Created using browser-solidity: Realtime Ethereum Contract Compiler and Runtime. Load this file by pasting this gists URL or ID at https://ethereum.github.io/browser-solidity/#version=soljson-v0.4.17+commit.bdeb9e52.js&optimize=undefined&gist=

group_payment.sol

pragma solidity ^0.4.0;

contract GroupPayment {
    enum Status { Open, Canceled, Finished }
    
    address owner;
    uint target;
    Status status;

    mapping(address => uint) investments;
    
    event Invested(
        address sender,
        uint value
    );
    event Finished();
    event Canceled();
    
    function GroupPayment(uint _target) public {
        require(_target > 0);
        owner = msg.sender;
        target = _target * 1 ether;
        status = Status.Open;
    }
    
    modifier onlyWhenOpen() {
        require(status == Status.Open);
        _;
    }
    
    modifier onlyWhenOpenOrCanceled() {
        require(status == Status.Open || status == Status.Canceled);
        _;
    }
    
    modifier onlyOwner() {
        require(msg.sender == owner);
        _;
    }
    
    function getTotal() public view returns (uint) {
        return this.balance;
    }
    
    function getTarget() public view returns (uint) {
        return target;
    }
    
    function cancel() public onlyOwner {
        status = Status.Canceled;
        Canceled();
    }
    
    function finish() internal {
        status = Status.Finished;
        Finished();
    }
    
    function invest() public payable onlyWhenOpen {
        investments[msg.sender] += msg.value;
        if (this.balance >= target) {
            finish();
            if (this.balance > target) {
                uint remain = this.balance - target;
                investments[msg.sender] -= remain;
                msg.sender.transfer(remain);
            }
        }
    }
    
    function takeBack() public onlyWhenOpenOrCanceled {
        var value = investments[msg.sender];
        investments[msg.sender] = 0;
        msg.sender.transfer(value);
    }
    
    function withdraw() public onlyOwner {
        require(status == Status.Finished);
        msg.sender.transfer(this.balance);
    }
}