ZIM file format
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 is 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=5, bytes 1-2: major, bytes 3-4: minor version of the ZIM file format |
| uuid | integer | 8 | 16 | unique id of this zim file |
| articleCount | integer | 24 | 4 | total number of articles |
| clusterCount | integer | 28 | 4 | total number of clusters |
| urlPtrPos | integer | 32 | 8 | position of the directory pointerlist orderes by URL |
| titlePtrPos | integer | 40 | 8 | position of the directory pointerlist ordered by Title |
| clusterPtrPos | integer | 48 | 8 | position of the cluster pointer list |
| mimeListPos | integer | 56 | 8 | position of the MIME type list (also header size) |
| mainPage | integer | 64 | 4 | main page or 0xffffffff if no main page |
| layoutPage | integer | 68 | 4 | layout page or 0xffffffffff if no layout page |
MIME Type List (mimeListPos)
The MIME type list always follows directly after the header, so the mimeListPos also defines the end and size of the ZIM file header.
The MIME types in this list are zero terminated strings. An empty string marks the end of the MIME type list.
| Field Name | Type | Offset | Length | Description |
|---|---|---|---|---|
| <1st MIME Type> | string | 0 | n/a | declaration of the <1st MIME Type> - zero terminated |
| <2nd MIME Type> | string | n/a | n/a | declaration of the <2nd MIME Type> - zero terminated |
| ... | string | ... | ... | ... |
| <last entry / end> | string | n/a | 0 | empty string - end of MIME type list - zero terminated |
URL Pointer List (urlPtrPos)
The URL pointer list is a list of 8 byte offsets to the directory entries.
The directory entries are always ordered by URL. Ordering is simply done by comparing the URL strings.
Since directory entries have variable sizes this is needed for random access.
| Field Name | Type | Offset | Length | Description |
|---|---|---|---|---|
| <1st URL> | integer | 0 | 8 | pointer to the directory entry of <1st URL> |
| <2nd URL> | integer | 8 | 8 | pointer to the directory entry of <2nd URL> |
| <nth URL> | integer | (n-1)*8 | 8 | pointer to the directory entry of <nth URL> |
| ... | integer | ... | 8 | ... |
Zimlib caches directory entries and references the cached entries via the URL pointers.
Title Pointer List (titlePtrPos)
The title pointer list is a list of article indeces ordered by title. The title pointer list actually points to entries in the URL pointer list. Note that the title pointers are only 4 bytes. They are not offsets in the file but article numbers. To get the offset of an article from the title pointer list, you have to look it up in the URL pointer list.
| Field Name | Type | Offset | Length | Description |
|---|---|---|---|---|
| <1st Title> | integer | 0 | 4 | Pointer to the URL pointer of <1st Title> |
| <2nd Title> | integer | 4 | 4 | Pointer to the URL pointer of <2nd Title> |
| <nth Title> | integer | (n-1)*4 | 4 | Pointer to the URL pointer of <nth Title> |
| ... | integer | ... | 4 | ... |
The indirection from titles via URLs to directory entries has two reasons:
- the pointer list is only half in size as 4 bytes are enough for each entry
- accessing directory entries by title also makes use of cached directory entries which are referenced by the URL pointers, as implemented in zimlib.
Directory Entries
Directory entries hold the meta information about all articles, images and other objects in a ZIM file.
There are two types of directory entries: article entries and redirect entries. If the first two bytes are 0xffff the directory entry is a redirect.
