After years of wanting to use COMPACT: in hosts.cfg but never getting it to work, I finally decided to have a closer look.
It turns out there are a few small problems that need attention. In particular, the documentation is wrong about delimiters. If you use the syntax shown in the man page then it will crash (or not depending on some undocumented behaviour with strlen and NULL input).
The correct format is actually:
COMPACT:newfield=oldfield1|oldfield2|oldfield3,anothernewfield=oldfield4|oldfield5
Etc.
These patches correct the manpage, make the software more robust against crashes when you try the bad syntax, and fix a problem with clicking on the summary icon.
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