Value compaction seems to do a lot of unnecessary work

[daenney]

While I was implementing value compaction as per https://www.w3.org/TR/json-ld11-api/#value-compaction there seems to be some unnecessary work being performed.

Steps 2 and 3 are unnecessary, the inverse context is never used during value compaction. I assume this is a left-over from a previous draft of the spec?

Compact value is only ever called once during Compaction, in step 7:

If element has an @value or @id entry and the result of using the Value Compaction algorithm, passing active context, active property, and element as value is a scalar, or the term definition for active property has a type mapping of @json, return that result.

With the exception of the @json entry, the result from value compaction is always discarded if it's not a scalar. This would suggest that every step in Value Compaction where we return anything other than a scalar is not necessary. For a value that's @json, we don't want to perform IRI compaction of the keys as far as I understand it, which would also make step 11 in value compaction unnecessary. I might be wrong about this part. Since we only ever care for the scalar return, creating the copy of result in step 1 isn't necessary either.

It seems like we can simplify things by:

• In Compaction step 7:
  • First check if the type mapping is @json and return value without going through Value Compaction.
  • Then do the value compaction and return it if it's not null.
• Update the Value Compaction as follows:
  • Drop 1), 2), 3).
  • In step 8) keep the branch but do nothing.
  • Drop 11).
  • Update 12) to return null

These changes seem to work in my own processor without it causing anything in the compaction test suite to fail.