An ex-manager of mine quoted some copyright law that applied to SW. "If there is less than 75% common with the old code, then you don't have to credit, etc..." I think he was wrong and I am so glad I don't work there anymore.

My feeling are thus:

If you benefited from the code you copied, give credit. If it's a simple algorithm that repeats what has been known before, you really don't have to.

I saw one Perl "coder" put a copyright notice of his own on a program that looked to be copied from the Camel book. I don't know why I didn't make an effort to slap him around.

The bottom line is a lot of coding is building on what came before so that the whole copyright issue is fuzzy, but it's better to be overly ethical than not.