A Game Winner

synopsis

Make sure you have all you need before proceeding:

• You understand the concepts of Protobuf.
• Have Go installed.
• The checkers blockchain codebase with the deadline field and its handling. You can get there by following the previous steps or checking out the relevant version (opens new window).

To be able to forcibly terminate games and avoid terminating one game twice, you need to identify games that have been terminated. A good field to add is one for the winner. It needs to contain:

• The rightful winner of a game that reaches completion.
• Or, the winner by forfeit, when a game is expired.
• Or a neutral value when the game is active.

In this exercise a draw is not handled and it would require yet another value to save in winner.

New information

In the StoredGame Protobuf definition file:

Copy message StoredGame { ... string winner = 11; } proto checkers stored_game.proto View source

To have Starport and Protobuf recompile this file use:

Copy $ starport generate proto-go

Add a helper function to get the winner's address, if it exists. A good place for it is in full_game.go:

Copy func (storedGame *StoredGame) GetPlayerAddress(color string) (address sdk.AccAddress, found bool, err error) { red, err := storedGame.GetRedAddress() if err != nil { return nil, false, err } black, err := storedGame.GetBlackAddress() if err != nil { return nil, false, err } address, found = map[string]sdk.AccAddress{ rules.RED_PLAYER.Color: red, rules.BLACK_PLAYER.Color: black, }[color] return address, found, nil } func (storedGame *StoredGame) GetWinnerAddress() (address sdk.AccAddress, found bool, err error) { address, found, err = storedGame.GetPlayerAddress(storedGame.Winner) return address, found, err } x checkers types full_game.go View source

Update and check for the winner

This is a two-part update. You set the winner where relevant but you also introduce new checks, so that a game with a winner cannot be acted upon.

Start with a new error that you define as a constant:

Copy ErrGameFinished = sdkerrors.Register(ModuleName, 1111, "game is already finished") x checkers types errors.go View source

Then at creation, in the create game message handler, you start with a neutral value:

Copy ... storedGame := types.StoredGame{ ... Winner: rules.NO_PLAYER.Color, } x checkers keeper msg_server_create_game.go View source

With further checks when handling a play in the handler:

1. Check that the game has not finished yet:

   Copy if storedGame.Winner != rules.NO_PLAYER.Color { return nil, types.ErrGameFinished } x checkers keeper msg_server_play_move.go View source

2. Update the winner field, which remains neutral (opens new window) if there is no winner yet:

   Copy storedGame.Winner = game.Winner().Color x checkers keeper msg_server_play_move.go View source

3. Handle the FIFO differently depending on whether the game is finished or not:

   Copy if storedGame.Winner == rules.NO_PLAYER.Color { k.Keeper.SendToFifoTail(ctx, &storedGame, &nextGame) } else { k.Keeper.RemoveFromFifo(ctx, &storedGame, &nextGame) } x checkers keeper msg_server_play_move.go View source

Just in case, when rejecting a game, in its handler:

Copy if storedGame.Winner != rules.NO_PLAYER.Color { return nil, types.ErrGameFinished } x checkers keeper msg_server_reject_game.go View source

Confirm it compiles and you are ready to handle the expiration of games.

Next up

You have introduced a game FIFO, a game deadline, and a game winner. Time to turn your attention to the next section.