# zstd 1.4.4 Manual

## Contents

1. Introduction
2. Version
3. Simple API
4. Explicit context
5. Advanced compression API
6. Advanced decompression API
7. Streaming
8. Streaming compression - HowTo
9. Streaming decompression - HowTo
10. Simple dictionary API
11. Bulk processing dictionary API
12. Dictionary helper functions
13. Advanced dictionary and prefix API
14. experimental API (static linking only)
15. Frame size functions
16. Memory management
17. Advanced compression functions
18. Advanced decompression functions
19. Advanced streaming functions
20. ! ZSTD_initCStream_usingDict() :
21. ! ZSTD_initCStream_advanced() :
22. ! ZSTD_initCStream_usingCDict() :
23. ! ZSTD_initCStream_usingCDict_advanced() :
24. This function is deprecated, and is equivalent to:
25. This function is deprecated, and is equivalent to:
26. Buffer-less and synchronous inner streaming functions
27. Buffer-less streaming compression (synchronous mode)
28. Buffer-less streaming decompression (synchronous mode)
29. Block level API

## Introduction

      zstd, short for Zstandard, is a fast lossless compression algorithm, targeting
      real-time compression scenarios at zlib-level and better compression ratios.
      The zstd compression library provides in-memory compression and decompression
      functions.

      The library supports regular compression levels from 1 up to ZSTD_maxCLevel(),
      which is currently 22. Levels >= 20, labeled `--ultra`, should be used with
      caution, as they require more memory. The library also offers negative
      compression levels, which extend the range of speed vs. ratio preferences.
      The lower the level, the faster the speed (at the cost of compression).

      Compression can be done in:
        - a single step (described as Simple API)
        - a single step, reusing a context (described as Explicit context)
        - unbounded multiple steps (described as Streaming compression)

      The compression ratio achievable on small data can be highly improved using
      a dictionary. Dictionary compression can be performed in:
        - a single step (described as Simple dictionary API)
        - a single step, reusing a dictionary (described as Bulk-processing
          dictionary API)

      Advanced experimental functions can be accessed using
      `#define ZSTD_STATIC_LINKING_ONLY` before including zstd.h.

      Advanced experimental APIs should never be used with a dynamically-linked
      library. They are not "stable"; their definitions or signatures may change in
      the future. Only static linking is allowed.

## Version

    unsigned ZSTD_versionNumber(void);   /**< to check runtime library version */

## Simple API

    size_t ZSTD_compress( void* dst, size_t dstCapacity,
                    const void* src, size_t srcSize,
                          int compressionLevel);
      Compresses `src` content as a single zstd compressed frame into already allocated `dst`.
      Hint : compression runs faster if `dstCapacity` >=  `ZSTD_compressBound(srcSize)`.
      @return : compressed size written into `dst` (<= `dstCapacity),
                or an error code if it fails (which can be tested using ZSTD_isError()).

    size_t ZSTD_decompress( void* dst, size_t dstCapacity,
                      const void* src, size_t compressedSize);
      `compressedSize` : must be the _exact_ size of some number of compressed and/or skippable frames.
      `dstCapacity` is an upper bound of originalSize to regenerate.
      If user cannot imply a maximum upper bound, it's better to use streaming mode to decompress data.
      @return : the number of bytes decompressed into `dst` (<= `dstCapacity`),
                or an errorCode if it fails (which can be tested using ZSTD_isError()).

    #define ZSTD_CONTENTSIZE_UNKNOWN (0ULL - 1)
    #define ZSTD_CONTENTSIZE_ERROR   (0ULL - 2)
    unsigned long long ZSTD_getFrameContentSize(const void *src, size_t srcSize);
      `src` should point to the start of a ZSTD encoded frame.
      `srcSize` must be at least as large as the frame header.
                hint : any size >= `ZSTD_frameHeaderSize_max` is large enough.
      @return : - decompressed size of `src` frame content, if known
                - ZSTD_CONTENTSIZE_UNKNOWN if the size cannot be determined
                - ZSTD_CONTENTSIZE_ERROR if an error occurred (e.g. invalid magic number, srcSize too small)
       note 1 : a 0 return value means the frame is valid but "empty".
       note 2 : decompressed size is an optional field, it may not be present, typically in streaming mode.
                When `return==ZSTD_CONTENTSIZE_UNKNOWN`, data to decompress could be any size.
                In which case, it's necessary to use streaming mode to decompress data.
                Optionally, application can rely on some implicit limit,
                as ZSTD_decompress() only needs an upper bound of decompressed size.
                (For example, data could be necessarily cut into blocks <= 16 KB).
       note 3 : decompressed size is always present when compression is completed using single-pass functions,
                such as ZSTD_compress(), ZSTD_compressCCtx() ZSTD_compress_usingDict() or ZSTD_compress_usingCDict().
       note 4 : decompressed size can be very large (64-bits value),
                potentially larger than what local system can handle as a single memory segment.
                In which case, it's necessary to use streaming mode to decompress data.
       note 5 : If source is untrusted, decompressed size could be wrong or intentionally modified.
                Always ensure return value fits within application's authorized limits.
                Each application can set its own limits.
       note 6 : This function replaces ZSTD_getDecompressedSize()

    unsigned long long ZSTD_getDecompressedSize(const void* src, size_t srcSize);
      NOTE: This function is now obsolete, in favor of ZSTD_getFrameContentSize().
      Both functions work the same way, but ZSTD_getDecompressedSize() blends
      "empty", "unknown" and "error" results to the same return value (0),
      while ZSTD_getFrameContentSize() gives them separate return values.
     @return : decompressed size of `src` frame content _if known and not empty_, 0 otherwise.

    size_t ZSTD_findFrameCompressedSize(const void* src, size_t srcSize);
     `src` should point to the start of a ZSTD frame or skippable frame.
     `srcSize` must be >= first frame size
     @return : the compressed size of the first frame starting at `src`,
               suitable to pass as `srcSize` to `ZSTD_decompress` or similar,
            or an error code if input is invalid

### Helper functions

    #define ZSTD_COMPRESSBOUND(srcSize)   ((srcSize) + ((srcSize)>>8) + (((srcSize) < (128<<10)) ? (((128<<10) - (srcSize)) >> 11)

/* margin, from 64 to 0 */ : 0)) /* this formula ensures that bound(A) + bound(B) <= bound(A+B) as long as A and B >= 128 KB */ size_t ZSTD_compressBound(size_t srcSize); /*!< maximum compressed size in worst case single-pass scenario */ unsigned ZSTD_isError(size_t code); /*!< tells if a `size_t` function result is an error code */ const char* ZSTD_getErrorName(size_t code); /*!< provides readable string from an error code */ int ZSTD_minCLevel(void); /*!< minimum negative compression level allowed */ int ZSTD_maxCLevel(void); /*!< maximum compression level available */

## Explicit context

### Compression context

      When compressing many times,
      it is recommended to allocate a context just once,
      and re-use it for each successive compression operation.
      This will make workload friendlier for system's memory.
      Note : re-using context is just a speed / resource optimization.
             It doesn't change the compression ratio, which remains identical.
      Note 2 : In multi-threaded environments,
             use one different context per thread for parallel execution.

    typedef struct ZSTD_CCtx_s ZSTD_CCtx;
    ZSTD_CCtx* ZSTD_createCCtx(void);
    size_t     ZSTD_freeCCtx(ZSTD_CCtx* cctx);

    size_t ZSTD_compressCCtx(ZSTD_CCtx* cctx,
                             void* dst, size_t dstCapacity,
                       const void* src, size_t srcSize,
                             int compressionLevel);
      Same as ZSTD_compress(), using an explicit ZSTD_CCtx.
      Important : in order to behave similarly to `ZSTD_compress()`,
      this function compresses at requested compression level,
      __ignoring any other parameter__ .
      If any advanced parameter was set using the advanced API,
      they will all be reset. Only `compressionLevel` remains.

### Decompression context

      When decompressing many times,
      it is recommended to allocate a context only once,
      and re-use it for each successive compression operation.
      This will make workload friendlier for system's memory.
      Use one context per thread for parallel execution.

    typedef struct ZSTD_DCtx_s ZSTD_DCtx;
    ZSTD_DCtx* ZSTD_createDCtx(void);
    size_t     ZSTD_freeDCtx(ZSTD_DCtx* dctx);

    size_t ZSTD_decompressDCtx(ZSTD_DCtx* dctx,
                               void* dst, size_t dstCapacity,
                         const void* src, size_t srcSize);
      Same as ZSTD_decompress(),
      requires an allocated ZSTD_DCtx.
      Compatible with sticky parameters.

## Advanced compression API

    typedef enum { ZSTD_fast=1,
                   ZSTD_dfast=2,
                   ZSTD_greedy=3,
                   ZSTD_lazy=4,
                   ZSTD_lazy2=5,
                   ZSTD_btlazy2=6,
                   ZSTD_btopt=7,
                   ZSTD_btultra=8,
                   ZSTD_btultra2=9
                   /* note : new strategies _might_ be added in the future.
                             Only the order (from fast to strong) is guaranteed */
    } ZSTD_strategy;

    typedef enum {

        /* compression parameters
         * Note: When compressing with a ZSTD_CDict these parameters are superseded
         * by the parameters used to construct the ZSTD_CDict.
         * See ZSTD_CCtx_refCDict() for more info (superseded-by-cdict). */
        ZSTD_c_compressionLevel=100, /* Set compression parameters according to pre-defined cLevel table.
                                  * Note that exact compression parameters are dynamically determined,
                                  * depending on both compression level and srcSize (when known).
                                  * 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 resets all other compression parameters to default */
        /* Advanced compression parameters :
         * It's possible to pin down compression parameters to some specific values.
         * In which case, these values are no longer dynamically selected by the compressor */
        ZSTD_c_windowLog=101,    /* Maximum allowed back-reference distance, expressed as power of 2.
                                  * This will set a memory budget for streaming decompression,
                                  * with larger values requiring more memory
                                  * and typically compressing more.
                                  * Must be clamped between ZSTD_WINDOWLOG_MIN and ZSTD_WINDOWLOG_MAX.
                                  * Special: value 0 means "use default windowLog".
                                  * Note: Using a windowLog greater than ZSTD_WINDOWLOG_LIMIT_DEFAULT
                                  *       requires explicitly allowing such size at streaming decompression stage. */
        ZSTD_c_hashLog=102,      /* Size of the initial probe table, as a power of 2.
                                  * Resulting memory usage is (1 << (hashLog+2)).
                                  * Must be clamped between ZSTD_HASHLOG_MIN and ZSTD_HASHLOG_MAX.
                                  * Larger tables improve compression ratio of strategies <= dFast,
                                  * and improve speed of strategies > dFast.
                                  * Special: value 0 means "use default hashLog". */
        ZSTD_c_chainLog=103,     /* Size of the multi-probe search table, as a power of 2.
                                  * Resulting memory usage is (1 << (chainLog+2)).
                                  * Must be clamped between ZSTD_CHAINLOG_MIN and ZSTD_CHAINLOG_MAX.
                                  * Larger tables result in better and slower compression.
                                  * This parameter is useless for "fast" strategy.
                                  * It's still useful when using "dfast" strategy,
                                  * in which case it defines a secondary probe table.
                                  * Special: value 0 means "use default chainLog". */
        ZSTD_c_searchLog=104,    /* Number of search attempts, as a power of 2.
                                  * More attempts result in better and slower compression.
                                  * This parameter is useless for "fast" and "dFast" strategies.
                                  * Special: value 0 means "use default searchLog". */
        ZSTD_c_minMatch=105,     /* Minimum size of searched matches.
                                  * Note that Zstandard can still find matches of smaller size,
                                  * it just tweaks its search algorithm to look for this size and larger.
                                  * Larger values increase compression and decompression speed, but decrease ratio.
                                  * Must be clamped between ZSTD_MINMATCH_MIN and ZSTD_MINMATCH_MAX.
                                  * Note that currently, for all strategies < btopt, effective minimum is 4.
                                  *                    , for all strategies > fast, effective maximum is 6.
                                  * Special: value 0 means "use default minMatchLength". */
        ZSTD_c_targetLength=106, /* Impact of this field depends on strategy.
                                  * For strategies btopt, btultra & btultra2:
                                  *     Length of Match considered "good enough" to stop search.
                                  *     Larger values make compression stronger, and slower.
                                  * For strategy fast:
                                  *     Distance between match sampling.
                                  *     Larger values make compression faster, and weaker.
                                  * Special: value 0 means "use default targetLength". */
        ZSTD_c_strategy=107,     /* See ZSTD_strategy enum definition.
                                  * The higher the value of selected strategy, the more complex it is,
                                  * resulting in stronger and slower compression.
                                  * Special: value 0 means "use default strategy". */

