# 1.8.1 Manual

## Contents

1. Introduction
2. Version
3. Tuning parameter
4. Simple Functions
5. Advanced Functions
6. Streaming Compression Functions
7. Streaming Decompression Functions
8. Private definitions
9. Obsolete Functions

## Introduction

      LZ4 is lossless compression algorithm, providing compression speed at 400 MB/s per core,
      scalable with multi-cores CPU. It features an extremely fast decoder, with speed in
      multiple GB/s per core, typically reaching RAM speed limits on multi-core systems.

      The LZ4 compression library provides in-memory compression and decompression functions.
      Compression can be done in:
        - a single step (described as Simple Functions)
        - a single step, reusing a context (described in Advanced Functions)
        - unbounded multiple steps (described as Streaming compression)

      lz4.h provides block compression functions. It gives full buffer control to user.
      Decompressing an lz4-compressed block also requires metadata (such as compressed size).
      Each application is free to encode such metadata in whichever way it wants.

      An additional format, called LZ4 frame specification (doc/lz4_Frame_format.md),
      take care of encoding standard metadata alongside LZ4-compressed blocks.
      If your application requires interoperability, it's recommended to use it.
      A library is provided to take care of it, see lz4frame.h.

## Version

    int LZ4_versionNumber (void);  /**< library version number; to be used when checking dll version */

    const char* LZ4_versionString (void);   /**< library version string; to be used when checking dll version */

## Tuning parameter

    #ifndef LZ4_MEMORY_USAGE
    # define LZ4_MEMORY_USAGE 14
    #endif
     Memory usage formula : N->2^N Bytes (examples : 10 -> 1KB; 12 -> 4KB ; 16 -> 64KB; 20 -> 1MB; etc.)
     Increasing memory usage improves compression ratio
     Reduced memory usage can improve speed, due to cache effect
     Default value is 14, for 16KB, which nicely fits into Intel x86 L1 cache

## Simple Functions

    int LZ4_compress_default(const char* src, char* dst, int srcSize, int dstCapacity);
        Compresses 'srcSize' bytes from buffer 'src'
        into already allocated 'dst' buffer of size 'dstCapacity'.
        Compression is guaranteed to succeed if 'dstCapacity' >= LZ4_compressBound(srcSize).
        It also runs faster, so it's a recommended setting.
        If the function cannot compress 'src' into a limited 'dst' budget,
        compression stops *immediately*, and the function result is zero.
        As a consequence, 'dst' content is not valid.
        This function never writes outside 'dst' buffer, nor read outside 'source' buffer.
            srcSize : supported max value is LZ4_MAX_INPUT_VALUE
            dstCapacity : full or partial size of buffer 'dst' (which must be already allocated)
            return  : the number of bytes written into buffer 'dst' (necessarily <= dstCapacity)
                      or 0 if compression fails

    int LZ4_decompress_safe (const char* src, char* dst, int compressedSize, int dstCapacity);
        compressedSize : is the exact complete size of the compressed block.
        dstCapacity : is the size of destination buffer, which must be already allocated.
        return : the number of bytes decompressed into destination buffer (necessarily <= dstCapacity)
                 If destination buffer is not large enough, decoding will stop and output an error code (negative value).
                 If the source stream is detected malformed, the function will stop decoding and return a negative result.
                 This function is protected against buffer overflow exploits, including malicious data packets.
                 It never writes outside output buffer, nor reads outside input buffer.

## Advanced Functions

    int LZ4_compressBound(int inputSize);
        Provides the maximum size that LZ4 compression may output in a "worst case" scenario (input data not compressible)
        This function is primarily useful for memory allocation purposes (destination buffer size).
        Macro LZ4_COMPRESSBOUND() is also provided for compilation-time evaluation (stack memory allocation for example).
        Note that LZ4_compress_default() compress faster when dest buffer size is >= LZ4_compressBound(srcSize)
            inputSize  : max supported value is LZ4_MAX_INPUT_SIZE
            return : maximum output size in a "worst case" scenario
                  or 0, if input size is too large ( > LZ4_MAX_INPUT_SIZE)

    int LZ4_compress_fast (const char* src, char* dst, int srcSize, int dstCapacity, int acceleration);
        Same as LZ4_compress_default(), but allows to select an "acceleration" factor.
        The larger the acceleration value, the faster the algorithm, but also the lesser the compression.
        It's a trade-off. It can be fine tuned, with each successive value providing roughly +~3% to speed.
        An acceleration value of "1" is the same as regular LZ4_compress_default()
        Values <= 0 will be replaced by ACCELERATION_DEFAULT (see lz4.c), which is 1.

