/* ----------
* pg_lzcompress.c -
*
- * $Header: /cvsroot/pgsql/src/backend/utils/adt/pg_lzcompress.c,v 1.13 2001/10/25 05:49:45 momjian Exp $
+ * $Header: /cvsroot/pgsql/src/backend/utils/adt/pg_lzcompress.c,v 1.14 2001/11/17 06:09:30 tgl Exp $
*
* This is an implementation of LZ compression for PostgreSQL.
* It uses a simple history table and generates 2-3 byte tags
* This limits the offset to 1-4095 (12 bits) and the length
* to 3-18 (4 bits) because 3 is allways added to it. To emit
* a tag of 2 bytes with a length of 2 only saves one control
- * bit. But we loose one byte in the possible length of a tag.
+ * 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
* 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
- * the allocation of the history table is expensive (it needs
- * 8 times the size of the input!).
+ * 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 posiiton remembered
+ * 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.
+ * 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
* Jan Wieck
* ----------
*/
-#include
-#include
+#include "postgres.h"
+
#include
#include
-#include
#include
-#include "postgres.h"
#include "utils/pg_lzcompress.h"
* Local definitions
* ----------
*/
-#define PGLZ_HISTORY_LISTS 8192
-#define PGLZ_HISTORY_MASK 0x1fff
+#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;
+ struct PGLZ_HistEntry *next; /* links for my hash key's list */
struct PGLZ_HistEntry *prev;
- char *pos;
+ int hindex; /* my current hash key */
+ char *pos; /* my input position */
} PGLZ_HistEntry;
PGLZ_Strategy *PGLZ_strategy_never = &strategy_never_data;
/* ----------
- * Global arrays for history
+ * Statically allocated work arrays for history
* ----------
*/
static PGLZ_HistEntry *hist_start[PGLZ_HISTORY_LISTS];
*
* 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.
* ----------
*/
-#if 1
-#define pglz_hist_idx(_s,_e) ( \
- (((_e) - (_s)) < 4) ? 0 : \
- ((((_s)[0] << 9) ^ ((_s)[1] << 6) ^ \
- ((_s)[2] << 3) ^ (_s)[3]) & (PGLZ_HISTORY_MASK)) \
- )
-#else
#define pglz_hist_idx(_s,_e) ( \
- (((_e) - (_s)) < 2) ? 0 : \
- ((((_s)[0] << 8) ^ (_s)[1]) & (PGLZ_HISTORY_MASK)) \
+ ((((_e) - (_s)) < 4) ? (int) (_s)[0] : \
+ (((_s)[0] << 9) ^ ((_s)[1] << 6) ^ \
+ ((_s)[2] << 3) ^ (_s)[3])) & (PGLZ_HISTORY_MASK) \
)
-#endif
/* ----------
* 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,_s,_e) \
+#define pglz_hist_add(_hs,_he,_hn,_recycle,_s,_e) \
do { \
int __hindex = pglz_hist_idx((_s),(_e)); \
- if ((_he)[(_hn)].prev == NULL) { \
- (_hs)[__hindex] = (_he)[(_hn)].next; \
- } else { \
- (_he)[(_hn)].prev->next = (_he)[(_hn)].next; \
- } \
- if ((_he)[(_hn)].next != NULL) { \
- (_he)[(_hn)].next->prev = (_he)[(_hn)].prev; \
- } \
- (_he)[(_hn)].next = (_hs)[__hindex]; \
- (_he)[(_hn)].prev = NULL; \
- (_he)[(_hn)].pos = (_s); \
- if ((_hs)[__hindex] != NULL) { \
- (_hs)[__hindex]->prev = &((_he)[(_hn)]); \
+ 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; \
} \
- (_hs)[__hindex] = &((_he)[(_hn)]); \
+ __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_HistEntry *hent;
int32 len = 0;
int32 off = 0;
- int32 thislen;
- int32 thisoff;
- char *ip;
- char *hp;
/*
* Traverse the linked history list until a good enough match is
* found.
*/
hent = hstart[pglz_hist_idx(input, end)];
- while (hent && len < good_match)
+ while (hent)
{
- /*
- * Be happy with lesser good matches the more entries we visited.
- */
- good_match -= (good_match * good_drop) / 100;
+ char *ip = input;
+ char *hp = hent->pos;
+ int32 thisoff;
+ int32 thislen;
/*
* Stop if the offset does not fit into our tag anymore.
*/
- thisoff = (ip = input) - (hp = hent->pos);
+ thisoff = ip - hp;
if (thisoff >= 0x0fff)
break;
* 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 comparision to know the
+ * fallback to character by character comparison to know the
* exact position where the diff occured.
*/
+ thislen = 0;
if (len >= 16)
{
- if (memcmp(ip, hp, len) != 0)
+ if (memcmp(ip, hp, len) == 0)
{
- hent = hent->next;
- continue;
+ thislen = len;
+ ip += len;
+ hp += len;
+ while (ip < end && *ip == *hp && thislen < PGLZ_MAX_MATCH)
+ {
+ thislen++;
+ ip++;
+ hp++;
+ }
}
- thislen = len;
- ip += len;
- hp += len;
}
else
- thislen = 0;
- while (ip < end && *ip == *hp && thislen < PGLZ_MAX_MATCH)
{
- thislen++;
- ip++;
- hp++;
+ while (ip < end && *ip == *hp && thislen < PGLZ_MAX_MATCH)
+ {
+ thislen++;
+ ip++;
+ hp++;
+ }
}
/*
* 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;
+ }
}
/*
int
pglz_compress(char *source, int32 slen, PGLZ_Header *dest, PGLZ_Strategy *strategy)
{
- int hist_next = 0;
-
unsigned char *bp = ((unsigned char *) dest) + sizeof(PGLZ_Header);
unsigned char *bstart = bp;
+ int hist_next = 0;
+ bool hist_recycle = false;
char *dp = source;
char *dend = source + slen;
unsigned char ctrl_dummy = 0;
good_drop = 100;
/*
- * Initialize the history tables. For inputs smaller than
- * PGLZ_HISTORY_SIZE, we already have a big enough history table on
- * the stack frame.
+ * 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((void *) hist_start, 0, sizeof(hist_start));
- memset((void *) hist_entries, 0, sizeof(hist_entries));
/*
* Compute the maximum result size allowed by the strategy. If the
pglz_out_tag(ctrlp, ctrlb, ctrl, bp, match_len, match_off);
while (match_len--)
{
- pglz_hist_add(hist_start, hist_entries, hist_next, dp, dend);
+ 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. */
}
* No match found. Copy one literal byte.
*/
pglz_out_literal(ctrlp, ctrlb, ctrl, bp, *dp);
- pglz_hist_add(hist_start, hist_entries, hist_next, dp, dend);
+ 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. */
}
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