        /* LDM mode parameters */
        ZSTD_c_enableLongDistanceMatching=160, /* Enable long distance matching.
                                         * This parameter is designed to improve compression ratio
                                         * for large inputs, by finding large matches at long distance.
                                         * It increases memory usage and window size.
                                         * Note: enabling this parameter increases default ZSTD_c_windowLog to 128 MB
                                         * except when expressly set to a different value. */
        ZSTD_c_ldmHashLog=161,   /* Size of the table for long distance matching, as a power of 2.
                                  * Larger values increase memory usage and compression ratio,
                                  * but decrease compression speed.
                                  * Must be clamped between ZSTD_HASHLOG_MIN and ZSTD_HASHLOG_MAX
                                  * default: windowlog - 7.
                                  * Special: value 0 means "automatically determine hashlog". */
        ZSTD_c_ldmMinMatch=162,  /* Minimum match size for long distance matcher.
                                  * Larger/too small values usually decrease compression ratio.
                                  * Must be clamped between ZSTD_LDM_MINMATCH_MIN and ZSTD_LDM_MINMATCH_MAX.
                                  * Special: value 0 means "use default value" (default: 64). */
        ZSTD_c_ldmBucketSizeLog=163, /* Log size of each bucket in the LDM hash table for collision resolution.
                                  * Larger values improve collision resolution but decrease compression speed.
                                  * The maximum value is ZSTD_LDM_BUCKETSIZELOG_MAX.
                                  * Special: value 0 means "use default value" (default: 3). */
        ZSTD_c_ldmHashRateLog=164, /* Frequency of inserting/looking up entries into the LDM hash table.
                                  * Must be clamped between 0 and (ZSTD_WINDOWLOG_MAX - ZSTD_HASHLOG_MIN).
                                  * Default is MAX(0, (windowLog - ldmHashLog)), optimizing hash table usage.
                                  * Larger values improve compression speed.
                                  * Deviating far from default value will likely result in a compression ratio decrease.
                                  * Special: value 0 means "automatically determine hashRateLog". */

        /* frame parameters */
        ZSTD_c_contentSizeFlag=200, /* Content size will be written into frame header _whenever known_ (default:1)
                                  * Content size must be known at the beginning of compression.
                                  * This is automatically the case when using ZSTD_compress2(),
                                  * For streaming scenarios, content size must be provided with ZSTD_CCtx_setPledgedSrcSize() */
        ZSTD_c_checksumFlag=201, /* A 32-bits checksum of content is written at end of frame (default:0) */
        ZSTD_c_dictIDFlag=202,   /* When applicable, dictionary's ID is written into frame header (default:1) */

        /* multi-threading parameters */
        /* These parameters are only useful if multi-threading is enabled (compiled with build macro ZSTD_MULTITHREAD).
         * They return an error otherwise. */
        ZSTD_c_nbWorkers=400,    /* Select how many threads will be spawned to compress in parallel.
                                  * When nbWorkers >= 1, triggers asynchronous mode when used with ZSTD_compressStream*() :
                                  * ZSTD_compressStream*() consumes input and flush output if possible, but immediately gives back control to caller,
                                  * while compression work is performed in parallel, within worker threads.
                                  * (note : a strong exception to this rule is when first invocation of ZSTD_compressStream2() sets ZSTD_e_end :
                                  *  in which case, ZSTD_compressStream2() delegates to ZSTD_compress2(), which is always a blocking call).
                                  * More workers improve speed, but also increase memory usage.
                                  * Default value is `0`, aka "single-threaded mode" : no worker is spawned, compression is performed inside Caller's thread, all invocations are blocking */
        ZSTD_c_jobSize=401,      /* Size of a compression job. This value is enforced only when nbWorkers >= 1.
                                  * Each compression job is completed in parallel, so this value can indirectly impact the nb of active threads.
                                  * 0 means default, which is dynamically determined based on compression parameters.
                                  * Job size must be a minimum of overlap size, or 1 MB, whichever is largest.
                                  * The minimum size is automatically and transparently enforced. */
        ZSTD_c_overlapLog=402,   /* Control the overlap size, as a fraction of window size.
                                  * The overlap size is an amount of data reloaded from previous job at the beginning of a new job.
                                  * It helps preserve compression ratio, while each job is compressed in parallel.
                                  * This value is enforced only when nbWorkers >= 1.
                                  * Larger values increase compression ratio, but decrease speed.
                                  * Possible values range from 0 to 9 :
                                  * - 0 means "default" : value will be determined by the library, depending on strategy
                                  * - 1 means "no overlap"
                                  * - 9 means "full overlap", using a full window size.
                                  * Each intermediate rank increases/decreases load size by a factor 2 :
                                  * 9: full window;  8: w/2;  7: w/4;  6: w/8;  5:w/16;  4: w/32;  3:w/64;  2:w/128;  1:no overlap;  0:default
                                  * default value varies between 6 and 9, depending on strategy */

        /* note : additional experimental parameters are also available
         * within the experimental section of the API.
         * At the time of this writing, they include :
         * ZSTD_c_rsyncable
         * ZSTD_c_format
         * ZSTD_c_forceMaxWindow
         * ZSTD_c_forceAttachDict
         * ZSTD_c_literalCompressionMode
         * ZSTD_c_targetCBlockSize
         * ZSTD_c_srcSizeHint
         * Because they are not stable, it's necessary to define ZSTD_STATIC_LINKING_ONLY to access them.
         * note : never ever use experimentalParam? names directly;
         *        also, the enums values themselves are unstable and can still change.
         */
         ZSTD_c_experimentalParam1=500,
         ZSTD_c_experimentalParam2=10,
         ZSTD_c_experimentalParam3=1000,
         ZSTD_c_experimentalParam4=1001,
         ZSTD_c_experimentalParam5=1002,
         ZSTD_c_experimentalParam6=1003,
         ZSTD_c_experimentalParam7=1004
    } ZSTD_cParameter;

    typedef struct {
        size_t error;
        int lowerBound;
        int upperBound;
    } ZSTD_bounds;

    ZSTD_bounds ZSTD_cParam_getBounds(ZSTD_cParameter cParam);
      All parameters must belong to an interval with lower and upper bounds,
      otherwise they will either trigger an error or be automatically clamped.
     @return : a structure, ZSTD_bounds, which contains
             - an error status field, which must be tested using ZSTD_isError()
             - lower and upper bounds, both inclusive

    size_t ZSTD_CCtx_setParameter(ZSTD_CCtx* cctx, ZSTD_cParameter param, int value);
      Set one compression parameter, selected by enum ZSTD_cParameter.
      All parameters have valid bounds. Bounds can be queried using ZSTD_cParam_getBounds().
      Providing a value beyond bound will either clamp it, or trigger an error (depending on parameter).
      Setting a parameter is generally only possible during frame initialization (before starting compression).
      Exception : when using multi-threading mode (nbWorkers >= 1),
                  the following parameters can be updated _during_ compression (within same frame):
                  => compressionLevel, hashLog, chainLog, searchLog, minMatch, targetLength and strategy.
                  new parameters will be active for next job only (after a flush()).
     @return : an error code (which can be tested using ZSTD_isError()).

    size_t ZSTD_CCtx_setPledgedSrcSize(ZSTD_CCtx* cctx, unsigned long long pledgedSrcSize);
      Total input data size to be compressed as a single frame.
      Value will be written in frame header, unless if explicitly forbidden using ZSTD_c_contentSizeFlag.
      This value will also be controlled at end of frame, and trigger an error if not respected.
     @result : 0, or an error code (which can be tested with ZSTD_isError()).
      Note 1 : pledgedSrcSize==0 actually means zero, aka an empty frame.
               In order to mean "unknown content size", pass constant ZSTD_CONTENTSIZE_UNKNOWN.
               ZSTD_CONTENTSIZE_UNKNOWN is default value for any new frame.
      Note 2 : pledgedSrcSize is only valid once, for the next frame.
               It's discarded at the end of the frame, and replaced by ZSTD_CONTENTSIZE_UNKNOWN.
      Note 3 : Whenever all input data is provided and consumed in a single round,
               for example with ZSTD_compress2(),
               or invoking immediately ZSTD_compressStream2(,,,ZSTD_e_end),
               this value is automatically overridden by srcSize instead.

    typedef enum {
        ZSTD_reset_session_only = 1,
        ZSTD_reset_parameters = 2,
        ZSTD_reset_session_and_parameters = 3
    } ZSTD_ResetDirective;

    size_t ZSTD_CCtx_reset(ZSTD_CCtx* cctx, ZSTD_ResetDirective reset);
      There are 2 different things that can be reset, independently or jointly :
      - The session : will stop compressing current frame, and make CCtx ready to start a new one.
                      Useful after an error, or to interrupt any ongoing compression.
                      Any internal data not yet flushed is cancelled.
                      Compression parameters and dictionary remain unchanged.
                      They will be used to compress next frame.
                      Resetting session never fails.
      - The parameters : changes all parameters back to "default".
                      This removes any reference to any dictionary too.
                      Parameters can only be changed between 2 sessions (i.e. no compression is currently ongoing)
                      otherwise the reset fails, and function returns an error value (which can be tested using ZSTD_isError())
      - Both : similar to resetting the session, followed by resetting parameters.

    size_t ZSTD_compress2( ZSTD_CCtx* cctx,
                           void* dst, size_t dstCapacity,
                     const void* src, size_t srcSize);
      Behave the same as ZSTD_compressCCtx(), but compression parameters are set using the advanced API.
      ZSTD_compress2() always starts a new frame.
      Should cctx hold data from a previously unfinished frame, everything about it is forgotten.
      - Compression parameters are pushed into CCtx before starting compression, using ZSTD_CCtx_set*()
      - The function is always blocking, returns when compression is completed.
      Hint : compression runs faster if `dstCapacity` >=  `ZSTD_compressBound(srcSize)`.
     @return : compressed size written into `dst` (<= `dstCapacity),
               or an error code if it fails (which can be tested using ZSTD_isError()).

