Difference between revisions of "ZIM file format"

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ZIM file format (view source)

Revision as of 13:45, 2 April 2024

54 bytes added ,  2 April
Rename Url to Path in spec.
(Enhance explanations around URLs encoding in ZIM / HTML document)
(Rename Url to Path in spec.)
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| clusterCount || integer || 28 || 4 || total number of clusters                   
| clusterCount || integer || 28 || 4 || total number of clusters                   
|-
|-
| urlPtrPos || integer || 32 || 8 || position of the directory pointerlist ordered by URL                      
| pathPtrPos || integer || 32 || 8 || position of the directory pointerlist ordered by Path                      
|-
|-
| titlePtrPos || integer || 40 || 8 || position of the directory pointerlist ordered by Title
| titlePtrPos || integer || 40 || 8 || position of the directory pointerlist ordered by Title
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|-
|-
| colspan="2" | 5 || yes || Introduces:
| colspan="2" | 5 || yes || Introduces:
- Url index (was only title indexed before)
- Path index (was only title indexed before)


- MimeList Pos  
- MimeList Pos  
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|}
|}


== URL Pointer List (urlPtrPos) ==
== Path Pointer List (pathPtrPos) ==
The URL pointer list is a list of 8 byte offsets to the directory entries.
The Path pointer list is a list of 8 byte offsets to the directory entries.


The directory entries are always ordered by "full" URL (<code><namespace><path></code>). Ordering is simply done by comparing the URL strings.
The directory entries are always ordered by "full" path (<code><namespace><path></code>). Ordering is simply done by comparing the path strings (utf8 encoded)


Since directory entries have variable sizes this is needed for random access.
Since directory entries have variable sizes this is needed for random access.
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! Field Name !! Type !!Offset!!Length!! Description                 
! Field Name !! Type !!Offset!!Length!! Description                 
|-
|-
| <1st URL> || integer || 0 || 8 || pointer to the directory entry of <1st URL>                       
| <1st path> || integer || 0 || 8 || pointer to the directory entry of <1st path>                       
|-
|-
| <2nd URL> || integer || 8 || 8 || pointer to the directory entry of <2nd URL>                       
| <2nd path> || integer || 8 || 8 || pointer to the directory entry of <2nd path>                       
|-
|-
| <nth URL> || integer ||(n-1)*8|| 8 || pointer to the directory entry of <nth URL>                 
| <nth path> || integer ||(n-1)*8|| 8 || pointer to the directory entry of <nth path>                 
|-
|-
| ... || integer || ... || 8 || ...                           
| ... || integer || ... || 8 || ...                           
|}
|}


Libzim caches directory entries and references the cached entries via the URL pointers.
Libzim caches directory entries and references the cached entries via the path pointers.


== Title Pointer List (titlePtrPos) ==
== Title Pointer List (titlePtrPos) ==
The title pointer list is a list of entry indices ordered by title (<code><namespace><title></code>). The title pointer list actually points to entries in the URL pointer list.
The title pointer list is a list of entry indices ordered by title (<code><namespace><title></code>). The title pointer list actually points to entries in the path pointer list.


Note that the title pointers are only 4 bytes. They are not offsets in the file but entry numbers.
Note that the title pointers are only 4 bytes. They are not offsets in the file but entry numbers.


To get the offset of an entry from the title pointer list, you have to look it up in the URL pointer list.
To get the offset of an entry from the title pointer list, you have to look it up in the path pointer list.


{| class="sortable" style="border-width:1px; border-style:solid; border-color:#888888; background-color:#eeeeee; border-collapse:collapse; empty-cells:show" cellspacing="0" cellpadding="4" {{Prettytable}}
{| class="sortable" style="border-width:1px; border-style:solid; border-color:#888888; background-color:#eeeeee; border-collapse:collapse; empty-cells:show" cellspacing="0" cellpadding="4" {{Prettytable}}
! Field Name !! Type !!Offset!!Length!! Description                 
! Field Name !! Type !!Offset!!Length!! Description                 
|-
|-
| <1st Title> || integer || 0 || 4 || pointer to the URL pointer of <1st Title>                     
| <1st Title> || integer || 0 || 4 || pointer to the path pointer of <1st Title>                     
|-
|-
| <2nd Title> || integer || 4 || 4 || pointer to the URL pointer of <2nd Title>                     
| <2nd Title> || integer || 4 || 4 || pointer to the path pointer of <2nd Title>                     
|-
|-
| <nth Title> || integer ||(n-1)*4|| 4 || pointer to the URL pointer of <nth Title>               
| <nth Title> || integer ||(n-1)*4|| 4 || pointer to the path pointer of <nth Title>               
|-
|-
| ... || integer || ... || 4 || ...                           
| ... || integer || ... || 4 || ...                           
|}
|}


