289 lines
15 KiB
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289 lines
15 KiB
HTML
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<meta http-equiv="Content-Type" content="text/html; charset=ISO-8859-1">
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<title>lz4 1.7.2 Manual</title>
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<h1>lz4 1.7.2 Manual</h1>
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<hr>
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<a name="Contents"></a><h2>Contents</h2>
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<ol>
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<li><a href="#Chapter1">Introduction</a></li>
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<li><a href="#Chapter2">Tuning parameter</a></li>
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<li><a href="#Chapter3">Private definitions</a></li>
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<li><a href="#Chapter4">Simple Functions</a></li>
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<li><a href="#Chapter5">Advanced Functions</a></li>
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<li><a href="#Chapter6">Streaming Compression Functions</a></li>
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<li><a href="#Chapter7">Streaming Decompression Functions</a></li>
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</ol>
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<hr>
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<a name="Chapter1"></a><h2>Introduction</h2><pre>
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LZ4 is lossless compression algorithm, providing compression speed at 400 MB/s per core,
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scalable with multi-cores CPU. It features an extremely fast decoder, with speed in
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multiple GB/s per core, typically reaching RAM speed limits on multi-core systems.
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The LZ4 compression library provides in-memory compression and decompression functions.
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Compression can be done in:
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- a single step (described as Simple Functions)
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- a single step, reusing a context (described in Advanced Functions)
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- unbounded multiple steps (described as Streaming compression)
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lz4.h provides block compression functions. It gives full buffer control to user.
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Block compression functions are not-enough to send information,
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since it's still necessary to provide metadata (such as compressed size),
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and each application can do it in whichever way it wants.
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For interoperability, there is LZ4 frame specification (doc/lz4_Frame_format.md).
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A library is provided to take care of it, see lz4frame.h.
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<BR></pre>
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<h3>Version</h3><pre><b>int LZ4_versionNumber (void);
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const char* LZ4_versionString (void);
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</b></pre><BR>
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<a name="Chapter2"></a><h2>Tuning parameter</h2><pre></pre>
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<pre><b>#define LZ4_MEMORY_USAGE 14
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</b><p> Memory usage formula : N->2^N Bytes (examples : 10 -> 1KB; 12 -> 4KB ; 16 -> 64KB; 20 -> 1MB; etc.)
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Increasing memory usage improves compression ratio
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Reduced memory usage can improve speed, due to cache effect
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Default value is 14, for 16KB, which nicely fits into Intel x86 L1 cache
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</p></pre><BR>
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<a name="Chapter3"></a><h2>Private definitions</h2><pre>
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Do not use these definitions.
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They are exposed to allow static allocation of `LZ4_stream_t` and `LZ4_streamDecode_t`.
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If you use these definitions in your code, it will break when you upgrade LZ4 to a new version.
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<BR></pre>
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<pre><b>typedef struct {
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uint32_t hashTable[LZ4_HASH_SIZE_U32];
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uint32_t currentOffset;
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uint32_t initCheck;
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const uint8_t* dictionary;
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uint8_t* bufferStart; </b>/* obsolete, used for slideInputBuffer */<b>
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uint32_t dictSize;
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} LZ4_stream_t_internal;
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</b></pre><BR>
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<pre><b>typedef struct {
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const uint8_t* externalDict;
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size_t extDictSize;
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const uint8_t* prefixEnd;
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size_t prefixSize;
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} LZ4_streamDecode_t_internal;
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</b></pre><BR>
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<pre><b>typedef struct {
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unsigned int hashTable[LZ4_HASH_SIZE_U32];
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unsigned int currentOffset;
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unsigned int initCheck;
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const unsigned char* dictionary;
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unsigned char* bufferStart; </b>/* obsolete, used for slideInputBuffer */<b>
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unsigned int dictSize;
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} LZ4_stream_t_internal;
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</b></pre><BR>
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<pre><b>typedef struct {
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const unsigned char* externalDict;
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size_t extDictSize;
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const unsigned char* prefixEnd;
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size_t prefixSize;
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} LZ4_streamDecode_t_internal;
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</b></pre><BR>
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<a name="Chapter4"></a><h2>Simple Functions</h2><pre></pre>
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<pre><b>int LZ4_compress_default(const char* source, char* dest, int sourceSize, int maxDestSize);
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</b><p> Compresses 'sourceSize' bytes from buffer 'source'
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into already allocated 'dest' buffer of size 'maxDestSize'.
