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[pgsql.git] / src / common / pg_lzcompress.c
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1 /* ----------
2 * pg_lzcompress.c -
3 *
4 * This is an implementation of LZ compression for PostgreSQL.
5 * It uses a simple history table and generates 2-3 byte tags
6 * capable of backward copy information for 3-273 bytes with
7 * a max offset of 4095.
8 *
9 * Entry routines:
10 *
11 * int32
12 * pglz_compress(const char *source, int32 slen, char *dest,
13 * const PGLZ_Strategy *strategy);
14 *
15 * source is the input data to be compressed.
16 *
17 * slen is the length of the input data.
18 *
19 * dest is the output area for the compressed result.
20 * It must be at least as big as PGLZ_MAX_OUTPUT(slen).
21 *
22 * strategy is a pointer to some information controlling
23 * the compression algorithm. If NULL, the compiled
24 * in default strategy is used.
25 *
26 * The return value is the number of bytes written in the
27 * buffer dest, or -1 if compression fails; in the latter
28 * case the contents of dest are undefined.
29 *
30 * int32
31 * pglz_decompress(const char *source, int32 slen, char *dest,
32 * int32 rawsize, bool check_complete)
33 *
34 * source is the compressed input.
35 *
36 * slen is the length of the compressed input.
37 *
38 * dest is the area where the uncompressed data will be
39 * written to. It is the callers responsibility to
40 * provide enough space.
41 *
42 * The data is written to buff exactly as it was handed
43 * to pglz_compress(). No terminating zero byte is added.
44 *
45 * rawsize is the length of the uncompressed data.
46 *
47 * check_complete is a flag to let us know if -1 should be
48 * returned in cases where we don't reach the end of the
49 * source or dest buffers, or not. This should be false
50 * if the caller is asking for only a partial result and
51 * true otherwise.
52 *
53 * The return value is the number of bytes written in the
54 * buffer dest, or -1 if decompression fails.
55 *
56 * The decompression algorithm and internal data format:
57 *
58 * It is made with the compressed data itself.
59 *
60 * The data representation is easiest explained by describing
61 * the process of decompression.
62 *
63 * If compressed_size == rawsize, then the data
64 * is stored uncompressed as plain bytes. Thus, the decompressor
65 * simply copies rawsize bytes to the destination.
66 *
67 * Otherwise the first byte tells what to do the next 8 times.
68 * We call this the control byte.
69 *
70 * An unset bit in the control byte means, that one uncompressed
71 * byte follows, which is copied from input to output.
72 *
73 * A set bit in the control byte means, that a tag of 2-3 bytes
74 * follows. A tag contains information to copy some bytes, that
75 * are already in the output buffer, to the current location in
76 * the output. Let's call the three tag bytes T1, T2 and T3. The
77 * position of the data to copy is coded as an offset from the
78 * actual output position.
79 *
80 * The offset is in the upper nibble of T1 and in T2.
81 * The length is in the lower nibble of T1.
82 *
83 * So the 16 bits of a 2 byte tag are coded as
84 *
85 * 7---T1--0 7---T2--0
86 * OOOO LLLL OOOO OOOO
87 *
88 * This limits the offset to 1-4095 (12 bits) and the length
89 * to 3-18 (4 bits) because 3 is always added to it. To emit
90 * a tag of 2 bytes with a length of 2 only saves one control
91 * bit. But we lose one byte in the possible length of a tag.
92 *
93 * In the actual implementation, the 2 byte tag's length is
94 * limited to 3-17, because the value 0xF in the length nibble
95 * has special meaning. It means, that the next following
96 * byte (T3) has to be added to the length value of 18. That
97 * makes total limits of 1-4095 for offset and 3-273 for length.
98 *
99 * Now that we have successfully decoded a tag. We simply copy
100 * the output that occurred <offset> bytes back to the current
101 * output location in the specified <length>. Thus, a
102 * sequence of 200 spaces (think about bpchar fields) could be
103 * coded in 4 bytes. One literal space and a three byte tag to
104 * copy 199 bytes with a -1 offset. Whow - that's a compression
105 * rate of 98%! Well, the implementation needs to save the
106 * original data size too, so we need another 4 bytes for it
107 * and end up with a total compression rate of 96%, what's still
108 * worth a Whow.