## Advanced decompression API

    typedef enum {

        ZSTD_d_windowLogMax=100, /* Select a size limit (in power of 2) beyond which
                                  * the streaming API will refuse to allocate memory buffer
                                  * in order to protect the host from unreasonable memory requirements.
                                  * This parameter is only useful in streaming mode, since no internal buffer is allocated in single-pass mode.
                                  * By default, a decompression context accepts window sizes <= (1 << ZSTD_WINDOWLOG_LIMIT_DEFAULT).
                                  * Special: value 0 means "use default maximum windowLog". */

        /* note : additional experimental parameters are also available
         * within the experimental section of the API.
         * At the time of this writing, they include :
         * ZSTD_c_format
         * Because they are not stable, it's necessary to define ZSTD_STATIC_LINKING_ONLY to access them.
         * note : never ever use experimentalParam? names directly
         */
         ZSTD_d_experimentalParam1=1000

    } ZSTD_dParameter;

    ZSTD_bounds ZSTD_dParam_getBounds(ZSTD_dParameter dParam);
      All parameters must belong to an interval with lower and upper bounds,
      otherwise they will either trigger an error or be automatically clamped.
     @return : a structure, ZSTD_bounds, which contains
             - an error status field, which must be tested using ZSTD_isError()
             - both lower and upper bounds, inclusive

    size_t ZSTD_DCtx_setParameter(ZSTD_DCtx* dctx, ZSTD_dParameter param, int value);
      Set one compression parameter, selected by enum ZSTD_dParameter.
      All parameters have valid bounds. Bounds can be queried using ZSTD_dParam_getBounds().
      Providing a value beyond bound will either clamp it, or trigger an error (depending on parameter).
      Setting a parameter is only possible during frame initialization (before starting decompression).
     @return : 0, or an error code (which can be tested using ZSTD_isError()).

    size_t ZSTD_DCtx_reset(ZSTD_DCtx* dctx, ZSTD_ResetDirective reset);
      Return a DCtx to clean state.
      Session and parameters can be reset jointly or separately.
      Parameters can only be reset when no active frame is being decompressed.
     @return : 0, or an error code, which can be tested with ZSTD_isError()

## Streaming

    typedef struct ZSTD_inBuffer_s {
      const void* src;    /**< start of input buffer */
      size_t size;        /**< size of input buffer */
      size_t pos;         /**< position where reading stopped. Will be updated. Necessarily 0 <= pos <= size */
    } ZSTD_inBuffer;

    typedef struct ZSTD_outBuffer_s {
      void*  dst;         /**< start of output buffer */
      size_t size;        /**< size of output buffer */
      size_t pos;         /**< position where writing stopped. Will be updated. Necessarily 0 <= pos <= size */
    } ZSTD_outBuffer;

## Streaming compression - HowTo

      A ZSTD_CStream object is required to track streaming operation.
      Use ZSTD_createCStream() and ZSTD_freeCStream() to create/release resources.
      ZSTD_CStream objects can be reused multiple times on consecutive compression operations.
      It is recommended to re-use ZSTD_CStream since it will play nicer with system's memory, by re-using already allocated memory.

      For parallel execution, use one separate ZSTD_CStream per thread.

      note : since v1.3.0, ZSTD_CStream and ZSTD_CCtx are the same thing.

      Parameters are sticky : when starting a new compression on the same context,
      it will re-use the same sticky parameters as previous compression session.
      When in doubt, it's recommended to fully initialize the context before usage.
      Use ZSTD_CCtx_reset() to reset the context and ZSTD_CCtx_setParameter(),
      ZSTD_CCtx_setPledgedSrcSize(), or ZSTD_CCtx_loadDictionary() and friends to
      set more specific parameters, the pledged source size, or load a dictionary.

      Use ZSTD_compressStream2() with ZSTD_e_continue as many times as necessary to
      consume input stream. The function will automatically update both `pos`
      fields within `input` and `output`.
      Note that the function may not consume the entire input, for example, because
      the output buffer is already full, in which case `input.pos < input.size`.
      The caller must check if input has been entirely consumed.
      If not, the caller must make some room to receive more compressed data,
      and then present again remaining input data.
      note: ZSTD_e_continue is guaranteed to make some forward progress when called,
            but doesn't guarantee maximal forward progress. This is especially relevant
            when compressing with multiple threads. The call won't block if it can
            consume some input, but if it can't it will wait for some, but not all,
            output to be flushed.
     @return : provides a minimum amount of data remaining to be flushed from internal buffers
               or an error code, which can be tested using ZSTD_isError().

      At any moment, it's possible to flush whatever data might remain stuck within internal buffer,
      using ZSTD_compressStream2() with ZSTD_e_flush. `output->pos` will be updated.
      Note that, if `output->size` is too small, a single invocation with ZSTD_e_flush might not be enough (return code > 0).
      In which case, make some room to receive more compressed data, and call again ZSTD_compressStream2() with ZSTD_e_flush.
      You must continue calling ZSTD_compressStream2() with ZSTD_e_flush until it returns 0, at which point you can change the
      operation.
      note: ZSTD_e_flush will flush as much output as possible, meaning when compressing with multiple threads, it will
            block until the flush is complete or the output buffer is full.
      @return : 0 if internal buffers are entirely flushed,
                >0 if some data still present within internal buffer (the value is minimal estimation of remaining size),
                or an error code, which can be tested using ZSTD_isError().

      Calling ZSTD_compressStream2() with ZSTD_e_end instructs to finish a frame.
      It will perform a flush and write frame epilogue.
      The epilogue is required for decoders to consider a frame completed.
      flush operation is the same, and follows same rules as calling ZSTD_compressStream2() with ZSTD_e_flush.
      You must continue calling ZSTD_compressStream2() with ZSTD_e_end until it returns 0, at which point you are free to
      start a new frame.
      note: ZSTD_e_end will flush as much output as possible, meaning when compressing with multiple threads, it will
            block until the flush is complete or the output buffer is full.
      @return : 0 if frame fully completed and fully flushed,
                >0 if some data still present within internal buffer (the value is minimal estimation of remaining size),
                or an error code, which can be tested using ZSTD_isError().

    typedef ZSTD_CCtx ZSTD_CStream;  /**< CCtx and CStream are now effectively same object (>= v1.3.0) */

### ZSTD_CStream management functions

    ZSTD_CStream* ZSTD_createCStream(void);
    size_t ZSTD_freeCStream(ZSTD_CStream* zcs);

### Streaming compression functions

    typedef enum {
        ZSTD_e_continue=0,

/* collect more data, encoder decides when to output compressed result, for optimal compression ratio */ ZSTD_e_flush=1, /* flush any data provided so far, * it creates (at least) one new block, that can be decoded immediately on reception; * frame will continue: any future data can still reference previously compressed data, improving compression. * note : multithreaded compression will block to flush as much output as possible. */ ZSTD_e_end=2 /* flush any remaining data _and_ close current frame. * note that frame is only closed after compressed data is fully flushed (return value == 0). * After that point, any additional data starts a new frame. * note : each frame is independent (does not reference any content from previous frame). : note : multithreaded compression will block to flush as much output as possible. */ } ZSTD_EndDirective;

    size_t ZSTD_compressStream2( ZSTD_CCtx* cctx,
                                 ZSTD_outBuffer* output,
                                 ZSTD_inBuffer* input,
                                 ZSTD_EndDirective endOp);
      Behaves about the same as ZSTD_compressStream, with additional control on end directive.
      - Compression parameters are pushed into CCtx before starting compression, using ZSTD_CCtx_set*()
      - Compression parameters cannot be changed once compression is started (save a list of exceptions in multi-threading mode)
      - output->pos must be <= dstCapacity, input->pos must be <= srcSize
      - output->pos and input->pos will be updated. They are guaranteed to remain below their respective limit.
      - When nbWorkers==0 (default), function is blocking : it completes its job before returning to caller.
      - When nbWorkers>=1, function is non-blocking : it just acquires a copy of input, and distributes jobs to internal worker threads, flush whatever is available,
                                                      and then immediately returns, just indicating that there is some data remaining to be flushed.
                                                      The function nonetheless guarantees forward progress : it will return only after it reads or write at least 1+ byte.
      - Exception : if the first call requests a ZSTD_e_end directive and provides enough dstCapacity, the function delegates to ZSTD_compress2() which is always blocking.
      - @return provides a minimum amount of data remaining to be flushed from internal buffers
                or an error code, which can be tested using ZSTD_isError().
                if @return != 0, flush is not fully completed, there is still some data left within internal buffers.
                This is useful for ZSTD_e_flush, since in this case more flushes are necessary to empty all buffers.
                For ZSTD_e_end, @return == 0 when internal buffers are fully flushed and frame is completed.
      - after a ZSTD_e_end directive, if internal buffer is not fully flushed (@return != 0),
                only ZSTD_e_end or ZSTD_e_flush operations are allowed.
                Before starting a new compression job, or changing compression parameters,
                it is required to fully flush internal buffers.

    size_t ZSTD_CStreamInSize(void);    /**< recommended size for input buffer */

    size_t ZSTD_CStreamOutSize(void);   /**< recommended size for output buffer. Guarantee to successfully flush at least one complete compressed block. */

    size_t ZSTD_initCStream(ZSTD_CStream* zcs, int compressionLevel);
    /*!
     * Alternative for ZSTD_compressStream2(zcs, output, input, ZSTD_e_continue).
     * NOTE: The return value is different. ZSTD_compressStream() returns a hint for
     * the next read size (if non-zero and not an error). ZSTD_compressStream2()
     * returns the minimum nb of bytes left to flush (if non-zero and not an error).
     */
    size_t ZSTD_compressStream(ZSTD_CStream* zcs, ZSTD_outBuffer* output, ZSTD_inBuffer* input);
    /*! Equivalent to ZSTD_compressStream2(zcs, output, &emptyInput, ZSTD_e_flush). */
    size_t ZSTD_flushStream(ZSTD_CStream* zcs, ZSTD_outBuffer* output);
    /*! Equivalent to ZSTD_compressStream2(zcs, output, &emptyInput, ZSTD_e_end). */
    size_t ZSTD_endStream(ZSTD_CStream* zcs, ZSTD_outBuffer* output);

         ZSTD_CCtx_reset(zcs, ZSTD_reset_session_only);
         ZSTD_CCtx_refCDict(zcs, NULL); // clear the dictionary (if any)
         ZSTD_CCtx_setParameter(zcs, ZSTD_c_compressionLevel, compressionLevel);

## Streaming decompression - HowTo

      A ZSTD_DStream object is required to track streaming operations.
      Use ZSTD_createDStream() and ZSTD_freeDStream() to create/release resources.
      ZSTD_DStream objects can be re-used multiple times.

      Use ZSTD_initDStream() to start a new decompression operation.
     @return : recommended first input size
      Alternatively, use advanced API to set specific properties.

