explanation of ccache stats

I wrote a few munin plugins for ccache this weekend, if anyone is
interested:  https://www.ccoderun.ca/#ccache

What I'm looking for now is an explanation of all the stats field when I
run --show-stats.

"cache miss" and "called for link" I can guess.

But I'm looking for explanations of the differences between things like
"cache hit direct" and "cache hit preprocessed".  And "no input file".

Didn't see these things explained in the documentation.  Is there a web
page that explains the statistics?

Stéphane Charette