109 *
110 * The compression algorithm
111 *
112 * The following uses numbers used in the default strategy.
113 *
114 * The compressor works best for attributes of a size between
115 * 1K and 1M. For smaller items there's not that much chance of
116 * redundancy in the character sequence (except for large areas
117 * of identical bytes like trailing spaces) and for bigger ones
118 * our 4K maximum look-back distance is too small.
119 *
120 * The compressor creates a table for lists of positions.
121 * For each input position (except the last 3), a hash key is
122 * built from the 4 next input bytes and the position remembered
123 * in the appropriate list. Thus, the table points to linked
124 * lists of likely to be at least in the first 4 characters
125 * matching strings. This is done on the fly while the input
126 * is compressed into the output area. Table entries are only
127 * kept for the last 4096 input positions, since we cannot use
128 * back-pointers larger than that anyway. The size of the hash
129 * table is chosen based on the size of the input - a larger table
130 * has a larger startup cost, as it needs to be initialized to
131 * zero, but reduces the number of hash collisions on long inputs.
132 *
133 * For each byte in the input, its hash key (built from this
134 * byte and the next 3) is used to find the appropriate list
135 * in the table. The lists remember the positions of all bytes
136 * that had the same hash key in the past in increasing backward
137 * offset order. Now for all entries in the used lists, the
138 * match length is computed by comparing the characters from the
139 * entries position with the characters from the actual input
140 * position.
141 *
142 * The compressor starts with a so called "good_match" of 128.
143 * It is a "prefer speed against compression ratio" optimizer.
144 * So if the first entry looked at already has 128 or more
145 * matching characters, the lookup stops and that position is
146 * used for the next tag in the output.
147 *
148 * For each subsequent entry in the history list, the "good_match"
149 * is lowered by 10%. So the compressor will be more happy with
150 * short matches the further it has to go back in the history.
151 * Another "speed against ratio" preference characteristic of
152 * the algorithm.
153 *
154 * Thus there are 3 stop conditions for the lookup of matches:
155 *
156 * - a match >= good_match is found
157 * - there are no more history entries to look at
158 * - the next history entry is already too far back
159 * to be coded into a tag.
160 *
161 * Finally the match algorithm checks that at least a match
162 * of 3 or more bytes has been found, because that is the smallest
163 * amount of copy information to code into a tag. If so, a tag
164 * is omitted and all the input bytes covered by that are just
165 * scanned for the history add's, otherwise a literal character
166 * is omitted and only his history entry added.
167 *
168 * Acknowledgments:
169 *
170 * Many thanks to Adisak Pochanayon, who's article about SLZ
171 * inspired me to write the PostgreSQL compression this way.
172 *
173 * Jan Wieck
174 *
175 * Copyright (c) 1999-2024, PostgreSQL Global Development Group
176 *
177 * src/common/pg_lzcompress.c
178 * ----------
179 */
180 #ifndef FRONTEND
181 #include "postgres.h"
182 #else
183 #include "postgres_fe.h"
184 #endif
185
186 #include <limits.h>
187
188 #include "common/pg_lzcompress.h"
189
190
191 /* ----------
192 * Local definitions
193 * ----------
194 */
195 #define PGLZ_MAX_HISTORY_LISTS 8192 /* must be power of 2 */
196 #define PGLZ_HISTORY_SIZE 4096
197 #define PGLZ_MAX_MATCH 273
198
199
200 /* ----------
201 * PGLZ_HistEntry -
202 *
203 * Linked list for the backward history lookup
204 *
205 * All the entries sharing a hash key are linked in a doubly linked list.
206 * This makes it easy to remove an entry when it's time to recycle it
207 * (because it's more than 4K positions old).
