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APT Cache File Format
Chapter 3 Notes on the Generator

The pkgCache::MergePackageFile function is currently the only generator of the cache file. It implements a conversion from the normal textual package file into the cache file.

The generator assumes any package declaration with a Status: line is a 'Status of the package' type of package declaration. A Package with a Target-Version field should also really have a status field. The processing of a Target-Version field can create a place-holder Version structure that is empty to refer to the specified version (See Version for info on what a empty Version looks like). The Target-Version syntax allows the specification of a specific version and a target distribution.

Different section names on different versions is supported, but I do not expect to use it. To simplify the GUI it will merely use the section in the Package structure. This should be okay as I hope sections do not change much.

The generator goes through a number of post processing steps after producing a disk file. It sorts all of the version lists to be in descending order and then generates the reverse dependency lists for all of the packages. ID numbers and count values are also generated in the post processing step.

It is possible to extend many of the structures in the cache with extra data. This is done by using the ID member. ID will be a unique number from 0 to Header->??Count. For example

     struct MyPkgData;
     MyPkgData *Data = new MyPkgData[Header->PackageCount];
     Data[Package->ID]->Item = 0;

This provides a one way reference between package structures and user data. To get a two way reference would require a member inside the MyPkgData structure.

The generators use of free space pools tend to make the package file quite large, and quite full of blank space. This could be fixed with sparse files.