| /* ---------- |
| * pg_lzcompress.c - |
| * |
| * This is an implementation of LZ compression for PostgreSQL. |
| * It uses a simple history table and generates 2-3 byte tags |
| * capable of backward copy information for 3-273 bytes with |
| * a max offset of 4095. |
| * |
| * Entry routines: |
| * |
| * bool |
| * pglz_compress(const char *source, int32 slen, PGLZ_Header *dest, |
| * const PGLZ_Strategy *strategy); |
| * |
| * source is the input data to be compressed. |
| * |
| * slen is the length of the input data. |
| * |
| * dest is the output area for the compressed result. |
| * It must be at least as big as PGLZ_MAX_OUTPUT(slen). |
| * |
| * strategy is a pointer to some information controlling |
| * the compression algorithm. If NULL, the compiled |
| * in default strategy is used. |
| * |
| * The return value is TRUE if compression succeeded, |
| * FALSE if not; in the latter case the contents of dest |
| * are undefined. |
| * |
| * void |
| * pglz_decompress(const PGLZ_Header *source, char *dest) |
| * |
| * source is the compressed input. |
| * |
| * dest is the area where the uncompressed data will be |
| * written to. It is the callers responsibility to |
| * provide enough space. The required amount can be |
| * obtained with the macro PGLZ_RAW_SIZE(source). |
| * |
| * The data is written to buff exactly as it was handed |
| * to pglz_compress(). No terminating zero byte is added. |
| * |
| * The decompression algorithm and internal data format: |
| * |
| * PGLZ_Header is defined as |
| * |
| * typedef struct PGLZ_Header { |
| * int32 vl_len_; |
| * int32 rawsize; |
| * } |
| * |
| * The header is followed by the compressed data itself. |
| * |
| * The data representation is easiest explained by describing |
| * the process of decompression. |
| * |
| * If VARSIZE(x) == rawsize + sizeof(PGLZ_Header), then the data |
| * is stored uncompressed as plain bytes. Thus, the decompressor |
| * simply copies rawsize bytes from the location after the |
| * header to the destination. |
| * |
| * Otherwise the first byte after the header tells what to do |
| * the next 8 times. We call this the control byte. |
| * |
| * An unset bit in the control byte means, that one uncompressed |
| * byte follows, which is copied from input to output. |
| * |
| * A set bit in the control byte means, that a tag of 2-3 bytes |
| * follows. A tag contains information to copy some bytes, that |
| * are already in the output buffer, to the current location in |
| * the output. Let's call the three tag bytes T1, T2 and T3. The |
| * position of the data to copy is coded as an offset from the |
| * actual output position. |
| * |
| * The offset is in the upper nibble of T1 and in T2. |
| * The length is in the lower nibble of T1. |
| * |
| * So the 16 bits of a 2 byte tag are coded as |
| * |
| * 7---T1--0 7---T2--0 |
| * OOOO LLLL OOOO OOOO |
| * |
| * This limits the offset to 1-4095 (12 bits) and the length |
| * to 3-18 (4 bits) because 3 is always added to it. To emit |
| * a tag of 2 bytes with a length of 2 only saves one control |
| * bit. But we lose one byte in the possible length of a tag. |
| * |
| * In the actual implementation, the 2 byte tag's length is |
| * limited to 3-17, because the value 0xF in the length nibble |
| * has special meaning. It means, that the next following |
| * byte (T3) has to be added to the length value of 18. That |
| * makes total limits of 1-4095 for offset and 3-273 for length. |
| * |
| * Now that we have successfully decoded a tag. We simply copy |
| * the output that occurred <offset> bytes back to the current |
| * output location in the specified <length>. Thus, a |
