DocumentFilter

A condition on a whole document, which limits a JsonPathQuery to the documents that meet it (JsonPathQuery.documentFilter, SPEC.md §10.5): an RFC 9535 logical expression, what a filter selector holds after its ?, evaluated once on each document with both @ and $ standing for the document's root.

@.kind == 'order' && @.version >= 2 keeps the documents whose root has those members, which a path alone can't say: RFC 9535 filters test the children of a node, never the root itself. The whole filter grammar applies: comparisons, existence tests, !, &&, ||, parentheses, queries with nested filters, and the functions of JsonPath. A comparison of a root member with a literal narrows the search through the index as the path's own filters do.

Compile once, then use it in any number of queries. Immutable and thread-safe; two filters are equal when their texts are.