Age Owner Branch data TLA Line data Source code
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
3940 heikki.linnakangas@i 399 :CBC 26081844 : 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;
8768 bruce@momjian.us 404 : 26081844 : int32 len = 0;
405 : 26081844 : int32 off = 0;
406 : :
407 : : /*
408 : : * Traverse the linked history list until a good enough match is found.
409 : : */
3940 heikki.linnakangas@i 410 [ + + ]: 26081844 : hentno = hstart[pglz_hist_idx(input, end, mask)];
411 : 26081844 : hent = &hist_entries[hentno];
412 [ + + ]: 96063578 : while (hent != INVALID_ENTRY_PTR)
413 : : {
6401 tgl@sss.pgh.pa.us 414 : 70882885 : const char *ip = input;
415 : 70882885 : 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 : : */
8184 422 : 70882885 : thisoff = ip - hp;
8915 JanWieck@Yahoo.com 423 [ + + ]: 70882885 : if (thisoff >= 0x0fff)
424 : 17485 : 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 : : */
8184 tgl@sss.pgh.pa.us 434 : 70865400 : thislen = 0;
8915 JanWieck@Yahoo.com 435 [ + + ]: 70865400 : if (len >= 16)
436 : : {
8184 tgl@sss.pgh.pa.us 437 [ + + ]: 5637566 : if (memcmp(ip, hp, len) == 0)
438 : : {
439 : 1951545 : thislen = len;
440 : 1951545 : ip += len;
441 : 1951545 : hp += len;
442 [ + + + + : 7346488 : while (ip < end && *ip == *hp && thislen < PGLZ_MAX_MATCH)
+ + ]
443 : : {
444 : 5394943 : thislen++;
445 : 5394943 : ip++;
446 : 5394943 : hp++;
447 : : }
448 : : }
449 : : }
450 : : else
451 : : {
452 [ + + + + : 417929865 : while (ip < end && *ip == *hp && thislen < PGLZ_MAX_MATCH)
+ + ]
453 : : {
454 : 352702031 : thislen++;
455 : 352702031 : ip++;
456 : 352702031 : hp++;
457 : : }
458 : : }
459 : :
460 : : /*
461 : : * Remember this match as the best (if it is)
462 : : */
8915 JanWieck@Yahoo.com 463 [ + + ]: 70865400 : if (thislen > len)
464 : : {
465 : 11782215 : len = thislen;
466 : 11782215 : off = thisoff;
467 : : }
468 : :
469 : : /*
470 : : * Advance to the next history entry
471 : : */
472 : 70865400 : 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 : : */
3924 heikki.linnakangas@i 478 [ + + ]: 70865400 : if (hent != INVALID_ENTRY_PTR)
479 : : {
8184 tgl@sss.pgh.pa.us 480 [ + + ]: 59494606 : if (len >= good_match)
481 : 883666 : break;
482 : 58610940 : 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 : : */
8915 JanWieck@Yahoo.com 490 [ + + ]: 26081844 : if (len > 2)
491 : : {
492 : 6881383 : *lenp = len;
493 : 6881383 : *offp = off;
494 : 6881383 : return 1;
495 : : }
496 : :
497 : 19200461 : 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
3352 fujii@postgresql.org 509 : 22033 : pglz_compress(const char *source, int32 slen, char *dest,
510 : : const PGLZ_Strategy *strategy)
511 : : {
512 : 22033 : unsigned char *bp = (unsigned char *) dest;
8768 bruce@momjian.us 513 : 22033 : unsigned char *bstart = bp;
3940 heikki.linnakangas@i 514 : 22033 : int hist_next = 1;
8184 tgl@sss.pgh.pa.us 515 : 22033 : bool hist_recycle = false;
6401 516 : 22033 : const char *dp = source;
517 : 22033 : const char *dend = source + slen;
8768 bruce@momjian.us 518 : 22033 : unsigned char ctrl_dummy = 0;
519 : 22033 : unsigned char *ctrlp = &ctrl_dummy;
520 : 22033 : unsigned char ctrlb = 0;
521 : 22033 : unsigned char ctrl = 0;
5882 tgl@sss.pgh.pa.us 522 : 22033 : 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 : : */
8915 JanWieck@Yahoo.com 536 [ + - ]: 22033 : if (strategy == NULL)
537 : 22033 : 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 : : */
6098 tgl@sss.pgh.pa.us 543 [ + - ]: 22033 : if (strategy->match_size_good <= 0 ||
5882 544 [ + - ]: 22033 : slen < strategy->min_input_size ||
545 [ - + ]: 22033 : slen > strategy->max_input_size)
3352 fujii@postgresql.org 546 :UBC 0 : return -1;
547 : :
548 : : /*
549 : : * Limit the match parameters to the supported range.
