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Diffstat (limited to 'src/common/pg_lzcompress.c')
| -rw-r--r-- | src/common/pg_lzcompress.c | 876 |
1 files changed, 876 insertions, 0 deletions
diff --git a/src/common/pg_lzcompress.c b/src/common/pg_lzcompress.c new file mode 100644 index 0000000..ad3970c --- /dev/null +++ b/src/common/pg_lzcompress.c @@ -0,0 +1,876 @@ +/* ---------- + * 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: + * + * int32 + * pglz_compress(const char *source, int32 slen, char *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 the number of bytes written in the + * buffer dest, or -1 if compression fails; in the latter + * case the contents of dest are undefined. + * + * int32 + * pglz_decompress(const char *source, int32 slen, char *dest, + * int32 rawsize, bool check_complete) + * + * source is the compressed input. + * + * slen is the length of 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 data is written to buff exactly as it was handed + * to pglz_compress(). No terminating zero byte is added. + * + * rawsize is the length of the uncompressed data. + * + * check_complete is a flag to let us know if -1 should be + * returned in cases where we don't reach the end of the + * source or dest buffers, or not. This should be false + * if the caller is asking for only a partial result and + * true otherwise. + * + * The return value is the number of bytes written in the + * buffer dest, or -1 if decompression fails. + * + * The decompression algorithm and internal data format: + * + * It is made with the compressed data itself. + * + * The data representation is easiest explained by describing + * the process of decompression. + * + * If compressed_size == rawsize, then the data + * is stored uncompressed as plain bytes. Thus, the decompressor + * simply copies rawsize bytes to the destination. + * + * Otherwise the first byte 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 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. The size of the hash + * table is chosen based on the size of the input - a larger table + * has a larger startup cost, as it needs to be initialized to + * zero, but reduces the number of hash collisions on long inputs. + * + * For each byte in the input, its 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 further 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 that is 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. + * + * Acknowledgments: + * + * Many thanks to Adisak Pochanayon, who's article about SLZ + * inspired me to write the PostgreSQL compression this way. + * + * Jan Wieck + * + * Copyright (c) 1999-2022, PostgreSQL Global Development Group + * + * src/common/pg_lzcompress.c + * ---------- + */ +#ifndef FRONTEND +#include "postgres.h" +#else +#include "postgres_fe.h" +#endif + +#include <limits.h> + +#include "common/pg_lzcompress.h" + + +/* ---------- + * Local definitions + * ---------- + */ +#define PGLZ_MAX_HISTORY_LISTS 8192 /* must be power of 2 */ +#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 int16 hist_start[PGLZ_MAX_HISTORY_LISTS]; +static PGLZ_HistEntry hist_entries[PGLZ_HISTORY_SIZE + 1]; + +/* + * Element 0 in hist_entries is unused, and means 'invalid'. Likewise, + * INVALID_ENTRY_PTR in next/prev pointers mean 'invalid'. + */ +#define INVALID_ENTRY 0 +#define INVALID_ENTRY_PTR (&hist_entries[INVALID_ENTRY]) + +/* ---------- + * 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, _mask) ( \ + ((((_e) - (_s)) < 4) ? (int) (_s)[0] : \ + (((_s)[0] << 6) ^ ((_s)[1] << 4) ^ \ + ((_s)[2] << 2) ^ (_s)[3])) & (_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, _mask) \ +do { \ + int __hindex = pglz_hist_idx((_s),(_e), (_mask)); \ + int16 *__myhsp = &(_hs)[__hindex]; \ + PGLZ_HistEntry *__myhe = &(_he)[_hn]; \ + if (_recycle) { \ + if (__myhe->prev == NULL) \ + (_hs)[__myhe->hindex] = __myhe->next - (_he); \ + else \ + __myhe->prev->next = __myhe->next; \ + if (__myhe->next != NULL) \ + __myhe->next->prev = __myhe->prev; \ + } \ + __myhe->next = &(_he)[*__myhsp]; \ + __myhe->prev = NULL; \ + __myhe->hindex = __hindex; \ + __myhe->pos = (_s); \ + /* If there was an existing entry in this hash slot, link */ \ + /* this new entry to it. However, the 0th entry in the */ \ + /* entries table is unused, so we can freely scribble on it. */ \ + /* So don't bother checking if the slot was used - we'll */ \ + /* scribble on the unused entry if it was not, but that's */ \ + /* harmless. Avoiding the branch in this critical path */ \ + /* speeds this up a little bit. */ \ + /* if (*__myhsp != INVALID_ENTRY) */ \ + (_he)[(*__myhsp)].prev = __myhe; \ + *__myhsp = _hn; \ + if (++(_hn) >= PGLZ_HISTORY_SIZE + 1) { \ + (_hn) = 1; \ + (_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(int16 *hstart, const char *input, const char *end, + int *lenp, int *offp, int good_match, int good_drop, int mask) +{ + PGLZ_HistEntry *hent; + int16 hentno; + int32 len = 0; + int32 off = 0; + + /* + * Traverse the linked history list until a good enough match is found. + */ + hentno = hstart[pglz_hist_idx(input, end, mask)]; + hent = &hist_entries[hentno]; + while (hent != INVALID_ENTRY_PTR) + { + 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 != INVALID_ENTRY_PTR) + { + 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. Returns the number of + * bytes written in buffer dest, or -1 if compression fails. + * ---------- + */ +int32 +pglz_compress(const char *source, int32 slen, char *dest, + const PGLZ_Strategy *strategy) +{ + unsigned char *bp = (unsigned char *) dest; + unsigned char *bstart = bp; + int hist_next = 1; + 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; + int hashsz; + int mask; + + /* + * 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 -1; + + /* + * 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; + + /* + * Experiments suggest that these hash sizes work pretty well. A large + * hash table minimizes collision, but has a higher startup cost. For a + * small input, the startup cost dominates. The table size must be a power + * of two. + */ + if (slen < 128) + hashsz = 512; + else if (slen < 256) + hashsz = 1024; + else if (slen < 512) + hashsz = 2048; + else if (slen < 1024) + hashsz = 4096; + else + hashsz = 8192; + mask = hashsz - 1; + + /* + * 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, hashsz * sizeof(int16)); + + /* + * 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 -1; + + /* + * 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 -1; + + /* + * Try to find a match in the history + */ + if (pglz_find_match(hist_start, dp, dend, &match_len, + &match_off, good_match, good_drop, mask)) + { + /* + * 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, mask); + 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, mask); + 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 -1; + + /* success */ + return result_size; +} + + +/* ---------- + * pglz_decompress - + * + * Decompresses source into dest. Returns the number of bytes + * decompressed into the destination buffer, or -1 if the + * compressed data is corrupted. + * + * If check_complete is true, the data is considered corrupted + * if we don't exactly fill the destination buffer. Callers that + * are extracting a slice typically can't apply this check. + * ---------- + */ +int32 +pglz_decompress(const char *source, int32 slen, char *dest, + int32 rawsize, bool check_complete) +{ + const unsigned char *sp; + const unsigned char *srcend; + unsigned char *dp; + unsigned char *destend; + + sp = (const unsigned char *) source; + srcend = ((const unsigned char *) source) + slen; + dp = (unsigned char *) dest; + destend = dp + 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 && dp < destend; ctrlc++) + { + if (ctrl & 1) + { + /* + * Set control bit means we must read a match tag. The match + * is coded with two bytes. First byte uses lower nibble to + * code length - 3. Higher nibble contains 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 corrupt data: if we fell off the end of the + * source, or if we obtained off = 0, or if off is more than + * the distance back to the buffer start, we have problems. + * (We must check for off = 0, else we risk an infinite loop + * below in the face of