      Use ZSTD_decompressStream() repetitively to consume your input.
      The function will update both `pos` fields.
      If `input.pos < input.size`, some input has not been consumed.
      It's up to the caller to present again remaining data.
      The function tries to flush all data decoded immediately, respecting output buffer size.
      If `output.pos < output.size`, decoder has flushed everything it could.
      But if `output.pos == output.size`, there might be some data left within internal buffers.,
      In which case, call ZSTD_decompressStream() again to flush whatever remains in the buffer.
      Note : with no additional input provided, amount of data flushed is necessarily <= ZSTD_BLOCKSIZE_MAX.
     @return : 0 when a frame is completely decoded and fully flushed,
            or an error code, which can be tested using ZSTD_isError(),
            or any other value > 0, which means there is still some decoding or flushing to do to complete current frame :
                                    the return value is a suggested next input size (just a hint for better latency)
                                    that will never request more than the remaining frame size.

    typedef ZSTD_DCtx ZSTD_DStream;  /**< DCtx and DStream are now effectively same object (>= v1.3.0) */

### ZSTD_DStream management functions

    ZSTD_DStream* ZSTD_createDStream(void);
    size_t ZSTD_freeDStream(ZSTD_DStream* zds);

### Streaming decompression functions

    size_t ZSTD_DStreamInSize(void);    /*!< recommended size for input buffer */

    size_t ZSTD_DStreamOutSize(void);   /*!< recommended size for output buffer. Guarantee to successfully flush at least one complete block in all circumstances. */

## Simple dictionary API

    size_t ZSTD_compress_usingDict(ZSTD_CCtx* ctx,
                                   void* dst, size_t dstCapacity,
                             const void* src, size_t srcSize,
                             const void* dict,size_t dictSize,
                                   int compressionLevel);
      Compression at an explicit compression level using a Dictionary.
      A dictionary can be any arbitrary data segment (also called a prefix),
      or a buffer with specified information (see dictBuilder/zdict.h).
      Note : This function loads the dictionary, resulting in significant startup delay.
             It's intended for a dictionary used only once.
      Note 2 : When `dict == NULL || dictSize < 8` no dictionary is used.

    size_t ZSTD_decompress_usingDict(ZSTD_DCtx* dctx,
                                     void* dst, size_t dstCapacity,
                               const void* src, size_t srcSize,
                               const void* dict,size_t dictSize);
      Decompression using a known Dictionary.
      Dictionary must be identical to the one used during compression.
      Note : This function loads the dictionary, resulting in significant startup delay.
             It's intended for a dictionary used only once.
      Note : When `dict == NULL || dictSize < 8` no dictionary is used.

## Bulk processing dictionary API

    ZSTD_CDict* ZSTD_createCDict(const void* dictBuffer, size_t dictSize,
                                 int compressionLevel);
      When compressing multiple messages or blocks using the same dictionary,
      it's recommended to digest the dictionary only once, since it's a costly operation.
      ZSTD_createCDict() will create a state from digesting a dictionary.
      The resulting state can be used for future compression operations with very limited startup cost.
      ZSTD_CDict can be created once and shared by multiple threads concurrently, since its usage is read-only.
     @dictBuffer can be released after ZSTD_CDict creation, because its content is copied within CDict.
      Note 1 : Consider experimental function `ZSTD_createCDict_byReference()` if you prefer to not duplicate @dictBuffer content.
      Note 2 : A ZSTD_CDict can be created from an empty @dictBuffer,
          in which case the only thing that it transports is the @compressionLevel.
          This can be useful in a pipeline featuring ZSTD_compress_usingCDict() exclusively,
          expecting a ZSTD_CDict parameter with any data, including those without a known dictionary.

    size_t      ZSTD_freeCDict(ZSTD_CDict* CDict);
      Function frees memory allocated by ZSTD_createCDict().

    size_t ZSTD_compress_usingCDict(ZSTD_CCtx* cctx,
                                    void* dst, size_t dstCapacity,
                              const void* src, size_t srcSize,
                              const ZSTD_CDict* cdict);
      Compression using a digested Dictionary.
      Recommended when same dictionary is used multiple times.
      Note : compression level is _decided at dictionary creation time_,
         and frame parameters are hardcoded (dictID=yes, contentSize=yes, checksum=no)

    ZSTD_DDict* ZSTD_createDDict(const void* dictBuffer, size_t dictSize);
      Create a digested dictionary, ready to start decompression operation without startup delay.
      dictBuffer can be released after DDict creation, as its content is copied inside DDict.

    size_t      ZSTD_freeDDict(ZSTD_DDict* ddict);
      Function frees memory allocated with ZSTD_createDDict()

    size_t ZSTD_decompress_usingDDict(ZSTD_DCtx* dctx,
                                      void* dst, size_t dstCapacity,
                                const void* src, size_t srcSize,
                                const ZSTD_DDict* ddict);
      Decompression using a digested Dictionary.
      Recommended when same dictionary is used multiple times.

## Dictionary helper functions

    unsigned ZSTD_getDictID_fromDict(const void* dict, size_t dictSize);
      Provides the dictID stored within dictionary.
      if @return == 0, the dictionary is not conformant with Zstandard specification.
      It can still be loaded, but as a content-only dictionary.

    unsigned ZSTD_getDictID_fromDDict(const ZSTD_DDict* ddict);
      Provides the dictID of the dictionary loaded into `ddict`.
      If @return == 0, the dictionary is not conformant to Zstandard specification, or empty.
      Non-conformant dictionaries can still be loaded, but as content-only dictionaries.

    unsigned ZSTD_getDictID_fromFrame(const void* src, size_t srcSize);
      Provides the dictID required to decompressed the frame stored within `src`.
      If @return == 0, the dictID could not be decoded.
      This could for one of the following reasons :
      - The frame does not require a dictionary to be decoded (most common case).
      - The frame was built with dictID intentionally removed. Whatever dictionary is necessary is a hidden information.
        Note : this use case also happens when using a non-conformant dictionary.
      - `srcSize` is too small, and as a result, the frame header could not be decoded (only possible if `srcSize < ZSTD_FRAMEHEADERSIZE_MAX`).
      - This is not a Zstandard frame.
      When identifying the exact failure cause, it's possible to use ZSTD_getFrameHeader(), which will provide a more precise error code.

## Advanced dictionary and prefix API

     This API allows dictionaries to be used with ZSTD_compress2(),
     ZSTD_compressStream2(), and ZSTD_decompress(). Dictionaries are sticky, and
     only reset with the context is reset with ZSTD_reset_parameters or
     ZSTD_reset_session_and_parameters. Prefixes are single-use.

    size_t ZSTD_CCtx_loadDictionary(ZSTD_CCtx* cctx, const void* dict, size_t dictSize);
      Create an internal CDict from `dict` buffer.
      Decompression will have to use same dictionary.
     @result : 0, or an error code (which can be tested with ZSTD_isError()).
      Special: Loading a NULL (or 0-size) dictionary invalidates previous dictionary,
               meaning "return to no-dictionary mode".
      Note 1 : Dictionary is sticky, it will be used for all future compressed frames.
               To return to "no-dictionary" situation, load a NULL dictionary (or reset parameters).
      Note 2 : Loading a dictionary involves building tables.
               It's also a CPU consuming operation, with non-negligible impact on latency.
               Tables are dependent on compression parameters, and for this reason,
               compression parameters can no longer be changed after loading a dictionary.
      Note 3 :`dict` content will be copied internally.
               Use experimental ZSTD_CCtx_loadDictionary_byReference() to reference content instead.
               In such a case, dictionary buffer must outlive its users.
      Note 4 : Use ZSTD_CCtx_loadDictionary_advanced()
               to precisely select how dictionary content must be interpreted.

    size_t ZSTD_CCtx_refCDict(ZSTD_CCtx* cctx, const ZSTD_CDict* cdict);
      Reference a prepared dictionary, to be used for all next compressed frames.
      Note that compression parameters are enforced from within CDict,
      and supersede any compression parameter previously set within CCtx.
      The parameters ignored are labled as "superseded-by-cdict" in the ZSTD_cParameter enum docs.
      The ignored parameters will be used again if the CCtx is returned to no-dictionary mode.
      The dictionary will remain valid for future compressed frames using same CCtx.
     @result : 0, or an error code (which can be tested with ZSTD_isError()).
      Special : Referencing a NULL CDict means "return to no-dictionary mode".
      Note 1 : Currently, only one dictionary can be managed.
               Referencing a new dictionary effectively "discards" any previous one.
      Note 2 : CDict is just referenced, its lifetime must outlive its usage within CCtx.

    size_t ZSTD_CCtx_refPrefix(ZSTD_CCtx* cctx,
                         const void* prefix, size_t prefixSize);
      Reference a prefix (single-usage dictionary) for next compressed frame.
      A prefix is **only used once**. Tables are discarded at end of frame (ZSTD_e_end).
      Decompression will need same prefix to properly regenerate data.
      Compressing with a prefix is similar in outcome as performing a diff and compressing it,
      but performs much faster, especially during decompression (compression speed is tunable with compression level).
     @result : 0, or an error code (which can be tested with ZSTD_isError()).
      Special: Adding any prefix (including NULL) invalidates any previous prefix or dictionary
      Note 1 : Prefix buffer is referenced. It **must** outlive compression.
               Its content must remain unmodified during compression.
      Note 2 : If the intention is to diff some large src data blob with some prior version of itself,
               ensure that the window size is large enough to contain the entire source.
               See ZSTD_c_windowLog.
      Note 3 : Referencing a prefix involves building tables, which are dependent on compression parameters.
               It's a CPU consuming operation, with non-negligible impact on latency.
               If there is a need to use the same prefix multiple times, consider loadDictionary instead.
      Note 4 : By default, the prefix is interpreted as raw content (ZSTD_dct_rawContent).
               Use experimental ZSTD_CCtx_refPrefix_advanced() to alter dictionary interpretation.

    size_t ZSTD_DCtx_loadDictionary(ZSTD_DCtx* dctx, const void* dict, size_t dictSize);
      Create an internal DDict from dict buffer,
      to be used to decompress next frames.
      The dictionary remains valid for all future frames, until explicitly invalidated.
     @result : 0, or an error code (which can be tested with ZSTD_isError()).
      Special : Adding a NULL (or 0-size) dictionary invalidates any previous dictionary,
                meaning "return to no-dictionary mode".
      Note 1 : Loading a dictionary involves building tables,
               which has a non-negligible impact on CPU usage and latency.
               It's recommended to "load once, use many times", to amortize the cost
      Note 2 :`dict` content will be copied internally, so `dict` can be released after loading.
               Use ZSTD_DCtx_loadDictionary_byReference() to reference dictionary content instead.
      Note 3 : Use ZSTD_DCtx_loadDictionary_advanced() to take control of
               how dictionary content is loaded and interpreted.

    size_t ZSTD_DCtx_refDDict(ZSTD_DCtx* dctx, const ZSTD_DDict* ddict);
      Reference a prepared dictionary, to be used to decompress next frames.
      The dictionary remains active for decompression of future frames using same DCtx.
     @result : 0, or an error code (which can be tested with ZSTD_isError()).
      Note 1 : Currently, only one dictionary can be managed.
               Referencing a new dictionary effectively "discards" any previous one.
      Special: referencing a NULL DDict means "return to no-dictionary mode".
      Note 2 : DDict is just referenced, its lifetime must outlive its usage from DCtx.

    size_t ZSTD_DCtx_refPrefix(ZSTD_DCtx* dctx,
                         const void* prefix, size_t prefixSize);
      Reference a prefix (single-usage dictionary) to decompress next frame.
      This is the reverse operation of ZSTD_CCtx_refPrefix(),
      and must use the same prefix as the one used during compression.
      Prefix is **only used once**. Reference is discarded at end of frame.
      End of frame is reached when ZSTD_decompressStream() returns 0.
     @result : 0, or an error code (which can be tested with ZSTD_isError()).
      Note 1 : Adding any prefix (including NULL) invalidates any previously set prefix or dictionary
      Note 2 : Prefix buffer is referenced. It **must** outlive decompression.
               Prefix buffer must remain unmodified up to the end of frame,
               reached when ZSTD_decompressStream() returns 0.
      Note 3 : By default, the prefix is treated as raw content (ZSTD_dct_rawContent).
               Use ZSTD_CCtx_refPrefix_advanced() to alter dictMode (Experimental section)
      Note 4 : Referencing a raw content prefix has almost no cpu nor memory cost.
               A full dictionary is more costly, as it requires building tables.

    size_t ZSTD_sizeof_CCtx(const ZSTD_CCtx* cctx);
    size_t ZSTD_sizeof_DCtx(const ZSTD_DCtx* dctx);
    size_t ZSTD_sizeof_CStream(const ZSTD_CStream* zcs);
    size_t ZSTD_sizeof_DStream(const ZSTD_DStream* zds);
    size_t ZSTD_sizeof_CDict(const ZSTD_CDict* cdict);
    size_t ZSTD_sizeof_DDict(const ZSTD_DDict* ddict);
      These functions give the _current_ memory usage of selected object.
      Note that object memory usage can evolve (increase or decrease) over time.

