Snake is a free, open-source, and cross-platform game developed with the intention of being friendly for developers (particularly inexperienced) to contribute to. This is a C++14 project whose only dependency is currently SFML.
To play the game (Windows 64-bit): https://github.com/FangfangFu/Snake
You can choose what different grib and move time; both of them decides the difficuty of the game.
Snake is heavily documented and uses the Github Wiki as the base for all documentation, including how to build the project and understanding the source code.