Experimental map-first preprocessing

DITA-OT 2.5 introduces an experimental map-first preprocessing option as an alternative to the default preprocess operation. The new method provides the same functionality, but takes a different approach.

Whereas the default preprocessing routine handles both maps and topics at the same time, the map-first approach begins by processing maps as far as possible, and only then starts processing topics. This simplifies the processing logic and creates cleaner module responsibilities, which makes it easier to process only those topics that are actually referenced after filtering, for example, or to only process the map to validate the map structure.

The current preprocessing architecture was established during the DITA 1.0 era when there were fewer DITA features that operated on the map level. Initially, the difference between processing modes was not that great, but as more map-level features such as keys and key scopes have been added to DITA, processing maps first to build the context makes sense. The original preprocessing operation already does this to a degree, but the new map-first preprocessing option will be even more formal about it.

To use map-first preprocessing, call the preprocess2 Ant target in your customizations instead of the default preprocess target.