CheckiO vs Codeforces

CheckiO has no write-up on Specifiction yet; its figures are above.

Codeforces is a programming-contest community and platform where regular contests give participants a way to solve problems and earn skill ratings. The site says it organizes approximately six contests each month, with free participation open to everybody. In contests following Codeforces rules, participants gain points for solving problems quickly and can test other contestants’ solutions through hacks. Ratings use a modified Elo system, and contestants are split into two divisions. The online judge checks submissions against prepared tests and tests suggested through hacks, then returns a verdict. Past contests are available for practice, while the site says learning content and training features are in development. Codeforces also offers an API that returns some data as machine-readable JSON over HTTP. Public data can be requested anonymously; private user data requires an API key. API requests are limited to one every two seconds, and faster calls receive a call-limit error. The service is free and available on web and through an API.