[Examples/Move] Tic-tac-toe

## Description

Port Move code for the tic tac toe example into the `sui/examples`
directory, modernising it in the process:

- Standardise the implementation between the owned, shared and
  multi-sig variants, so that the only differences are relevant to what
  they need to do differently. In doing this, the owned and multi-sig
  variants become identical (at least at the level of Move code).
- Use transfer-to-object in the "owned" example, to send potential
  moves to the `Game` object, instead of the admin that owns the game.
- Use more Move 2024 features (`syntax(index)`, receiver function
  aliases, etc).
- Improve test coverage

## Test plan

New unit tests:

```
tic_tac_toe/move$ sui move test
```

examples/tic-tac-toe/move/Move.toml

@@ -0,0 +1,9 @@
+[package]
+name = "tic_tac_toe"
+edition = "2024.beta"
+
+[dependencies]
+Sui = { local = "../../../crates/sui-framework/packages/sui-framework" }
+
+[addresses]
+tic_tac_toe = "0x0"

examples/tic-tac-toe/move/sources/owned.move

@@ -0,0 +1,326 @@
+// Copyright (c) Mysten Labs, Inc.
+// SPDX-License-Identifier: Apache-2.0
+
+/// An implementation of Tic Tac Toe, using owned objects.
+///
+/// The `Game` object is owned by an admin, so players cannot mutate the game
+/// board directly. Instead, they convey their intention to place a mark by
+/// transferring a `Mark` object to the `Game`.
+///
+/// This means that every move takes two owned object fast path operations --
+/// one by the player, and one by the admin. The admin could be a third party
+/// running a centralized service that monitors marker placement events and
+/// responds to them, or it could be a 1-of-2 multisig address shared between
+/// the two players, as demonstrated in the demo app.
+///
+/// The `shared` module shows a variant of this game implemented using shared
+/// objects, which provides different trade-offs: Using shared objects is more
+/// expensive, however the implementation is more straightforward and each move
+/// only requires one transaction.
+module tic_tac_toe::owned {
+    use sui::event;
+    use sui::transfer::Receiving;
+
+    // === Object Types ===
+
+    /// The state of an active game of tic-tac-toe.
+    public struct Game has key, store {
+        id: UID,
+
+        /// Marks on the board.
+        board: vector<u8>,
+
+        /// The next turn to be played.
+        turn: u8,
+
+        /// The address expected to send moves on behalf of X.
+        x: address,
+
+        /// The address expected to send moves on behalf of O.
+        o: address,
+
+        /// Public key of the admin address.
+        admin: vector<u8>,
+    }
+
+    /// The player that the next turn is expected from is given a `TurnCap`.
+    public struct TurnCap has key {
+        id: UID,
+        game: ID,
+    }
+
+    /// A request to make a play -- only the player with the `TurnCap` can
+    /// create and send `Mark`s.
+    public struct Mark has key, store {
+        id: UID,
+        player: address,
+        row: u8,
+        col: u8,
+    }
+
+    /// An NFT representing a finished game. Sent to the winning player if there
+    /// is one, or to both players in the case of a draw.
+    public struct Trophy has key {
+        id: UID,
+
+        /// Whether the game was won or drawn.
+        status: u8,
+
+        /// The state of the board at the end of the game.
+        board: vector<u8>,
+
+        /// The number of turns played
+        turn: u8,
+
+        /// The other player (relative to the player who owns this Trophy).
+        other: address,
+    }
+
+    // === Event Types ===
+
+    public struct MarkSent has copy, drop {
+        game: ID,
+        mark: ID,
+    }
+
+    public struct GameEnd has copy, drop {
+        game: ID,
+    }
+
+    // === Constants ===
+
+    // Marks
