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updated documentation for ZSTD_estimate*()

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make it clearer that tighter memory estimation
can be provided using advanced functions
on the condition of a defined input size bound.
This commit is contained in:
Yann Collet 2020-05-11 19:21:50 -07:00
parent 91ad01218e
commit 76e726e3be
1 changed files with 23 additions and 18 deletions
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41
lib/zstd.h
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@ -274,9 +274,9 @@ typedef enum {
* Default level is ZSTD_CLEVEL_DEFAULT==3.
* Special: value 0 means default, which is controlled by ZSTD_CLEVEL_DEFAULT.
* Note 1 : it's possible to pass a negative compression level.
* Note 2 : setting a level does not automatically set all other compression parameters
* to default. Setting this will however eventually dynamically impact the compression
* parameters which have not been manually set. The manually set
* Note 2 : setting a level does not automatically set all other compression parameters
* to default. Setting this will however eventually dynamically impact the compression
* parameters which have not been manually set. The manually set
* ones will 'stick'. */
/* Advanced compression parameters :
* It's possible to pin down compression parameters to some specific values.
@ -1268,23 +1268,28 @@ ZSTDLIB_API size_t ZSTD_getSequences(ZSTD_CCtx* zc, ZSTD_Sequence* outSeqs,
***************************************/
/*! ZSTD_estimate*() :
* These functions make it possible to estimate memory usage of a future
* {D,C}Ctx, before its creation.
* These functions make it possible to estimate memory usage
* of a future {D,C}Ctx, before its creation.
*
* ZSTD_estimateCCtxSize() will provide a budget large enough for any
* compression level up to selected one. Unlike ZSTD_estimateCStreamSize*(),
* this estimate does not include space for a window buffer, so this estimate
* is guaranteed to be enough for single-shot compressions, but not streaming
* compressions. It will however assume the input may be arbitrarily large,
* which is the worst case. If srcSize is known to always be small,
* ZSTD_estimateCCtxSize_usingCParams() can provide a tighter estimation.
* ZSTD_estimateCCtxSize_usingCParams() can be used in tandem with
* ZSTD_getCParams() to create cParams from compressionLevel.
* ZSTD_estimateCCtxSize_usingCCtxParams() can be used in tandem with
* ZSTD_CCtxParams_setParameter().
* ZSTD_estimateCCtxSize() will provide a memory budget large enough
* for any compression level up to selected one.
* Note : Unlike ZSTD_estimateCStreamSize*(), this estimate
* does not include space for a window buffer.
* Therefore, the estimation is only guaranteed for single-shot compressions, not streaming.
* The estimate will assume the input may be arbitrarily large,
* which is the worst case.
*
* Note: only single-threaded compression is supported. This function will
* return an error code if ZSTD_c_nbWorkers is >= 1. */
* When srcSize can be bound by a known and rather "small" value,
* this fact can be used to provide a tighter estimation
* because the CCtx compression context will need less memory.
* This tighter estimation can be provided by more advanced functions
* ZSTD_estimateCCtxSize_usingCParams(), which can be used in tandem with ZSTD_getCParams(),
* and ZSTD_estimateCCtxSize_usingCCtxParams(), which can be used in tandem with ZSTD_CCtxParams_setParameter().
* Both can be used to estimate memory using custom compression parameters and arbitrary srcSize limits.
*
* Note 2 : only single-threaded compression is supported.
* ZSTD_estimateCCtxSize_usingCCtxParams() will return an error code if ZSTD_c_nbWorkers is >= 1.
*/
ZSTDLIB_API size_t ZSTD_estimateCCtxSize(int compressionLevel);
ZSTDLIB_API size_t ZSTD_estimateCCtxSize_usingCParams(ZSTD_compressionParameters cParams);
ZSTDLIB_API size_t ZSTD_estimateCCtxSize_usingCCtxParams(const ZSTD_CCtx_params* params);