How do you modify the bytes in a `Hash` in Parity Substrate?

Question

Given some Hash value that is generated within a substrate runtime, how do I modify or access the individual bytes of that hash?

Answer 1

The Hash trait Output has the AsRef and AsMut traits which allows you to interact with the hash as a byteslice ([u8]):

pub trait Hash: 'static + MaybeSerializeDebug + Clone + Eq + PartialEq {
    type Output: Member + MaybeSerializeDebug + AsRef<[u8]> + AsMut<[u8]>;

    // ... removed for brevity
}

Using the as_ref() or as_mut() on a hash will return a slice of bytes which you can use as normal:

For example:

// Iterate over a hash
let hash1 = <T as system::Trait>::Hashing::hash(1);
for hash_byte in hash1.as_ref().iter() {
    // ... do something
}

or

// Add one to the first byte of a hash
let mut hash2 = <T as system::Trait>::Hashing::hash(2);
hash2.as_mut()[0] += 1;