Didn't you (or someone else) say that some old patches only adds a little to trunk (since they might be small), so not a lot of the patch code needs updating? Or is it much harder than that?
I did say some, you recall correctly, but that was refering to single patches.
The difficulty with patchpacks is that instead of two sources to merge (trunk's source and the patch's source) you get as many 'sources' as there are patches.
Most of them do not interfere with eachother at all, while some patches only need to be merged with one other source.
As it is now ... I think about 25% of the patches had a conflict with another in at least one place.
50% of them has code merged for at least three patches or more in a single funtion.
The remaining 25% are mostly oneliners or very small patches which do not cause to much effort.
eg.
Updating the old zoom patch may turn out quite easy but If I add it now I have at least three patches to merge, CargoDist, MoreHeightlevels, and zoomzoom in at least two very big and somewhat complicated areas, namely the viewport code and the smallmap code.
That is three features that need merging in about every single file in the source.
Then the smaller patches have to be fitted in there, sometimes causing extra merging work sometimes none.
Slapping some patches toghether is not that hard at all.
It is making sure everything works (testing), balancing (tuning) and maintaining afterwards (bumping and updating patches) that consumes most of the time required.
http://en.wikipedia.org/wiki/Ninety-ninety_rule
Just google "programming 90-90", there is a much better page then the one I linked somewhere but I can not find it ...
EDIT:
http://hg.openttd.org/openttd/trunk.hg/rev/006dfecf1559
Autoseparation is in trunk yiehaa.

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Playing with my patchpack? Ask questions on usage and report bugs in the correct thread first, please.
All included patches have been modified and are no longer 100% original.