Doubler.sol

contract Doubler {

    struct Participant {
        address etherAddress;
        uint amount;
    }

    Participant[] public participants;

    uint public payoutIdx = 0;
    uint public collectedFees;
    uint public balance = 0;

    address public owner;

    // simple single-sig function modifier
    modifier onlyowner { if (msg.sender == owner) _ }

    // this function is executed at initialization and sets the owner of the contract
    function Doubler() {
        owner = msg.sender;
    }

    // fallback function - simple transactions trigger this
    function() {
        enter();
    }
    
    function enter() {
        if (msg.value < 1 ether) {
            msg.sender.send(msg.value);
            return;
        }

      	// add a new participant to array
        uint idx = participants.length;
        participants.length += 1;
        participants[idx].etherAddress = msg.sender;
        participants[idx].amount = msg.value;
        
        // collect fees and update contract balance
        if (idx != 0) {
            collectedFees += msg.value / 10;
            balance += msg.value;
        } 
        else {
            // first participant has no one above him,
            // so it goes all to fees
            collectedFees += msg.value;
        }

	// if there are enough ether on the balance we can pay out to an earlier participant
        if (balance > participants[payoutIdx].amount * 2) {
            uint transactionAmount = 2 * (participants[payoutIdx].amount - participants[payoutIdx].amount / 10);
            participants[payoutIdx].etherAddress.send(transactionAmount);

            balance -= participants[payoutIdx].amount * 2;
            payoutIdx += 1;
        }
    }

    function collectFees() onlyowner {
        if (collectedFees == 0) return;

        owner.send(collectedFees);
        collectedFees = 0;
    }

    function setOwner(address _owner) onlyowner {
        owner = _owner;
    }
}
ABI:
[
  {
    "constant": false,
    "inputs": [
      {
        "name": "_owner",
        "type": "address"
      }
    ],
    "name": "setOwner",
    "outputs": [],
    "type": "function"
  },
  {
    "constant": true,
    "inputs": [
      {
        "name": "",
        "type": "uint256"
      }
    ],
    "name": "participants",
    "outputs": [
      {
        "name": "etherAddress",
        "type": "address"
      },
      {
        "name": "amount",
        "type": "uint256"
      }
    ],
    "type": "function"
  },
  {
    "constant": true,
    "inputs": [],
    "name": "owner",
    "outputs": [
      {
        "name": "",
        "type": "address"
      }
    ],
    "type": "function"
  },
  {
    "constant": true,
    "inputs": [],
    "name": "collectedFees",
    "outputs": [
      {
        "name": "",
        "type": "uint256"
      }
    ],
    "type": "function"
  },
  {
    "constant": true,
    "inputs": [],
    "name": "payoutIdx",
    "outputs": [
      {
        "name": "",
        "type": "uint256"
      }
    ],
    "type": "function"
  },
  {
    "constant": true,
    "inputs": [],
    "name": "balance",
    "outputs": [
      {
        "name": "",
        "type": "uint256"
      }
    ],
    "type": "function"
  },
  {
    "constant": false,
    "inputs": [],
    "name": "collectFees",
    "outputs": [],
    "type": "function"
  },
  {
    "constant": false,
    "inputs": [],
    "name": "enter",
    "outputs": [],
    "type": "function"
  },
  {
    "inputs": [],
    "type": "constructor"
  }
]