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Diffstat (limited to 'doc/lz4_manual.html')
| -rw-r--r-- | doc/lz4_manual.html | 59 |
1 files changed, 57 insertions, 2 deletions
diff --git a/doc/lz4_manual.html b/doc/lz4_manual.html index 3a9e0db..1480089 100644 --- a/doc/lz4_manual.html +++ b/doc/lz4_manual.html @@ -40,9 +40,9 @@ Blocks are different from Frames (doc/lz4_Frame_format.md). Frames bundle both blocks and metadata in a specified manner. - This are required for compressed data to be self-contained and portable. + Embedding metadata is required for compressed data to be self-contained and portable. Frame format is delivered through a companion API, declared in lz4frame.h. - Note that the `lz4` CLI can only manage frames. + The `lz4` CLI can only manage frames. <BR></pre> <a name="Chapter2"></a><h2>Version</h2><pre></pre> @@ -357,6 +357,61 @@ int LZ4_freeStreamDecode (LZ4_streamDecode_t* LZ4_stream); </p></pre><BR> +<pre><b></b><p> + It's possible to have input and output sharing the same buffer, + for highly contrained memory environments. + In both cases, it requires input to lay at the end of the buffer, + and decompression to start at beginning of the buffer. + Buffer size must feature some margin, hence be larger than final size. + + |<------------------------buffer--------------------------------->| + |<-----------compressed data--------->| + |<-----------decompressed size------------------>| + |<----margin---->| + + This technique is more useful for decompression, + since decompressed size is typically larger, + and margin is short. + + In-place decompression will work inside any buffer + which size is >= LZ4_DECOMPRESS_INPLACE_BUFFER_SIZE(decompressedSize). + This presumes that decompressedSize > compressedSize. + Otherwise, it means compression actually expanded data, + and it would be more efficient to store such data with a flag indicating it's not compressed. + This can happen when data is not compressible (already compressed, or encrypted). + + For in-place compression, margin is larger, as it must be able to cope with both + history preservation, requiring input data to remain unmodified up to LZ4_DISTANCE_MAX, + and data expansion, which can happen when input is not compressible. + As a consequence, buffer size requirements are much higher, + and memory savings offered by in-place compression are more limited. + + There are ways to limit this cost for compression : + - Reduce history size, by modifying LZ4_DISTANCE_MAX. + Note that it is a compile-time constant, so all compressions will apply this limit. + Lower values will reduce compression ratio, except when input_size < LZ4_DISTANCE_MAX, + so it's a reasonable trick when inputs are known to be small. + - Require the compressor to deliver a "maximum compressed size". + This is the `dstCapacity` parameter in `LZ4_compress*()`. + When this size is < LZ4_COMPRESSBOUND(inputSize), then compression can fail, + in which case, the return code will be 0 (zero). + The caller must be ready for these cases to happen, + and typically design a backup scheme to send data uncompressed. + The combination of both techniques can significantly reduce + the amount of margin required for in-place compression. + + In-place compression can work in any buffer + which size is >= (maxCompressedSize) + with maxCompressedSize == LZ4_COMPRESSBOUND(srcSize) for guaranteed compression success. + LZ4_COMPRESS_INPLACE_BUFFER_SIZE() depends on both maxCompressedSize and LZ4_DISTANCE_MAX, + so it's possible to reduce memory requirements by playing with them. + +</p></pre><BR> + +<pre><b>#define LZ4_DECOMPRESS_INPLACE_BUFFER_SIZE(decompressedSize) ((decompressedSize) + LZ4_DECOMPRESS_INPLACE_MARGIN(decompressedSize)) </b>/**< note: presumes that compressedSize < decompressedSize. note2: margin is overestimated a bit, since it could use compressedSize instead */<b> +</b></pre><BR> +<pre><b>#define LZ4_COMPRESS_INPLACE_BUFFER_SIZE(maxCompressedSize) ((maxCompressedSize) + LZ4_COMPRESS_INPLACE_MARGIN) </b>/**< maxCompressedSize is generally LZ4_COMPRESSBOUND(inputSize), but can be set to any lower value, with the risk that compression can fail (return code 0(zero)) */<b> +</b></pre><BR> <a name="Chapter9"></a><h2>PRIVATE DEFINITIONS</h2><pre> Do not use these definitions directly. They are only exposed to allow static allocation of `LZ4_stream_t` and `LZ4_streamDecode_t`. |