_IMPL *, uint8_t *, uint16_t, WT_HUFFMAN_CODE *, u_int);
static void node_queue_close(WT_SESSION_IMPL *, NODE_QUEUE *);
static void node_queue_dequeue(
	WT_SESSION_IMPL *, NODE_QUEUE *, WT_FREQTREE_NODE **);
static int  node_queue_enqueue(
	WT_SESSION_IMPL *, NODE_QUEUE *, WT_FREQTREE_NODE *);
static uint32_t profile_tree(
	WT_FREQTREE_NODE *, uint16_t, uint16_t *, uint16_t *);
static void recursive_free_node(WT_SESSION_IMPL *, WT_FREQTREE_NODE *);
static void set_codes(WT_FREQTREE_NODE *, WT_HUFFMAN_CODE *, uint16_t, uint8_t);

#define	node_queue_is_empty(queue)					\
	((queue) == NULL || (queue)->first == NULL)

/*
 * indexed_symbol_compare --
 *	Qsort comparator to order the table by symbol, lowest to highest.
 */
static int WT_CDECL
indexed_symbol_compare(const void *a, const void *b)
{
	return (((INDEXED_SYMBOL *)a)->symbol >
	    ((INDEXED_SYMBOL *)b)->symbol ? 1 :
	    (((INDEXED_SYMBOL *)a)->symbol <
	    ((INDEXED_SYMBOL *)b)->symbol ? -1 : 0));
}

/*
 * indexed_freq_compare --
 *	Qsort comparator to order the table by frequency (the most frequent
 * symbols will be at the end of the array).
 */
static int WT_CDECL
indexed_freq_compare(const void *a, const void *b)
{
	return (((INDEXED_SYMBOL *)a)->frequency >
	    ((INDEXED_SYMBOL *)b)->frequency ? 1 :
	    (((INDEXED_SYMBOL *)a)->frequency <
	    ((INDEXED_SYMBOL *)b)->frequency ? -1 : 0));
}

/*
 * profile_tree --
 *	Traverses tree to determine #leaves under each node, max depth, min
 *	depth of leaf.
 */
static uint32_t
profile_tree(WT_FREQTREE_NODE *node,
    uint16_t len, uint16_t *max_depth, uint16_t *min_depth)
{
	uint32_t leaf_cnt;

	if (node->left == NULL && node->right == NULL) {	/* leaf */
		leaf_cnt = 1;
		if (*max_depth < len)
			*max_depth = len;
		if (*min_depth > len)
			*min_depth = len;
	} else {
		/*
		 * internal node -- way tree constructed internal always has
		 * left and right children
		 */
		leaf_cnt =
		    profile_tree(node->left, len + 1, max_depth, min_depth) +
		    profile_tree(node->right, len + 1, max_depth, min_depth);
	}
	node->weight = leaf_cnt;		/* abuse weight field */
	return (leaf_cnt);
}

/*
 * set_codes --
 *	Computes Huffman code for each symbol in tree.
 *
 * Method is standard way in the literature, except that limits maximum code
 * length.  A known max code length is important for limiting memory use by
 * the tables and for knowing how large data types need to be such as the field
 * that holds the code pattern.
 */
static void
set_codes(WT_FREQTREE_NODE *node,
    WT_HUFFMAN_CODE *codes, uint16_t pattern, uint8_t len)
{
	WT_HUFFMAN_CODE *code;
	uint16_t patternleft, patternright, half;
	uint8_t remaining;

	if (node->left == NULL && node->right == NULL) {
		code = &codes[node->symbol];
		code->pattern = pattern;
		code->length = len;
#if __HUFFMAN_DETAIL
		printf("%" PRIx16 ": code %" PRIx16 ", len %" PRIu8 "\n",
		    node->symbol, pattern, len);
#endif
	} else {
		/*
		 * Check each subtree individually to see if can afford to split
		 * up bits into possibly shorter codes, or if need to employ all
		 * remaining bits up to MAX_CODE_LENGTH to consecutively number
		 * leaves.
		 */
		remaining = MAX_CODE_LENGTH - len;
		/*
		 * If not already in "low-bit mode", but need to be, open up
		 * lower-order bits for consecutive numbering.
		 */
		if (len < MAX_CODE_LENGTH &&
		    ((half = (uint16_t)(1 << (remaining - 1))) <
		    node->left->weight || half < node->right->weight)) {
			pattern = (uint16_t)(pattern << remaining);
			len = MAX_CODE_LENGTH;
		}