| * sequence of 200 spaces (think about bpchar fields) could be |
| * coded in 4 bytes. One literal space and a three byte tag to |
| * copy 199 bytes with a -1 offset. Whow - that's a compression |
| * rate of 98%! Well, the implementation needs to save the |
| * original data size too, so we need another 4 bytes for it |
| * and end up with a total compression rate of 96%, what's still |
| * worth a Whow. |
| * |
| * The compression algorithm |
| * |
| * The following uses numbers used in the default strategy. |
| * |
| * The compressor works best for attributes of a size between |
| * 1K and 1M. For smaller items there's not that much chance of |
| * redundancy in the character sequence (except for large areas |
| * of identical bytes like trailing spaces) and for bigger ones |
| * our 4K maximum look-back distance is too small. |
| * |
| * The compressor creates a table for 8192 lists of positions. |
| * For each input position (except the last 3), a hash key is |
| * built from the 4 next input bytes and the position remembered |
| * in the appropriate list. Thus, the table points to linked |
| * lists of likely to be at least in the first 4 characters |
| * matching strings. This is done on the fly while the input |
| * is compressed into the output area. Table entries are only |
| * kept for the last 4096 input positions, since we cannot use |
| * back-pointers larger than that anyway. |
| * |
| * For each byte in the input, it's hash key (built from this |
| * byte and the next 3) is used to find the appropriate list |
| * in the table. The lists remember the positions of all bytes |
| * that had the same hash key in the past in increasing backward |
| * offset order. Now for all entries in the used lists, the |
| * match length is computed by comparing the characters from the |
| * entries position with the characters from the actual input |
| * position. |
| * |
| * The compressor starts with a so called "good_match" of 128. |
| * It is a "prefer speed against compression ratio" optimizer. |
| * So if the first entry looked at already has 128 or more |
| * matching characters, the lookup stops and that position is |
| * used for the next tag in the output. |
| * |
| * For each subsequent entry in the history list, the "good_match" |
| * is lowered by 10%. So the compressor will be more happy with |
| * short matches the farer it has to go back in the history. |
| * Another "speed against ratio" preference characteristic of |
| * the algorithm. |
| * |
| * Thus there are 3 stop conditions for the lookup of matches: |
| * |
| * - a match >= good_match is found |
| * - there are no more history entries to look at |
| * - the next history entry is already too far back |
| * to be coded into a tag. |
| * |
| * Finally the match algorithm checks that at least a match |
| * of 3 or more bytes has been found, because thats the smallest |
| * amount of copy information to code into a tag. If so, a tag |
| * is omitted and all the input bytes covered by that are just |
| * scanned for the history add's, otherwise a literal character |
| * is omitted and only his history entry added. |
| * |
| * Acknowledgements: |
| * |
| * Many thanks to Adisak Pochanayon, who's article about SLZ |
| * inspired me to write the PostgreSQL compression this way. |
| * |
| * Jan Wieck |
| * |
| * Copyright (c) 1999-2009, PostgreSQL Global Development Group |
| * |
| * $PostgreSQL: pgsql/src/backend/utils/adt/pg_lzcompress.c,v 1.34 2009/06/11 14:49:03 momjian Exp $ |
| * ---------- |
| */ |
| #include "postgres.h" |
| |
| #include <limits.h> |
| |
| #include "utils/pg_lzcompress.h" |
| |
| |
| /* ---------- |
| * Local definitions |
