能夠壓縮我們的檔案以便更容易地共享和/或傳輸它們可以使我們的電子生活更容易，但有時我們在壓縮檔案後可能會看到奇怪或意外的大小調整結果。為什麼？今天的超級使用者問答帖子回答了一位困惑讀者的問題。

今天的問答環節是由SuperUser提供的，SuperUser是Stack Exchange的一個分支，是一個由社群驅動的問答網站分組。

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

問題

超級使用者讀者sixtyfootersdude想知道為什麼zip能夠比具有相同內容型別的多個檔案更好地壓縮單個檔案：

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.

為什麼zip壓縮單個檔案比壓縮具有相同內容型別的多個檔案更好？

答案

超級使用者貢獻者Alan Shutko和Aganju為我們提供了答案。首先，艾倫·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.

接下來是阿甘朱的回答：

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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