## experimental API (static linking only)

     The following symbols and constants
     are not planned to join "stable API" status in the near future.
     They can still change in future versions.
     Some of them are planned to remain in the static_only section indefinitely.
     Some of them might be removed in the future (especially when redundant with existing stable functions)

    typedef struct {
        unsigned int matchPos; /* Match pos in dst */
        /* If seqDef.offset > 3, then this is seqDef.offset - 3
         * If seqDef.offset < 3, then this is the corresponding repeat offset
         * But if seqDef.offset < 3 and litLength == 0, this is the
         *   repeat offset before the corresponding repeat offset
         * And if seqDef.offset == 3 and litLength == 0, this is the
         *   most recent repeat offset - 1
         */
        unsigned int offset;
        unsigned int litLength; /* Literal length */
        unsigned int matchLength; /* Match length */
        /* 0 when seq not rep and seqDef.offset otherwise
         * when litLength == 0 this will be <= 4, otherwise <= 3 like normal
         */
        unsigned int rep;
    } ZSTD_Sequence;

    typedef struct {
        unsigned windowLog;       /**< largest match distance : larger == more compression, more memory needed during decompression */
        unsigned chainLog;        /**< fully searched segment : larger == more compression, slower, more memory (useless for fast) */
        unsigned hashLog;         /**< dispatch table : larger == faster, more memory */
        unsigned searchLog;       /**< nb of searches : larger == more compression, slower */
        unsigned minMatch;        /**< match length searched : larger == faster decompression, sometimes less compression */
        unsigned targetLength;    /**< acceptable match size for optimal parser (only) : larger == more compression, slower */
        ZSTD_strategy strategy;   /**< see ZSTD_strategy definition above */
    } ZSTD_compressionParameters;

    typedef struct {
        int contentSizeFlag; /**< 1: content size will be in frame header (when known) */
        int checksumFlag;    /**< 1: generate a 32-bits checksum using XXH64 algorithm at end of frame, for error detection */
        int noDictIDFlag;    /**< 1: no dictID will be saved into frame header (dictID is only useful for dictionary compression) */
    } ZSTD_frameParameters;

    typedef struct {
        ZSTD_compressionParameters cParams;
        ZSTD_frameParameters fParams;
    } ZSTD_parameters;

    typedef enum {
        ZSTD_dct_auto = 0,       /* dictionary is "full" when starting with ZSTD_MAGIC_DICTIONARY, otherwise it is "rawContent" */
        ZSTD_dct_rawContent = 1, /* ensures dictionary is always loaded as rawContent, even if it starts with ZSTD_MAGIC_DICTIONARY */
        ZSTD_dct_fullDict = 2    /* refuses to load a dictionary if it does not respect Zstandard's specification, starting with ZSTD_MAGIC_DICTIONARY */
    } ZSTD_dictContentType_e;

    typedef enum {
        ZSTD_dlm_byCopy = 0,  /**< Copy dictionary content internally */
        ZSTD_dlm_byRef = 1    /**< Reference dictionary content -- the dictionary buffer must outlive its users. */
    } ZSTD_dictLoadMethod_e;

    typedef enum {
        ZSTD_f_zstd1 = 0,           /* zstd frame format, specified in zstd_compression_format.md (default) */
        ZSTD_f_zstd1_magicless = 1  /* Variant of zstd frame format, without initial 4-bytes magic number.
                                     * Useful to save 4 bytes per generated frame.
                                     * Decoder cannot recognise automatically this format, requiring this instruction. */
    } ZSTD_format_e;

    typedef enum {
        /* Note: this enum and the behavior it controls are effectively internal
         * implementation details of the compressor. They are expected to continue
         * to evolve and should be considered only in the context of extremely
         * advanced performance tuning.
         *
         * Zstd currently supports the use of a CDict in three ways:
         *
         * - The contents of the CDict can be copied into the working context. This
         *   means that the compression can search both the dictionary and input
         *   while operating on a single set of internal tables. This makes
         *   the compression faster per-byte of input. However, the initial copy of
         *   the CDict's tables incurs a fixed cost at the beginning of the
         *   compression. For small compressions (< 8 KB), that copy can dominate
         *   the cost of the compression.
         *
         * - The CDict's tables can be used in-place. In this model, compression is
         *   slower per input byte, because the compressor has to search two sets of
         *   tables. However, this model incurs no start-up cost (as long as the
         *   working context's tables can be reused). For small inputs, this can be
         *   faster than copying the CDict's tables.
         *
         * - The CDict's tables are not used at all, and instead we use the working
         *   context alone to reload the dictionary and use params based on the source
         *   size. See ZSTD_compress_insertDictionary() and ZSTD_compress_usingDict().
         *   This method is effective when the dictionary sizes are very small relative
         *   to the input size, and the input size is fairly large to begin with.
         *
         * Zstd has a simple internal heuristic that selects which strategy to use
         * at the beginning of a compression. However, if experimentation shows that
         * Zstd is making poor choices, it is possible to override that choice with
         * this enum.
         */
        ZSTD_dictDefaultAttach = 0, /* Use the default heuristic. */
        ZSTD_dictForceAttach   = 1, /* Never copy the dictionary. */
        ZSTD_dictForceCopy     = 2, /* Always copy the dictionary. */
        ZSTD_dictForceLoad     = 3  /* Always reload the dictionary */
    } ZSTD_dictAttachPref_e;

    typedef enum {
      ZSTD_lcm_auto = 0,          /**< Automatically determine the compression mode based on the compression level.
                                   *   Negative compression levels will be uncompressed, and positive compression
                                   *   levels will be compressed. */
      ZSTD_lcm_huffman = 1,       /**< Always attempt Huffman compression. Uncompressed literals will still be
                                   *   emitted if Huffman compression is not profitable. */
      ZSTD_lcm_uncompressed = 2   /**< Always emit uncompressed literals. */
    } ZSTD_literalCompressionMode_e;

## Frame size functions

    unsigned long long ZSTD_findDecompressedSize(const void* src, size_t srcSize);
      `src` should point to the start of a series of ZSTD encoded and/or skippable frames
      `srcSize` must be the _exact_ size of this series
           (i.e. there should be a frame boundary at `src + srcSize`)
      @return : - decompressed size of all data in all successive frames
                - if the decompressed size cannot be determined: ZSTD_CONTENTSIZE_UNKNOWN
                - if an error occurred: ZSTD_CONTENTSIZE_ERROR

       note 1 : decompressed size is an optional field, that may not be present, especially in streaming mode.
                When `return==ZSTD_CONTENTSIZE_UNKNOWN`, data to decompress could be any size.
                In which case, it's necessary to use streaming mode to decompress data.
       note 2 : decompressed size is always present when compression is done with ZSTD_compress()
       note 3 : decompressed size can be very large (64-bits value),
                potentially larger than what local system can handle as a single memory segment.
                In which case, it's necessary to use streaming mode to decompress data.
       note 4 : If source is untrusted, decompressed size could be wrong or intentionally modified.
                Always ensure result fits within application's authorized limits.
                Each application can set its own limits.
       note 5 : ZSTD_findDecompressedSize handles multiple frames, and so it must traverse the input to
                read each contained frame header.  This is fast as most of the data is skipped,
                however it does mean that all frame data must be present and valid.

    unsigned long long ZSTD_decompressBound(const void* src, size_t srcSize);
      `src` should point to the start of a series of ZSTD encoded and/or skippable frames
      `srcSize` must be the _exact_ size of this series
           (i.e. there should be a frame boundary at `src + srcSize`)
      @return : - upper-bound for the decompressed size of all data in all successive frames
                - if an error occured: ZSTD_CONTENTSIZE_ERROR

      note 1  : an error can occur if `src` contains an invalid or incorrectly formatted frame.
      note 2  : the upper-bound is exact when the decompressed size field is available in every ZSTD encoded frame of `src`.
                in this case, `ZSTD_findDecompressedSize` and `ZSTD_decompressBound` return the same value.
      note 3  : when the decompressed size field isn't available, the upper-bound for that frame is calculated by:
                  upper-bound = # blocks * min(128 KB, Window_Size)

    size_t ZSTD_frameHeaderSize(const void* src, size_t srcSize);
      srcSize must be >= ZSTD_FRAMEHEADERSIZE_PREFIX.
     @return : size of the Frame Header,
               or an error code (if srcSize is too small)

    size_t ZSTD_getSequences(ZSTD_CCtx* zc, ZSTD_Sequence* outSeqs,
        size_t outSeqsSize, const void* src, size_t srcSize);
     Extract sequences from the sequence store
     zc can be used to insert custom compression params.
     This function invokes ZSTD_compress2
     @return : number of sequences extracted

## Memory management

    size_t ZSTD_estimateCCtxSize(int compressionLevel);
    size_t ZSTD_estimateCCtxSize_usingCParams(ZSTD_compressionParameters cParams);
    size_t ZSTD_estimateCCtxSize_usingCCtxParams(const ZSTD_CCtx_params* params);
    size_t ZSTD_estimateDCtxSize(void);
      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().

