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Difference between revisions of "ZIM file format"
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→Clusters: Clarification around the cluster compression types
Mgautierfr (talk | contribs) (Remove the idea that titlePtrPos may be set to zero.W) |
(→Clusters: Clarification around the cluster compression types) |
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The first byte of the cluster identifies some information about the cluster. | The first byte of the cluster identifies some information about the cluster. | ||
The first fourth low bits identifies if the cluster | The first fourth low bits identifies if the cluster compression type: | ||
* | * No compression is indicated by a value of 1 | ||
* Compressed clusters are indicated by a value of 4 ([[LZMA2 compression]] (or more precisely XZ, since there is a XZ header)) | * Compressed clusters are indicated by a value of 4 ([[LZMA2 compression]] (or more precisely XZ, since there is a XZ header)) or 5 (Zstandard compression). | ||
* There have been other compression algorithms used before | * There have been other compression algorithms used before which have been removed: 2 for zlib and 3 for bzip2. | ||
The | * 0 is an obselete code for no compression (inhereted from the Zeno) | ||
The fifth bit identifies 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. | * 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. | * 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. | ||
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! Field Name !! Type !!Offset!!Length!! Description | ! Field Name !! Type !!Offset!!Length!! Description | ||
|- | |- | ||
| cluster information || integer || 0 || 1 || Fourth low bits : | | cluster information || integer || 0 || 1 || Fourth low bits : 1: no compression, 4: LZMA2 compressed, 5: zstd compressed | ||
Firth bits : 0: normal (OFFSET_SIZE=4) 1: extended (OFFSET_SIZE=8) | Firth bits : 0: normal (OFFSET_SIZE=4) 1: extended (OFFSET_SIZE=8) | ||
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