		if (len < MAX_CODE_LENGTH) {
			patternleft = (uint16_t)((pattern << 1) | 0);
			patternright = (uint16_t)((pattern << 1) | 1);
			len++;
		} else {			/* "low bit mode" */
			patternleft = pattern;
			patternright = (uint16_t)(pattern + node->left->weight);
						/* len unchanged */
		}

		set_codes(node->left, codes, patternleft, len);
		set_codes(node->right, codes, patternright, len);
	}
}

/*
 * make_table --
 *	Computes Huffman table used for subsequent lookups in encoding and
 * decoding.  With the table, encoding from a symbol to Huffman code and
 * decoding from a code to a symbol are simple array lookups.
 */
static void
make_table(WT_SESSION_IMPL *session, uint8_t *code2symbol,
    uint16_t max_depth, WT_HUFFMAN_CODE *codes, u_int symcnt)
{
	uint32_t j, c1, c2;	/* Exceeds uint16_t bounds at loop boundary. */
	uint16_t c, i;
	uint8_t len, shift;

	/* Zero out, for assertion below. */
	for (j = 0, c2 = (1U << max_depth); j < c2; j++)
		code2symbol[j] = 0;

	/*
	 * Here's the magic: flood all bit patterns for lower-order bits to
	 * point to same symbol.
	 */
	for (i = 0; i < symcnt; i++) {
		if ((len = codes[i].length) == 0)
			continue;

		/*
		 * The size of the array index should be enough to hold largest
		 * index into symbol table.  Pre-existing symbols were packed
		 * 0-255, so 8 bits is enough.  Don't want to make it larger
		 * than necessary, we allocate (2 ^ max-code-length) of them.
		 */
		c = codes[i].pattern;
		shift = (uint8_t)(max_depth - len);
		c1 = (uint32_t)c << shift;
		c2 = (uint32_t)(c + 1) << shift;
		for (j = c1; j < c2; j++) {
			WT_ASSERT(session, code2symbol[j] == 0);
			code2symbol[j] = (uint8_t)i;
		}
	}
}

/*
 * recursive_free_node --
 *	Recursively free the huffman frequency tree's nodes.
 */
static void
recursive_free_node(WT_SESSION_IMPL *session, WT_FREQTREE_NODE *node)
{
	if (node != NULL) {
		recursive_free_node(session, node->left);
		recursive_free_node(session, node->right);
		__wt_free(session, node);
	}
}

/*
 * __wt_huffman_open --
 *	Take a frequency table and return a pointer to a descriptor object.
 */
int
__wt_huffman_open(WT_SESSION_IMPL *session,
    void *symbol_frequency_array, u_int symcnt, u_int numbytes, void *retp)
{
	INDEXED_SYMBOL *indexed_freqs, *sym;
	NODE_QUEUE *combined_nodes, *leaves;
	WT_DECL_RET;
	WT_FREQTREE_NODE *node, *node2, **refnode, *tempnode;
	WT_HUFFMAN_OBJ *huffman;
	uint64_t w1, w2;
	uint16_t i;

	indexed_freqs = NULL;
	combined_nodes = leaves = NULL;
	node = node2 = tempnode = NULL;

	WT_RET(__wt_calloc_one(session, &huffman));

	/*
	 * The frequency table is 4B pairs of symbol and frequency.  The symbol
	 * is either 1 or 2 bytes and the frequency ranges from 1 to UINT32_MAX
	 * (a frequency of 0 means the value is never expected to appear in the
	 * input).  Validate the symbols are within range.
	 */
	if (numbytes != 1 && numbytes != 2)
		WT_ERR_MSG(session, EINVAL,
		    "illegal number of symbol bytes specified for a huffman "
		    "table");

	if (symcnt == 0)
		WT_ERR_MSG(session, EINVAL,
		    "illegal number of symbols specified for a huffman table");

	huffman->numSymbols = numbytes == 2 ? UINT16_MAX : UINT8_MAX;

