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ZIM file format

3,109 bytes added, 18:00, 6 March 2018
Update zim format about extended cluster.
Length in bytes, all types are little-endian.
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! Field Name !! Type !!Offset!!Length!! Description
|-
| magicNumber || integer || 0 || 4 || Magic number to recognise the file format, must be 72173914
|-
| majorVersion|integer|4|2|Major version of the ZIM file format (5 or 6)|-| minorVersion || integer || 4 6 || 4 2 || ZIM=5, bytes 1-2: major, bytes 3-4: minor Minor version of the ZIM file format
|-
| uuid || integer || 8 || 16 || unique id of this zim file
| checksumPos || integer || 72 || 8 || pointer to the md5checksum of this file without the checksum itself. This points always 16 bytes before the end of the file.
|}
 
Major version is updated when an incompatible change is integrated in the format (a lib made for a version N will probably not be able to read a version N+1)
 
Minor version is updated when an compatible change is integrated (a lib made for a minor version n will be able to read a version n+1)
 
There are currently 2 major versions :
* The version 5
* The version 6 (the same that version 5 + potential extended cluster)
== MIME Type List (mimeListPos) ==
The MIME types in this list are zero terminated strings. An empty string marks the end of the MIME type list.
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! Field Name !! Type !!Offset!!Length!! Description
|-
Since directory entries have variable sizes this is needed for random access.
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! Field Name !! Type !!Offset!!Length!! Description
|-
To get the offset of an article from the title pointer list, you have to look it up in the URL pointer list.
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! Field Name !! Type !!Offset!!Length!! Description
|-
=== Article Entry ===
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! Field Name !! Type !!Offset!!Length!! Description
|-
=== Redirect Entry ===
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! Field Name !! Type !!Offset!!Length!! Description
|-
=== Linktarget or deleted Entry ===
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! Field Name !! Type !!Offset!!Length!! Description
|-
The cluster pointer list is a list of 8 byte offsets which point to all data clusters in a ZIM file.
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! Field Name !! Type !!Offset!!Length!! Description
|-
The clusters contain the actual data of the directory entries. Clusters can be compressed or uncompressed. The purpose of the clusters are that data of more than one directory entry can be compressed inside one cluster, making the compression much more efficient. Typically clusters have a size of about 1 MB.
The first byte of the cluster identifies some information about the cluster. The first fourth low bits identifies if it the cluster is compressed (4) or not (0). The default is uncompressed indicated by a value of 0 or 1 (obsoleted, inherited by Zeno) while compressed clusters are indicated by a value of 4 which indicates [[LZMA2 compression]] (or more precisely XZ, since there is a XZ header). There have been other compression algorithms used before (2: zlib, 3: bzip2) which have been removed. The zimlib uses [http://tukaani.org/xz/ xz-utils] as a C++ implementation of lzma2, for Java see [http://tukaani.org/xz/java.html XZ-Java]. The firth bit identifies if the cluster is extended or not :* By default (5th bit == 0) the cluster is not extended. It means that the offsets are stored in a 4 bytes length integer. Thus contents stored in the cluster cannot exceed 4Go.* If the cluster is extended (5th bit == 1), the offsets are stored in 8 bytes length integer. Thus contents stored in the cluster can exceed 4Go.A cluster can be extended only if the zim major version is 6. Else (major version == 5) cluster will always be not extended.
To find the data of a specific directory entry within a cluster the uncompressed cluster has a list of pointers to blobs within the uncompressed cluster after the first byte.
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! Field Name !! Type !!Offset!!Length!! Description
|-
| compression type cluster information || integer || 0 || 1 || Fourth low bits : 0: default (no compression), 1: none (inherited from Zeno), 4: LZMA2 compressed Firth bits : 0: normal (OFFSET_SIZE=4) 1: extended (OFFSET_SIZE=8)
|-
|colspan="5" | The following data bytes have to be uncompressed!
|-
| <1st Blob> || integer || 1 || 4 OFFSET_SIZE || offset to the <1st Blob>
|-
| <2nd Blob> || integer || 5 1+OFFSET_SIZE || 4 OFFSET_SIZE || offset to the <2nd Blob>
|-
| <nth Blob> || integer ||(n-1)*4OFFSET_SIZE+1|| 4 OFFSET_SIZE || offset to the <nth Blob>
|-
| ... || integer || ... || 4 OFFSET_SIZE || ...
|-
| <last blob / end> || integer || n/a || 4 OFFSET_SIZE || offset to the end of the cluster
|-
| <1st Blob> || data || n/a || n/a || data of the <1st Blob>
|}
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 4OFFSET_SIZE. The size of one blob is calculated by the difference of two consecutive offsets.
== Namespaces ==
They can be distinguished by prepending the article namespace before the article name in the URL path, eg. ''http://localhost/A/Articlename''.
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! Namespace !! Description
|-
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