The problem is hard to describe. So I only explain how to trigger this bug here.
1.Choose multiplayer game and refuse one's request. But do not disconnect in the other side.
2.Choose singleplayer game and start game.
3.Request again in the other side.
4.Accept the request.

Although only the frontend fail after these operation, we should fix this bug in backend.

1. Write test cases.
2. Integrate into GitHub Actions.
3. Display the real-time coverage rate on README, and ensure it is 100% all the time.

Only after putting 2 stones, the game ends.

This may relate to some fragile codes in rules.

When an exception such as Player Already Exists is thrown, the server will simply exit without leaving any logs.

White players giving up will cause logic_error(Black is not allowed to concede).

This class is now like a pile of rubbish.

We need to support finding the player by its property, like role or participant.
Like:

auto player = players.get({Role::NONE, participant}); //  which means finding by participant.

I think we can use std::set or such things here, and support properties as indexes.

A logger could be extremely useful for bug finding and error troubleshooting.

Logging from different modules, including rule, contest, and network, and with different levels, like fatal, info, and so on.

Serialize State to Message instead and then send it to the front end.

Consider applying Better Enums or Magic Enum to simplify our codes with enums, such as printing their field name. Find a way to turn Role into an enum(or such things) without drastically changing its usage.

Then consider moving these enums into a new place. These enums represent the constants in the Message and the result of State calculation as well.

A brief introduction to the project structure.

Add CI icon and coverage rate, test passing icon (#11) to README.