Function hdk::hash::hash_entry

pub fn hash_entry<I, E>(input: I) -> Result<HoloHash<Entry>, WasmError> where
    Entry: TryFrom<I>,
    WasmError: From<E>,
    <Entry as TryFrom<I>>::Error == E, 

Hash anything that implements TryInto<Entry>.

Hashes are typed in Holochain, e.g. ActionHash and EntryHash are different and yield different bytes for a given value. This ensures correctness and allows type based dispatch in various areas of the codebase.

Usually you want to hash a value that you want to reference on the DHT with must_get_entry etc. because it represents some domain-specific data sourced externally or generated within the wasm. ActionHash hashes are always generated by the process of committing something to a local chain. Every host function that commits an entry returns the new ActionHash. The ActionHash can also be used with must_get_action etc. to retreive a specific record from the DHT rather than the oldest live record. However there is no way to generate an action hash directly from an action from inside wasm. Record values (entry+action pairs returned by must_get_action etc.) contain prehashed action structs called ActionHashed, which is composed of a ActionHash alongside the “raw” Action value. Generally the pre-hashing is more efficient than hashing actions ad-hoc as hashing always needs to be done at the database layer, so we want to re-use that as much as possible. The action hash can be extracted from the Record as record.action_hashed().as_hash().

@todo is there any use-case that can’t be satisfied by the action_hashed approach?

Anything that is annotated with #[hdk_entry( .. )] or entry_def!( .. ) implements this so is compatible automatically.

hash_entry is “dumb” in that it doesn’t check that the entry is defined, committed, on the DHT or any other validation, it simply generates the hash for the serialized representation of something in the same way that the DHT would.

It is strongly recommended that you use the hash_entry function to calculate hashes to avoid inconsistencies between hashes in the wasm guest and the host. For example, a lot of the crypto crates in rust compile to wasm so in theory could generate the hash in the guest, but there is the potential that the serialization logic could be slightly different, etc.

#[hdk_entry(id="foo")]
struct Foo;

let foo_hash = hash_entry(Foo)?;