Index

Chapter 13: Bonus: Kitty Genes

Our function logic is now complete... but let's add in one bonus feature.
Let's make it so zombies made from kitties have some unique feature that shows they're cat-zombies.
To do this, we can add some special kitty code in the zombie's DNA.
If you recall from lesson 1, we're currently only using the first 12 digits of our 16 digit DNA to determine the zombie's appearance. So let's use the last 2 unused digits to handle "special" characteristics.
We'll say that cat-zombies have 99 as their last two digits of DNA (since cats have 9 lives). So in our code, we'll say if a zombie comes from a cat, then set the last two digits of DNA to 99.

If statements

If statements in Solidity look just like javascript:

function eatBLT(string sandwich) public {
  // Remember with strings, we have to compare their keccak256 hashes
  // to check equality
  if (keccak256(abi.encodePacked(sandwich)) == keccak256(abi.encodePacked("BLT"))) {
    eat();
  }
}

Put it to the test

Let's implement cat genes in our zombie code.
1. First, let's change the function definition for feedAndMultiply so it takes a 3rd argument: a string named _species

2. Next, after we calculate the new zombie's DNA, let's add an if statement comparing the keccak256 hashes of _species and the string "kitty". We can't directly pass strings to keccak256. Instead, we will pass abi.encodePacked(_species) as an argument on the left side and abi.encodePacked("kitty") as an argument on the right side.

3. Inside the if statement, we want to replace the last 2 digits of DNA with 99. One way to do this is using the logic: newDna = newDna - newDna % 100 + 99;.
Explanation: Assume newDna is 334455. Then newDna % 100 is 55, so newDna - newDna % 100 is 334400. Finally add 99 to get 334499.

4. Lastly, we need to change the function call inside feedOnKitty. When it calls feedAndMultiply, add the parameter "kitty" to the end.

pragma solidity ^0.4.25;

import "./zombiefactory.sol";

contract KittyInterface {
  function getKitty(uint256 _id) external view returns (
    bool isGestating,
    bool isReady,
    uint256 cooldownIndex,
    uint256 nextActionAt,
    uint256 siringWithId,
    uint256 birthTime,
    uint256 matronId,
    uint256 sireId,
    uint256 generation,
    uint256 genes
  );
}

contract ZombieFeeding is ZombieFactory {

  address ckAddress = 0x06012c8cf97BEaD5deAe237070F9587f8E7A266d;
  KittyInterface kittyContract = KittyInterface(ckAddress);

  // Modify function definition here:
  function feedAndMultiply(uint _zombieId, uint _targetDna, string _species) public {
    require(msg.sender == zombieToOwner[_zombieId]);
    Zombie storage myZombie = zombies[_zombieId];
    _targetDna = _targetDna % dnaModulus;
    uint newDna = (myZombie.dna + _targetDna) / 2;
    // Add an if statement here
    if (keccak256(abi.encodePacked(_species)) == keccak256(abi.encodePacked("kitty"))){
      newDna = newDna - newDna % 100 + 99;
    }
    _createZombie("NoName", newDna);
  }

  function feedOnKitty(uint _zombieId, uint _kittyId) public {
    uint kittyDna;
    (,,,,,,,,,kittyDna) = kittyContract.getKitty(_kittyId);
    // And modify function call here:
    feedAndMultiply(_zombieId, kittyDna, "kitty");
  }

}