ZIM File Format/4

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Schema File Format.png

The ZIM file format is based on the Zeno File Format. It starts with a header, which is described here:

Header

A ZIM file starts with a header. This starts always at offset 0.

Length in byte, all types are littlendian

Field Name Type Offset Length Description
magicNumber integer 0 4 Magic number to recognise the file format, must be 72173914
version integer 4 4 ZIM=4, version of the file format
uuid integer 8 16 unique id of this zim file
count integer 24 4 total number of articles
indexPtrPos integer 28 8 position of the directory pointerlist
clusterCount integer 36 4 number of data clusters
clusterPtrPos integer 40 8 position of the cluster pointer list
mainPage integer 48 4 article index of main page or 0xffffffff if no main page
layoutPage integer 52 4 article index of layout page or 0xffffffffff if no layout page

Each article in the zim file has a directory entry. Since the directory entry has a variable size we have an index pointerlist which is a list of 4-byte offsets. The pointers points to the directory entries.

Index pointer list

The index pointer list is a list of 8 byte offsets to the directory entries. Since directory entries have variable sizes this is needed for random access.

The directory entries are always sorted by title. The title is encoded as QUnicode, which is a custom utf-variant, which supports fast ordering.

Cluster pointer list

The cluster pointer list is a list of 8 byte offsets which point to the data clusters.

Directory entries

length in byte, all types are littlendian

article entry

Field Name Type Offset Length Description
redirectFlag boolean 0 1 0 for article
mime integer 1 1 mime type code
empty 2 1 was compression flag, this is now in the cluster header
namespace char 3 1
cluster number integer 4 4
blob number integer 8 4
extraLen integer 12 2 length of extra bytes (title and parameter)
title + parameter separated by 0-byte QUnicode (title) + custom (parameter, not used in articles) 14 specified by extraLen actual title of article; when parameter is empty, the 0-byte is omitted

redirect entry

Field Name Type Offset Length Description
redirectFlag boolean 0 1 1 for redirect
mime integer 1 1 unused for redirects
empty 2 1 was compression flag, this is now in the cluster header
namespace char 3 1
redirect index integer 4 4
extraLen integer 8 2 length of extra bytes (title and parameter)
title + parameter separated by 0-byte QUnicode (title) + custom (parameter, not used in articles) 10 specified by extraLen actual title of article; when parameter is empty, the 0-byte is omitted

Clusters

The clusters contain the actual article data. This file section contain a list of clusters, which contain a list of blobs each. The blob is the data of one specific article. So this blob is adressed by the cluster number and the blob number in this cluster. The cluster number is used to look up the file offset in the cluster pointer list.

The cluster has a starting byte, which indicated, which compresion is used. After this byte, all other data is compressed. Possible values are:

  • 0 default (no compression)
  • 1 none also no compression (inherited from zeno)
  • 2 zip (zlib)
  • 3 bzip2 (currently used in writer)
  • 4 lzma (not implemented in reader or writer due to lack of compression library)

The data area has a list of 4 byte offsets to the blobs counting from the first offset. The offset addresses uncompressed data. The last pointer points to the end of the data area. So there is always one more offset than blobs. Since the first offset points to the start of the first data, the number of offsets can be determined by dividing this offset by 4. The size of one blob is calculated by the difference of two consecutive offsets.

Mime types

Currently these mime types are assigned:

number mime-type
0 text/html
1 text/plain
2 image/jpeg
3 image/png
4 image/tiff
5 text/css
6 image/gif
7 index page
8 application/x-javascript
9 image/x-icon
10 text/xml

Namespaces

Namespaces seperate different types of data stored in the ZIM File Format.

They can be distinguished by prepending the article namespace before the article name in the URL path, eg. http://localhost/A/Articlename.

Namespace Description
- template data
A articles
I images, files - see Image Handling
J images, text - see Image Handling
U categories, text - see Category Handling
V categories, article list - see Category Handling
W deprecated - see Zeno File Format
X fulltext index - see Zim Index Format
Y deprecated - see Zeno File Format
Z deprecated - see Zeno File Format