| * ---------- |
| */ |
| #define PGLZ_HISTORY_LISTS 8192 /* must be power of 2 */ |
| #define PGLZ_HISTORY_MASK (PGLZ_HISTORY_LISTS - 1) |
| #define PGLZ_HISTORY_SIZE 4096 |
| #define PGLZ_MAX_MATCH 273 |
| |
| |
| /* ---------- |
| * PGLZ_HistEntry - |
| * |
| * Linked list for the backward history lookup |
| * |
| * All the entries sharing a hash key are linked in a doubly linked list. |
| * This makes it easy to remove an entry when it's time to recycle it |
| * (because it's more than 4K positions old). |
| * ---------- |
| */ |
| typedef struct PGLZ_HistEntry |
| { |
| struct PGLZ_HistEntry *next; /* links for my hash key's list */ |
| struct PGLZ_HistEntry *prev; |
| int hindex; /* my current hash key */ |
| const char *pos; /* my input position */ |
| } PGLZ_HistEntry; |
| |
| |
| /* ---------- |
| * The provided standard strategies |
| * ---------- |
| */ |
| static const PGLZ_Strategy strategy_default_data = { |
| 32, /* Data chunks less than 32 bytes are not |
| * compressed */ |
| INT_MAX, /* No upper limit on what we'll try to |
| * compress */ |
| 25, /* Require 25% compression rate, or not worth |
| * it */ |
| 1024, /* Give up if no compression in the first 1KB */ |
| 128, /* Stop history lookup if a match of 128 bytes |
| * is found */ |
| 10 /* Lower good match size by 10% at every loop |
| * iteration */ |
| }; |
| const PGLZ_Strategy *const PGLZ_strategy_default = &strategy_default_data; |
| |
| |
| static const PGLZ_Strategy strategy_always_data = { |
| 0, /* Chunks of any size are compressed */ |
| INT_MAX, |
| 0, /* It's enough to save one single byte */ |
| INT_MAX, /* Never give up early */ |
| 128, /* Stop history lookup if a match of 128 bytes |
| * is found */ |
| 6 /* Look harder for a good match */ |
| }; |
| const PGLZ_Strategy *const PGLZ_strategy_always = &strategy_always_data; |
| |
| |
| /* ---------- |
| * Statically allocated work arrays for history |
| * ---------- |
| */ |
| static PGLZ_HistEntry *hist_start[PGLZ_HISTORY_LISTS]; |
| static PGLZ_HistEntry hist_entries[PGLZ_HISTORY_SIZE]; |
| |
| |
| /* ---------- |
| * pglz_hist_idx - |
| * |
| * Computes the history table slot for the lookup by the next 4 |
| * characters in the input. |
| * |
| * NB: because we use the next 4 characters, we are not guaranteed to |
| * find 3-character matches; they very possibly will be in the wrong |
| * hash list. This seems an acceptable tradeoff for spreading out the |
| * hash keys more. |
| * ---------- |
| */ |
| #define pglz_hist_idx(_s,_e) ( \ |
| ((((_e) - (_s)) < 4) ? (int) (_s)[0] : \ |
| (((_s)[0] << 9) ^ ((_s)[1] << 6) ^ \ |
| ((_s)[2] << 3) ^ (_s)[3])) & (PGLZ_HISTORY_MASK) \ |
| ) |
| |
| |
| /* ---------- |
| * pglz_hist_add - |
| * |
| * Adds a new entry to the history table. |
| * |
| * If _recycle is true, then we are recycling a previously used entry, |
| * and must first delink it from its old hashcode's linked list. |
| * |
| * NOTE: beware of multiple evaluations of macro's arguments, and note that |
| * _hn and _recycle are modified in the macro. |
| * ---------- |
| */ |
| #define pglz_hist_add(_hs,_he,_hn,_recycle,_s,_e) \ |
| do { \ |
| int __hindex = pglz_hist_idx((_s),(_e)); \ |
| PGLZ_HistEntry **__myhsp = &(_hs)[__hindex]; \ |
| PGLZ_HistEntry *__myhe = &(_he)[_hn]; \ |
| if (_recycle) { \ |
| if (__myhe->prev == NULL) \ |
| (_hs)[__myhe->hindex] = __myhe->next; \ |
| else \ |
| __myhe->prev->next = __myhe->next; \ |
| if (__myhe->next != NULL) \ |
| __myhe->next->prev = __myhe->prev; \ |
| } \ |
| __myhe->next = *__myhsp; \ |
| __myhe->prev = NULL; \ |
| __myhe->hindex = __hindex; \ |
| __myhe->pos = (_s); \ |
| if (*__myhsp != NULL) \ |