      Note: only single-threaded compression is supported. This function will
      return an error code if ZSTD_c_nbWorkers is >= 1.

    size_t ZSTD_estimateCStreamSize(int compressionLevel);
    size_t ZSTD_estimateCStreamSize_usingCParams(ZSTD_compressionParameters cParams);
    size_t ZSTD_estimateCStreamSize_usingCCtxParams(const ZSTD_CCtx_params* params);
    size_t ZSTD_estimateDStreamSize(size_t windowSize);
    size_t ZSTD_estimateDStreamSize_fromFrame(const void* src, size_t srcSize);
      ZSTD_estimateCStreamSize() will provide a budget large enough for any compression level up to selected one.
      It will also consider src size to be arbitrarily "large", which is worst case.
      If srcSize is known to always be small, ZSTD_estimateCStreamSize_usingCParams() can provide a tighter estimation.
      ZSTD_estimateCStreamSize_usingCParams() can be used in tandem with ZSTD_getCParams() to create cParams from compressionLevel.
      ZSTD_estimateCStreamSize_usingCCtxParams() can be used in tandem with ZSTD_CCtxParams_setParameter(). Only single-threaded compression is supported. This function will return an error code if ZSTD_c_nbWorkers is >= 1.
      Note : CStream size estimation is only correct for single-threaded compression.
      ZSTD_DStream memory budget depends on window Size.
      This information can be passed manually, using ZSTD_estimateDStreamSize,
      or deducted from a valid frame Header, using ZSTD_estimateDStreamSize_fromFrame();
      Note : if streaming is init with function ZSTD_init?Stream_usingDict(),
             an internal ?Dict will be created, which additional size is not estimated here.
             In this case, get total size by adding ZSTD_estimate?DictSize

    size_t ZSTD_estimateCDictSize(size_t dictSize, int compressionLevel);
    size_t ZSTD_estimateCDictSize_advanced(size_t dictSize, ZSTD_compressionParameters cParams, ZSTD_dictLoadMethod_e dictLoadMethod);
    size_t ZSTD_estimateDDictSize(size_t dictSize, ZSTD_dictLoadMethod_e dictLoadMethod);
      ZSTD_estimateCDictSize() will bet that src size is relatively "small", and content is copied, like ZSTD_createCDict().
      ZSTD_estimateCDictSize_advanced() makes it possible to control compression parameters precisely, like ZSTD_createCDict_advanced().
      Note : dictionaries created by reference (`ZSTD_dlm_byRef`) are logically smaller.

    ZSTD_CCtx*    ZSTD_initStaticCCtx(void* workspace, size_t workspaceSize);
    ZSTD_CStream* ZSTD_initStaticCStream(void* workspace, size_t workspaceSize);    /**< same as ZSTD_initStaticCCtx() */
      Initialize an object using a pre-allocated fixed-size buffer.
      workspace: The memory area to emplace the object into.
                 Provided pointer *must be 8-bytes aligned*.
                 Buffer must outlive object.
      workspaceSize: Use ZSTD_estimate*Size() to determine
                     how large workspace must be to support target scenario.
     @return : pointer to object (same address as workspace, just different type),
               or NULL if error (size too small, incorrect alignment, etc.)
      Note : zstd will never resize nor malloc() when using a static buffer.
             If the object requires more memory than available,
             zstd will just error out (typically ZSTD_error_memory_allocation).
      Note 2 : there is no corresponding "free" function.
               Since workspace is allocated externally, it must be freed externally too.
      Note 3 : cParams : use ZSTD_getCParams() to convert a compression level
               into its associated cParams.
      Limitation 1 : currently not compatible with internal dictionary creation, triggered by
                     ZSTD_CCtx_loadDictionary(), ZSTD_initCStream_usingDict() or ZSTD_initDStream_usingDict().
      Limitation 2 : static cctx currently not compatible with multi-threading.
      Limitation 3 : static dctx is incompatible with legacy support.

    ZSTD_DStream* ZSTD_initStaticDStream(void* workspace, size_t workspaceSize);    /**< same as ZSTD_initStaticDCtx() */

    typedef void* (*ZSTD_allocFunction) (void* opaque, size_t size);
    typedef void  (*ZSTD_freeFunction) (void* opaque, void* address);
    typedef struct { ZSTD_allocFunction customAlloc; ZSTD_freeFunction customFree; void* opaque; } ZSTD_customMem;
    static ZSTD_customMem const ZSTD_defaultCMem = { NULL, NULL, NULL };  /**< this constant defers to stdlib's functions */
      These prototypes make it possible to pass your own allocation/free functions.
      ZSTD_customMem is provided at creation time, using ZSTD_create*_advanced() variants listed below.
      All allocation/free operations will be completed using these custom variants instead of regular  ones.

## Advanced compression functions

    ZSTD_CDict* ZSTD_createCDict_byReference(const void* dictBuffer, size_t dictSize, int compressionLevel);
      Create a digested dictionary for compression
      Dictionary content is just referenced, not duplicated.
      As a consequence, `dictBuffer` **must** outlive CDict,
      and its content must remain unmodified throughout the lifetime of CDict.
      note: equivalent to ZSTD_createCDict_advanced(), with dictLoadMethod==ZSTD_dlm_byRef

    ZSTD_compressionParameters ZSTD_getCParams(int compressionLevel, unsigned long long estimatedSrcSize, size_t dictSize);
     @return ZSTD_compressionParameters structure for a selected compression level and estimated srcSize.
     `estimatedSrcSize` value is optional, select 0 if not known

    ZSTD_parameters ZSTD_getParams(int compressionLevel, unsigned long long estimatedSrcSize, size_t dictSize);
      same as ZSTD_getCParams(), but @return a full `ZSTD_parameters` object instead of sub-component `ZSTD_compressionParameters`.
      All fields of `ZSTD_frameParameters` are set to default : contentSize=1, checksum=0, noDictID=0

    size_t ZSTD_checkCParams(ZSTD_compressionParameters params);
      Ensure param values remain within authorized range.
     @return 0 on success, or an error code (can be checked with ZSTD_isError())

    ZSTD_compressionParameters ZSTD_adjustCParams(ZSTD_compressionParameters cPar, unsigned long long srcSize, size_t dictSize);
      optimize params for a given `srcSize` and `dictSize`.
     `srcSize` can be unknown, in which case use ZSTD_CONTENTSIZE_UNKNOWN.
     `dictSize` must be `0` when there is no dictionary.
      cPar can be invalid : all parameters will be clamped within valid range in the @return struct.
      This function never fails (wide contract)

    size_t ZSTD_compress_advanced(ZSTD_CCtx* cctx,
                                  void* dst, size_t dstCapacity,
                            const void* src, size_t srcSize,
                            const void* dict,size_t dictSize,
                                  ZSTD_parameters params);
      Note : this function is now DEPRECATED.
             It can be replaced by ZSTD_compress2(), in combination with ZSTD_CCtx_setParameter() and other parameter setters.
      This prototype will be marked as deprecated and generate compilation warning on reaching v1.5.x

    size_t ZSTD_compress_usingCDict_advanced(ZSTD_CCtx* cctx,
                                      void* dst, size_t dstCapacity,
                                const void* src, size_t srcSize,
                                const ZSTD_CDict* cdict,
                                      ZSTD_frameParameters fParams);
      Note : this function is now REDUNDANT.
             It can be replaced by ZSTD_compress2(), in combination with ZSTD_CCtx_loadDictionary() and other parameter setters.
      This prototype will be marked as deprecated and generate compilation warning in some future version

    size_t ZSTD_CCtx_loadDictionary_byReference(ZSTD_CCtx* cctx, const void* dict, size_t dictSize);
      Same as ZSTD_CCtx_loadDictionary(), but dictionary content is referenced, instead of being copied into CCtx.
      It saves some memory, but also requires that `dict` outlives its usage within `cctx`

    size_t ZSTD_CCtx_loadDictionary_advanced(ZSTD_CCtx* cctx, const void* dict, size_t dictSize, ZSTD_dictLoadMethod_e dictLoadMethod, ZSTD_dictContentType_e dictContentType);
      Same as ZSTD_CCtx_loadDictionary(), but gives finer control over
      how to load the dictionary (by copy ? by reference ?)
      and how to interpret it (automatic ? force raw mode ? full mode only ?)

    size_t ZSTD_CCtx_refPrefix_advanced(ZSTD_CCtx* cctx, const void* prefix, size_t prefixSize, ZSTD_dictContentType_e dictContentType);
      Same as ZSTD_CCtx_refPrefix(), but gives finer control over
      how to interpret prefix content (automatic ? force raw mode (default) ? full mode only ?)

    size_t ZSTD_CCtx_getParameter(ZSTD_CCtx* cctx, ZSTD_cParameter param, int* value);
      Get the requested compression parameter value, selected by enum ZSTD_cParameter,
      and store it into int* value.
     @return : 0, or an error code (which can be tested with ZSTD_isError()).

    ZSTD_CCtx_params* ZSTD_createCCtxParams(void);
    size_t ZSTD_freeCCtxParams(ZSTD_CCtx_params* params);
      Quick howto :
      - ZSTD_createCCtxParams() : Create a ZSTD_CCtx_params structure
      - ZSTD_CCtxParams_setParameter() : Push parameters one by one into
                                         an existing ZSTD_CCtx_params structure.
                                         This is similar to
                                         ZSTD_CCtx_setParameter().
      - ZSTD_CCtx_setParametersUsingCCtxParams() : Apply parameters to
                                        an existing CCtx.
                                        These parameters will be applied to
                                        all subsequent frames.
      - ZSTD_compressStream2() : Do compression using the CCtx.
      - ZSTD_freeCCtxParams() : Free the memory.

      This can be used with ZSTD_estimateCCtxSize_advanced_usingCCtxParams()
      for static allocation of CCtx for single-threaded compression.

    size_t ZSTD_CCtxParams_reset(ZSTD_CCtx_params* params);
      Reset params to default values.

    size_t ZSTD_CCtxParams_init(ZSTD_CCtx_params* cctxParams, int compressionLevel);
      Initializes the compression parameters of cctxParams according to
      compression level. All other parameters are reset to their default values.

    size_t ZSTD_CCtxParams_init_advanced(ZSTD_CCtx_params* cctxParams, ZSTD_parameters params);
      Initializes the compression and frame parameters of cctxParams according to
      params. All other parameters are reset to their default values.

    size_t ZSTD_CCtxParams_setParameter(ZSTD_CCtx_params* params, ZSTD_cParameter param, int value);
      Similar to ZSTD_CCtx_setParameter.
      Set one compression parameter, selected by enum ZSTD_cParameter.
      Parameters must be applied to a ZSTD_CCtx using ZSTD_CCtx_setParametersUsingCCtxParams().
     @result : 0, or an error code (which can be tested with ZSTD_isError()).

    size_t ZSTD_CCtxParams_getParameter(ZSTD_CCtx_params* params, ZSTD_cParameter param, int* value);
     Similar to ZSTD_CCtx_getParameter.
     Get the requested value of one compression parameter, selected by enum ZSTD_cParameter.
     @result : 0, or an error code (which can be tested with ZSTD_isError()).

    size_t ZSTD_CCtx_setParametersUsingCCtxParams(
            ZSTD_CCtx* cctx, const ZSTD_CCtx_params* params);
      Apply a set of ZSTD_CCtx_params to the compression context.
      This can be done even after compression is started,
        if nbWorkers==0, this will have no impact until a new compression is started.
        if nbWorkers>=1, new parameters will be picked up at next job,
           with a few restrictions (windowLog, pledgedSrcSize, nbWorkers, jobSize, and overlapLog are not updated).

    size_t ZSTD_compressStream2_simpleArgs (
                    ZSTD_CCtx* cctx,
                    void* dst, size_t dstCapacity, size_t* dstPos,
              const void* src, size_t srcSize, size_t* srcPos,
                    ZSTD_EndDirective endOp);
      Same as ZSTD_compressStream2(),
      but using only integral types as arguments.
      This variant might be helpful for binders from dynamic languages
      which have troubles handling structures containing memory pointers.