	/*
	 * Order the array by symbol and check for invalid symbols and
	 * duplicates.
	 */
	sym = symbol_frequency_array;
	qsort(sym, symcnt, sizeof(INDEXED_SYMBOL), indexed_symbol_compare);
	for (i = 0; i < symcnt; ++i) {
		if (i > 0 && sym[i].symbol == sym[i - 1].symbol)
			WT_ERR_MSG(session, EINVAL,
			    "duplicate symbol %" PRIu32 " (%#" PRIx32 ") "
			    "specified in a huffman table",
			    sym[i].symbol, sym[i].symbol);
		if (sym[i].symbol > huffman->numSymbols)
			WT_ERR_MSG(session, EINVAL,
			    "out-of-range symbol %" PRIu32 " (%#" PRIx32 ") "
			    "specified in a huffman table",
			    sym[i].symbol, sym[i].symbol);
	}

	/*
	 * Massage frequencies.
	 */
	WT_ERR(__wt_calloc_def(session, 256, &indexed_freqs));

	/*
	 * Minimum of frequency==1 so everybody gets a Huffman code, in case
	 * data evolves and we need to represent this value.
	 */
	for (i = 0; i < 256; i++) {
		sym = &indexed_freqs[i];
		sym->symbol = i;
		sym->frequency = 1;
	}
	/*
	 * Avoid large tables by splitting UTF-16 frequencies into high byte
	 * and low byte.
	 */
	for (i = 0; i < symcnt; i++) {
		sym = &((INDEXED_SYMBOL *)symbol_frequency_array)[i];
		indexed_freqs[sym->symbol & 0xff].frequency += sym->frequency;
		if (numbytes == 2)
			indexed_freqs[(sym->symbol >> 8) & 0xff].frequency +=
			    sym->frequency;
	}
	huffman->numSymbols = symcnt = 256;

	/*
	 * The array must be sorted by frequency to be able to use a linear time
	 * construction algorithm.
	 */
	qsort((void *)indexed_freqs,
	    symcnt, sizeof(INDEXED_SYMBOL), indexed_freq_compare);

	/* We need two node queues to build the tree. */
	WT_ERR(__wt_calloc_one(session, &leaves));
	WT_ERR(__wt_calloc_one(session, &combined_nodes));

	/*
	 * Adding the leaves to the queue.
	 *
	 * Discard symbols with a frequency of 0; this assumes these symbols
	 * never occur in the source stream, and the purpose is to reduce the
	 * huffman tree's size.
	 */
	for (i = 0; i < symcnt; ++i)
		if (indexed_freqs[i].frequency > 0) {
			WT_ERR(__wt_calloc_one(session, &tempnode));
			tempnode->symbol = (uint8_t)indexed_freqs[i].symbol;
			tempnode->weight = indexed_freqs[i].frequency;
			WT_ERR(node_queue_enqueue(session, leaves, tempnode));
			tempnode = NULL;
		}

	while (!node_queue_is_empty(leaves) ||
	    !node_queue_is_empty(combined_nodes)) {
		/*
		 * We have to get the node with the smaller weight, examining
		 * both queues' first element.  We are collecting pairs of these
		 * items, by alternating between node and node2:
		 */
		refnode = !node ? &node : &node2;

		/*
		 * To decide which queue must be used, we get the weights of
		 * the first items from both:
		 */
		w1 = node_queue_is_empty(leaves) ?
		    UINT64_MAX : leaves->first->node->weight;
		w2 = node_queue_is_empty(combined_nodes) ?
		    UINT64_MAX : combined_nodes->first->node->weight;

		/*
		 * Based on the two weights we finally can dequeue the smaller
		 * element and place it to the alternating target node pointer:
		 */
		if (w1 < w2)
			node_queue_dequeue(session, leaves, refnode);
		else
			node_queue_dequeue(session, combined_nodes, refnode);

		/*
		 * In every second run, we have both node and node2 initialized.
		 */
		if (node != NULL && node2 != NULL) {
			WT_ERR(__wt_calloc_one(session, &tempnode));