    int LZ4_sizeofState(void);
    int LZ4_compress_fast_extState (void* state, const char* src, char* dst, int srcSize, int dstCapacity, int acceleration);
        Same compression function, just using an externally allocated memory space to store compression state.
        Use LZ4_sizeofState() to know how much memory must be allocated,
        and allocate it on 8-bytes boundaries (using malloc() typically).
        Then, provide it as 'void* state' to compression function.

    int LZ4_compress_destSize (const char* src, char* dst, int* srcSizePtr, int targetDstSize);
        Reverse the logic : compresses as much data as possible from 'src' buffer
        into already allocated buffer 'dst' of size 'targetDestSize'.
        This function either compresses the entire 'src' content into 'dst' if it's large enough,
        or fill 'dst' buffer completely with as much data as possible from 'src'.
            *srcSizePtr : will be modified to indicate how many bytes where read from 'src' to fill 'dst'.
                          New value is necessarily <= old value.
            return : Nb bytes written into 'dst' (necessarily <= targetDestSize)
                     or 0 if compression fails

    int LZ4_decompress_fast (const char* src, char* dst, int originalSize);
        originalSize : is the original uncompressed size
        return : the number of bytes read from the source buffer (in other words, the compressed size)
                 If the source stream is detected malformed, the function will stop decoding and return a negative result.
                 Destination buffer must be already allocated. Its size must be >= 'originalSize' bytes.
        note : This function respects memory boundaries for *properly formed* compressed data.
               It is a bit faster than LZ4_decompress_safe().
               However, it does not provide any protection against intentionally modified data stream (malicious input).
               Use this function in trusted environment only (data to decode comes from a trusted source).

    int LZ4_decompress_safe_partial (const char* src, char* dst, int srcSize, int targetOutputSize, int dstCapacity);
        This function decompress a compressed block of size 'srcSize' at position 'src'
        into destination buffer 'dst' of size 'dstCapacity'.
        The function will decompress a minimum of 'targetOutputSize' bytes, and stop after that.
        However, it's not accurate, and may write more than 'targetOutputSize' (but <= dstCapacity).
       @return : the number of bytes decoded in the destination buffer (necessarily <= dstCapacity)
           Note : this number can be < 'targetOutputSize' should the compressed block contain less data.
                 Always control how many bytes were decoded.
                 If the source stream is detected malformed, the function will stop decoding and return a negative result.
                 This function never writes outside of output buffer, and never reads outside of input buffer. It is therefore protected against malicious data packets.

## Streaming Compression Functions

    LZ4_stream_t* LZ4_createStream(void);
    int           LZ4_freeStream (LZ4_stream_t* streamPtr);
      LZ4_createStream() will allocate and initialize an `LZ4_stream_t` structure.
      LZ4_freeStream() releases its memory.

    void LZ4_resetStream (LZ4_stream_t* streamPtr);
      An LZ4_stream_t structure can be allocated once and re-used multiple times.
      Use this function to start compressing a new stream.

    int LZ4_loadDict (LZ4_stream_t* streamPtr, const char* dictionary, int dictSize);
      Use this function to load a static dictionary into LZ4_stream_t.
      Any previous data will be forgotten, only 'dictionary' will remain in memory.
      Loading a size of 0 is allowed, and is the same as reset.
     @return : dictionary size, in bytes (necessarily <= 64 KB)

    int LZ4_compress_fast_continue (LZ4_stream_t* streamPtr, const char* src, char* dst, int srcSize, int dstCapacity, int acceleration);
      Compress content into 'src' using data from previously compressed blocks, improving compression ratio.
      'dst' buffer must be already allocated.
      If dstCapacity >= LZ4_compressBound(srcSize), compression is guaranteed to succeed, and runs faster.