## Advanced decompression functions

    unsigned ZSTD_isFrame(const void* buffer, size_t size);
      Tells if the content of `buffer` starts with a valid Frame Identifier.
      Note : Frame Identifier is 4 bytes. If `size < 4`, @return will always be 0.
      Note 2 : Legacy Frame Identifiers are considered valid only if Legacy Support is enabled.
      Note 3 : Skippable Frame Identifiers are considered valid.

    ZSTD_DDict* ZSTD_createDDict_byReference(const void* dictBuffer, size_t dictSize);
      Create a digested dictionary, ready to start decompression operation without startup delay.
      Dictionary content is referenced, and therefore stays in dictBuffer.
      It is important that dictBuffer outlives DDict,
      it must remain read accessible throughout the lifetime of DDict

    size_t ZSTD_DCtx_loadDictionary_byReference(ZSTD_DCtx* dctx, const void* dict, size_t dictSize);
      Same as ZSTD_DCtx_loadDictionary(),
      but references `dict` content instead of copying it into `dctx`.
      This saves memory if `dict` remains around.,
      However, it's imperative that `dict` remains accessible (and unmodified) while being used, so it must outlive decompression.

    size_t ZSTD_DCtx_loadDictionary_advanced(ZSTD_DCtx* dctx, const void* dict, size_t dictSize, ZSTD_dictLoadMethod_e dictLoadMethod, ZSTD_dictContentType_e dictContentType);
      Same as ZSTD_DCtx_loadDictionary(),
      but gives direct control over
      how to load the dictionary (by copy ? by reference ?)
      and how to interpret it (automatic ? force raw mode ? full mode only ?).

    size_t ZSTD_DCtx_refPrefix_advanced(ZSTD_DCtx* dctx, const void* prefix, size_t prefixSize, ZSTD_dictContentType_e dictContentType);
      Same as ZSTD_DCtx_refPrefix(), but gives finer control over
      how to interpret prefix content (automatic ? force raw mode (default) ? full mode only ?)

    size_t ZSTD_DCtx_setMaxWindowSize(ZSTD_DCtx* dctx, size_t maxWindowSize);
      Refuses allocating internal buffers for frames requiring a window size larger than provided limit.
      This protects a decoder context from reserving too much memory for itself (potential attack scenario).
      This parameter is only useful in streaming mode, since no internal buffer is allocated in single-pass mode.
      By default, a decompression context accepts all window sizes <= (1 << ZSTD_WINDOWLOG_LIMIT_DEFAULT)
     @return : 0, or an error code (which can be tested using ZSTD_isError()).

    size_t ZSTD_DCtx_setFormat(ZSTD_DCtx* dctx, ZSTD_format_e format);
      Instruct the decoder context about what kind of data to decode next.
      This instruction is mandatory to decode data without a fully-formed header,
      such ZSTD_f_zstd1_magicless for example.
     @return : 0, or an error code (which can be tested using ZSTD_isError()).

    size_t ZSTD_decompressStream_simpleArgs (
                    ZSTD_DCtx* dctx,
                    void* dst, size_t dstCapacity, size_t* dstPos,
              const void* src, size_t srcSize, size_t* srcPos);
      Same as ZSTD_decompressStream(),
      but using only integral types as arguments.
      This can be helpful for binders from dynamic languages
      which have troubles handling structures containing memory pointers.

## Advanced streaming functions

      Warning : most of these functions are now redundant with the Advanced API.
      Once Advanced API reaches "stable" status,
      redundant functions will be deprecated, and then at some point removed.

### Advanced Streaming compression functions

/**! ZSTD_initCStream_srcSize() : * This function is deprecated, and equivalent to: * ZSTD_CCtx_reset(zcs, ZSTD_reset_session_only); * ZSTD_CCtx_refCDict(zcs, NULL); // clear the dictionary (if any) * ZSTD_CCtx_setParameter(zcs, ZSTD_c_compressionLevel, compressionLevel); * ZSTD_CCtx_setPledgedSrcSize(zcs, pledgedSrcSize); * * pledgedSrcSize must be correct. If it is not known at init time, use * ZSTD_CONTENTSIZE_UNKNOWN. Note that, for compatibility with older programs, * "0" also disables frame content size field. It may be enabled in the future. * Note : this prototype will be marked as deprecated and generate compilation warnings on reaching v1.5.x */ size_t ZSTD_initCStream_srcSize(ZSTD_CStream* zcs, int compressionLevel, unsigned long long pledgedSrcSize);

## ! ZSTD_initCStream_usingDict() :

     This function is deprecated, and is equivalent to:
         ZSTD_CCtx_reset(zcs, ZSTD_reset_session_only);
         ZSTD_CCtx_setParameter(zcs, ZSTD_c_compressionLevel, compressionLevel);
         ZSTD_CCtx_loadDictionary(zcs, dict, dictSize);

     Creates of an internal CDict (incompatible with static CCtx), except if
     dict == NULL or dictSize < 8, in which case no dict is used.
     Note: dict is loaded with ZSTD_dct_auto (treated as a full zstd dictionary if
     it begins with ZSTD_MAGIC_DICTIONARY, else as raw content) and ZSTD_dlm_byCopy.
     Note : this prototype will be marked as deprecated and generate compilation warnings on reaching v1.5.x

## ! ZSTD_initCStream_advanced() :

     This function is deprecated, and is approximately equivalent to:
         ZSTD_CCtx_reset(zcs, ZSTD_reset_session_only);
         // Pseudocode: Set each zstd parameter and leave the rest as-is.
         for ((param, value) : params) {
             ZSTD_CCtx_setParameter(zcs, param, value);
         }
         ZSTD_CCtx_setPledgedSrcSize(zcs, pledgedSrcSize);
         ZSTD_CCtx_loadDictionary(zcs, dict, dictSize);

     dict is loaded with ZSTD_dct_auto and ZSTD_dlm_byCopy.
     pledgedSrcSize must be correct.
     If srcSize is not known at init time, use value ZSTD_CONTENTSIZE_UNKNOWN.
     Note : this prototype will be marked as deprecated and generate compilation warnings on reaching v1.5.x

## ! ZSTD_initCStream_usingCDict() :

     This function is deprecated, and equivalent to:
         ZSTD_CCtx_reset(zcs, ZSTD_reset_session_only);
         ZSTD_CCtx_refCDict(zcs, cdict);

     note : cdict will just be referenced, and must outlive compression session
     Note : this prototype will be marked as deprecated and generate compilation warnings on reaching v1.5.x

## ! ZSTD_initCStream_usingCDict_advanced() :

       This function is DEPRECATED, and is approximately equivalent to:
         ZSTD_CCtx_reset(zcs, ZSTD_reset_session_only);
         // Pseudocode: Set each zstd frame parameter and leave the rest as-is.
         for ((fParam, value) : fParams) {
             ZSTD_CCtx_setParameter(zcs, fParam, value);
         }
         ZSTD_CCtx_setPledgedSrcSize(zcs, pledgedSrcSize);
         ZSTD_CCtx_refCDict(zcs, cdict);

     same as ZSTD_initCStream_usingCDict(), with control over frame parameters.
     pledgedSrcSize must be correct. If srcSize is not known at init time, use
     value ZSTD_CONTENTSIZE_UNKNOWN.
     Note : this prototype will be marked as deprecated and generate compilation warnings on reaching v1.5.x

    size_t ZSTD_resetCStream(ZSTD_CStream* zcs, unsigned long long pledgedSrcSize);
     This function is deprecated, and is equivalent to:
         ZSTD_CCtx_reset(zcs, ZSTD_reset_session_only);
         ZSTD_CCtx_setPledgedSrcSize(zcs, pledgedSrcSize);

      start a new frame, using same parameters from previous frame.
      This is typically useful to skip dictionary loading stage, since it will re-use it in-place.
      Note that zcs must be init at least once before using ZSTD_resetCStream().
      If pledgedSrcSize is not known at reset time, use macro ZSTD_CONTENTSIZE_UNKNOWN.
      If pledgedSrcSize > 0, its value must be correct, as it will be written in header, and controlled at the end.
      For the time being, pledgedSrcSize==0 is interpreted as "srcSize unknown" for compatibility with older programs,
      but it will change to mean "empty" in future version, so use macro ZSTD_CONTENTSIZE_UNKNOWN instead.
     @return : 0, or an error code (which can be tested using ZSTD_isError())
      Note : this prototype will be marked as deprecated and generate compilation warnings on reaching v1.5.x

    typedef struct {
        unsigned long long ingested;   /* nb input bytes read and buffered */
        unsigned long long consumed;   /* nb input bytes actually compressed */
        unsigned long long produced;   /* nb of compressed bytes generated and buffered */
        unsigned long long flushed;    /* nb of compressed bytes flushed : not provided; can be tracked from caller side */
        unsigned currentJobID;         /* MT only : latest started job nb */
        unsigned nbActiveWorkers;      /* MT only : nb of workers actively compressing at probe time */
    } ZSTD_frameProgression;

    size_t ZSTD_toFlushNow(ZSTD_CCtx* cctx);
      Tell how many bytes are ready to be flushed immediately.
      Useful for multithreading scenarios (nbWorkers >= 1).
      Probe the oldest active job, defined as oldest job not yet entirely flushed,
      and check its output buffer.
     @return : amount of data stored in oldest job and ready to be flushed immediately.
      if @return == 0, it means either :
      + there is no active job (could be checked with ZSTD_frameProgression()), or
      + oldest job is still actively compressing data,
        but everything it has produced has also been flushed so far,
        therefore flush speed is limited by production speed of oldest job
        irrespective of the speed of concurrent (and newer) jobs.

### Advanced Streaming decompression functions

/** * This function is deprecated, and is equivalent to: * * ZSTD_DCtx_reset(zds, ZSTD_reset_session_only); * ZSTD_DCtx_loadDictionary(zds, dict, dictSize); * * note: no dictionary will be used if dict == NULL or dictSize < 8 * Note : this prototype will be marked as deprecated and generate compilation warnings on reaching v1.5.x */ size_t ZSTD_initDStream_usingDict(ZSTD_DStream* zds, const void* dict, size_t dictSize);

## This function is deprecated, and is equivalent to:

         ZSTD_DCtx_reset(zds, ZSTD_reset_session_only);
         ZSTD_DCtx_refDDict(zds, ddict);

     note : ddict is referenced, it must outlive decompression session
     Note : this prototype will be marked as deprecated and generate compilation warnings on reaching v1.5.x

## This function is deprecated, and is equivalent to:

         ZSTD_DCtx_reset(zds, ZSTD_reset_session_only);

     re-use decompression parameters from previous init; saves dictionary loading
     Note : this prototype will be marked as deprecated and generate compilation warnings on reaching v1.5.x

## Buffer-less and synchronous inner streaming functions

      This is an advanced API, giving full control over buffer management, for users which need direct control over memory.
      But it's also a complex one, with several restrictions, documented below.
      Prefer normal streaming API for an easier experience.

## Buffer-less streaming compression (synchronous mode)

      A ZSTD_CCtx object is required to track streaming operations.
      Use ZSTD_createCCtx() / ZSTD_freeCCtx() to manage resource.
      ZSTD_CCtx object can be re-used multiple times within successive compression operations.

