/src/postgres/src/common/pg_lzcompress.c

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/* ----------
 * 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) 1999-2025, 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;
}

Coverage Report

Created: 2025-06-13 06:06