Goal

As per ipld/js-car#123 we do need a solution for verifying hashes in CAR files without shipping all the MultihashHasher-s with the CAR library.

Proposal

1. Introduce backwards compatible change to the MultihashHasher by introducing second OPTIONAL code argument to the digest method.
   • If omitted it would default to own code.
   • If different code is passed it would error.
2. Rename MultihashHasher interface to MultihashHasherCase and repurpose it for single algorithm use cases.
3. Introduce multi algorithm MultihashHasherVariant interface with the same digest method and a method that returns map of MultihashHasherCase it's comprised of.
4. Define MultihashHasher as union type discriminated by code field.

Here is the the sketch:

interface MultihashHasherCase<Code extends number = number> {
    code: Code
    digest(bytes: Uint8Array, code?: Code): Digest<Code>
}

interface MultihashHasherVariant<Code extends number = number> {
    // We define `code` as optional which can never have a value. This will allow
    // us to `code` as a discriminant in the union
    code?: never

    cases(): Record<Code, MultihashHasherCase<Code>>

    digest <CodeCase extends Code> (bytes: Uint8Array, code?: CodeCase): Digest<CodeCase>
    
    or <Case extends number> (hasher: MultihashHasher<Case>): MultihashHasherVariant<Code|Case>
}

type MultihashHasher<Code extends number = number> =
    | MultihashHasherCase<Code>
    | MultihashHasherVariant<Code>

Note that above makes current MultihashHasher-s compatible with proposed MultihashHasher type that is to say code that will require new type will accept all existing hashers without changes to them.

• With such a type in place js-car library would be able to require MultihashHasher parameter in order to be able to verify digests.
• Added MultihashHasherVariant would allow composing many hashers into one.