We want our _generateRandomDna function to return a (semi) random uint. How can we accomplish this?
Ethereum has the hash function keccak256 built in, which is a version of SHA3. A hash function basically maps an input into a random 256-bit hexadecimal number. A slight change in the input will cause a large change in the hash.
It's useful for many purposes in Ethereum, but for right now we're just going to use it for pseudo-random number generation.
Also important, keccak256 expects a single parameter of type bytes. This means that we have to "pack" any parameters before calling keccak256:
Example:
//6e91ec6b618bb462a4a6ee5aa2cb0e9cf30f7a052bb467b0ba58b8748c00d2e5keccak256(abi.encodePacked("aaaab"));
//b1f078126895a1424524de5321b339ab00408010b7cf0e6ed451514981e58aa9keccak256(abi.encodePacked("aaaac"));
As you can see, the returned values are totally different despite only a 1 character change in the input.
Note: Secure random-number generation in blockchain is a very difficult problem. Our method here is insecure, but since security isn't top priority for our Zombie DNA, it will be good enough for our purposes.
Sometimes you need to convert between data types. Take the following example:
uint8 a = 5;
uint b = 6;
// throws an error because a * b returns a uint, not uint8:uint8 c = a * b;
// we have to typecast b as a uint8 to make it work:uint8 c = a * uint8(b);
In the above, a * b returns a uint, but we were trying to store it as a uint8, which could cause potential problems. By casting it as a uint8, it works and the compiler won't throw an error.