Article Entry
| Field Name | Type | Offset | Length | Description |
|---|---|---|---|---|
| mimetype | integer | 0 | 2 | MIME type number as defined in the MIME type list |
| parameter len | byte | 2 | 1 | (not used) length of extra paramters |
| namespace | char | 3 | 1 | defines to which namespace this directory entry belongs |
| revision | integer | 4 | 4 | (optional) identifies a revision of the contents of this directory entry, needed to identify updates or revisions in the original history |
| cluster number | integer | 8 | 4 | cluster number in which the data of this directory entry is stored |
| blob number | integer | 12 | 4 | blob number inside the compressed cluster where the contents are stored |
| url | string | 16 | zero terminated | string with the URL as refered in the URL pointer list |
| title | string | n/a | zero terminated | string with an title as refered in the Title pointer list or empty; in case it is empty, the URL is used as title |
| parameter | data | see parameter len | (not used) extra parameters |
Redirect Entry
| Field Name | Type | Offset | Length | Description |
|---|---|---|---|---|
| mimetype | integer | 0 | 2 | 0xffff for redirect |
| parameter len | byte | 2 | 1 | (not used) length of extra paramters |
| namespace | char | 3 | 1 | defines to which namespace this directory entry belongs |
| revision | integer | 4 | 4 | (optional) identifies a revision of the contents of this directory entry, needed to identify updates or revisions in the original history |
| redirect index | integer | 8 | 4 | pointer to the directory entry of the redirect target |
| url | string | 16 | zero terminated | string with the URL as refered in the URL pointer list |
| title | string | n/a | zero terminated | string with an title as refered in the Title pointer list or empty; in case it is empty, the URL is used as title |
| parameter | data | see parameter len | (not used) extra parameters |
Cluster Pointer List (clusterPtrPos)
The cluster pointer list is a list of 8 byte offsets which point to all data clusters in a ZIM file.
| Field Name | Type | Offset | Length | Description |
|---|---|---|---|---|
| <1st Cluster> | integer | 0 | 8 | pointer to the <1st Cluster> |
| <1st Cluster> | integer | 8 | 8 | pointer to the <2nd Cluster> |
| <nth Cluster> | integer | (n-1)*8 | 8 | pointer to the <nth Cluster> |
| ... | integer | ... | 8 | ... |
Clusters
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 if it 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. There have been other compression algorithms used before (2: zlib, 3: bzip2) which have been removed. The zimlib uses xz-utils as a C++ implementation of lzma2, for Java see LZMA2-java.
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.
| Field Name | Type | Offset | Length | Description |
|---|---|---|---|---|
| compression type | integer | 0 | 1 | 0: default (no compression), 1: none (inherited from Zeno), 4: LZMA2 compressed |
| The following data bytes have to be uncompressed! | ||||
| <1st Blob> | integer | 1 | 4 | pointer to the <1st Blob> |
| <2nd Blob> | integer | 5 | 4 | pointer to the <2nd Blob> |
| <nth Blob> | integer | (n-1)*4+1 | 4 | pointer to the <nth Blob> |
| ... | integer | ... | 4 | ... |
| <last blob / end> | integer | n/a | 4 | pointer to the end of the cluster |
| <1st Blob> | data | n/a | n/a | data of the <1st Blob> |
| <2nd Blob> | data | n/a | n/a | data of the <2nd Blob> |
| ... | data | ... | n/a | ... |
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.
Namespaces
Namespaces seperate different types of directory entries - which might have the same title - 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 |
|---|---|
| - | layout, eg. the LayoutPage, CSS, JavaScript and images not related to the articles |
| A | articles - see Article Format |
| B | article meta data - see Article Format |
| I | images, files - see Image Handling |
| J | images, text - see Image Handling |
| M | ZIM metadata - see Metadata |
| U | categories, text - see Category Handling |
| V | categories, article list - see Category Handling |
| W | categories per article, category list - see Category Handling |
| X | fulltext index - see ZIM Index Format |
URLs
ZIM contents are addressed using URLs: /<namespace>/<article_url>.
The references in Article Text (<a href=""></a>, <img src=""> etc.) are URL-encoded (RFC 1738).
The URLs in the UrlPointerlist are not encoded. Some readers process the requests that already do the decoding internally whereas most readers will handle the URLs directly. In this case you have to do the decoding before you pass the parameter to zimlib, but zimlib already provides a method to do so.