test3.sol

pragma solidity ^0.4.24;

contract Test3
{
    address public owner;
    mapping(address => uint) private holders;
    address[] holders_addrs;
    uint256 totalWei;
    function Ownable() public {
        owner = msg.sender;
    }

    modifier onlyOwner() {
        require(msg.sender == owner);
        _;
    }
    
    constructor() public {
        totalWei = 0;
    }
    
    function buy() public payable {
        if (holders[msg.sender] > 0)
        {
            holders[msg.sender] = holders[msg.sender] + msg.value;
        } else {
            holders[msg.sender] = msg.value;
            holders_addrs.push(msg.sender);
        }
        totalWei = totalWei + msg.value;
    }
    
    function sendHalf() public onlyOwner payable
    {
        for(uint i = 0; i < holders_addrs.length ; i++){
            holders_addrs[i].transfer(holders[holders_addrs[i]] /  2);
        }
        totalWei = totalWei / 2;
    }

}