How is the "unique visitor" hashtag built?



the Piwik FAQ says:
“If the visitor does not accept the cookie (if they have disabled, blocked or deleted cookies), a simple rule of thumb is used to try to match the visitor to a previous visitor with the same features (IP, resolution, browser, plugins, OS, etc).”

We want to analyse the building of the hash in detail. The listing ends with “etc”.
In which function or file of piwik can I find these?

Thanks for your help.