[rkangel]

What you basically do is take all of the inputs (source files, compiler, exactly what command you're going to run) and then produce a hash of all of that together. Then the result of that becomes the input to the next thing etc.

For caches you just say 'here's the SHA1 of the inputs and the cache server can just give you the output.

Basel does this for each build command, e.g. for each object in a big C program. Nix does it for each package.