The indirection from titles via URLs to directory entries has two reasons:
The indirection from titles via Paths to directory entries has two reasons:
* the pointer list is only half in size as 4 bytes are enough for each entry
* 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 libzim.
* accessing directory entries by title also makes use of cached directory entries which are referenced by the path pointers, as implemented in libzim.


== Directory Entries ==
== Directory Entries ==
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| blob number || integer || 12 || 4 || blob number inside the compressed cluster where the contents are 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                         
| path || string || 16 ||zero terminated|| string with the path as referred in the path 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                       
| title || string || n/a ||zero terminated|| string with an title as referred in the Title pointer list or empty; in case it is empty, the path is used as title                       
|-
|-
| parameter || data || ||see parameter len|| (not used) extra parameters                         
| parameter || data || ||see parameter len|| (not used) extra parameters                         
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| redirect index || integer || 8 || 4 || pointer to the directory entry of the redirect target                 
| redirect index || integer || 8 || 4 || pointer to the directory entry of the redirect target                 
|-
|-
| url || string || 12 ||zero terminated|| string with the URL as refered in the URL pointer list                           
| path || string || 12 ||zero terminated|| string with the path as referred in the path 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                       
| title || string || n/a ||zero terminated|| string with an title as referred in the Title pointer list or empty; in case it is empty, the path is used as title                       
|-
|-
| parameter || data || ||see parameter len|| (not used) extra parameters                         
| parameter || data || ||see parameter len|| (not used) extra parameters                         
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== Namespaces ==
== Namespaces ==
Namespaces separate different types of directory entries - which might have the same title or url - stored in the ZIM archive Format.
Namespaces separate different types of directory entries - which might have the same title or path - stored in the ZIM archive Format.


The new namespace usage put a strong semantics on the namespaces. The libzim uses this semantics and provide different kind of API to access the different kind of entries.
The new namespace usage put a strong semantics on the namespaces. The libzim uses this semantics and provide different kind of API to access the different kind of entries.
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|}
|}


== URLs ==
== Paths ==


=== URL Encoding in the ZIM ===
=== Path Encoding in the ZIM ===
The URLs in the UrlPointerlist are encoded in utf-8 and are '''not''' url encoded.
The Path in the PathPointerlist are encoded in utf-8 and are '''not''' url encoded.


For instance, if you store in the ZIM an HTML document with a href pointing to `characters%20%C3%A9ncoding.html`, you have to store the corresponding ZIM entry at `characters éncoding.html` URL.
For instance, if you store in the ZIM an HTML document with a href pointing to `characters%20%C3%A9ncoding.html`, you have to store the corresponding ZIM entry at `characters éncoding.html` Path.


Or if you want to store a ZIM entry at `index.html?param=value`, the HTML document pointing to it will have to use the `index.html%3Fparam%3Dvalue` href.
Or if you want to store a ZIM entry at `index.html?param=value`, the HTML document pointing to it will have to use the `index.html%3Fparam%3Dvalue` href.
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The reason behind it is that libzim is agnostic of which kind of content and which kind of readers will be used. Everything around URL encoding is purely linked to HTTP / HTML / Web standards.
The reason behind it is that libzim is agnostic of which kind of content and which kind of readers will be used. Everything around URL encoding is purely linked to HTTP / HTML / Web standards.


When serving web content (which is usually the case), some readers process the requests and already do the url decoding internally, whereas most readers will handle the URLs directly.
When serving web content (which is usually the case), some readers process the requests and already do the url decoding internally, whereas most readers will handle the paths directly.


The same applies to querystring which might be absorbed by some webservers and not passed to the libzim.
The same applies to querystring which might be absorbed by some webservers and not passed to the libzim.
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== Encodings ==
== Encodings ==
=== Character Encoding ===
=== Character Encoding ===
The standard encoding for ZIM archive content is UTF-8. So both article data and URLs should be handled accordingly.
The standard encoding for ZIM archive content is UTF-8. So both article data and Path should be handled accordingly.


=== Integer Encoding ===
=== Integer Encoding ===
Mgautierfr
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