| (*__myhsp)->prev = __myhe; \ |
| *__myhsp = __myhe; \ |
| if (++(_hn) >= PGLZ_HISTORY_SIZE) { \ |
| (_hn) = 0; \ |
| (_recycle) = true; \ |
| } \ |
| } while (0) |
| |
| |
| /* ---------- |
| * pglz_out_ctrl - |
| * |
| * Outputs the last and allocates a new control byte if needed. |
| * ---------- |
| */ |
| #define pglz_out_ctrl(__ctrlp,__ctrlb,__ctrl,__buf) \ |
| do { \ |
| if ((__ctrl & 0xff) == 0) \ |
| { \ |
| *(__ctrlp) = __ctrlb; \ |
| __ctrlp = (__buf)++; \ |
| __ctrlb = 0; \ |
| __ctrl = 1; \ |
| } \ |
| } while (0) |
| |
| |
| /* ---------- |
| * pglz_out_literal - |
| * |
| * Outputs a literal byte to the destination buffer including the |
| * appropriate control bit. |
| * ---------- |
| */ |
| #define pglz_out_literal(_ctrlp,_ctrlb,_ctrl,_buf,_byte) \ |
| do { \ |
| pglz_out_ctrl(_ctrlp,_ctrlb,_ctrl,_buf); \ |
| *(_buf)++ = (unsigned char)(_byte); \ |
| _ctrl <<= 1; \ |
| } while (0) |
| |
| |
| /* ---------- |
| * pglz_out_tag - |
| * |
| * Outputs a backward reference tag of 2-4 bytes (depending on |
| * offset and length) to the destination buffer including the |
| * appropriate control bit. |
| * ---------- |
| */ |
| #define pglz_out_tag(_ctrlp,_ctrlb,_ctrl,_buf,_len,_off) \ |
| do { \ |
| pglz_out_ctrl(_ctrlp,_ctrlb,_ctrl,_buf); \ |
| _ctrlb |= _ctrl; \ |
| _ctrl <<= 1; \ |
| if (_len > 17) \ |
| { \ |
| (_buf)[0] = (unsigned char)((((_off) & 0xf00) >> 4) | 0x0f); \ |
| (_buf)[1] = (unsigned char)(((_off) & 0xff)); \ |
| (_buf)[2] = (unsigned char)((_len) - 18); \ |
| (_buf) += 3; \ |
| } else { \ |
| (_buf)[0] = (unsigned char)((((_off) & 0xf00) >> 4) | ((_len) - 3)); \ |
| (_buf)[1] = (unsigned char)((_off) & 0xff); \ |
| (_buf) += 2; \ |
| } \ |
| } while (0) |
| |
| |
| /* ---------- |
| * pglz_find_match - |
| * |
| * Lookup the history table if the actual input stream matches |
| * another sequence of characters, starting somewhere earlier |
| * in the input buffer. |
| * ---------- |
| */ |
| static inline int |
| pglz_find_match(PGLZ_HistEntry **hstart, const char *input, const char *end, |
| int *lenp, int *offp, int good_match, int good_drop) |
| { |
| PGLZ_HistEntry *hent; |
| int32 len = 0; |
| int32 off = 0; |
| |
| /* |
| * Traverse the linked history list until a good enough match is found. |
| */ |
| hent = hstart[pglz_hist_idx(input, end)]; |
| while (hent) |
| { |
| const char *ip = input; |
| const char *hp = hent->pos; |
| int32 thisoff; |
| int32 thislen; |
| |
| /* |
| * Stop if the offset does not fit into our tag anymore. |
| */ |
| thisoff = ip - hp; |
| if (thisoff >= 0x0fff) |
| break; |
| |
| /* |
| * Determine length of match. A better match must be larger than the |
| * best so far. And if we already have a match of 16 or more bytes, |
| * it's worth the call overhead to use memcmp() to check if this match |
| * is equal for the same size. After that we must fallback to |
| * character by character comparison to know the exact position where |
| * the diff occurred. |
| */ |
| thislen = 0; |
| if (len >= 16) |
| { |
| if (memcmp(ip, hp, len) == 0) |
| { |
| thislen = len; |
| ip += len; |
| hp += len; |
| while (ip < end && *ip == *hp && thislen < PGLZ_MAX_MATCH) |
| { |
| thislen++; |
| ip++; |
| hp++; |
| } |
| } |
| } |
| else |
| { |
| while (ip < end && *ip == *hp && thislen < PGLZ_MAX_MATCH) |
| { |
| thislen++; |
| ip++; |
| hp++; |
| } |
| } |
| |
| /* |
| * Remember this match as the best (if it is) |
| */ |
| if (thislen > len) |
| { |
| len = thislen; |
| off = thisoff; |
| } |
| |
| /* |
| * Advance to the next history entry |
| */ |
| hent = hent->next; |
| |
| /* |
| * Be happy with lesser good matches the more entries we visited. But |