208 * ----------
209 */
210 typedef struct PGLZ_HistEntry
211 {
212 struct PGLZ_HistEntry *next; /* links for my hash key's list */
213 struct PGLZ_HistEntry *prev;
214 int hindex; /* my current hash key */
215 const char *pos; /* my input position */
216 } PGLZ_HistEntry;
217
218
219 /* ----------
220 * The provided standard strategies
221 * ----------
222 */
223 static const PGLZ_Strategy strategy_default_data = {
224 32, /* Data chunks less than 32 bytes are not
225 * compressed */
226 INT_MAX, /* No upper limit on what we'll try to
227 * compress */
228 25, /* Require 25% compression rate, or not worth
229 * it */
230 1024, /* Give up if no compression in the first 1KB */
231 128, /* Stop history lookup if a match of 128 bytes
232 * is found */
233 10 /* Lower good match size by 10% at every loop
234 * iteration */
235 };
236 const PGLZ_Strategy *const PGLZ_strategy_default = &strategy_default_data;
237
238
239 static const PGLZ_Strategy strategy_always_data = {
240 0, /* Chunks of any size are compressed */
241 INT_MAX,
242 0, /* It's enough to save one single byte */
243 INT_MAX, /* Never give up early */
244 128, /* Stop history lookup if a match of 128 bytes
245 * is found */
246 6 /* Look harder for a good match */
247 };
248 const PGLZ_Strategy *const PGLZ_strategy_always = &strategy_always_data;
249
250
251 /* ----------
252 * Statically allocated work arrays for history
253 * ----------
254 */
255 static int16 hist_start[PGLZ_MAX_HISTORY_LISTS];
256 static PGLZ_HistEntry hist_entries[PGLZ_HISTORY_SIZE + 1];
257
258 /*
259 * Element 0 in hist_entries is unused, and means 'invalid'. Likewise,
260 * INVALID_ENTRY_PTR in next/prev pointers mean 'invalid'.
261 */
262 #define INVALID_ENTRY 0
263 #define INVALID_ENTRY_PTR (&hist_entries[INVALID_ENTRY])
264
265 /* ----------
266 * pglz_hist_idx -
267 *
268 * Computes the history table slot for the lookup by the next 4
269 * characters in the input.
270 *
271 * NB: because we use the next 4 characters, we are not guaranteed to
272 * find 3-character matches; they very possibly will be in the wrong
273 * hash list. This seems an acceptable tradeoff for spreading out the
274 * hash keys more.
275 * ----------
276 */
277 #define pglz_hist_idx(_s,_e, _mask) ( \
278 ((((_e) - (_s)) < 4) ? (int) (_s)[0] : \
279 (((_s)[0] << 6) ^ ((_s)[1] << 4) ^ \
280 ((_s)[2] << 2) ^ (_s)[3])) & (_mask) \
281 )
282
283
284 /* ----------
285 * pglz_hist_add -
286 *
287 * Adds a new entry to the history table.
288 *
289 * If _recycle is true, then we are recycling a previously used entry,
290 * and must first delink it from its old hashcode's linked list.
291 *
292 * NOTE: beware of multiple evaluations of macro's arguments, and note that
293 * _hn and _recycle are modified in the macro.
294 * ----------
295 */
296 #define pglz_hist_add(_hs,_he,_hn,_recycle,_s,_e, _mask) \
297 do { \
298 int __hindex = pglz_hist_idx((_s),(_e), (_mask)); \
299 int16 *__myhsp = &(_hs)[__hindex]; \
300 PGLZ_HistEntry *__myhe = &(_he)[_hn]; \
301 if (_recycle) { \
302 if (__myhe->prev == NULL) \
303 (_hs)[__myhe->hindex] = __myhe->next - (_he); \
304 else \
305 __myhe->prev->next = __myhe->next; \
306 if (__myhe->next != NULL) \
307 __myhe->next->prev = __myhe->prev; \
308 } \
309 __myhe->next = &(_he)[*__myhsp]; \
310 __myhe->prev = NULL; \
311 __myhe->hindex = __hindex; \
312 __myhe->pos = (_s); \
313 /* If there was an existing entry in this hash slot, link */ \
314 /* this new entry to it. However, the 0th entry in the */ \
315 /* entries table is unused, so we can freely scribble on it. */ \
316 /* So don't bother checking if the slot was used - we'll */ \
317 /* scribble on the unused entry if it was not, but that's */ \
318 /* harmless. Avoiding the branch in this critical path */ \
319 /* speeds this up a little bit. */ \
320 /* if (*__myhsp != INVALID_ENTRY) */ \
321 (_he)[(*__myhsp)].prev = __myhe; \
322 *__myhsp = _hn; \
323 if (++(_hn) >= PGLZ_HISTORY_SIZE + 1) { \
324 (_hn) = 1; \
325 (_recycle) = true; \
326 } \
327 } while (0)
328
329
330 /* ----------
331 * pglz_out_ctrl -
332 *
333 * Outputs the last and allocates a new control byte if needed.
