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378c79dc78
remaining functions, simplify pglz_compress's API to not require a useless data copy when compression fails. Also add a check in pglz_decompress that the expected amount of data was decompressed.
123 lines
3.5 KiB
C
123 lines
3.5 KiB
C
/* ----------
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* pg_lzcompress.h -
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*
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* Definitions for the builtin LZ compressor
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*
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* $PostgreSQL: pgsql/src/include/utils/pg_lzcompress.h,v 1.13 2006/10/05 23:33:33 tgl Exp $
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* ----------
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*/
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#ifndef _PG_LZCOMPRESS_H_
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#define _PG_LZCOMPRESS_H_
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/* ----------
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* PGLZ_Header -
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*
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* The information at the top of the compressed data.
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* The varsize must be kept the same data type as the value
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* in front of all variable size data types in PostgreSQL.
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* ----------
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*/
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typedef struct PGLZ_Header
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{
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int32 varsize;
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int32 rawsize;
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} PGLZ_Header;
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/* ----------
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* PGLZ_MAX_OUTPUT -
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*
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* Macro to compute the buffer size required by pglz_compress().
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* We allow 4 bytes for overrun before detecting compression failure.
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* ----------
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*/
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#define PGLZ_MAX_OUTPUT(_dlen) ((_dlen) + 4 + sizeof(PGLZ_Header))
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/* ----------
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* PGLZ_RAW_SIZE -
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*
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* Macro to determine the uncompressed data size contained
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* in the entry.
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* ----------
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*/
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#define PGLZ_RAW_SIZE(_lzdata) ((_lzdata)->rawsize)
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/* ----------
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* PGLZ_Strategy -
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*
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* Some values that control the compression algorithm.
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*
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* min_input_size Minimum input data size to start compression.
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*
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* force_input_size Input data size at which compressed storage is
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* forced even if the compression rate drops below
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* min_comp_rate (but not below 0).
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*
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* min_comp_rate Minimum compression rate (0-99%), the output
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* must be smaller than the input. If that isn't
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* the case, the compressor will throw away its
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* output and copy the original, uncompressed data
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* to the output buffer.
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*
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* match_size_good The initial GOOD match size when starting history
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* lookup. When looking up the history to find a
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* match that could be expressed as a tag, the
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* algorithm does not always walk back entirely.
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* A good match fast is usually better than the
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* best possible one very late. For each iteration
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* in the lookup, this value is lowered so the
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* longer the lookup takes, the smaller matches
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* are considered good.
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*
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* match_size_drop The percentage, match_size_good is lowered
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* at each history check. Allowed values are
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* 0 (no change until end) to 100 (only check
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* latest history entry at all).
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* ----------
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*/
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typedef struct PGLZ_Strategy
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{
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int32 min_input_size;
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int32 force_input_size;
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int32 min_comp_rate;
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int32 match_size_good;
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int32 match_size_drop;
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} PGLZ_Strategy;
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/* ----------
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* The standard strategies
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*
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* PGLZ_strategy_default Starts compression only if input is
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* at least 256 bytes large. Stores output
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* uncompressed if compression does not
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* gain at least 20% size reducture but
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* input does not exceed 6K. Stops history
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* lookup if at least a 128 byte long
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* match has been found.
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*
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* This is the default strategy if none
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* is given to pglz_compress().
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*
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* PGLZ_strategy_always Starts compression on any infinitely
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* small input and does fallback to
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* uncompressed storage only if output
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* would be larger than input.
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* ----------
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*/
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extern const PGLZ_Strategy * const PGLZ_strategy_default;
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extern const PGLZ_Strategy * const PGLZ_strategy_always;
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/* ----------
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* Global function declarations
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* ----------
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*/
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extern bool pglz_compress(const char *source, int32 slen, PGLZ_Header *dest,
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const PGLZ_Strategy *strategy);
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extern void pglz_decompress(const PGLZ_Header *source, char *dest);
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#endif /* _PG_LZCOMPRESS_H_ */
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