+    const MARK__: u8 = 0;
+    const MARK_X: u8 = 1;
+    const MARK_O: u8 = 2;
+
+    // Trophy status
+    const TROPHY_NONE: u8 = 0;
+    const TROPHY_DRAW: u8 = 1;
+    const TROPHY_WIN: u8 = 2;
+
+    // === Errors ===
+
+    /// Move was for a position that doesn't exist on the board.
+    const EInvalidLocation: u64 = 0;
+
+    /// Game expected a move from another player.
+    const EWrongPlayer: u64 = 1;
+
+    /// Game has not reached an end condition.
+    const ENotFinished: u64 = 2;
+
+    /// Can't place a mark on a finished game.
+    const EAlreadyFinished: u64 = 3;
+
+    /// Game reached an end state that wasn't expected.
+    const EInvalidEndState: u64 = 4;
+
+    // === Public Functions ===
+
+    /// Create a new game, played by `x` and `o`. The game should be
+    /// transfered to the address that will administrate the game. If
+    /// that address is a multi-sig of the two players, its public key
+    /// should be passed as `admin`.
+    public fun new(x: address, o: address, admin: vector<u8>, ctx: &mut TxContext): Game {
+        let game = Game {
+            id: object::new(ctx),
+            board: vector[
+                MARK__, MARK__, MARK__,
+                MARK__, MARK__, MARK__,
+                MARK__, MARK__, MARK__,
+            ],
+
+            turn: 0,
+            x,
+            o,
+            admin,
+        };
+
+        let turn = TurnCap {
+            id: object::new(ctx),
+            game: object::id(&game),
+        };
+
+        // X is the first player, so send the capability to them.
+        transfer::transfer(turn, x);
+        game
+    }
+
+    /// Called by the active player to express their intention to make a move.
+    /// This consumes the `TurnCap` to prevent a player from making more than
+    /// one move on their turn.
+    public fun send_mark(cap: TurnCap, row: u8, col: u8, ctx: &mut TxContext) {
+        assert!(row < 3 && col < 3, EInvalidLocation);
+
+        let TurnCap { id, game } = cap;
+        id.delete();
+
+        let mark = Mark {
+            id: object::new(ctx),
+            player: ctx.sender(),
+            row,
+            col,
+        };
+
+        event::emit(MarkSent { game, mark: object::id(&mark) });
+        transfer::transfer(mark, game.to_address());
+    }
+
+    /// Called by the admin (who owns the `Game`), to commit a player's
+    /// intention to make a move. If the game should end, `Trophy`s are sent to
+    /// the appropriate players, if the game should continue, a new `TurnCap` is
+    /// sent to the player who should make the next move.
+    public fun place_mark(
+        game: &mut Game,
+        mark: Receiving<Mark>,
+        ctx: &mut TxContext,
+    ) {
+        assert!(game.ended() == TROPHY_NONE, EAlreadyFinished);
+
+        // Fetch the mark on behalf of the game -- only works if the mark in
+        // question was sent to this game.
+        let Mark { id, row, col, player } = transfer::receive(&mut game.id, mark);
+        id.delete();
+
+        // Confirm that the mark is from the player we expect -- it should not
+        // be possible to hit this assertion, because the `Mark`s can only be
+        // created by the address that owns the `TurnCap` which cannot be
+        // transferred, and is always held by `game.next_player()`.
+        let (me, them, sentinel) = game.next_player();
+        assert!(me == player, EWrongPlayer);
+
+        if (game[row, col] == MARK__) {
+            *(&mut game[row, col]) = sentinel;
+            game.turn = game.turn + 1;
+        };
+
+        // Check win condition -- if there is a winner, send them the trophy,
+        // otherwise, create a new turn cap and send that to the next player.
+        let end = game.ended();
+        if (end == TROPHY_WIN) {
+            transfer::transfer(game.mint_trophy(end, them, ctx), me);