334 * ----------
335 */
336 #define pglz_out_ctrl(__ctrlp,__ctrlb,__ctrl,__buf) \
337 do { \
338 if ((__ctrl & 0xff) == 0) \
339 { \
340 *(__ctrlp) = __ctrlb; \
341 __ctrlp = (__buf)++; \
342 __ctrlb = 0; \
343 __ctrl = 1; \
344 } \
345 } while (0)
346
347
348 /* ----------
349 * pglz_out_literal -
350 *
351 * Outputs a literal byte to the destination buffer including the
352 * appropriate control bit.
353 * ----------
354 */
355 #define pglz_out_literal(_ctrlp,_ctrlb,_ctrl,_buf,_byte) \
356 do { \
357 pglz_out_ctrl(_ctrlp,_ctrlb,_ctrl,_buf); \
358 *(_buf)++ = (unsigned char)(_byte); \
359 _ctrl <<= 1; \
360 } while (0)
361
362
363 /* ----------
364 * pglz_out_tag -
365 *
366 * Outputs a backward reference tag of 2-4 bytes (depending on
367 * offset and length) to the destination buffer including the
368 * appropriate control bit.
369 * ----------
370 */
371 #define pglz_out_tag(_ctrlp,_ctrlb,_ctrl,_buf,_len,_off) \
372 do { \
373 pglz_out_ctrl(_ctrlp,_ctrlb,_ctrl,_buf); \
374 _ctrlb |= _ctrl; \
375 _ctrl <<= 1; \
376 if (_len > 17) \
377 { \
378 (_buf)[0] = (unsigned char)((((_off) & 0xf00) >> 4) | 0x0f); \
379 (_buf)[1] = (unsigned char)(((_off) & 0xff)); \
380 (_buf)[2] = (unsigned char)((_len) - 18); \
381 (_buf) += 3; \
382 } else { \
383 (_buf)[0] = (unsigned char)((((_off) & 0xf00) >> 4) | ((_len) - 3)); \
384 (_buf)[1] = (unsigned char)((_off) & 0xff); \
385 (_buf) += 2; \
386 } \
387 } while (0)
388
389
390 /* ----------
391 * pglz_find_match -
392 *
393 * Lookup the history table if the actual input stream matches
394 * another sequence of characters, starting somewhere earlier
395 * in the input buffer.
396 * ----------
397 */
398 static inline int
399 pglz_find_match(int16 *hstart, const char *input, const char *end,
400 int *lenp, int *offp, int good_match, int good_drop, int mask)
401 {
402 PGLZ_HistEntry *hent;
403 int16 hentno;
404 int32 len = 0;
405 int32 off = 0;
406
407 /*
408 * Traverse the linked history list until a good enough match is found.
409 */
410 hentno = hstart[pglz_hist_idx(input, end, mask)];
411 hent = &hist_entries[hentno];
412 while (hent != INVALID_ENTRY_PTR)
413 {
414 const char *ip = input;
415 const char *hp = hent->pos;
416 int32 thisoff;
417 int32 thislen;
418
419 /*
420 * Stop if the offset does not fit into our tag anymore.
421 */
422 thisoff = ip - hp;
423 if (thisoff >= 0x0fff)
424 break;
425
426 /*
427 * Determine length of match. A better match must be larger than the
428 * best so far. And if we already have a match of 16 or more bytes,
429 * it's worth the call overhead to use memcmp() to check if this match
430 * is equal for the same size. After that we must fallback to
431 * character by character comparison to know the exact position where
432 * the diff occurred.
433 */
434 thislen = 0;
435 if (len >= 16)
436 {
437 if (memcmp(ip, hp, len) == 0)
438 {
439 thislen = len;
440 ip += len;
441 hp += len;
442 while (ip < end && *ip == *hp && thislen < PGLZ_MAX_MATCH)
443 {
444 thislen++;
445 ip++;
446 hp++;
447 }
448 }
449 }
450 else
451 {
452 while (ip < end && *ip == *hp && thislen < PGLZ_MAX_MATCH)
453 {
454 thislen++;
455 ip++;
456 hp++;
457 }
458 }
459
460 /*
461 * Remember this match as the best (if it is)
462 */
463 if (thislen > len)
464 {
465 len = thislen;
466 off = thisoff;
467 }
468
469 /*
470 * Advance to the next history entry
471 */
472 hent = hent->next;
473
474 /*
475 * Be happy with lesser good matches the more entries we visited. But
476 * no point in doing calculation if we're at end of list.
