src/include/utils/pg_lzcompress.h

/* ----------
 * pg_lzcompress.h -
 *
 *	Definitions for the builtin LZ compressor
 *
 * $PostgreSQL: pgsql/src/include/utils/pg_lzcompress.h,v 1.17 2008/03/07 23:20:21 tgl Exp $
 * ----------
 */

#ifndef _PG_LZCOMPRESS_H_
#define _PG_LZCOMPRESS_H_


/* ----------
 * PGLZ_Header -
 *
 *		The information at the start of the compressed data.
 * ----------
 */
typedef struct PGLZ_Header
{
	int32		vl_len_;		/* varlena header (do not touch directly!) */
	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 consider compression.
 *
 *		max_input_size		Maximum input data size to consider compression.
 *
 *		min_comp_rate		Minimum compression rate (0-99%) to require.
 *							Regardless of min_comp_rate, the output must be
 *							smaller than the input, else we don't store
 *							compressed.
 *
 *		first_success_by	Abandon compression if we find no compressible
 *							data within the first this-many bytes.
 *
 *		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 by which match_size_good is lowered
 *							after 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		max_input_size;
	int32		min_comp_rate;
	int32		first_success_by;
	int32		match_size_good;
	int32		match_size_drop;
} PGLZ_Strategy;


/* ----------
 * The standard strategies
 *
 *		PGLZ_strategy_default		Recommended default strategy for TOAST.
 *
 *		PGLZ_strategy_always		Try to compress inputs of any length.
 *									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_ */