      Important : Up to 64KB of previously compressed data is assumed to remain present and unmodified in memory !
      Special 1 : If input buffer is a double-buffer, it can have any size, including < 64 KB.
      Special 2 : If input buffer is a ring-buffer, it can have any size, including < 64 KB.

     @return : size of compressed block
               or 0 if there is an error (typically, compressed data cannot fit into 'dst')
      After an error, the stream status is invalid, it can only be reset or freed.

    int LZ4_saveDict (LZ4_stream_t* streamPtr, char* safeBuffer, int dictSize);
      If previously compressed data block is not guaranteed to remain available at its current memory location,
      save it into a safer place (char* safeBuffer).
      Note : it's not necessary to call LZ4_loadDict() after LZ4_saveDict(), dictionary is immediately usable.
      @return : saved dictionary size in bytes (necessarily <= dictSize), or 0 if error.

## Streaming Decompression Functions

      Bufferless synchronous API

    LZ4_streamDecode_t* LZ4_createStreamDecode(void);
    int                 LZ4_freeStreamDecode (LZ4_streamDecode_t* LZ4_stream);
      creation / destruction of streaming decompression tracking structure.
      A tracking structure can be re-used multiple times sequentially.

    int LZ4_setStreamDecode (LZ4_streamDecode_t* LZ4_streamDecode, const char* dictionary, int dictSize);
      An LZ4_streamDecode_t structure can be allocated once and re-used multiple times.
      Use this function to start decompression of a new stream of blocks.
      A dictionary can optionnally be set. Use NULL or size 0 for a simple reset order.
     @return : 1 if OK, 0 if error

    int LZ4_decompress_safe_continue (LZ4_streamDecode_t* LZ4_streamDecode, const char* src, char* dst, int srcSize, int dstCapacity);
    int LZ4_decompress_fast_continue (LZ4_streamDecode_t* LZ4_streamDecode, const char* src, char* dst, int originalSize);
      These decoding functions allow decompression of consecutive blocks in "streaming" mode.
      A block is an unsplittable entity, it must be presented entirely to a decompression function.
      Decompression functions only accept one block at a time.
      Previously decoded blocks *must* remain available at the memory position where they were decoded (up to 64 KB).

      Special : if application sets a ring buffer for decompression, it must respect one of the following conditions :
      - Exactly same size as encoding buffer, with same update rule (block boundaries at same positions)
        In which case, the decoding & encoding ring buffer can have any size, including very small ones ( < 64 KB).
      - Larger than encoding buffer, by a minimum of maxBlockSize more bytes.
        maxBlockSize is implementation dependent. It's the maximum size of any single block.
        In which case, encoding and decoding buffers do not need to be synchronized,
        and encoding ring buffer can have any size, including small ones ( < 64 KB).
      - _At least_ 64 KB + 8 bytes + maxBlockSize.
        In which case, encoding and decoding buffers do not need to be synchronized,
        and encoding ring buffer can have any size, including larger than decoding buffer.
      Whenever these conditions are not possible, save the last 64KB of decoded data into a safe buffer,
      and indicate where it is saved using LZ4_setStreamDecode() before decompressing next block.

    int LZ4_decompress_safe_usingDict (const char* src, char* dst, int srcSize, int dstCapcity, const char* dictStart, int dictSize);
    int LZ4_decompress_fast_usingDict (const char* src, char* dst, int originalSize, const char* dictStart, int dictSize);
      These decoding functions work the same as
      a combination of LZ4_setStreamDecode() followed by LZ4_decompress_*_continue()
      They are stand-alone, and don't need an LZ4_streamDecode_t structure.