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Compression is guaranteed to succeed if 'maxDestSize' >= LZ4_compressBound(sourceSize).
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It also runs faster, so it's a recommended setting.
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If the function cannot compress 'source' into a more limited 'dest' budget,
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compression stops *immediately*, and the function result is zero.
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As a consequence, 'dest' content is not valid.
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This function never writes outside 'dest' buffer, nor read outside 'source' buffer.
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sourceSize : Max supported value is LZ4_MAX_INPUT_VALUE
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maxDestSize : full or partial size of buffer 'dest' (which must be already allocated)
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return : the number of bytes written into buffer 'dest' (necessarily <= maxOutputSize)
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or 0 if compression fails
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</p></pre><BR>
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<pre><b>int LZ4_decompress_safe (const char* source, char* dest, int compressedSize, int maxDecompressedSize);
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</b><p> compressedSize : is the precise full size of the compressed block.
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maxDecompressedSize : is the size of destination buffer, which must be already allocated.
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return : the number of bytes decompressed into destination buffer (necessarily <= maxDecompressedSize)
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If destination buffer is not large enough, decoding will stop and output an error code (<0).
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If the source stream is detected malformed, the function will stop decoding and return a negative result.
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This function is protected against buffer overflow exploits, including malicious data packets.
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It never writes outside output buffer, nor reads outside input buffer.
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</p></pre><BR>
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<a name="Chapter5"></a><h2>Advanced Functions</h2><pre></pre>
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<pre><b>int LZ4_compressBound(int inputSize);
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</b><p> Provides the maximum size that LZ4 compression may output in a "worst case" scenario (input data not compressible)
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This function is primarily useful for memory allocation purposes (destination buffer size).
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Macro LZ4_COMPRESSBOUND() is also provided for compilation-time evaluation (stack memory allocation for example).
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Note that LZ4_compress_default() compress faster when dest buffer size is >= LZ4_compressBound(srcSize)
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inputSize : max supported value is LZ4_MAX_INPUT_SIZE
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return : maximum output size in a "worst case" scenario
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or 0, if input size is too large ( > LZ4_MAX_INPUT_SIZE)
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</p></pre><BR>
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<pre><b>int LZ4_compress_fast (const char* source, char* dest, int sourceSize, int maxDestSize, int acceleration);
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</b><p> Same as LZ4_compress_default(), but allows to select an "acceleration" factor.
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The larger the acceleration value, the faster the algorithm, but also the lesser the compression.
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It's a trade-off. It can be fine tuned, with each successive value providing roughly +~3% to speed.
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An acceleration value of "1" is the same as regular LZ4_compress_default()
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Values <= 0 will be replaced by ACCELERATION_DEFAULT (see lz4.c), which is 1.
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</p></pre><BR>
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<pre><b>int LZ4_sizeofState(void);
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int LZ4_compress_fast_extState (void* state, const char* source, char* dest, int inputSize, int maxDestSize, int acceleration);
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</b><p> Same compression function, just using an externally allocated memory space to store compression state.
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Use LZ4_sizeofState() to know how much memory must be allocated,
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and allocate it on 8-bytes boundaries (using malloc() typically).
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Then, provide it as 'void* state' to compression function.
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</p></pre><BR>
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<pre><b>int LZ4_compress_destSize (const char* source, char* dest, int* sourceSizePtr, int targetDestSize);
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</b><p> Reverse the logic, by compressing as much data as possible from 'source' buffer
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into already allocated buffer 'dest' of size 'targetDestSize'.
