Fix end-of-loop optimization in pglz_find_match() function.

[postgresql] / src / backend / utils / adt / 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 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 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) 1999-2013, PostgreSQL Global Development Group
 *
 * src/backend/utils/adt/pg_lzcompress.c
 * ----------
 */
#include "postgres.h"

#include <limits.h>

#include "utils/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.
 * ----------
 */
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 = 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 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;

        /*
         * 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 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, 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 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.
         */
}