## Private definitions

     Do not use these definitions.
     They are exposed to allow static allocation of `LZ4_stream_t` and `LZ4_streamDecode_t`.
     Using these definitions will expose code to API and/or ABI break in future versions of the library.

    typedef struct {
        uint32_t hashTable[LZ4_HASH_SIZE_U32];
        uint32_t currentOffset;
        uint32_t initCheck;
        const uint8_t* dictionary;
        uint8_t* bufferStart;   /* obsolete, used for slideInputBuffer */
        uint32_t dictSize;
    } LZ4_stream_t_internal;

    typedef struct {
        const uint8_t* externalDict;
        size_t extDictSize;
        const uint8_t* prefixEnd;
        size_t prefixSize;
    } LZ4_streamDecode_t_internal;

    typedef struct {
        unsigned int hashTable[LZ4_HASH_SIZE_U32];
        unsigned int currentOffset;
        unsigned int initCheck;
        const unsigned char* dictionary;
        unsigned char* bufferStart;   /* obsolete, used for slideInputBuffer */
        unsigned int dictSize;
    } LZ4_stream_t_internal;

    typedef struct {
        const unsigned char* externalDict;
        size_t extDictSize;
        const unsigned char* prefixEnd;
        size_t prefixSize;
    } LZ4_streamDecode_t_internal;

    #define LZ4_STREAMSIZE_U64 ((1 << (LZ4_MEMORY_USAGE-3)) + 4)
    #define LZ4_STREAMSIZE     (LZ4_STREAMSIZE_U64 * sizeof(unsigned long long))
    union LZ4_stream_u {
        unsigned long long table[LZ4_STREAMSIZE_U64];
        LZ4_stream_t_internal internal_donotuse;
    } ;  /* previously typedef'd to LZ4_stream_t */
     information structure to track an LZ4 stream.
     init this structure before first use.
     note : only use in association with static linking !
            this definition is not API/ABI safe,
            it may change in a future version !

    #define LZ4_STREAMDECODESIZE_U64  4
    #define LZ4_STREAMDECODESIZE     (LZ4_STREAMDECODESIZE_U64 * sizeof(unsigned long long))
    union LZ4_streamDecode_u {
        unsigned long long table[LZ4_STREAMDECODESIZE_U64];
        LZ4_streamDecode_t_internal internal_donotuse;
    } ;   /* previously typedef'd to LZ4_streamDecode_t */
     information structure to track an LZ4 stream during decompression.
     init this structure  using LZ4_setStreamDecode (or memset()) before first use
     note : only use in association with static linking !
            this definition is not API/ABI safe,
            and may change in a future version !

## Obsolete Functions

    #ifdef LZ4_DISABLE_DEPRECATE_WARNINGS
    #  define LZ4_DEPRECATED(message)   /* disable deprecation warnings */
    #else
    #  define LZ4_GCC_VERSION (__GNUC__ * 100 + __GNUC_MINOR__)
    #  if defined(__clang__) /* clang doesn't handle mixed C++11 and CNU attributes */
    #    define LZ4_DEPRECATED(message) __attribute__((deprecated(message)))
    #  elif defined (__cplusplus) && (__cplusplus >= 201402) /* C++14 or greater */
    #    define LZ4_DEPRECATED(message) [[deprecated(message)]]
    #  elif (LZ4_GCC_VERSION >= 405)
    #    define LZ4_DEPRECATED(message) __attribute__((deprecated(message)))
    #  elif (LZ4_GCC_VERSION >= 301)
    #    define LZ4_DEPRECATED(message) __attribute__((deprecated))
    #  elif defined(_MSC_VER)
    #    define LZ4_DEPRECATED(message) __declspec(deprecated(message))
    #  else
    #    pragma message("WARNING: You need to implement LZ4_DEPRECATED for this compiler")
    #    define LZ4_DEPRECATED(message)
    #  endif
    #endif /* LZ4_DISABLE_DEPRECATE_WARNINGS */
       Should deprecation warnings be a problem,
       it is generally possible to disable them,
       typically with -Wno-deprecated-declarations for gcc
       or _CRT_SECURE_NO_WARNINGS in Visual.
       Otherwise, it's also possible to define LZ4_DISABLE_DEPRECATE_WARNINGS