      Start by initializing a context.
      Use ZSTD_compressBegin(), or ZSTD_compressBegin_usingDict() for dictionary compression,
      or ZSTD_compressBegin_advanced(), for finer parameter control.
      It's also possible to duplicate a reference context which has already been initialized, using ZSTD_copyCCtx()

      Then, consume your input using ZSTD_compressContinue().
      There are some important considerations to keep in mind when using this advanced function :
      - ZSTD_compressContinue() has no internal buffer. It uses externally provided buffers only.
      - Interface is synchronous : input is consumed entirely and produces 1+ compressed blocks.
      - Caller must ensure there is enough space in `dst` to store compressed data under worst case scenario.
        Worst case evaluation is provided by ZSTD_compressBound().
        ZSTD_compressContinue() doesn't guarantee recover after a failed compression.
      - ZSTD_compressContinue() presumes prior input ***is still accessible and unmodified*** (up to maximum distance size, see WindowLog).
        It remembers all previous contiguous blocks, plus one separated memory segment (which can itself consists of multiple contiguous blocks)
      - ZSTD_compressContinue() detects that prior input has been overwritten when `src` buffer overlaps.
        In which case, it will "discard" the relevant memory section from its history.

      Finish a frame with ZSTD_compressEnd(), which will write the last block(s) and optional checksum.
      It's possible to use srcSize==0, in which case, it will write a final empty block to end the frame.
      Without last block mark, frames are considered unfinished (hence corrupted) by compliant decoders.

      `ZSTD_CCtx` object can be re-used (ZSTD_compressBegin()) to compress again.

### Buffer-less streaming compression functions

    size_t ZSTD_compressBegin(ZSTD_CCtx* cctx, int compressionLevel);
    size_t ZSTD_compressBegin_usingDict(ZSTD_CCtx* cctx, const void* dict, size_t dictSize, int compressionLevel);
    size_t ZSTD_compressBegin_advanced(ZSTD_CCtx* cctx, const void* dict, size_t dictSize, ZSTD_parameters params, unsigned long long pledgedSrcSize);

/**< pledgedSrcSize : If srcSize is not known at init time, use ZSTD_CONTENTSIZE_UNKNOWN */ size_t ZSTD_compressBegin_usingCDict(ZSTD_CCtx* cctx, const ZSTD_CDict* cdict); /**< note: fails if cdict==NULL */ size_t ZSTD_compressBegin_usingCDict_advanced(ZSTD_CCtx* const cctx, const ZSTD_CDict* const cdict, ZSTD_frameParameters const fParams, unsigned long long const pledgedSrcSize); /* compression parameters are already set within cdict. pledgedSrcSize must be correct. If srcSize is not known, use macro ZSTD_CONTENTSIZE_UNKNOWN */ size_t ZSTD_copyCCtx(ZSTD_CCtx* cctx, const ZSTD_CCtx* preparedCCtx, unsigned long long pledgedSrcSize); /**< note: if pledgedSrcSize is not known, use ZSTD_CONTENTSIZE_UNKNOWN */

## Buffer-less streaming decompression (synchronous mode)

      A ZSTD_DCtx object is required to track streaming operations.
      Use ZSTD_createDCtx() / ZSTD_freeDCtx() to manage it.
      A ZSTD_DCtx object can be re-used multiple times.

      First typical operation is to retrieve frame parameters, using ZSTD_getFrameHeader().
      Frame header is extracted from the beginning of compressed frame, so providing only the frame's beginning is enough.
      Data fragment must be large enough to ensure successful decoding.
     `ZSTD_frameHeaderSize_max` bytes is guaranteed to always be large enough.
      @result : 0 : successful decoding, the `ZSTD_frameHeader` structure is correctly filled.
               >0 : `srcSize` is too small, please provide at least @result bytes on next attempt.
               errorCode, which can be tested using ZSTD_isError().

      It fills a ZSTD_frameHeader structure with important information to correctly decode the frame,
      such as the dictionary ID, content size, or maximum back-reference distance (`windowSize`).
      Note that these values could be wrong, either because of data corruption, or because a 3rd party deliberately spoofs false information.
      As a consequence, check that values remain within valid application range.
      For example, do not allocate memory blindly, check that `windowSize` is within expectation.
      Each application can set its own limits, depending on local restrictions.
      For extended interoperability, it is recommended to support `windowSize` of at least 8 MB.

      ZSTD_decompressContinue() needs previous data blocks during decompression, up to `windowSize` bytes.
      ZSTD_decompressContinue() is very sensitive to contiguity,
      if 2 blocks don't follow each other, make sure that either the compressor breaks contiguity at the same place,
      or that previous contiguous segment is large enough to properly handle maximum back-reference distance.
      There are multiple ways to guarantee this condition.

      The most memory efficient way is to use a round buffer of sufficient size.
      Sufficient size is determined by invoking ZSTD_decodingBufferSize_min(),
      which can @return an error code if required value is too large for current system (in 32-bits mode).
      In a round buffer methodology, ZSTD_decompressContinue() decompresses each block next to previous one,
      up to the moment there is not enough room left in the buffer to guarantee decoding another full block,
      which maximum size is provided in `ZSTD_frameHeader` structure, field `blockSizeMax`.
      At which point, decoding can resume from the beginning of the buffer.
      Note that already decoded data stored in the buffer should be flushed before being overwritten.

      There are alternatives possible, for example using two or more buffers of size `windowSize` each, though they consume more memory.

      Finally, if you control the compression process, you can also ignore all buffer size rules,
      as long as the encoder and decoder progress in "lock-step",
      aka use exactly the same buffer sizes, break contiguity at the same place, etc.

      Once buffers are setup, start decompression, with ZSTD_decompressBegin().
      If decompression requires a dictionary, use ZSTD_decompressBegin_usingDict() or ZSTD_decompressBegin_usingDDict().

      Then use ZSTD_nextSrcSizeToDecompress() and ZSTD_decompressContinue() alternatively.
      ZSTD_nextSrcSizeToDecompress() tells how many bytes to provide as 'srcSize' to ZSTD_decompressContinue().
      ZSTD_decompressContinue() requires this _exact_ amount of bytes, or it will fail.

     @result of ZSTD_decompressContinue() is the number of bytes regenerated within 'dst' (necessarily <= dstCapacity).
      It can be zero : it just means ZSTD_decompressContinue() has decoded some metadata item.
      It can also be an error code, which can be tested with ZSTD_isError().

      A frame is fully decoded when ZSTD_nextSrcSizeToDecompress() returns zero.
      Context can then be reset to start a new decompression.

      Note : it's possible to know if next input to present is a header or a block, using ZSTD_nextInputType().
      This information is not required to properly decode a frame.

      == Special case : skippable frames

      Skippable frames allow integration of user-defined data into a flow of concatenated frames.
      Skippable frames will be ignored (skipped) by decompressor.
      The format of skippable frames is as follows :
      a) Skippable frame ID - 4 Bytes, Little endian format, any value from 0x184D2A50 to 0x184D2A5F
      b) Frame Size - 4 Bytes, Little endian format, unsigned 32-bits
      c) Frame Content - any content (User Data) of length equal to Frame Size
      For skippable frames ZSTD_getFrameHeader() returns zfhPtr->frameType==ZSTD_skippableFrame.
      For skippable frames ZSTD_decompressContinue() always returns 0 : it only skips the content.

### Buffer-less streaming decompression functions

    typedef enum { ZSTD_frame, ZSTD_skippableFrame } ZSTD_frameType_e;
    typedef struct {
        unsigned long long frameContentSize;

/* if == ZSTD_CONTENTSIZE_UNKNOWN, it means this field is not available. 0 means "empty" */ unsigned long long windowSize; /* can be very large, up to <= frameContentSize */ unsigned blockSizeMax; ZSTD_frameType_e frameType; /* if == ZSTD_skippableFrame, frameContentSize is the size of skippable content */ unsigned headerSize; unsigned dictID; unsigned checksumFlag; } ZSTD_frameHeader;

    size_t ZSTD_getFrameHeader(ZSTD_frameHeader* zfhPtr, const void* src, size_t srcSize);   /**< doesn't consume input */
    /*! ZSTD_getFrameHeader_advanced() :
     *  same as ZSTD_getFrameHeader(),
     *  with added capability to select a format (like ZSTD_f_zstd1_magicless) */
    size_t ZSTD_getFrameHeader_advanced(ZSTD_frameHeader* zfhPtr, const void* src, size_t srcSize, ZSTD_format_e format);
    size_t ZSTD_decodingBufferSize_min(unsigned long long windowSize, unsigned long long frameContentSize);  /**< when frame content size is not known, pass in frameContentSize == ZSTD_CONTENTSIZE_UNKNOWN */
      decode Frame Header, or requires larger `srcSize`.
     @return : 0, `zfhPtr` is correctly filled,
              >0, `srcSize` is too small, value is wanted `srcSize` amount,
               or an error code, which can be tested using ZSTD_isError()

    typedef enum { ZSTDnit_frameHeader, ZSTDnit_blockHeader, ZSTDnit_block, ZSTDnit_lastBlock, ZSTDnit_checksum, ZSTDnit_skippableFrame } ZSTD_nextInputType_e;

## Block level API

        Frame metadata cost is typically ~12 bytes, which can be non-negligible for very small blocks (< 100 bytes).
        But users will have to take in charge needed metadata to regenerate data, such as compressed and content sizes.

        A few rules to respect :
        - Compressing and decompressing require a context structure
          + Use ZSTD_createCCtx() and ZSTD_createDCtx()
        - It is necessary to init context before starting
          + compression : any ZSTD_compressBegin*() variant, including with dictionary
          + decompression : any ZSTD_decompressBegin*() variant, including with dictionary
          + copyCCtx() and copyDCtx() can be used too
        - Block size is limited, it must be <= ZSTD_getBlockSize() <= ZSTD_BLOCKSIZE_MAX == 128 KB
          + If input is larger than a block size, it's necessary to split input data into multiple blocks
          + For inputs larger than a single block, consider using regular ZSTD_compress() instead.
            Frame metadata is not that costly, and quickly becomes negligible as source size grows larger than a block.
        - When a block is considered not compressible enough, ZSTD_compressBlock() result will be 0 (zero) !
          ===> In which case, nothing is produced into `dst` !
          + User __must__ test for such outcome and deal directly with uncompressed data
          + A block cannot be declared incompressible if ZSTD_compressBlock() return value was != 0.
            Doing so would mess up with statistics history, leading to potential data corruption.
          + ZSTD_decompressBlock() _doesn't accept uncompressed data as input_ !!
          + In case of multiple successive blocks, should some of them be uncompressed,
            decoder must be informed of their existence in order to follow proper history.
            Use ZSTD_insertBlock() for such a case.

### Raw zstd block functions

    size_t ZSTD_getBlockSize   (const ZSTD_CCtx* cctx);
    size_t ZSTD_compressBlock  (ZSTD_CCtx* cctx, void* dst, size_t dstCapacity, const void* src, size_t srcSize);
    size_t ZSTD_decompressBlock(ZSTD_DCtx* dctx, void* dst, size_t dstCapacity, const void* src, size_t srcSize);
    size_t ZSTD_insertBlock    (ZSTD_DCtx* dctx, const void* blockStart, size_t blockSize);

/**< insert uncompressed block into `dctx` history. Useful for multi-blocks decompression. */