477 */
478 if (hent != INVALID_ENTRY_PTR)
479 {
480 if (len >= good_match)
481 break;
482 good_match -= (good_match * good_drop) / 100;
483 }
484 }
485
486 /*
487 * Return match information only if it results at least in one byte
488 * reduction.
489 */
490 if (len > 2)
491 {
492 *lenp = len;
493 *offp = off;
494 return 1;
495 }
496
497 return 0;
498 }
499
500
501 /* ----------
502 * pglz_compress -
503 *
504 * Compresses source into dest using strategy. Returns the number of
505 * bytes written in buffer dest, or -1 if compression fails.
506 * ----------
507 */
508 int32
509 pglz_compress(const char *source, int32 slen, char *dest,
510 const PGLZ_Strategy *strategy)
511 {
512 unsigned char *bp = (unsigned char *) dest;
513 unsigned char *bstart = bp;
514 int hist_next = 1;
515 bool hist_recycle = false;
516 const char *dp = source;
517 const char *dend = source + slen;
518 unsigned char ctrl_dummy = 0;
519 unsigned char *ctrlp = &ctrl_dummy;
520 unsigned char ctrlb = 0;
521 unsigned char ctrl = 0;
522 bool found_match = false;
523 int32 match_len;
524 int32 match_off;
525 int32 good_match;
526 int32 good_drop;
527 int32 result_size;
528 int32 result_max;
529 int32 need_rate;
530 int hashsz;
531 int mask;
532
533 /*
534 * Our fallback strategy is the default.
535 */
536 if (strategy == NULL)
537 strategy = PGLZ_strategy_default;
538
539 /*
540 * If the strategy forbids compression (at all or if source chunk size out
541 * of range), fail.
542 */
543 if (strategy->match_size_good <= 0 ||
544 slen < strategy->min_input_size ||
545 slen > strategy->max_input_size)
546 return -1;
547
548 /*
549 * Limit the match parameters to the supported range.
550 */
551 good_match = strategy->match_size_good;
552 if (good_match > PGLZ_MAX_MATCH)
553 good_match = PGLZ_MAX_MATCH;
554 else if (good_match < 17)
555 good_match = 17;
556
557 good_drop = strategy->match_size_drop;
558 if (good_drop < 0)
559 good_drop = 0;
560 else if (good_drop > 100)
561 good_drop = 100;
562
563 need_rate = strategy->min_comp_rate;
564 if (need_rate < 0)
565 need_rate = 0;
566 else if (need_rate > 99)
567 need_rate = 99;
568
569 /*
570 * Compute the maximum result size allowed by the strategy, namely the
571 * input size minus the minimum wanted compression rate. This had better
572 * be <= slen, else we might overrun the provided output buffer.
573 */
574 if (slen > (INT_MAX / 100))
575 {
576 /* Approximate to avoid overflow */
577 result_max = (slen / 100) * (100 - need_rate);
578 }
579 else
580 result_max = (slen * (100 - need_rate)) / 100;
581
582 /*
583 * Experiments suggest that these hash sizes work pretty well. A large
584 * hash table minimizes collision, but has a higher startup cost. For a
585 * small input, the startup cost dominates. The table size must be a power
586 * of two.
587 */
588 if (slen < 128)
589 hashsz = 512;
590 else if (slen < 256)
591 hashsz = 1024;
592 else if (slen < 512)
593 hashsz = 2048;
594 else if (slen < 1024)
595 hashsz = 4096;
596 else
597 hashsz = 8192;
598 mask = hashsz - 1;
599
600 /*
601 * Initialize the history lists to empty. We do not need to zero the
602 * hist_entries[] array; its entries are initialized as they are used.
603 */
604 memset(hist_start, 0, hashsz * sizeof(int16));
605
606 /*
607 * Compress the source directly into the output buffer.
608 */
609 while (dp < dend)
610 {
611 /*
612 * If we already exceeded the maximum result size, fail.
613 *
614 * We check once per loop; since the loop body could emit as many as 4
615 * bytes (a control byte and 3-byte tag), PGLZ_MAX_OUTPUT() had better
616 * allow 4 slop bytes.
