Add tic-tac-toe program flow concept

rfcs/rfc-004-tictactoe-program.md

@@ -0,0 +1,77 @@
+
+Two players want to play tic-tac-toe with each other on Solana.
+
+The tic-tac-toe program has already been provisioned on the network, and the
+program author has advertised the following information to potential gamers:
+* `tictactoe_publickey` - the program's public key
+* `tictactoe_gamestate_size` - the number of bytes needed to maintain the game state
+
+The game state is a well-documented data structure consisting of:
+- Player 1's public key
+- Player 2's public key
+- Game status. An 8-bit value where:
+  * 0 = game uninitialized
+  * 1 = Player 1's turn
+  * 2 = Player 2's turn
+  * 3 = Player 1 won
+  * 4 = Player 2 won
+- Current board configuration.  A 3x3 character array containing the values '\0', 'X' or 'O'
+
+### Game Setup
+
+1. Two players want to start a game.  Player 2 sends Player 1 their public key,
+`player2_publickey` off-chain (IM, email, etc)
+
+2. Player 1 creates a new keypair to represent the game state, `(gamestate_publickey,
+gamestate_privatekey)`.
+
+3. Player 1 issues an allocate_memory transaction, assigning that memory page to the
+tic-tac-toe program.  The `memory_fee` is used to *rent* the memory page for the
+duration of the game and is subtracted from current account balance of Player
+1:
+```
+allocate_memory(gamestate_publickey, tictactoe_publickey, tictactoe_gamestate_size, memory_fee)
+```
+
+
+4. Game state is then initialized by issuing a *new* call transaction to the
+tic-tac-toe program.  This transaction is signed by `gamestate_privatekey`, known only
+to Player 1.
+```
+call(tictactoe_publickey, gamestate_publickey, 'new', player1_publickey, player2_publickey)
+```
+
+5. Once the game is initialized, Player 1 shares `gamestate_publickey` with
+Player 2 off-chain (IM, email, etc)
+
+Note that it's likely each player prefer to generate a game-specific keypair
+rather than sharing their primary public key (`player1_publickey`,
+`player2_publickey`) with each other and the tic-tac-toe program.
+
+### Game Play
+
+Both players poll the network, via a **TBD off-chain RPC API**, to read the
+current game state from the `gamestate_publickey` memory page.
+
+When the *Game status* field indicates it's their turn, the player issues a
+*move* call transaction passing in the board position (1..9) that they want to
+mark as X or O:
+```
+call(tictactoe_publickey, gamestate_publickey, 'move', position)
+```
+The program will reject the transaction if it was not signed by the player whose
+turn it is.
+
+The outcome of the *move* call is also observed by polling the current game state via
+the **TBD off-chain RPC API**.
+
+### Game Cancellation
+
+At any time Player 1 may conclude the game by issuing:
+```
+call(tictactoe_publickey, gamestate_publickey, 'abort')
+```
+causing any remaining *rent* tokens assigned to the `gamestate_publickey` page
+to be transferred back to Player 1 by the tic-tac-toe program.  Lastly, the
+network recognizes the empty account and frees the `gamestate_publickey` memory
+page.