			/* The new weight is the sum of the two weights. */
			tempnode->weight = node->weight + node2->weight;
			tempnode->left = node;
			tempnode->right = node2;

			/* Enqueue it to the combined nodes queue */
			WT_ERR(node_queue_enqueue(
			    session, combined_nodes, tempnode));
			tempnode = NULL;

			/* Reset the state pointers */
			node = node2 = NULL;
		}
	}

	/*
	 * The remaining node is in the node variable, this is the root of the
	 * tree. Calculate how many bytes it takes to hold numSymbols bytes
	 * bits.
	 */
	huffman->max_depth = 0;
	huffman->min_depth = MAX_CODE_LENGTH;
	(void)profile_tree(node, 0, &huffman->max_depth, &huffman->min_depth);
	if (huffman->max_depth > MAX_CODE_LENGTH)
		huffman->max_depth = MAX_CODE_LENGTH;

	WT_ERR(__wt_calloc_def(session, huffman->numSymbols, &huffman->codes));
	set_codes(node, huffman->codes, 0, 0);

	WT_ERR(__wt_calloc_def(
	    session, (size_t)1U << huffman->max_depth, &huffman->code2symbol));
	make_table(session, huffman->code2symbol,
	    huffman->max_depth, huffman->codes, huffman->numSymbols);

#if __HUFFMAN_DETAIL
	{
	uint8_t symbol;
	uint32_t weighted_length;

	printf("leaf depth %" PRIu16 "..%" PRIu16
	    ", memory use: codes %u# * %" WT_SIZET_FMT
	    "B + code2symbol %u# * %" WT_SIZET_FMT "B\n",
	    huffman->min_depth, huffman->max_depth,
	    huffman->numSymbols, sizeof(WT_HUFFMAN_CODE),
	    1U << huffman->max_depth, sizeof(uint16_t));

	/*
	 * measure quality of computed Huffman codes, for different max bit
	 * lengths (say, 16 vs 24 vs 32)
	 */
	weighted_length = 0;
	for (i = 0; i < symcnt; i++) {
		symbol = indexed_freqs[i].symbol;
		weighted_length +=
		    indexed_freqs[i].frequency * huffman->codes[symbol].length;
		printf(
		    "\t%" PRIu16 "->%" PRIu16 ". %" PRIu32 " * %" PRIu8 "\n",
		    i, symbol,
		    indexed_freqs[i].frequency, huffman->codes[symbol].length);
	}
	printf("weighted length of all codes (the smaller the better): "
	    "%" PRIu32 "\n", weighted_length);
	}
#endif

	*(void **)retp = huffman;

	if (0) {
err:		if (ret == 0)
			ret = WT_ERROR;
	}
	__wt_free(session, indexed_freqs);
	if (leaves != NULL)
		node_queue_close(session, leaves);
	if (combined_nodes != NULL)
		node_queue_close(session, combined_nodes);
	if (node != NULL)
		recursive_free_node(session, node);
	if (node2 != NULL)
		recursive_free_node(session, node2);
	__wt_free(session, tempnode);
	if (ret != 0)
		__wt_huffman_close(session, huffman);
	return (ret);
}

/*
 * __wt_huffman_close --
 *	Discard a Huffman descriptor object.
 */
void
__wt_huffman_close(WT_SESSION_IMPL *session, void *huffman_arg)
{
	WT_HUFFMAN_OBJ *huffman;

	huffman = huffman_arg;

	__wt_free(session, huffman->code2symbol);
	__wt_free(session, huffman->codes);
	__wt_free(session, huffman);
}

#if __HUFFMAN_DETAIL
/*
 * __wt_print_huffman_code --
 *	Prints a symbol's Huffman code.
 */
void
__wt_print_huffman_code(void *huffman_arg, uint16_t symbol)
{
	WT_HUFFMAN_CODE code;
	WT_HUFFMAN_OBJ *huffman;

	huffman = huffman_arg;

	if (symbol >= huffman->numSymbols)
		printf("symbol %" PRIu16 " out of range\n", symbol);
	else {
		code = huffman->codes[symbol];
		if (code.length == 0)
			printf(
			    "symbol %" PRIu16 " not defined -- 0 frequency\n",
			    symbol);
		else
			/* should print code as binary */
			printf(
			    "%" PRIu16 " -> code pattern "
			    "%" PRIx16 ", length %" PRIu8 "\n",
				symbol, code.pattern, code.length);
	}
}
#endif