617 */
618 if (bp - bstart >= result_max)
619 return -1;
620
621 /*
622 * If we've emitted more than first_success_by bytes without finding
623 * anything compressible at all, fail. This lets us fall out
624 * reasonably quickly when looking at incompressible input (such as
625 * pre-compressed data).
626 */
627 if (!found_match && bp - bstart >= strategy->first_success_by)
628 return -1;
629
630 /*
631 * Try to find a match in the history
632 */
633 if (pglz_find_match(hist_start, dp, dend, &match_len,
634 &match_off, good_match, good_drop, mask))
635 {
636 /*
637 * Create the tag and add history entries for all matched
638 * characters.
639 */
640 pglz_out_tag(ctrlp, ctrlb, ctrl, bp, match_len, match_off);
641 while (match_len--)
642 {
643 pglz_hist_add(hist_start, hist_entries,
644 hist_next, hist_recycle,
645 dp, dend, mask);
646 dp++; /* Do not do this ++ in the line above! */
647 /* The macro would do it four times - Jan. */
648 }
649 found_match = true;
650 }
651 else
652 {
653 /*
654 * No match found. Copy one literal byte.
655 */
656 pglz_out_literal(ctrlp, ctrlb, ctrl, bp, *dp);
657 pglz_hist_add(hist_start, hist_entries,
658 hist_next, hist_recycle,
659 dp, dend, mask);
660 dp++; /* Do not do this ++ in the line above! */
661 /* The macro would do it four times - Jan. */
662 }
663 }
664
665 /*
666 * Write out the last control byte and check that we haven't overrun the
667 * output size allowed by the strategy.
668 */
669 *ctrlp = ctrlb;
670 result_size = bp - bstart;
671 if (result_size >= result_max)
672 return -1;
673
674 /* success */
675 return result_size;
676 }
677
678
679 /* ----------
680 * pglz_decompress -
681 *
682 * Decompresses source into dest. Returns the number of bytes
683 * decompressed into the destination buffer, or -1 if the
684 * compressed data is corrupted.
685 *
686 * If check_complete is true, the data is considered corrupted
687 * if we don't exactly fill the destination buffer. Callers that
688 * are extracting a slice typically can't apply this check.
689 * ----------
690 */
691 int32
692 pglz_decompress(const char *source, int32 slen, char *dest,
693 int32 rawsize, bool check_complete)
694 {
695 const unsigned char *sp;
696 const unsigned char *srcend;
697 unsigned char *dp;
698 unsigned char *destend;
699
700 sp = (const unsigned char *) source;
701 srcend = ((const unsigned char *) source) + slen;
702 dp = (unsigned char *) dest;
703 destend = dp + rawsize;
704
705 while (sp < srcend && dp < destend)
706 {
707 /*
708 * Read one control byte and process the next 8 items (or as many as
709 * remain in the compressed input).
710 */
711 unsigned char ctrl = *sp++;
712 int ctrlc;
713
714 for (ctrlc = 0; ctrlc < 8 && sp < srcend && dp < destend; ctrlc++)
715 {
716 if (ctrl & 1)
717 {
718 /*
719 * Set control bit means we must read a match tag. The match
720 * is coded with two bytes. First byte uses lower nibble to
721 * code length - 3. Higher nibble contains upper 4 bits of the
722 * offset. The next following byte contains the lower 8 bits
723 * of the offset. If the length is coded as 18, another
724 * extension tag byte tells how much longer the match really
725 * was (0-255).
726 */
727 int32 len;
728 int32 off;
729
730 len = (sp[0] & 0x0f) + 3;
731 off = ((sp[0] & 0xf0) << 4) | sp[1];
732 sp += 2;
733 if (len == 18)
734 len += *sp++;
735
736 /*
737 * Check for corrupt data: if we fell off the end of the
738 * source, or if we obtained off = 0, or if off is more than
739 * the distance back to the buffer start, we have problems.
740 * (We must check for off = 0, else we risk an infinite loop
741 * below in the face of corrupt data. Likewise, the upper
742 * limit on off prevents accessing outside the buffer
743 * boundaries.)
744 */
745 if (unlikely(sp > srcend || off == 0 ||
746 off > (dp - (unsigned char *) dest)))
747 return -1;
748
749 /*
750 * Don't emit more data than requested.
