10_math_trick.sol

/*
 * Title: Math Trick
 * Trick: Since 50*(1/2)^0 + 50*(1/2)^1 + 50*(1/2)^2 + 50*(1/2)^3 + ... will always less than 50*2. We can never withdraw the money.
 * Reference: https://github.com/misterch0c/Solidlity-Vulnerable/blob/master/honeypots/RACEFORETH.sol
*/
pragma solidity 0.4.21;

// How fast can you get to 100 points and win the prize?
// First person to deposit 0.1 eth (100 finney) wins the entire prize!
// 1 finney = 1 point

contract RACEFORETH {
    // 100 points to win!
    uint256 public SCORE_TO_WIN = 100 finney;
    uint256 public PRIZE;

    // 100 points = 0.1 ether
    // Speed limit: 0.05 eth to prevent insta-win
    // Prevents people from going too fast!
    uint256 public speed_limit = 50 finney;

    // Keep track of everyone's score
    mapping (address => uint256) racerScore;
    mapping (address => uint256) racerSpeedLimit;

    uint256 latestTimestamp;
    address owner;

    function RACEFORETH () public payable {
        PRIZE = msg.value;
        owner = msg.sender;
    }

    function race() public payable {
        if (racerSpeedLimit[msg.sender] == 0) { racerSpeedLimit[msg.sender] = speed_limit; }
        require(msg.value <= racerSpeedLimit[msg.sender] && msg.value > 1 wei);

        racerScore[msg.sender] += msg.value;
        racerSpeedLimit[msg.sender] = (racerSpeedLimit[msg.sender] / 2);

        latestTimestamp = now;

        // YOU WON
        if (racerScore[msg.sender] >= SCORE_TO_WIN) {
            msg.sender.transfer(PRIZE);
        }
    }

    function () public payable {
        race();
    }

    // Pull the prize if no one has raced in 3 days :(
    function endRace() public {
        require(msg.sender == owner);
        require(now > latestTimestamp + 3 days);

        msg.sender.transfer(this.balance);
    }
}