/*
 * __wt_huffman_encode --
 *	Take a byte string, encode it into the target.
 *
 * Translation from symbol to Huffman code is a simple array lookup.
 *
 * WT_HUFFMAN_OBJ contains an array called 'codes' with one WT_HUFFMAN_CODE per
 * symbol.  Then, given a symbol:
 *	pattern = codes[symbol].pattern;
 *	length = codes[symbol].length;
 *
 * To encode byte-string, we iterate over the input symbols.  For each symbol,
 * look it up via table, shift bits onto a shift register (an int long enough
 * to hold the longest code word + up to 7 bits remaining from the previous),
 * then drain out full bytes.  Finally, at the end flush remaining bits
 * and write header bits.
 */
int
__wt_huffman_encode(WT_SESSION_IMPL *session, void *huffman_arg,
    const uint8_t *from_arg, size_t from_len, WT_ITEM *to_buf)
{
	WT_DECL_RET;
	WT_HUFFMAN_CODE code;
	WT_HUFFMAN_OBJ *huffman;
	WT_ITEM *tmp;
	size_t max_len, outlen, bytes;
	uint64_t bitpos;
	const uint8_t *from;
	uint8_t len, *out, padding_info, symbol;

	/*
	 * Shift register to accumulate bits from input.
	 * Should be >= (MAX_CODE_LENGTH + 7), but also efficient to shift bits
	 * and preferably in a machine register.
	 */
	uint32_t bits;

	/* Count of bits in shift register ('bits' above). */
	uint8_t valid;

	huffman = huffman_arg;
	from = from_arg;
	tmp = NULL;

	/*
	 * We don't want to find all of our callers and ensure they don't pass
	 * 0-length byte strings, but there's no reason to do any work.
	 */
	if (from_len == 0) {
		to_buf->size = 0;
		return (0);
	}

	/*
	 * Compute the largest compressed output size, which is if all symbols
	 * are least frequent and so have largest Huffman codes, and compressed
	 * output may be larger than the input size.  This way we don't have to
	 * worry about resizing the buffer during compression.  Use the shared
	 * system buffer while compressing, then allocate a new buffer of the
	 * right size and copy the result into it.
	 */
	max_len = (WT_HUFFMAN_HEADER +
	    from_len * huffman->max_depth + 7 /* round up to full byte */) / 8;
	WT_ERR(__wt_scr_alloc(session, max_len, &tmp));

	/*
	 * Leave the first 3 bits of the encoded value empty, it holds the
	 * number of bits actually used in the last byte of the encoded value.
	 */
	bits = 0;
	bitpos = WT_HUFFMAN_HEADER;
	valid = WT_HUFFMAN_HEADER;
	out = tmp->mem;
	for (bytes = 0; bytes < from_len; bytes++) {
		WT_ASSERT(session, WT_PTR_IN_RANGE(from, from_arg, from_len));

		symbol = *from++;

		/* Translate symbol into Huffman code and stuff into buffer. */
		code = huffman->codes[symbol];
		len = code.length;
		bits = (bits << len) | code.pattern;
		valid += len;
		bitpos += len;
		while (valid >= 8) {
			WT_ASSERT(session,
			    WT_PTR_IN_RANGE(out, tmp->mem, tmp->memsize));
			*out++ = (uint8_t)(bits >> (valid - 8));
			valid -= 8;
		}
	}
	if (valid > 0) {		/* Flush shift register. */
		WT_ASSERT(session,
		    WT_PTR_IN_RANGE(out, tmp->mem, tmp->memsize));
		*out = (uint8_t)(bits << (8 - valid));
	}

	/*
	 * At this point, bitpos is the total number of used bits (including
	 * the 3 bits at the beginning of the buffer, which we'll set now to
	 * the number of bits used in the last byte). Note if the number of
	 * bits used in the last byte is 8, we set the 3 bits to 0, in other
	 * words, the first 3 bits of the encoded value are the number of bits
	 * used in the last byte, unless they're 0, in which case there are 8
	 * bits used in the last byte.
	 */
	padding_info = (uint8_t)((bitpos % 8) << (8 - WT_HUFFMAN_HEADER));
	((uint8_t *)tmp->mem)[0] |= padding_info;