751 */
752 len = Min(len, destend - dp);
753
754 /*
755 * Now we copy the bytes specified by the tag from OUTPUT to
756 * OUTPUT (copy len bytes from dp - off to dp). The copied
757 * areas could overlap, so to avoid undefined behavior in
758 * memcpy(), be careful to copy only non-overlapping regions.
759 *
760 * Note that we cannot use memmove() instead, since while its
761 * behavior is well-defined, it's also not what we want.
762 */
763 while (off < len)
764 {
765 /*
766 * We can safely copy "off" bytes since that clearly
767 * results in non-overlapping source and destination.
768 */
769 memcpy(dp, dp - off, off);
770 len -= off;
771 dp += off;
772
773 /*----------
774 * This bit is less obvious: we can double "off" after
775 * each such step. Consider this raw input:
776 * 112341234123412341234
777 * This will be encoded as 5 literal bytes "11234" and
778 * then a match tag with length 16 and offset 4. After
779 * memcpy'ing the first 4 bytes, we will have emitted
780 * 112341234
781 * so we can double "off" to 8, then after the next step
782 * we have emitted
783 * 11234123412341234
784 * Then we can double "off" again, after which it is more
785 * than the remaining "len" so we fall out of this loop
786 * and finish with a non-overlapping copy of the
787 * remainder. In general, a match tag with off < len
788 * implies that the decoded data has a repeat length of
789 * "off". We can handle 1, 2, 4, etc repetitions of the
790 * repeated string per memcpy until we get to a situation
791 * where the final copy step is non-overlapping.
792 *
793 * (Another way to understand this is that we are keeping
794 * the copy source point dp - off the same throughout.)
795 *----------
796 */
797 off += off;
798 }
799 memcpy(dp, dp - off, len);
800 dp += len;
801 }
802 else
803 {
804 /*
805 * An unset control bit means LITERAL BYTE. So we just copy
806 * one from INPUT to OUTPUT.
807 */
808 *dp++ = *sp++;
809 }
810
811 /*
812 * Advance the control bit
813 */
814 ctrl >>= 1;
815 }
816 }
817
818 /*
819 * If requested, check we decompressed the right amount.
820 */
821 if (check_complete && (dp != destend || sp != srcend))
822 return -1;
823
824 /*
825 * That's it.
826 */
827 return (char *) dp - dest;
828 }
829
830
831 /* ----------
832 * pglz_maximum_compressed_size -
833 *
834 * Calculate the maximum compressed size for a given amount of raw data.
835 * Return the maximum size, or total compressed size if maximum size is
836 * larger than total compressed size.
837 *
838 * We can't use PGLZ_MAX_OUTPUT for this purpose, because that's used to size
839 * the compression buffer (and abort the compression). It does not really say
840 * what's the maximum compressed size for an input of a given length, and it
841 * may happen that while the whole value is compressible (and thus fits into
842 * PGLZ_MAX_OUTPUT nicely), the prefix is not compressible at all.
843 * ----------
844 */
845 int32
846 pglz_maximum_compressed_size(int32 rawsize, int32 total_compressed_size)
847 {
848 int64 compressed_size;
849
850 /*
851 * pglz uses one control bit per byte, so if the entire desired prefix is
852 * represented as literal bytes, we'll need (rawsize * 9) bits. We care
853 * about bytes though, so be sure to round up not down.
854 *
855 * Use int64 here to prevent overflow during calculation.
856 */
857 compressed_size = ((int64) rawsize * 9 + 7) / 8;
858
859 /*
860 * The above fails to account for a corner case: we could have compressed
861 * data that starts with N-1 or N-2 literal bytes and then has a match tag
862 * of 2 or 3 bytes. It's therefore possible that we need to fetch 1 or 2
863 * more bytes in order to have the whole match tag. (Match tags earlier
864 * in the compressed data don't cause a problem, since they should
865 * represent more decompressed bytes than they occupy themselves.)
866 */
867 compressed_size += 2;
868
869 /*
870 * Maximum compressed size can't be larger than total compressed size.
871 * (This also ensures that our result fits in int32.)
872 */
873 compressed_size = Min(compressed_size, total_compressed_size);
874
875 return (int32) compressed_size;
876 }
Postgresql Clone from git://git.postgresql.org/git/postgresql.git