Difference between revisions of "ZIM file format"

Revision as of 10:49, 4 April 2009

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

Header

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 a index pointerlist, which is a list of 4-byte offsets, which points to the directory entries.

Index pointer list

The index pointer list is a list of 4 byte offsets to the directory entries. Since directory entries has variable size, this is needed for random access.

Due to the fact, that these pointers are only 4 byte, the directory entries have to be in the first 4GB of the zim file. The pointers to data in the directory entries are 8 bytes, so that the zim file may be much larger than the 4GB.

Directory entries

length in byte, all types are littlendian

Field Name	Type	Offset	Length	Description
filePos	integer	0	8	position of data chunk
fileLen	integer	8	4	length of data chunk
compression	integer	12	1	1=none; 2=zlib; 3=bzip2; 4=lzma
mime	integer	13	1	code of mimetype
redirectFlag	integer	14	1	1 if this is a redirect
namespace	char	15	1	namespace of article
articleOffset	integer	16	4	offset insize data chunk
articleSize	integer	20	4	uncompressed size of article
redirectIndex	integer	16	4	index of redirected article (field shared with articleOffset)
extraLen	integer	24	2	length of extra bytes (title and parameter)

Data

Mime types

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
Y	deprecated - see Zeno File Format
Z	deprecated - see Zeno File Format

@@ Line 18: / Line 18: @@
 | indexPtrPos            || integer || 28 || 8 || position of the directory pointerlist
 |-
-| mainPage               || integer || 32 || 4 || article index of main page or 0xffff if no main page
+| clusterCount    || integer || 36 || 4 || number of data clusters
 |-
-| layoutPage            || integer || 36 || 4 || article index of layout page or 0xffff if no layout page
+| 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
 |}