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This function either compresses the entire 'source' content into 'dest' if it's large enough,
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or fill 'dest' buffer completely with as much data as possible from 'source'.
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*sourceSizePtr : will be modified to indicate how many bytes where read from 'source' to fill 'dest'.
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New value is necessarily <= old value.
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return : Nb bytes written into 'dest' (necessarily <= targetDestSize)
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or 0 if compression fails
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</p></pre><BR>
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<pre><b>int LZ4_decompress_fast (const char* source, char* dest, int originalSize);
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</b><p> originalSize : is the original and therefore uncompressed size
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return : the number of bytes read from the source buffer (in other words, the compressed size)
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If the source stream is detected malformed, the function will stop decoding and return a negative result.
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Destination buffer must be already allocated. Its size must be a minimum of 'originalSize' bytes.
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note : This function fully respect memory boundaries for properly formed compressed data.
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It is a bit faster than LZ4_decompress_safe().
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However, it does not provide any protection against intentionally modified data stream (malicious input).
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Use this function in trusted environment only (data to decode comes from a trusted source).
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</p></pre><BR>
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<pre><b>int LZ4_decompress_safe_partial (const char* source, char* dest, int compressedSize, int targetOutputSize, int maxDecompressedSize);
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</b><p> This function decompress a compressed block of size 'compressedSize' at position 'source'
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into destination buffer 'dest' of size 'maxDecompressedSize'.
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The function tries to stop decompressing operation as soon as 'targetOutputSize' has been reached,
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reducing decompression time.
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return : the number of bytes decoded in the destination buffer (necessarily <= maxDecompressedSize)
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Note : this number can be < 'targetOutputSize' should the compressed block to decode be smaller.
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Always control how many bytes were decoded.
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If the source stream is detected malformed, the function will stop decoding and return a negative result.
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This function never writes outside of output buffer, and never reads outside of input buffer. It is therefore protected against malicious data packets
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</p></pre><BR>
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<a name="Chapter6"></a><h2>Streaming Compression Functions</h2><pre></pre>
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<pre><b>typedef struct {
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union {
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long long table[LZ4_STREAMSIZE_U64];
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LZ4_stream_t_internal internal_donotuse;
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};
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} LZ4_stream_t;
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</b><p> information structure to track an LZ4 stream.
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important : init this structure content before first use !
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note : only allocated directly the structure if you are statically linking LZ4
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If you are using liblz4 as a DLL, please use below construction methods instead.
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</p></pre><BR>
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<pre><b>void LZ4_resetStream (LZ4_stream_t* streamPtr);
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</b><p> Use this function to init an allocated `LZ4_stream_t` structure
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</p></pre><BR>
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<pre><b>LZ4_stream_t* LZ4_createStream(void);
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int LZ4_freeStream (LZ4_stream_t* streamPtr);
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</b><p> LZ4_createStream() will allocate and initialize an `LZ4_stream_t` structure.
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LZ4_freeStream() releases its memory.
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In the context of a DLL (liblz4), please use these methods rather than the static struct.
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They are more future proof, in case of a change of `LZ4_stream_t` size.
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</p></pre><BR>
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<pre><b>int LZ4_loadDict (LZ4_stream_t* streamPtr, const char* dictionary, int dictSize);
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</b><p> Use this function to load a static dictionary into LZ4_stream.
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Any previous data will be forgotten, only 'dictionary' will remain in memory.
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Loading a size of 0 is allowed.
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Return : dictionary size, in bytes (necessarily <= 64 KB)
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</p></pre><BR>
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<pre><b>int LZ4_compress_fast_continue (LZ4_stream_t* streamPtr, const char* src, char* dst, int srcSize, int maxDstSize, int acceleration);
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</b><p> Compress buffer content 'src', using data from previously compressed blocks as dictionary to improve compression ratio.
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Important : Previous data blocks are assumed to still be present and unmodified !
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'dst' buffer must be already allocated.
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If maxDstSize >= LZ4_compressBound(srcSize), compression is guaranteed to succeed, and runs faster.
