Formula for CacheDir MD5 calculation

Hi,

In https://bitbucket.org/scons/scons/pull-requests/302/change-the-cache-to-use-2-character/diff
it looks like CacheDir() can grow endlessly and needs to be purged
from time to time. The PR propose a migration script for build cache,
but I think it is useless if the cache needs to be purged anyway.

The problems seems actual, because of presence of this page -
https://bitbucket.org/scons/scons/wiki/LimitCacheSizeWithProgress

So, the question are - how does SCons decide when a new file should be
added into cache? How does it actually calculate MD5 to find file that
already exists in the cache? Docs don't explain this
http://www.scons.org/doc/HTML/scons-user/ch24.html#idp1403343476

-- 
anatoly t.