10_HeadTail.sol

// Hidden head or tail.
// If the party B guess the choice of party A, it wins 2 ethers, otherwise party A does.
contract HeadTail {
    address public partyA;
    address public partyB;
    bytes32 public commitmentA;
    bool public chooseHeadB;
    uint public timeB;
    
    
    
    /** @dev Constructor, commit head or tail.
     *  @param _commitmentA is keccak256(chooseHead,randomNumber);
     */
    function HeadTail(bytes32 _commitmentA) payable {
        require(msg.value == 1 ether);
        
        commitmentA=_commitmentA;
        partyA=msg.sender;
    }
    
    /** @dev Guess the choice of party A.
     *  @param _chooseHead True if the guess is head, false otherwize.
     */
    function guess(bool _chooseHead) payable {
        require(msg.value == 1 ether);
        
        chooseHeadB=_chooseHead;
        timeB=now;
    }
    
    /** @dev Reveal the commited value and send ETH to the winner.
     *  @param _chooseHead True if head was chose.
     *  @param _randomNumber The random number chosen to obfuscate the commitment.
     */
    function resolve(bool _chooseHead, uint _randomNumber) {
        require(msg.sender == partyA);
        require(keccak256(_chooseHead, _randomNumber) == commitmentA);
        require(this.balance >= 2 ether);
        
        if (_chooseHead == chooseHeadB)
            partyB.transfer(2 ether);
        else
            partyA.transfer(2 ether);
    }
    
    /** @dev Time out party A if it takes more than 1 day to reveal.
     *  Send ETH to party B.
     * */
    function timeOut() {
        require(now > timeB + 1 days);
        require(this.balance>=2 ether);
        partyB.transfer(2 ether);
    }
    
}