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/* ----------
* pg_lzcompress.h -
*
* Definitions for the builtin LZ compressor
*
* $PostgreSQL: pgsql/src/include/utils/pg_lzcompress.h,v 1.13 2006/10/05 23:33:33 tgl Exp $
* ----------
*/
#ifndef _PG_LZCOMPRESS_H_
#define _PG_LZCOMPRESS_H_
/* ----------
* PGLZ_Header -
*
* The information at the top of the compressed data.
* The varsize must be kept the same data type as the value
* in front of all variable size data types in PostgreSQL.
* ----------
*/
typedef struct PGLZ_Header
{
int32 varsize;
int32 rawsize;
} PGLZ_Header;
/* ----------
* PGLZ_MAX_OUTPUT -
*
* Macro to compute the buffer size required by pglz_compress().
* We allow 4 bytes for overrun before detecting compression failure.
* ----------
*/
#define PGLZ_MAX_OUTPUT(_dlen) ((_dlen) + 4 + sizeof(PGLZ_Header))
/* ----------
* PGLZ_RAW_SIZE -
*
* Macro to determine the uncompressed data size contained
* in the entry.
* ----------
*/
#define PGLZ_RAW_SIZE(_lzdata) ((_lzdata)->rawsize)
/* ----------
* PGLZ_Strategy -
*
* Some values that control the compression algorithm.
*
* min_input_size Minimum input data size to start compression.
*
* force_input_size Input data size at which compressed storage is
* forced even if the compression rate drops below
* min_comp_rate (but not below 0).
*
* min_comp_rate Minimum compression rate (0-99%), the output
* must be smaller than the input. If that isn't
* the case, the compressor will throw away its
* output and copy the original, uncompressed data
* to the output buffer.
*
* match_size_good The initial GOOD match size when starting history
* lookup. When looking up the history to find a
* match that could be expressed as a tag, the
* algorithm does not always walk back entirely.
* A good match fast is usually better than the
* best possible one very late. For each iteration
* in the lookup, this value is lowered so the
* longer the lookup takes, the smaller matches
* are considered good.
*
* match_size_drop The percentage, match_size_good is lowered
* at each history check. Allowed values are
* 0 (no change until end) to 100 (only check
* latest history entry at all).
* ----------
*/
typedef struct PGLZ_Strategy
{
int32 min_input_size;
int32 force_input_size;
int32 min_comp_rate;
int32 match_size_good;
int32 match_size_drop;
} PGLZ_Strategy;
/* ----------
* The standard strategies
*
* PGLZ_strategy_default Starts compression only if input is
* at least 256 bytes large. Stores output
* uncompressed if compression does not
* gain at least 20% size reducture but
* input does not exceed 6K. Stops history
* lookup if at least a 128 byte long
* match has been found.
*
* This is the default strategy if none
* is given to pglz_compress().
*
* PGLZ_strategy_always Starts compression on any infinitely
* small input and does fallback to
* uncompressed storage only if output
* would be larger than input.
* ----------
*/
extern const PGLZ_Strategy * const PGLZ_strategy_default;
extern const PGLZ_Strategy * const PGLZ_strategy_always;
/* ----------
* Global function declarations
* ----------
*/
extern bool pglz_compress(const char *source, int32 slen, PGLZ_Header *dest,
const PGLZ_Strategy *strategy);
extern void pglz_decompress(const PGLZ_Header *source, char *dest);
#endif /* _PG_LZCOMPRESS_H_ */
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