	/* Copy result of exact known size into caller's buffer. */
	outlen = (uint32_t)((bitpos + 7) / 8);
	WT_ERR(__wt_buf_initsize(session, to_buf, outlen));
	memcpy(to_buf->mem, tmp->mem, outlen);

#if __HUFFMAN_DETAIL
	printf("encode: worst case %" PRIu32 " bytes -> actual %" PRIu32 "\n",
	    max_len, outlen);
#endif

err:	__wt_scr_free(session, &tmp);
	return (ret);

}

/*
 * __wt_huffman_decode --
 *	Take a byte string, decode it into the target.
 *
 * Translation from Huffman code to symbol is a simple array lookup.
 *
 * WT_HUFFMAN_OBJ contains an array called 'code2symbol' indexed by code word
 * and whose value is the corresponding symbol.
 * From the symbol, we index into the 'codes' array to get the code length.
 *
 * When decoding a message, we don't know where the boundaries are between
 * codes.  The trick is that we collect enough bits for the longest code word,
 * and construct the table such that for codes with fewer bits we flood the
 * table with all of the bit patterns in the lower order bits.  This works
 * because the Huffman code is a unique prefix, and by the flooding we are
 * treating bits beyond the unique prefix as don't care bits.
 *
 * For example, we have table of length 2^max_code_length (1<<max_code_length).
 * For a code of length, max_code_length, the position code2symbol[code] =
 *	symbol.
 * For a code word of (max_length - 1), we fill code2symbol[code << 1] = symbol,
 * as well as code2symbol[(code << 1) | 1] = symbol.
 * And so on, so in general we fill:
 * 	code2symbol[(code) << shift inclusive .. (code+1) << shift exclusive].
 *
 * To decode a message, we read in enough bits from input to fill the shift
 * register with at least MAX_CODE_LENGTH bits.
 * We look up in the table code2symbol to obtain the symbol.
 * We look up the symbol in 'codes' to obtain the code length
 * Finally, subtract off these bits from the shift register.
 */
int
__wt_huffman_decode(WT_SESSION_IMPL *session, void *huffman_arg,
    const uint8_t *from_arg, size_t from_len, WT_ITEM *to_buf)
{
	WT_DECL_RET;
	WT_ITEM *tmp;
	WT_HUFFMAN_OBJ *huffman;
	size_t from_bytes, len, max_len, outlen;
	uint64_t from_len_bits;
	uint32_t bits, mask, max;
	uint16_t pattern;
	const uint8_t *from;
	uint8_t padding_info, symbol, *to, valid;

	huffman = huffman_arg;
	from = from_arg;
	tmp = NULL;

	/*
	 * We don't want to find all of our callers and ensure they don't pass
	 * 0-length byte strings, but there's no reason to do any work.
	 */
	if (from_len == 0) {
		to_buf->size = 0;
		return (0);
	}

	/*
	 * The first 3 bits are the number of used bits in the last byte, unless
	 * they're 0, in which case there are 8 bits used in the last byte.
	 */
	padding_info = (*from & 0xE0) >> (8 - WT_HUFFMAN_HEADER);
	from_len_bits = from_len * 8;
	if (padding_info != 0)
		from_len_bits -= 8U - padding_info;

	/* Number of bits that have codes. */
	from_len_bits -= WT_HUFFMAN_HEADER;

	/*
	 * Compute largest uncompressed output size, which is if all symbols are
	 * most frequent and so have smallest Huffman codes and therefore
	 * largest expansion.  Use the shared system buffer while uncompressing,
	 * then allocate a new buffer of exactly the right size and copy the
	 * result into it.
	 */
	max_len = (uint32_t)(from_len_bits / huffman->min_depth);
	WT_ERR(__wt_scr_alloc(session, max_len, &tmp));
	to = tmp->mem;