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If not, and if compressed data cannot fit into 'dst' buffer size, compression stops, and function returns a zero.
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</p></pre><BR>
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<pre><b>int LZ4_saveDict (LZ4_stream_t* streamPtr, char* safeBuffer, int dictSize);
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</b><p> If previously compressed data block is not guaranteed to remain available at its memory location,
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save it into a safer place (char* safeBuffer).
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Note : you don't need to call LZ4_loadDict() afterwards,
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dictionary is immediately usable, you can therefore call LZ4_compress_fast_continue().
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Return : saved dictionary size in bytes (necessarily <= dictSize), or 0 if error.
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</p></pre><BR>
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<a name="Chapter7"></a><h2>Streaming Decompression Functions</h2><pre></pre>
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<pre><b>typedef struct {
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union {
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unsigned long long table[LZ4_STREAMDECODESIZE_U64];
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LZ4_streamDecode_t_internal internal_donotuse;
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};
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</b></pre><BR>
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<pre><b>LZ4_streamDecode_t* LZ4_createStreamDecode(void);
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int LZ4_freeStreamDecode (LZ4_streamDecode_t* LZ4_stream);
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</b><p> information structure to track an LZ4 stream.
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init this structure content using LZ4_setStreamDecode or memset() before first use !
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In the context of a DLL (liblz4) please prefer usage of construction methods below.
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They are more future proof, in case of a change of LZ4_streamDecode_t size in the future.
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LZ4_createStreamDecode will allocate and initialize an LZ4_streamDecode_t structure
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LZ4_freeStreamDecode releases its memory.
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</p></pre><BR>
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<pre><b>int LZ4_setStreamDecode (LZ4_streamDecode_t* LZ4_streamDecode, const char* dictionary, int dictSize);
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</b><p> Use this function to instruct where to find the dictionary.
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Setting a size of 0 is allowed (same effect as reset).
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@return : 1 if OK, 0 if error
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</p></pre><BR>
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<pre><b>int LZ4_decompress_safe_continue (LZ4_streamDecode_t* LZ4_streamDecode, const char* source, char* dest, int compressedSize, int maxDecompressedSize);
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int LZ4_decompress_fast_continue (LZ4_streamDecode_t* LZ4_streamDecode, const char* source, char* dest, int originalSize);
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</b><p> These decoding functions allow decompression of multiple blocks in "streaming" mode.
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Previously decoded blocks *must* remain available at the memory position where they were decoded (up to 64 KB)
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In the case of a ring buffers, decoding buffer must be either :
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- Exactly same size as encoding buffer, with same update rule (block boundaries at same positions)
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In which case, the decoding & encoding ring buffer can have any size, including very small ones ( < 64 KB).
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- Larger than encoding buffer, by a minimum of maxBlockSize more bytes.
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maxBlockSize is implementation dependent. It's the maximum size you intend to compress into a single block.
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In which case, encoding and decoding buffers do not need to be synchronized,
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and encoding ring buffer can have any size, including small ones ( < 64 KB).
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- _At least_ 64 KB + 8 bytes + maxBlockSize.
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In which case, encoding and decoding buffers do not need to be synchronized,
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and encoding ring buffer can have any size, including larger than decoding buffer.
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Whenever these conditions are not possible, save the last 64KB of decoded data into a safe buffer,
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and indicate where it is saved using LZ4_setStreamDecode()
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</p></pre><BR>
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<pre><b>int LZ4_decompress_safe_usingDict (const char* source, char* dest, int compressedSize, int maxDecompressedSize, const char* dictStart, int dictSize);
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int LZ4_decompress_fast_usingDict (const char* source, char* dest, int originalSize, const char* dictStart, int dictSize);
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</b><p>Advanced decoding functions :
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These decoding functions work the same as
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a combination of LZ4_setStreamDecode() followed by LZ4_decompress_x_continue()
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They are stand-alone. They don't need nor update an LZ4_streamDecode_t structure.
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</p></pre><BR>
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</html>
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</body>
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