 ### sCrypt by Example ## Tic-Tac-Toe

To initiate a game, Alice and Bob each lock up X amount of bitcoins into a contract UTXO. They take turns to play the game by sending signed transactions interacting with the stateful contract. If one of them wins, the winner claims the 2X bitcoins. If there is a draw, they take back their bitcoins.

``````import "arrayUtil.scrypt";

contract TicTacToe {

PubKey alice;
PubKey bob;

@state
bool is_alice_turn;

@state
bytes board;

static const int TURNLEN = 1;
static const int BOARDLEN = 9;
static const bytes EMPTY = b'00';
static const bytes ALICE = b'01';
static const bytes BOB = b'02';

public function move(int n, Sig sig, int amount, SigHashPreimage txPreimage) {
require(Tx.checkPreimage(txPreimage));
require(n >= 0 && n < BOARDLEN);

bytes play = this.is_alice_turn ? ALICE : BOB;
PubKey player = this.is_alice_turn ? this.alice : this.bob;

// ensure it's player's turn
require(checkSig(sig, player));
// make the move
this.board = ArrayUtil.setElemAt(this.board, n, play);
this.is_alice_turn = !this.is_alice_turn;

bytes outputs = b'';
if (this.won(this.board, play)) {
// winner takes all
bytes outputScript = Utils.buildPublicKeyHashScript(hash160(player));
bytes output = Utils.buildOutput(outputScript, amount);
outputs = output;
}
else if (this.full(this.board)) {
// draw: equally split, i.e., both outputs have the same amount
bytes aliceScript = Utils.buildPublicKeyHashScript(hash160(this.alice));
bytes aliceOutput = Utils.buildOutput(aliceScript, amount);

bytes bobScript = Utils.buildPublicKeyHashScript(hash160(this.bob));
bytes bobOutput = Utils.buildOutput(bobScript, amount);

outputs = aliceOutput + bobOutput;
}
else {
// update state
bytes scriptCode_ = this.getStateScript();
bytes output = Utils.buildOutput(scriptCode_, amount);
outputs = output;
}

require(hash256(outputs) == SigHash.hashOutputs(txPreimage));
}

// does play win after current move?
function won(bytes board, bytes play) : bool {
// three in a row, a column, or a diagnoal
int lines = [[0, 1, 2], [3, 4, 5], [6, 7, 8], [0, 3, 6], [1, 4, 7], [2, 5, 8], [0, 4, 8], [2, 4, 6]];

bool anyLine = false;
loop (8) : i {
bool line = true;
loop (3) : j {
line = line && ArrayUtil.getElemAt(board, lines[i][j]) == play;
}

anyLine = anyLine || line;
}

return anyLine;
}

// is board full?
function full(bytes board) : bool {
bool full = true;

loop (BOARDLEN) : i {
full = full && ArrayUtil.getElemAt(board, i) != EMPTY;
}

return full;
}
}
``````