550 : : */
5882 tgl@sss.pgh.pa.us 551 :CBC 22033 : good_match = strategy->match_size_good;
552 [ - + ]: 22033 : if (good_match > PGLZ_MAX_MATCH)
8915 JanWieck@Yahoo.com 553 :UBC 0 : good_match = PGLZ_MAX_MATCH;
5882 tgl@sss.pgh.pa.us 554 [ - + ]:CBC 22033 : else if (good_match < 17)
8915 JanWieck@Yahoo.com 555 :UBC 0 : good_match = 17;
556 : :
5882 tgl@sss.pgh.pa.us 557 :CBC 22033 : good_drop = strategy->match_size_drop;
558 [ - + ]: 22033 : if (good_drop < 0)
8915 JanWieck@Yahoo.com 559 :UBC 0 : good_drop = 0;
5882 tgl@sss.pgh.pa.us 560 [ - + ]:CBC 22033 : else if (good_drop > 100)
8915 JanWieck@Yahoo.com 561 :UBC 0 : good_drop = 100;
562 : :
5882 tgl@sss.pgh.pa.us 563 :CBC 22033 : need_rate = strategy->min_comp_rate;
564 [ - + ]: 22033 : if (need_rate < 0)
5882 tgl@sss.pgh.pa.us 565 :UBC 0 : need_rate = 0;
5882 tgl@sss.pgh.pa.us 566 [ - + ]:CBC 22033 : else if (need_rate > 99)
5882 tgl@sss.pgh.pa.us 567 :UBC 0 : 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 : : */
5421 bruce@momjian.us 574 [ - + ]:CBC 22033 : if (slen > (INT_MAX / 100))
575 : : {
576 : : /* Approximate to avoid overflow */
5882 tgl@sss.pgh.pa.us 577 :UBC 0 : result_max = (slen / 100) * (100 - need_rate);
578 : : }
579 : : else
5882 tgl@sss.pgh.pa.us 580 :CBC 22033 : 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 : : */
3940 heikki.linnakangas@i 588 [ + + ]: 22033 : if (slen < 128)
589 : 115 : hashsz = 512;
590 [ + + ]: 21918 : else if (slen < 256)
591 : 12 : hashsz = 1024;
592 [ + + ]: 21906 : else if (slen < 512)
593 : 466 : hashsz = 2048;
594 [ + + ]: 21440 : else if (slen < 1024)
595 : 3511 : hashsz = 4096;
596 : : else
597 : 17929 : hashsz = 8192;
598 : 22033 : 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 : 22033 : memset(hist_start, 0, hashsz * sizeof(int16));
605 : :
606 : : /*
607 : : * Compress the source directly into the output buffer.
608 : : */
8915 JanWieck@Yahoo.com 609 [ + + ]: 26103877 : 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 [ + + ]: 26087308 : if (bp - bstart >= result_max)
3352 fujii@postgresql.org 619 : 5463 : 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 : : */
5882 tgl@sss.pgh.pa.us 627 [ + + + + ]: 26081845 : if (!found_match && bp - bstart >= strategy->first_success_by)
3352 fujii@postgresql.org 628 :GBC 1 : return -1;
629 : :
630 : : /*
631 : : * Try to find a match in the history
632 : : */
8768 bruce@momjian.us 633 [ + + ]:CBC 26081844 : 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 : : */
8915 JanWieck@Yahoo.com 640 [ + + + + ]: 6881383 : pglz_out_tag(ctrlp, ctrlb, ctrl, bp, match_len, match_off);
8768 bruce@momjian.us 641 [ + + ]: 170560631 : while (match_len--)
642 : : {
8184 tgl@sss.pgh.pa.us 643 [ + + + + : 163679248 : pglz_hist_add(hist_start, hist_entries,
+ + + - +
+ ]
644 : : hist_next, hist_recycle,
645 : : dp, dend, mask);
5882 646 : 163679248 : dp++; /* Do not do this ++ in the line above! */
647 : : /* The macro would do it four times - Jan. */
648 : : }
649 : 6881383 : found_match = true;
650 : : }
651 : : else
652 : : {
653 : : /*
654 : : * No match found. Copy one literal byte.