+            event::emit(GameEnd { game: object::id(game) });
+        } else if (end == TROPHY_DRAW) {
+            transfer::transfer(game.mint_trophy(end, them, ctx), me);
+            transfer::transfer(game.mint_trophy(end, me, ctx), them);
+            event::emit(GameEnd { game: object::id(game) });
+        } else if (end == TROPHY_NONE) {
+            let cap = TurnCap { id: object::new(ctx), game: object::id(game) };
+            let (to, _, _) = game.next_player();
+            transfer::transfer(cap, to);
+        } else {
+            abort EInvalidEndState
+        }
+    }
+
+    public fun burn(game: Game) {
+        assert!(game.ended() != TROPHY_NONE, ENotFinished);
+        let Game { id, .. } = game;
+        id.delete();
+    }
+
+    /// Test whether the game has reached an end condition or not.
+    public fun ended(game: &Game): u8 {
+        if (
+            // Test rows
+            test_triple(game, 0, 1, 2) ||
+            test_triple(game, 3, 4, 5) ||
+            test_triple(game, 6, 7, 8) ||
+            // Test columns
+            test_triple(game, 0, 3, 6) ||
+            test_triple(game, 1, 4, 7) ||
+            test_triple(game, 2, 5, 8) ||
+            // Test diagonals
+            test_triple(game, 0, 4, 8) ||
+            test_triple(game, 2, 4, 6)
+        ) {
+            TROPHY_WIN
+        } else if (game.turn == 9) {
+            TROPHY_DRAW
+        } else {
+            TROPHY_NONE
+        }
+    }
+
+    #[syntax(index)]
+    public fun mark(game: &Game, row: u8, col: u8): &u8 {
+        &game.board[(row * 3 + col) as u64]
+    }
+
+    #[syntax(index)]
+    fun mark_mut(game: &mut Game, row: u8, col: u8): &mut u8 {
+        &mut game.board[(row * 3 + col) as u64]
+    }
+
+    // === Private Helpers ===
+
+    /// Address of the player the move is expected from, the address of the
+    /// other player, and the mark to use for the upcoming move.
+    fun next_player(game: &Game): (address, address, u8) {
+        if (game.turn % 2 == 0) {
+            (game.x, game.o, MARK_X)
+        } else {
+            (game.o, game.x, MARK_O)
+        }
+    }
+
+    /// Test whether the values at the triple of positions all match each other
+    /// (and are not all EMPTY).
+    fun test_triple(game: &Game, x: u8, y: u8, z: u8): bool {
+        let x = game.board[x as u64];
+        let y = game.board[y as u64];
+        let z = game.board[z as u64];
+
+        MARK__ != x && x == y && y == z
+    }
+
+    /// Create a trophy from the current state of the `game`, that indicates
+    /// that a player won or drew against `other` player.
+    fun mint_trophy(
+        game: &Game,
+        status: u8,
+        other: address,
+        ctx: &mut TxContext,
+    ): Trophy {
+        Trophy {
+            id: object::new(ctx),
+            status,
+            board: game.board,
+            turn: game.turn,
+            other,
+        }
+    }
+
+    // === Test Helpers ===
+    #[test_only] public use fun game_board as Game.board;
+    #[test_only] public use fun trophy_status as Trophy.status;
+    #[test_only] public use fun trophy_board as Trophy.board;
+    #[test_only] public use fun trophy_turn as Trophy.turn;
+    #[test_only] public use fun trophy_other as Trophy.other;
+
+    #[test_only]
+    public fun game_board(game: &Game): vector<u8> {
+        game.board
+    }
+
+    #[test_only]
+    public fun trophy_status(trophy: &Trophy): u8 {
+        trophy.status
+    }
+
+    #[test_only]
+    public fun trophy_board(trophy: &Trophy): vector<u8> {
+        trophy.board
+    }
+
+    #[test_only]
+    public fun trophy_turn(trophy: &Trophy): u8 {
+        trophy.turn
+    }
+
+    #[test_only]
+    public fun trophy_other(trophy: &Trophy): address {
+        trophy.other
+    }
+}