Refine Valid action helper

[peter1591]

Refines on state::State:

• PrepareValidActions() --> returns ValidActionHelper()
  • Do some process
  • Selection stage can save the board after this is called
• ApplyAction(ValidActionHelper const* = nullptr)

Requirement

• No overhead. Keep the simulation quick.

Notes

• Even in selection stage, the board after 'PrepareValidActions()' is not saved in memory
  • Only save the BoardView
• Since there's hidden information, a determination phase runs before each episode. So even the board is saved, we will not run that exactly board at following episodes.
• But, in fact, the hidden information should get nothing to do with the prepare action.
• So, maybe the valid action helper should not be implemented within the state::State. It should be related to BoardView.

[peter1591]

Another way:
Refine the simulation engine. If an action is invalid...

1. Return kResultInvalid
2. and, do not modify the game state

[peter1591]

It makes logic too complicated if we want to enforce the constant-state at compile time
If we just leave it as a mandatory, things might blow up after a long-term maintenance

Could we made the game state to be copy-on-write?

1. Make things copy-on-write
2. Investigate which fields are possibly changed for an invalid action

[peter1591]

Close this issue.
Open another issue to profile the time of state copying