655 : : */
8915 JanWieck@Yahoo.com 656 [ + + ]: 19200461 : pglz_out_literal(ctrlp, ctrlb, ctrl, bp, *dp);
8184 tgl@sss.pgh.pa.us 657 [ + + + + : 19200461 : pglz_hist_add(hist_start, hist_entries,
+ + + - +
+ ]
658 : : hist_next, hist_recycle,
659 : : dp, dend, mask);
5882 660 : 19200461 : 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 : : */
6401 669 : 16569 : *ctrlp = ctrlb;
670 : 16569 : result_size = bp - bstart;
671 [ + + ]: 16569 : if (result_size >= result_max)
3352 fujii@postgresql.org 672 :GBC 1 : return -1;
673 : :
674 : : /* success */
3352 fujii@postgresql.org 675 :CBC 16568 : 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 : 65532 : 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 : 65532 : sp = (const unsigned char *) source;
701 : 65532 : srcend = ((const unsigned char *) source) + slen;
5881 tgl@sss.pgh.pa.us 702 : 65532 : dp = (unsigned char *) dest;
3352 fujii@postgresql.org 703 : 65532 : destend = dp + rawsize;
704 : :
5881 tgl@sss.pgh.pa.us 705 [ + + + + ]: 7818047 : 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 : 7752515 : unsigned char ctrl = *sp++;
712 : : int ctrlc;
713 : :
1839 sfrost@snowman.net 714 [ + + + + : 69548270 : for (ctrlc = 0; ctrlc < 8 && sp < srcend && dp < destend; ctrlc++)
+ + ]
715 : : {
8915 JanWieck@Yahoo.com 716 [ + + ]: 61795755 : 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 : :
5881 tgl@sss.pgh.pa.us 730 : 20487864 : len = (sp[0] & 0x0f) + 3;
731 : 20487864 : off = ((sp[0] & 0xf0) << 4) | sp[1];
732 : 20487864 : sp += 2;
8915 JanWieck@Yahoo.com 733 [ + + ]: 20487864 : if (len == 18)
5881 tgl@sss.pgh.pa.us 734 : 3466601 : 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 : : */
179 745 [ + - + - : 20487864 : if (unlikely(sp > srcend || off == 0 ||
- + - + -
+ ]
746 : : off > (dp - (unsigned char *) dest)))
1259 tgl@sss.pgh.pa.us 747 :UBC 0 : return -1;
748 : :
749 : : /*
750 : : * Don't emit more data than requested.
751 : : */
1839 sfrost@snowman.net 752 :CBC 20487864 : 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 : : */
1599 tomas.vondra@postgre 763 [ + + ]: 29214006 : 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 : 8726142 : memcpy(dp, dp - off, off);
770 : 8726142 : len -= off;
771 : 8726142 : 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 : 8726142 : off += off;
798 : : }
799 : 20487864 : memcpy(dp, dp - off, len);
800 : 20487864 : 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 : : */
5881 tgl@sss.pgh.pa.us 808 : 41307891 : *dp++ = *sp++;
809 : : }
810 : :
811 : : /*
812 : : * Advance the control bit
813 : : */
8915 JanWieck@Yahoo.com 814 : 61795755 : ctrl >>= 1;
815 : : }
816 : : }
817 : :
818 : : /*
819 : : * If requested, check we decompressed the right amount.
820 : : */
1839 sfrost@snowman.net 821 [ + + + - : 65532 : if (check_complete && (dp != destend || sp != srcend))
- + ]
3352 fujii@postgresql.org 822 :UBC 0 : return -1;
823 : :
824 : : /*
825 : : * That's it.
826 : : */
1789 tgl@sss.pgh.pa.us 827 :CBC 65532 : 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
1657 tomas.vondra@postgre 846 : 18 : 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 : : */
1260 tgl@sss.pgh.pa.us 857 : 18 : 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 : 18 : 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 : : */
1657 tomas.vondra@postgre 873 : 18 : compressed_size = Min(compressed_size, total_compressed_size);
874 : :
1260 tgl@sss.pgh.pa.us 875 : 18 : return (int32) compressed_size;
876 : : }
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