pg_lzcompress.h

来自「PostgreSQL 8.2中增加了很多企业用户所需要的功能和性能上的提高,其开」· C头文件 代码 · 共 124 行

/* ----------
 * 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_ */