Name- and number-tree handling of Name and Num array "keys" which are null

[petervwyatt]

See veraPDF/veraPDF-library#1355 and pdf-association/arlington-pdf-model#90.

One of the issues in veraPDF/veraPDF-library#1355 is that the key in the name-tree is null and name tree keys are defined as "where each keyi shall be a string" and "The keys shall be sorted in lexical order". number-tree use slightly different wording for the key type ("each keyi is an integer" - "is" vs "shall") but the same wording for the sorting requirement ("The keys shall be sorted in numerical order").

So name-tree/number-tree keys cannot be null (fails "shall be a string"/"is an integer") and no allowance is made in either the lexical or numeric sorting algorithms for null keys - for example, whether they need to come first, last, or are skipped over entirely. And what would it mean if you had a null key but a non-null value??? Is that meaningful? (I note in veraPDF/veraPDF-library#1355 that both the key and value are both null).

So referring to Tables 36 and 37:

• can name-tree and number-trees have keys that are null?

I think the answer is yes (it's relatively common as @bdoubrov points out) so we need to allow null as an explicit key type for both. And name and number trees should both use the same "shall"-style wording for consistency.

• what are the requirements (if any) for the lexical and numeric sorting for keys that are null?

I think the answer is that keys that are null can appear anywhere but are ignored (not involved/skipped over) when considering lexical or numeric sorting. Not sure how to best phrase this - either we can say that only keys that are string/integer are sorted, or we say something about keys that are null...

• does a null key with a non-null value mean anything special?

I think no. It's syntactically valid but stupid as the value is "inaccessible" via a name/number tree lookup - so no real difference to a key and value both being null. This is probably worth a short informative NOTE.

[petervwyatt]

My misunderstanding about veraPDF/veraPDF-library#1355 and pdf-association/arlington-pdf-model#90. That is not related.

But my questions are still valid - can name-/number-trees have keys (even array elements 0, 2, 4, ...) that are null in Names and Nums arrays?

[bdoubrov]

I'd say no, same as for dictionaries:

• The keys in name-tree are always of string type
• The keys in number-tree are always of integer type

[petervwyatt]

And makes it very easy to then state "... and shall not be null".

[petervwyatt]

PDF TWG agree