Project Euler

They don't have an actual online judge system, all problems simply have a single small string solution, almost always integer or fixed precision floating point, and they just check that you've found the value.

The only metric that matters is who solved the problem first after publication. This is visible e.g. at: projecteuler.net/fastest=454 but only for logged in users... Lol it is ridiculous. The "language" in which problems were solved is just whatever the user put in their profile, they can't actually confirm that.

Problems are under CC BY-NC-SA: projecteuler.net/copyright

Once you solve a problem, you can then access its "private" forum thread: projecteuler.net/thread=950 and people will post a bunch of code solutions in there.

How problems are chosen:

projecteuler.net says it started as a subsection in mathschallenge.net, and in 2006 moved to its own domain. WhoisXMLAPI WHOIS history says it was registered by domainmonster.com but details are anonymous. TODO: sample problem on mathschallenge.net on Wayback Machine? Likely wouldn't reveal much anyways though as there is no attribution to problem authors on that site.

www.hackerrank.com/contests/projecteuler/challenges holds challenges with an actual judge and sometimes multiple test cases so just printing the final solution number is not enough.