| * no point in doing calculation if we're at end of list. |
| */ |
| if (hent) |
| { |
| if (len >= good_match) |
| break; |
| good_match -= (good_match * good_drop) / 100; |
| } |
| } |
| |
| /* |
| * Return match information only if it results at least in one byte |
| * reduction. |
| */ |
| if (len > 2) |
| { |
| *lenp = len; |
| *offp = off; |
| return 1; |
| } |
| |
| return 0; |
| } |
| |
| |
| /* ---------- |
| * pglz_compress - |
| * |
| * Compresses source into dest using strategy. |
| * ---------- |
| */ |
| bool |
| pglz_compress(const char *source, int32 slen, PGLZ_Header *dest, |
| const PGLZ_Strategy *strategy) |
| { |
| unsigned char *bp = ((unsigned char *) dest) + sizeof(PGLZ_Header); |
| unsigned char *bstart = bp; |
| int hist_next = 0; |
| bool hist_recycle = false; |
| const char *dp = source; |
| const char *dend = source + slen; |
| unsigned char ctrl_dummy = 0; |
| unsigned char *ctrlp = &ctrl_dummy; |
| unsigned char ctrlb = 0; |
| unsigned char ctrl = 0; |
| bool found_match = false; |
| int32 match_len; |
| int32 match_off; |
| int32 good_match; |
| int32 good_drop; |
| int32 result_size; |
| int32 result_max; |
| int32 need_rate; |
| |
| /* |
| * Our fallback strategy is the default. |
| */ |
| if (strategy == NULL) |
| strategy = PGLZ_strategy_default; |
| |
| /* |
| * If the strategy forbids compression (at all or if source chunk size out |
| * of range), fail. |
| */ |
| if (strategy->match_size_good <= 0 || |
| slen < strategy->min_input_size || |
| slen > strategy->max_input_size) |
| return false; |
| |
| /* |
| * Save the original source size in the header. |
| */ |
| dest->rawsize = slen; |
| |
| /* |
| * Limit the match parameters to the supported range. |
| */ |
| good_match = strategy->match_size_good; |
| if (good_match > PGLZ_MAX_MATCH) |
| good_match = PGLZ_MAX_MATCH; |
| else if (good_match < 17) |
| good_match = 17; |
| |
| good_drop = strategy->match_size_drop; |
| if (good_drop < 0) |
| good_drop = 0; |
| else if (good_drop > 100) |
| good_drop = 100; |
| |
| need_rate = strategy->min_comp_rate; |
| if (need_rate < 0) |
| need_rate = 0; |
| else if (need_rate > 99) |
| need_rate = 99; |
| |
| /* |
| * Compute the maximum result size allowed by the strategy, namely the |
| * input size minus the minimum wanted compression rate. This had better |
| * be <= slen, else we might overrun the provided output buffer. |
| */ |
| if (slen > (INT_MAX / 100)) |
| { |
| /* Approximate to avoid overflow */ |
| result_max = (slen / 100) * (100 - need_rate); |
| } |
| else |
| result_max = (slen * (100 - need_rate)) / 100; |
| |
| /* |
| * Initialize the history lists to empty. We do not need to zero the |
| * hist_entries[] array; its entries are initialized as they are used. |
| */ |
| memset(hist_start, 0, sizeof(hist_start)); |
| |
| /* |
| * Compress the source directly into the output buffer. |
| */ |
| while (dp < dend) |
| { |
| /* |
| * If we already exceeded the maximum result size, fail. |
| * |
| * We check once per loop; since the loop body could emit as many as 4 |
| * bytes (a control byte and 3-byte tag), PGLZ_MAX_OUTPUT() had better |
| * allow 4 slop bytes. |
| */ |
| if (bp - bstart >= result_max) |
| return false; |
| |
| /* |
| * If we've emitted more than first_success_by bytes without finding |
| * anything compressible at all, fail. This lets us fall out |
| * reasonably quickly when looking at incompressible input (such as |
| * pre-compressed data). |
| */ |
| if (!found_match && bp - bstart >= strategy->first_success_by) |
| return false; |
| |
| /* |
| * Try to find a match in the history |
| */ |
| if (pglz_find_match(hist_start, dp, dend, &match_len, |