corrupt data. Likewise, the upper + * limit on off prevents accessing outside the buffer + * boundaries.) + */ + if (unlikely(sp > srcend || off == 0 || + off > (dp - (unsigned char *) dest))) + return -1; + + /* + * Don't emit more data than requested. + */ + len = Min(len, destend - dp); + + /* + * Now we copy the bytes specified by the tag from OUTPUT to + * OUTPUT (copy len bytes from dp - off to dp). The copied + * areas could overlap, so to avoid undefined behavior in + * memcpy(), be careful to copy only non-overlapping regions. + * + * Note that we cannot use memmove() instead, since while its + * behavior is well-defined, it's also not what we want. + */ + while (off < len) + { + /* + * We can safely copy "off" bytes since that clearly + * results in non-overlapping source and destination. + */ + memcpy(dp, dp - off, off); + len -= off; + dp += off; + + /*---------- + * This bit is less obvious: we can double "off" after + * each such step. Consider this raw input: + * 112341234123412341234 + * This will be encoded as 5 literal bytes "11234" and + * then a match tag with length 16 and offset 4. After + * memcpy'ing the first 4 bytes, we will have emitted + * 112341234 + * so we can double "off" to 8, then after the next step + * we have emitted + * 11234123412341234 + * Then we can double "off" again, after which it is more + * than the remaining "len" so we fall out of this loop + * and finish with a non-overlapping copy of the + * remainder. In general, a match tag with off < len + * implies that the decoded data has a repeat length of + * "off". We can handle 1, 2, 4, etc repetitions of the + * repeated string per memcpy until we get to a situation + * where the final copy step is non-overlapping. + * + * (Another way to understand this is that we are keeping + * the copy source point dp - off the same throughout.) + *---------- + */ + off += off; + } + memcpy(dp, dp - off, len); + dp += len; + } + else + { + /* + * An unset control bit means LITERAL BYTE. So we just copy + * one from INPUT to OUTPUT. + */ + *dp++ = *sp++; + } + + /* + * Advance the control bit + */ + ctrl >>= 1; + } + } + + /* + * If requested, check we decompressed the right amount. + */ + if (check_complete && (dp != destend || sp != srcend)) + return -1; + + /* + * That's it. + */ + return (char *) dp - dest; +} + + +/* ---------- + * pglz_maximum_compressed_size - + * + * Calculate the maximum compressed size for a given amount of raw data. + * Return the maximum size, or total compressed size if maximum size is + * larger than total compressed size. + * + * We can't use PGLZ_MAX_OUTPUT for this purpose, because that's used to size + * the compression buffer (and abort the compression). It does not really say + * what's the maximum compressed size for an input of a given length, and it + * may happen that while the whole value is compressible (and thus fits into + * PGLZ_MAX_OUTPUT nicely), the prefix is not compressible at all. + * ---------- + */ +int32 +pglz_maximum_compressed_size(int32 rawsize, int32 total_compressed_size) +{ + int64 compressed_size; + + /* + * pglz uses one control bit per byte, so if the entire desired prefix is + * represented as literal bytes, we'll need (rawsize * 9) bits. We care + * about bytes though, so be sure to round up not down. + * + * Use int64 here to prevent overflow during calculation. + */ + compressed_size = ((int64) rawsize * 9 + 7) / 8; + + /* + * The above fails to account for a corner case: we could have compressed + * data that starts with N-1 or N-2 literal bytes and then has a match tag + * of 2 or 3 bytes. It's therefore possible that we need to fetch 1 or 2 + * more bytes in order to have the whole match tag. (Match tags earlier + * in the compressed data don't cause a problem, since they should + * represent more decompressed bytes than they occupy themselves.) + */ + compressed_size += 2; + + /* + * Maximum compressed size can't be larger than total compressed size. + * (This also ensures that our result fits in int32.) + */ + compressed_size = Min(compressed_size, total_compressed_size); + + return (int32) compressed_size; +} |