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I'd like to call a function on another contract from my own contract but do not want to sign the transaction as a user but rather let the contract sign the transaction itself.

contract A

contract EtheremonLite {
    function initMonster(string _monsterName) public;
    function battle() public returns(uint256);
    function getName(address _monsterAddress) public view returns(string);
    function getNumWins(address _monsterAddress) public view returns(uint);
    function getNumLosses(address _monsterAddress) public view returns(uint);
}

contract B

contract Monster {
    string name;
    uint win;
    uint loss;
    event EmitName(string name);

    address constant public ethermonster = 0xf3259eec5b4a46748a1f608ec3d74b89058bb3ad;
    EtheremonLite monsters_contract;


    constructor() public {
        monsters_contract = EtheremonLite(ethermonster);        
    }

    function create_moster(string monster_name) public returns (bool) {
        monsters_contract.initMonster(monster_name);
    }

    function get_monster_name() public {
        name = monsters_contract.getName(address(this));
        emit EmitName(name);
    }

    function get_last_block_hash() public view returns(uint) {
        return uint(blockhash(block.number -1));
    }

    function get_game_stats() public view returns(string) {
        win = monsters_contract.getNumWins(address(this));
        loss = monsters_contract.getNumLosses(address(this));
        return string(abi.encodePacked("win: ", win, " loss: ", loss));

    }

    function check_attack() public view returns(uint) {
        uint result;
        uint _dice = get_last_block_hash();
        uint dice = _dice / 85;
        return dice % 3;
    }

    function attack() public returns(uint256) {
        return monsters_contract.battle();   
    }
}

I want to call the attack function internally without needing to confirm the transaction using metamask.

My goal is to win everytime I call battle. To do that I need to wait until the last blockhash is divisible by 3. So need to create a function that continuously checks for the previous block and if it is divisible by 3 then call battle and win the game.

Here is the battle function on that other smart contract:

   function battle() public returns(uint256){
       address challenger = msg.sender;
       require(monsters[challenger].created  && monsters[Ogre].created);
       bool challengerWins = false;

       uint battleRatio = monsters[Ogre].weight / monsters[challenger].weight;

       uint dice = uint(blockhash(block.number - 1));

       dice = dice / 85; // Divide the dice by 85 to add obfuscation
       if (dice % battleRatio == 0) {
           monsters[challenger].wins += 1;
           monsters[Ogre].losses += 1;
           challengerWins = true;
       }
       else {
           monsters[challenger].losses += 1;
           monsters[Ogre].wins += 1;
       }
       emit battleOutcome(challenger, challengerWins);
   }
1

A contract can't just do something by itself; it needs to be invoked in a transaction. Transactions can only be sent by externally owned accounts (EOAs).

But you can write code in your contract like this:

function tryit() external {
    if (check_attack()) {
        attack();
    }
}

and then send a transaction every few seconds to call tryit(). You'll pay for a small amount of gas for each transaction, but the only time you'll call attack is when you're guaranteed to win.

|improve this answer|||||
  • Thanks for that. How does DAO work then if you contracts can't call each other without a user signing the transaction? – Cyzanfar Apr 7 '19 at 18:58
  • 1
    A user signs a transaction. :-) You'd have to look at the specific code, but I would imagine there's typically some function called execute that anyone can call when, e.g., a vote is complete, and it carries out the result of the vote. – user19510 Apr 7 '19 at 18:59
  • Makes total sense. thanks for pointing that out! – Cyzanfar Apr 7 '19 at 19:11

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