We want our _generateRandomDna function to return a (semi) random uint256. 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 bytes32, Bytes or String.
keccak256("aaaab")
# returns 6e91ec6b618bb462a4a6ee5aa2cb0e9cf30f7a052bb467b0ba58b8748c00d2e5
keccak256("aaaac")
# returns b1f078126895a1424524de5321b339ab00408010b7cf0e6ed451514981e58aa9As you can see, the returned values are totally different despite only 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 Pokemon DNA, it will be good enough for our purposes.
Sometimes you need to convert between data types. Take the following example:
a: uint256 = 10
b: int128 = 10
# The following operation will throw error
# as a and b have different types.
# hence it cannot be a uint256
c: int128 = a * b
# converting a from uint256 to int128 will
# remove this error
c: int128 = convert(a, int128) * bHere we mitigated the error by converting a from uint256 to int128.
All type conversions in Vyper must be made explicitly using the built-in convert(a: atype, btype) function. Here is the list of supported type converstions.
Let's fill in the body of our _generateRandomDna function! Here's what it should do:
- Generate
keccak256hash of the_nameargument. - Convert this hash into a
uint256and assign its value to auint256variable namedrandom. - We want our DNA to only be 16 digits long (remember our
DNA_MODULUS?). Returnrandommodulus (%)DNA_MODULUS.