= Project Euler
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https://projecteuler.net
They don't have an actual online judge system, all problems simply have an integer or floating point solution and they just check that you've found the value.
The only metric that matters is who solved the problem first after publication, e.g.: https://projecteuler.net/fastest=454[]. The "language" in which problems were solved is just whatever the user put in their profile, they can't actually confirm that.
<Project Euler> problems typically involve finding or proving and then using a <lemma (mathematics)> that makes computation of the solution feasible without brute force. As such, they live in the intersection of mathematics and computer science.
Repositories of numerical solutions:
* https://github.com/lucky-bai/projecteuler-solutions
* https://www.kaggle.com/datasets/dheerajmpai/projecteuler
Repositories of code solutions:
* https://euler.stephan-brumme.com/ large number of solutions in <C++>, stopped around 600. Informal permissive license, e.g. at: https://euler.stephan-brumme.com/243/
> I hope you enjoy my code and learn something - or give me feedback how I can improve my solutions.
All of my solutions can be used for any purpose and I am in no way liable for any damages caused.
You can even remove my name and claim it's yours. But then you shall burn in hell.
Asked for a more formal open license at: https://github.com/stbrumme/euler/issues/7
* https://www.ivl-projecteuler.com/home 330+ solutions in <Python> as of 2025. Random looking problem selection. On GitHub: https://github.com/igorvanloo/Project-Euler-Explained[] under <Unlicense> license, a <public domain> license.
* https://www.nayuki.io/page/project-euler-solutions[]. Large number of solutions, mostly in <Java> and <Python> primarily but also <Mathematica> and <Haskell> sometimes. Proprietary license.
Basically no one ever had the patience to solve them all. What we need is a collaborative solution.
Problems are under <CC BY-NC-SA>: https://projecteuler.net/copyright
Once you solve a problem, you can then access its "private" forum thread: https://projecteuler.net/thread=950 and people will post a bunch of code solutions in there.
How problems are chosen:
* https://matheducators.stackexchange.com/questions/12087/how-does-project-euler-come-up-with-such-good-problems-so-rapidly
https://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.
https://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.
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