ZIM File Format/4

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

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

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:

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.