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pg_lzcompress.c

/* ----------
 * 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
 *
 * $PostgreSQL$
 * ----------
 */
#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.
       */
}

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