| &match_off, good_match, good_drop)) |
| { |
| /* |
| * Create the tag and add history entries for all matched |
| * characters. |
| */ |
| pglz_out_tag(ctrlp, ctrlb, ctrl, bp, match_len, match_off); |
| while (match_len--) |
| { |
| pglz_hist_add(hist_start, hist_entries, |
| hist_next, hist_recycle, |
| dp, dend); |
| dp++; /* Do not do this ++ in the line above! */ |
| /* The macro would do it four times - Jan. */ |
| } |
| found_match = true; |
| } |
| else |
| { |
| /* |
| * No match found. Copy one literal byte. |
| */ |
| pglz_out_literal(ctrlp, ctrlb, ctrl, bp, *dp); |
| pglz_hist_add(hist_start, hist_entries, |
| hist_next, hist_recycle, |
| dp, dend); |
| dp++; /* Do not do this ++ in the line above! */ |
| /* The macro would do it four times - Jan. */ |
| } |
| } |
| |
| /* |
| * Write out the last control byte and check that we haven't overrun the |
| * output size allowed by the strategy. |
| */ |
| *ctrlp = ctrlb; |
| result_size = bp - bstart; |
| if (result_size >= result_max) |
| return false; |
| |
| /* |
| * Success - need only fill in the actual length of the compressed datum. |
| */ |
| SET_VARSIZE_COMPRESSED(dest, result_size + sizeof(PGLZ_Header)); |
| |
| return true; |
| } |
| |
| |
| /* ---------- |
| * pglz_decompress - |
| * |
| * Decompresses source into dest. |
| * ---------- |
| */ |
| void |
| pglz_decompress(const PGLZ_Header *source, char *dest) |
| { |
| const unsigned char *sp; |
| const unsigned char *srcend; |
| unsigned char *dp; |
| unsigned char *destend; |
| |
| sp = ((const unsigned char *) source) + sizeof(PGLZ_Header); |
| srcend = ((const unsigned char *) source) + VARSIZE(source); |
| dp = (unsigned char *) dest; |
| destend = dp + source->rawsize; |
| |
| while (sp < srcend && dp < destend) |
| { |
| /* |
| * Read one control byte and process the next 8 items (or as many as |
| * remain in the compressed input). |
| */ |
| unsigned char ctrl = *sp++; |
| int ctrlc; |
| |
| for (ctrlc = 0; ctrlc < 8 && sp < srcend; ctrlc++) |
| { |
| if (ctrl & 1) |
| { |
| /* |
| * Otherwise it contains the match length minus 3 and the |
| * upper 4 bits of the offset. The next following byte |
| * contains the lower 8 bits of the offset. If the length is |
| * coded as 18, another extension tag byte tells how much |
| * longer the match really was (0-255). |
| */ |
| int32 len; |
| int32 off; |
| |
| len = (sp[0] & 0x0f) + 3; |
| off = ((sp[0] & 0xf0) << 4) | sp[1]; |
| sp += 2; |
| if (len == 18) |
| len += *sp++; |
| |
| /* |
| * Check for output buffer overrun, to ensure we don't clobber |
| * memory in case of corrupt input. Note: we must advance dp |
| * here to ensure the error is detected below the loop. We |
| * don't simply put the elog inside the loop since that will |
| * probably interfere with optimization. |
| */ |
| if (dp + len > destend) |
| { |
| dp += len; |
| break; |
| } |
| |
| /* |
| * Now we copy the bytes specified by the tag from OUTPUT to |
| * OUTPUT. It is dangerous and platform dependent to use |
| * memcpy() here, because the copied areas could overlap |
| * extremely! |
| */ |
| while (len--) |
| { |
| *dp = dp[-off]; |
| dp++; |
| } |
| } |
| else |
| { |
| /* |
| * An unset control bit means LITERAL BYTE. So we just copy |
| * one from INPUT to OUTPUT. |
| */ |
| if (dp >= destend) /* check for buffer overrun */ |
| break; /* do not clobber memory */ |
| |
| *dp++ = *sp++; |
| } |
| |
| /* |
| * Advance the control bit |
| */ |
| ctrl >>= 1; |
| } |
| } |
| |
| /* |
| * Check we decompressed the right amount. |
| */ |
| if (dp != destend || sp != srcend) |
| elog(ERROR, "compressed data is corrupt"); |
| |
| /* |
| * That's it. |
| */ |
| } |