	/* The first byte of input is a special case because of header bits. */
	bits = *from++;
	valid = 8 - WT_HUFFMAN_HEADER;
	from_bytes = from_len - 1;

	max = huffman->max_depth;
	mask = (1U << max) - 1;
	for (outlen = 0; from_len_bits > 0; outlen++) {
		while (valid < max && from_bytes > 0) {
			WT_ASSERT(session,
			    WT_PTR_IN_RANGE(from, from_arg, from_len));
			bits = (bits << 8) | *from++;
			valid += 8;
			from_bytes--;
		}
		pattern = (uint16_t)
		    (valid >= max ?	/* short patterns near end */
		    (bits >> (valid - max)) : (bits << (max - valid)));
		symbol = huffman->code2symbol[pattern & mask];
		len = huffman->codes[symbol].length;
		valid -= (uint8_t)len;

		/*
		 * from_len_bits is the total number of input bits, reduced by
		 * the number of bits we consume from input at each step.  For
		 * all but the last step from_len_bits > len, then at the last
		 * step from_len_bits == len (in other words, from_len_bits -
		 * len = 0 input bits remaining). Generally, we cannot detect
		 * corruption during huffman decompression, this is one place
		 * where that's not true.
		 */
		if (from_len_bits < len)	/* corrupted */
			WT_ERR_MSG(session, EINVAL,
			    "huffman decompression detected input corruption");
		from_len_bits -= len;

		WT_ASSERT(session,
		    WT_PTR_IN_RANGE(to, tmp->mem, tmp->memsize));
		*to++ = symbol;
	}

	/* Return the number of bytes used. */
	WT_ERR(__wt_buf_initsize(session, to_buf, outlen));
	memcpy(to_buf->mem, tmp->mem, outlen);

#if __HUFFMAN_DETAIL
	printf("decode: worst case %" PRIu32 " bytes -> actual %" PRIu32 "\n",
	    max_len, outlen);
#endif

err:	__wt_scr_free(session, &tmp);
	return (ret);
}

/*
 * node_queue_close --
 *	Delete a queue from memory.
 *
 * It does not delete the pointed huffman tree nodes!
 */
static void
node_queue_close(WT_SESSION_IMPL *session, NODE_QUEUE *queue)
{
	NODE_QUEUE_ELEM *elem, *next_elem;

	/* Freeing each element of the queue's linked list. */
	for (elem = queue->first; elem != NULL; elem = next_elem) {
		next_elem = elem->next;
		__wt_free(session, elem);
	}

	/* Freeing the queue record itself. */
	__wt_free(session, queue);
}

/*
 * node_queue_enqueue --
 *	Push a tree node to the end of the queue.
 */
static int
node_queue_enqueue(
    WT_SESSION_IMPL *session, NODE_QUEUE *queue, WT_FREQTREE_NODE *node)
{
	NODE_QUEUE_ELEM *elem;

	/* Allocating a new linked list element */
	WT_RET(__wt_calloc_one(session, &elem));

	/* It holds the tree node, and has no next element yet */
	elem->node = node;
	elem->next = NULL;

	/* If the queue is empty, the first element will be the new one. */
	if (queue->first == NULL)
		queue->first = elem;

	/*
	 * If the queue is not empty, the last element's next pointer must be
	 * updated.
	 */
	if (queue->last != NULL)
		queue->last->next = elem;

	/* The last element is the new one */
	queue->last = elem;

	return (0);
}

/*
 * node_queue_dequeue --
 *	Removes a node from the beginning of the queue and copies the node's
 *	pointer to the location referred by the retp parameter.
 */
static void
node_queue_dequeue(
    WT_SESSION_IMPL *session, NODE_QUEUE *queue, WT_FREQTREE_NODE **retp)
{
	NODE_QUEUE_ELEM *first_elem;

	/*
	 * Getting the first element of the queue and updating it to point to
	 * the next element as first.
	 */
	first_elem = queue->first;
	*retp = first_elem->node;
	queue->first = first_elem->next;

	/*
	 * If the last element was the dequeued element, we have to update it
	 * to NULL.
	 */
	if (queue->last == first_elem)
		queue->last = NULL;

	/* Freeing the linked list element that has been dequeued */
	__wt_free(session, first_elem);
}