能够压缩我们的文件以便更容易地共享和/或传输它们可以使我们的电子生活更容易，但有时我们在压缩文件后可能会看到奇怪或意外的大小调整结果。为什么？今天的超级用户问答帖子回答了一位困惑读者的问题。

今天的问答环节是由SuperUser提供的，SuperUser是Stack Exchange的一个分支，是一个由社区驱动的问答网站分组。

图片由Jean-Etienne Minh Duy Poirrier（Flickr）提供。

问题

超级用户读者sixtyfootersdude想知道为什么zip能够比具有相同内容类型的多个文件更好地压缩单个文件：

Suppose that I have 10,000 XML files and want to send them to a friend. Before sending them, I would like to compress them.

Method 1: Do Not Compress Them

Results:

Method 2: Zip Every File Separately and Send Him 10,000 Zipped XML Files

Command:

Results:

Method 3: Create a Single Zip File Containing All 10,000 XML Files

Command:

Results:

Method 4: Concatenate the Files Into a Single File and Zip It

Command:

Results:

Questi***

• Why do I get such dramatically better results when I am just zipping a single file?
• I was expecting to get drastically better results using method 3 rather than method 2, but I do not. Why is this?
• Is this behaviour specific to zip? If I tried using Gzip, would I get different results?

Additional Info

Meta Data

One of the answers given suggests that the difference is the system meta data that is stored in the zip file. I do not believe that this can be the case. To test it, I did the following:

The resulting zip file is 1.4 MB. This means that there is still approximately ten MB of unexplained space.

为什么zip压缩单个文件比压缩具有相同内容类型的多个文件更好？

答案

超级用户贡献者Alan Shutko和Aganju为我们提供了答案。首先，艾伦·舒特科：

Zip compression is based on repetitive patterns in the data to be compressed, and the compression gets better the longer the file is, as more and longer patterns can be found and used.

Simplified, if you compress one file, the dictionary that maps (short) codes to (longer) patterns is necessarily contained in each resulting zip file; if you zip one long file, the dictionary is ‘reused’ and grows even more effective across all content.

If your files are even a bit similar (as text always is), reuse of the ‘dictionary’ becomes very efficient and the result is a much **aller total zip file.

接下来是阿甘朱的回答：

In zip, each file is compressed separately. The opposite is solid compression, that is, files are compressed together. 7-zip and Rar use solid compression by default. Gzip and Bzip2 cannot compress multiple files, so Tar is used first, having the same effect as solid compression.

As xml files have similar structure (and probably similar content), if the files are compressed together then the compression will be higher.

For example, if a file contains the string “<content><element name=” and the compressor has already found that string in another file, it will replace it with a **all pointer to the previous match. If the compressor does not use solid compression, the first occurrence of the string in the file will be recorded as a literal, which is larger.

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