LZ4库解压缩数据上限大小估计 [英] LZ4 library decompressed data upper bound size estimation

问题描述

我正在使用 LZ4 库，并在使用

解压缩数据时使用

I'm using LZ4 library and when decompressing data with

int LZ4_decompress_safe (const char* source, char* dest, int compressedSize, int maxDecompressedSize);

我想估计最大的解压缩数据大小.但是我找不到

I want to estimate maximum decompressed data size. But I can not find reverse function of

int LZ4_compressBound(int isize);

通过它我可以确定解压缩数据的上限，该上限将提供给解压缩功能的最后一个参数maxDecompressedSize.

with which I can determine the upper bound for decompressed data, which to provide to last parameter maxDecompressedSize, of decompressing function.

例如其他压缩库(例如 snappy )提供了此类功能.

Other compression libraries like snappy for example, provides such function.

bool GetUncompressedLength(Source* source, uint32* result);

如果我无法保存初始数据大小(在压缩之前)，并且我不想对必须分配的缓冲区大小过于悲观，该怎么办?

What can I do if I have not capability to save initial data size (before compression), and if I don't want to be over pessimistic for the size of the buffer which I must allocate?

推荐答案

LZ4的最大压缩率是255，因此，保证解压缩数据大小的过高估计是输入大小的255倍.

The maximum compression ratio of LZ4 is 255, so a guaranteed over-estimation of decompressed data size is 255 times input size.

这显然太多了，不能真正有用，因此没有"reverse LZ4_compressBound()"函数的原因.

That's obviously too much to be really useful, hence the reason why there is no "reverse LZ4_compressBound()" function available.

恐怕没有其他方法可以保存或知道未压缩的大小. LZ4原始"压缩格式没有定义保存此类信息的方法，因为最佳选择是特定于应用程序的.例如，某些应用程序预先知道任何块都不能大于16KB，因此在调用LZ4_decompress_safe()时，它们可以使用maxDecompressedSize = 16 KB.

I'm afraid there is no other way than to save, or know, the uncompressed size. The LZ4 "raw" compression format doesn't define a way to save such information, because optimal choice is application specific. For example, some application know in advance that no block can be > 16KB, so they can use maxDecompressedSize = 16 KB when calling LZ4_decompress_safe().

现在，如果您正在寻找一种负责这种责任的信封格式，则可以创建自己的自定义格式，也可以使用LZ4取景格式: https://github. com/Cyan4973/lz4/tree/frame )

Now, if you are looking for an envelope format that will take in charge such responsibility, you could either create your own custom one, or use the LZ4 Framing format : http://fastcompression.blogspot.fr/2013/04/lz4-streaming-format-final.html (also present as LZ4_Framing_Format.html within source package). Alas, the library able to generate and read this format is currently in beta stage (https://github.com/Cyan4973/lz4/tree/frame)

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